FT-757 (GX or GXII) page (service notes) (Dec 12-2019)

 

there are plenty of tips for Anyone who is still working with and on FT757 type radios. Thanks to the ham in Ontario who submitted this information.

 

This ham in Southern Ontario did pass me some very useful information and Service NOTES. Here below they are written out for those hams who still service radios.


Any comments can be sent to me (I am not the author of the article (however)) .. earl@nettyelectronics.com

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FT-757GX Service Notes
~~~~~~~~~~~~~~~~~~~~~~

last update Feb 21, 2021

Some folks call this the ‘GX I, but there is no official model 1 of any
thing. Its just a FT-757GX.

We’ll start with a quick review.

The radio is noted to be ground breaking at the time as having every
option included – keyer, metering, IF Shift/Width, calibrator, all mode
and full general coverage too. The TX Finals are near bullet proof.
Metering is done by a real meter also.

The downside is that it is quite compact and is a challenge to fix in
some ways. There are no parts to speak of and while many are being parted
out on eBay these parts too are 30 years old. Overall this isn’t too bad,
but parts that wear like shaft encoder are unavailable and any from a
parted out radio are likely to be worn also.

In the author’s opinion, the biggest problem with this radio is the “Local
Unit” otherwise known as the synthesizer. Its noisy and therefore reduces
the receiver performance some. It does not render the radio useless;
its just that the FT-757GX is readily beaten by more modern HF radio
examples.

No one makes a radio that looks like this any more. Modern radios are all
SMD these days and not easily serviced at home. Neither is this radio
cost effective to send out for repair. Its not worth much and any real
repair that requires a board to be pulled to replace a part will exceed
in labor at $70/hr what the radio is worth.

For the few hams left that are capable this radio can be serviced at home.
Its still a good performer for general use and to those Radio Amateurs
that regard this as a hobby, its a good radio for mobile, cottage or
portable. In other words, a perfect General Use radio.

Authors Note
~~~~~~~~~~~~

To date I have received two pieces of feed back. This piece has been up
for two years.

One was from a fellow ham in W6. Thank you David H for your suggestions
to include the signal generator and the emphasis regarding the -8V supply.

The other was from a CB’r and technician berating me for my sarcasm
towards those that CB. He considers the modern CB person much more skilled
than I allude to. He specifically asked me to “tone it down”.

NO way.

First my opinion of those that CB is…. well, I don’t have one. I’ve
never bothered with it personally and to me it comes across as a cult. I
don’t mean that negatively. The folks that engage in CB seem to me to be
more interested in results than radio as a hobby.

Last and in specic relation to the FT-757GX I’ve bought a few of them in
what might be referred to as “anonymous sources”. Read: eBay and flea
markets. In almost EVERY case there are traces of a CB history. The
“magic switch” has been thrown, the radio reeks of stale ciggies, the
knobs are yellow and there are streaks of goo running down the sides.
The ALC is backed off and anything that looks like a “drive” adjustmet
is fully clock wise. Any soldering is done in blobs. In one case the
radio arrived and displayed “27165.0” upon first power up. It “swung”
close to 150 watts with modj and, as expected, sounded like crap.

So no. I am not going to edit out one word.

Thank God for CLR.

Last and for the record Netty Electronics is just hosting this article.
It is NOT authored by Netty Electronics.

A Used FT-757GX
~~~~~~~~~~~~~~~

The problem with 30 year old HF radios is that they can have 25 years of
neglect. The first 5 years of a radios life is where it is respected and
has resale or trade in value. After that it appears to the market as a
“back up radio” and after that it starts to acquire the status of “beater”.
People stop to care anymore.

The FT-757GX is a perfect CB radio. It can be made to go on 27 MHz with
the flick of a switch… literally. Now all the attributes that made this
radio attractive at the Dealer in 1983 apply to the CB crowd in Year 2000.
The CB crowd has little to no radio knowledge other than mutual support
and that is often driven by rumor, myth and innuendo. Radios get modified
for more “swing”, punch, modj or horror – RF output (a modified FT757 will
“swing” 150 watts). These folks have little skills to do repair and will
crack a radio covers at the drop of a hat and proceed to modify a radio
with nothing more than vice grips and a 300 watt Black Beauty iron.

NO wonder 27 MHz sounds so horrible.

Simply put – if you are looking for an FT757 be very careful of where it
can come from.

The FT757 has a row of adjustment pots along the rear panel. The knobs
are often missing and the knobs are unobtainium.

The original model did not have that many factory changes although there
are a few. In particular Yaesu improved the CW QSK that was incorporated
by Series 5 and some Series 4.

The shaft encoder being a mechanical device that has been in constant use
for 35 years will show various extremes of wear. Usually the tuning knob
will wobble “some” without ill effect. In extreme cases the radio will
frequency jump. Don’t call Yaesu – they don’t have any.

The most common fault with these radios is the failure of the front end
PIN diodes. Usually they will be found shorted. Failure of these diodes
(depending on which ones, of course) often is illustrated by the FM RX
being dead or the Squelch control acting “funny”.

The S Meter is lit by 2 wired in bulbs. The illumination is inadequate
even on a new radio. Today all of them have burnt out at least once and
what they get replaced with can be very inventive. No bulbs are available
from Yaesu. Their orientation is critical for proper illumination. But
bulbs – reasonable replacements – ARE available.

The top cover is the Final heat sink. It is large, heavy and has a thermo
controlled blower. It also has gaps that allow small items to enter.
Thats often where I find the missing screws.

This is a 30+ year old radio and around now the electrolytic are starting
to dry out. Unlike the TS-50 these capacitors are not leaking out, they’re
just drying out. Relatively speaking they are easy to replace and not
worth troubleshooting beyond the first one that goes. Modern capacitors
are smaller and have a lower ESR. Once the board is out, may as well have
at as many as you can.

The FT757 has a memory back up battery. Chances are its near dead if it
original or possibly replaced by some interesting methods involving a small
soldering iron. I’ve also seen original memory batteries still soldiering
on and providing back up sitting at 2.65 volts.

The above is not in any way to represent Service Notes, but rather is a
short list of what are the nuances of a used FT757GX.

WHY the FT-757GX?
~~~~~~~~~~~~~~~~~

WHY would anyone in their right mind laud the virtues of the FT-757GX? It
is a 35 year old radio. Just about anything – even the cheapest dealer HF
radio – will eat it.

First… its true. I have an FT-891 here and a for $600 USD radio the
FT-891 will leave it in the dust functionally. I would hope so.

BUT…. consider this –

– with the exception of DATA, the 757 does all the modes an 891 does

– real knobs

– no menus

– a real S Meter and metering

– the FT-757 has a keyer

– speach processor

– RX IF Width and Shift

– a really nice illuminescent display, but only to 100 Hz

– built like a tank. The PA transistors are WAY over rated for 100W.

– carrying handle

– through hole parts. You CAN fix it yourself

Note that no where am I saying the FT-757GX is a better radio. What I
have listed here are the high lights of what makes it such a fun radio
to have around.

Thats all – just fun.

Absolutely, there is a downside too which I won’t list. Its an OLD radio.
There is NO dealer support, parts, or in many cases folks that will fix
this radio on a cost effective basis.

Note also it is an HF radio. Signals are usually quite strong and over
95% of the places you would use this radio will be limited by the back
ground noise anyway. If you get your radio – any radio – and connect an
antenna to it and the noise goes up in the speaker you have all the
sensitivity you can use. Are there some radios where this doesn’t happen?
You bet. A Heathkit HR-10 is stone deaf on 15M.

An FT-757GX will pass this test easy…. on any band and without the pre
amp. The pre amp is useless anyway. Details to follow as to why.

To be honest, I would not have the FT-757GX as my only radio. It is though
one of my radios that isn’t going any where. Its just too much damn fun.

The FT-757GX II
~~~~~~~~~~~~~~~

Obviously this radio followed the original model. It looks quite similar
and is exactly the same size. It shares all of the above cautions and
features of the original model, including part numbers.

Its main difference is the ‘II does not have Shift/Width. Instead, this
model has IF Shift and IF Notch. The latter is completely manual.

Aside from that, I think one of the major changes with the second model
was the improved computer control. Its still crude and don’t forget that
“Computer Control” meant DOS and at best a 286. This radio was 10 years
before any version of Windows.

In the authors opinion the original version with IF Shift/Width is much
more useful. To each their own.

Your “new” FT-757GX
~~~~~~~~~~~~~~~~~~~

You bought this used radio at a flea market or perhaps on eBay. Here you
are in the late afternoon and you can’t wait to power this puppy up. It
really doesn’t matter just what it is. It could be a Drake or a Kenwood.
The seller told you on his mother’s grave that this was his back up radio
and was only used lightly.

And you believed him… or you wanted to.

There is nothing wrong with what you are about to do – that is power it
up and play radio. But if you think for a moment this is where it stops
you are in for a surprise. What you have at this moment is an orphan and
guess what you have adopted? It has the status of “stray” and right now
your new acquisition needs to be vetted.

First thing to do is make notes. You need to go over the radio physically
and electrically. Read the manual. If you didn’t get one then down load
it from the Web. You need to play with it either on an antenna or into
a dummy load. As you do, make notes of what works and what doesn’t. This
should be good for about 2 hours. When you are done, you have a base line
of where to start to get this puppy back on line.

Unhook the radio and CLEAN it. Pull the knobs and get rid of the grime in
the knob grip areas. Clean the controls. Do what you can to get rid of
30 years of ciggy and cigar smoke. Make note of all the stripped cabinet
screws. Check it for power out and transmit range. Specifically the
FT-757GX is a prime candidate for open transmit and transmit output in
excess of 100W.

Issues and Repair
~~~~~~~~~~~~~~~~~

Certain traits and faults are unique to the FT757. Specifically this
radio used steering diodes to switch signals, audio and the front end
BPF. It is these diodes and specifically the BPF where a lot of strange
operation from diode failure can take place.

The diodes in the BPF are severely under rated and prone to failure in a
strong signal environment such as Field Day. Their failure is often as a
short, and its this that leads to some interesting failure modes in the
radio itself. The good news is that a common 1N4148 is a good replacement.
These are NOT PIN diodes, but they serve well and provide adequate
isolation and low loss. This is after all HF. The bad news is I’ve seen
Yaesu use Schottky diodes in some production runs and you should not
mix them. 1N4148 have a forward voltage drop of about .65V and the
Schottky diodes are around .27 V. A simple VOM reading will tell you
which is used. In the case of the Schottky diode, use 1N5711. Now it
could be that I have encountered a radio thats been badly repaired, but
I doubt it.

This only seems to be in the radio’s front end. Similar diodes are used
in many other places in the radio but their failure rate is much less.

The specifics of the above will be addressed in further detail.

As you go through your FT-757GX you may encounter faults that are
difficult to run down completely. You get enough fault info but not quite
enough to nail it. This is the dangerous area of “repair”. You run through
scenarios in your head and say “Yup – its gotta be….”. This is where you
reach that Y in the road. It leads to fragging the radio or making things
worse. That is, finding the REAL fault more difficult.

You NEVER fix anything by modification. Thats both philosophical and
“Golden Rule”. As part of level setting the radio to nominal if your new
to you FT-757GX has had “mods” you need to figure out why they are there
and then get them out. There are very few mods out there that will make
an FT-757GX better.

Many folks have the knowledge to address repair at some level or another
but lack the equipment to do so. You can address a lot of repair using the
Marker and a good voltmeter. NEVER attempt alignment using signals from
the antenna. It simply does not work.

Specific FT757GX Failures
~~~~~~~~~~~~~~~~~~~~~~~~~

1) Service manual errors

2) Intermittent

3) Disassembly

4) No FM; FM permanently squelched.

5) Drift

6) Memory battery

7) Low RF output on some bands

8) Poor or no receive on some bands

9) PLL alignment

10) Pre Amplifier Gain

11) Attenuator

12) CPU Reset

13) Reverse Voltage

14) Display Problems

15) Meter Lighting

16) Power On Problems

17) Pulling the Circuit Boards

18) Final Amplifier and the “MARS Mod”

19) Capacitors

20) Cooling Fan Grinding

21) Top Cover won’t fit

22) Radio Dead

23) RX Alignment Check

1) Service Manual Errors

Setting the CW BFO offset crystal. The manual wants you to set the CW
trimmer TC2004 to 8.215.900. Ignore this adjustment. TC2004 is for
LSB. The actual CW adjustment is covered under Point I in the Service
Manual.

The manual tells you to adjust VR2006 for 100W out. On some RF Boards
its marked as VR05. Its near VR07. Until I figured this out I spent
a lot of time looking for VR2006. There isn’t one on some circuit
boards.

Some.

2) Intermittent

This is quite common in many old radios. In the case of the FT757 it
is often caused by oxidized pin headers. This is especially evident if
the display sometimes goes weird on you. The fix is simple and this
should be a common practice for every flea market and eBay refugee.

CLEAN THE RADIO

You need to do this as soon as the radio is in your hot little hands.

You need to pull out every pin header, hit it with some deOxit and
reseat. EVERY pin header. It will take you an afternoon. There are a lot
of screws to undo – especially to get to the PA deck.

Pull the knobs and get them into some warm water and dish detergent.

Windex, or better…. Invisible Glass.

I once had an FT757 that must have been in a barn with a heavy cigarette
smoker. This thing stank and was covered in brown dusty goo. The dial
skirt had smears of yellowy brown guck. The heat sink channels were
filled with a thick layer of brown gunk. There was only one thing I knew
of that would touch this…. CLR. I spent hours on it. I powered it up
the moment I took it out of the delivery box and knew immediately what I
was in for when the display read 27.165.0. It was advertised on eBay as
“works well but the display is intermittent”. I needed rubber gloves to
tune it.

The display problem was fixed with deOxit.

CLEAN THE RADIO

3) Disassembly

Improper disassembly can ruin the radio.

The top cover is heavy and held in place with just 4 – 3mm screws.
Attached to this top cover are many small wires and a few coax cable
runs.

The bottom cover screw removal is straight forward, but the bottom
cover itself will not come off without some GENTLE expansion.

The top cover will allow itself to be flipped over the back, but there
are a few wires and cables that are attached. They are completely
unforgiving if the top cover ever gets away on you. There is a large
cable harness tucked into the rear chassis joined to the top cover,
plus a number of wires connected to the back of the front panel. If you
have the bottom cover removed, the top cover will flip out on to a large
book about 1/2″ thick nicely. From this point on you need to move the top
cover and the rest of the radio as one unit.

The front panel is on a short rail. It is held in place by 2 screws on
either side of the main chassis. Once loosened and NOT removed, the
front panel may be slid forward about 3/4″. Don’t forget to undo the
ring around the Key Jack. If its missing, some one has been dancing
on the RF Deck.

When you go to put the radio back together note the large exposed harness
at the back of the radio coming out of the PA Deck. You will not be able
to get the top cover back on unless this harness is tucked away properly.
If the top cover will not settle such that you can screw it down without
force, then you need to have a look and see whats going on and why.

A SPECIAL NOTE ON THE COAX CABLES

NEVER remove a chassis mounted coax cable by pulling on the cable. EVER.

The coax cable is just press fitted into the little connector at the end.
Remove the cable from its socket by pulling on the “ears” that are
actually part of initial assembly crimp. Once you rip the connector off,
you’ll never get it back on.

There is an aluminum cover over the PA unit. Note that there are THREE
countersunk 3 mm screws. Two are in the center near the front and one is
at the back in the corner right on the edge. If the top cover will not
seat properly when you go to put the radio back together, you have a
normal screw where there should be the counter sunk screw, most likely.

4) No FM RX

This is VERY common to this model radio and usually is indicative of
other faults too such as poor receive on some bands. Measure Pin 15
on the MC3357 on the IF board while in receive. It should be near zero.
Chances are its high – perhaps 5 volts. DO NOT adjust the squelch level
pots. The shorted PIN diode is squelching the FM IC and providing some
low level TX voltage on RX to really confuse things.

You will likely be drawn to believe that Q68 the TX 8V switch is leaky.
Chances are its fine. The fault is a shorted PIN diode in the BPF. There
are a number that it could be, but do not despair. The fix will take some
time but it will not be expensive.

Proceed to Point 8) for PIN diode detail.

The squelch circuit for FM does not use the same method for the IC that
the manufacturer documents. There is an adjustment for the squelch
control, and there is also a master trim pot for recovered audio. The
radio has an all mode squelch thats AGC controlled, more or less. The
same for the scanning function as it too is driven from the AGC.

5) Drift

Its usually caused by the 15 MHz master oscillator. Part of the fix
for this involves changing the PLL crystals. Yaesu has no stock on
these items. You are reduced to changing the trimmers for ceramic
models. Mouser is your best choice and sometimes Dan’s. You want the
2 pin 6 mm variety. Yaesu no longer has the trimmers available.

There have been reports of these radios drifting, but I have yet to
experience it despite a number of FT-757GX that I have had.

6) Memory Battery

The radio is very inconvenient to use without a memory back up. Upon
powering up it will default to 7.000.0 MHz if the battery has gone South.
Every time.

There is a trick repair on the Web that allows for its replacement
without pulling the Local Unit. On the other hand while the original
memory battery is available and expensive, there is a common Lithium
battery that you can use in its place if you are not afraid of doing
a proper repair and pulling the Local Unit.

A near drop in replacement is a CR1220 Lithium battery. It comes with
solder tabs. Its possible, at least in theory, to snap the tabs off the
existing dead battery and solder the new battery to the stubs. Chances
are you’ll melt the solder on the circuit board but it might be worth a
shot.

7) Low RF Out On Some Bands

This is similar to 8) that follows but the symptoms are that the receive
is OK but the TX output is down either on a band basis or every where.
Once again, depending on what diodes are done the FM mode on RX could be
permanently squelched.

More or less, if a particular band is good on RX, it should be good on
TX. That is the BPF is common on both RX and TX. This is the circuit
block that has all the coils and is diode switched. The diodes switch
the appropriate BPF, but additional diodes steer or connect the selected
BPF between RX and TX.

Before attempting to fix low or no TX on a particular band you should
determine the health of the RX. Fix the RX first. If you determine that
the RX is fine, then your potential diode switching problem is limited
and much easier to find.

Part of evaluating the RX is seeing if the RX is squelched on FM. Does
the pre amp work?  If the RX is squelched on FM, quite likely one or more
diodes in the BPF and area are bad…. usually in a shorted condition.

If a particular band is dead on RX and TX then there is a good chance
that the BPF selector diode(s) are gone plus the BPF input coil. Not only
does the coil tune, it is also a DC path for the select diodes such as
D07 and D08. In this case L04 and L08.

You can use an ohm meter to test the coils. In most cases the diode
will show a short. That is instantly a bad diode but it can also be in
an open state. The diodes in question are 1SS83 for the BPF rated for
300 Vrev and 1SS53 elsewhere rated for 25 Vrev. Don’t let these ratings
fool you. Either diode has been found in a failed state in this radio.

8) Poor Or No Receive

In situations where the receiver is just simply dead, check the lamp fuse
on the RF Board. It can be replaced by just about any bulb of the same
size. Its not critical. Either you were in a strong RF environment, or
you used the transceiver for a dummy load or perhaps the bulb just died
from old age – who knows why it opened. Unconfirmed specs on the bulb
are 8V at 100 mA.

But before you put the covers back on, check the radio out. It could be
that whatever took the bulb out could have also cleaned out some of the
front end PIN diodes. Also check that you have FM RX and it squelches
properly.

If you find a band thats dead, but its diodes are good then you need to
ohm out the BPF coils, for they are the ones that complete the diode
conduction path. For instance for the 40M Band diodes D11 and D12 switch
the BPF array for 40M. Coils L14 and L15 not only are tuned to 7 MHz,
but they are the conducting path for the diodes.

Diodes D112 and D113 protect the RX during transmit. If the bulb is open,
these diodes are immediately suspect.

Diodes D03, D04 and D06 form signal routing to and from the diode
switched BPF. If the bulb is open, D03 and D04 could be bad. Consequential
damage could have D06 compromised and therefore have weak RX and TX
exhibited.

I have never seen D01 fail, but if it has you’ll need to replace it to
find out if Q01 and Q02 have been taken out.

Some of these diodes, in particular D112 and D113 may be located on the
bottom of the board. You’ll need to pull the board to get access. You
may see a blank component at the marked location on the circuit board.
If you need to pull the board, you may as well make the pre amp mod.

Before you start replacing PIN diodes you’ll need to see if they are
just silicon diodes or Schottky. If you encounter the latter you need
to use 1N5711 for PIN replacement diodes. Mixing them up may turn out
bad as the usual silicon diodes have a higher voltage drop and may stop
the others from conducting.

8) Part Two

The FT-757GX uses an MC3357 as the NBFM receiver. Referencing the Motorola
Data Sheet will give you a good idea on how the chip works, but Yaesu
does not use this chip exactly as per the Data Sheet. Yaesu uses Pin 15
to mute the IC. Almost ANY voltage on the pin will mute it. This is
likely happening if there is no white noise in FM RX with the squelch
open. Its likely happening because the TX 8V line has some voltage on it.

Measure Pin 15 of the FM chip in RX. It should be zero or even slightly
negative. If its more than a volt, the chip is being muted, most likely.
The “usual” cause is a bad diode (or even diodes) in the BPF. In some
cases its an “age thing” and I’ve fixed it by putting a 680 ohm 1/4 W
resistor. The TX 8V line uses little current and this resistor will force
an 11 mA load on the line, hauling any “leakage” to ground.

9) PLL Alignment

Is more or less straight forward from the Service manual. If there is a
crystal that won’t align, replace its trimmer with a 7mm ceramic. It DID
adjust at one time, so its either the crystal or the trimmer, most likely.
May as well blame the trimmer as replacement crystals are not to be had
any way.

The 15 MHz crystal frequency adjustment is critical for the display to be
accurate.

The IF Shift and Width controls must be centered. That is the lines on the
knobs must be pointing straight up, and this is only possible if the knobs
are positioned properly on the control shafts.

10) Pre Amplifier Gain

The pre amplifier has far too much gain to be useful. I question its
value to begin with since all it appears to do is just increase the back
ground noise. This is not the fault of the amplifier but rather is a
fault of the synthesizer.

You can reduce the gain of the pre amplifier and improve its AGC action
by removing R22 and installing a 1N4148 in its place. Cathode to ground
(bar end).

11) Attenuator

The attenuator has a 20 dB loss, which in use to me is excessive. Change
R04 to 220 ohms for a 10 dB loss or 360 ohms for about 15 dB. 360 ohms is
not common but is a common value in 5% tolerance. Feel free to use a 330
or 390 instead.

The board in question is the TX LPF Board.

12) CPU Reset

Sometimes after repair the CPU will lock up. Sometimes you might want to
clear memories.

Turn the radio off. Push in both black square buttons on the rear panel
(linear and Marker). Turn the radio on. Turn the radio off. Reset both
buttons to their out position.

This DOES NOT APPLY to the FT757GX II.

13) Reverse Voltage

Many Yaesu HF radios of this vintage have a master relay thats engaged
when power is applied. The power on relay is on the RF deck and has a
steering diode in series with the relay coil. The radio simply will not
power up with reverse voltage applied.

No damage to this radio with reverse voltage.

14) Display Problems

The radio uses a custom display electro luminescent vacuum tube. I have
found the tube itself to be robust and encountering one that is dim by
it being at fault is rare. More often than not its a fault of the voltage
inverter and that is usually a fault of the inverter or the filter caps
for the negative supply.

The radio does have a negative 8V supply line driven by the display
inverter. Failure to have this -8V line is the responsibility of many
operational problems but is most likely observed in a weak display. The
inverter is unlikely intermittent, Dealing with the display and the
inverter is a bitch as it involves mucking with the front panel.

A lot of display problems are resolved simply by cleaning the radio.

I have had intermittent displays. In this case it was caused by the
solder failing on a pin header socket on the synthesizer. The wiring at
the front of the synthesizer board can get compressed by the top cover
and over time the pressure fatigues the solder joints on the PIN header.

15) Meter Lighting

Illumination of the front panel meter is poor. Its always been poor. It
was poor when the radio was new. Now, 35 years later it probably isn’t
any better, if not worse.

The radio originally had these bulbs in little blue booties that were
press fit into the clear plastic ears on either side of the meter. As
the years have gone by there have been many inventive ways of mounting
new bulbs. I have tried clear LEDs and the success is at best moderate.

It seems that there is no substitute for a real light bulb in this
application. Yaesu no longer stocks these bulbs. Grain of Wheat bulbs
will not fit properly. One needs to be careful to use a low wattage
bulb as the heat from the bulb will do damage to the plastic mounting
receptacle. A 12V bulb at 40 mA looks to be ideal and a good place to
start is a 2174 from Bulb Town. Its a T1 3/4 size bulb – 5.5 mm dia
by 8.6 mm long. Something like .22″ by .35″.

The meter lights up from the bottom and is never very bright.

16) Power On Problems

The radio will not power on with the MOX button pushed in. There is no
indication that anything is amiss, and you are lead to believe that the
power on relay or its switching logic has failed.

The master power on switch is not directly involved with powering the
radio on. That is done through a fairly sophisticated transistor switch,
10A relay and some TTL logic. The switching transistor is a Darlington.
The whole thing is steered through a silicon diode that prevents the
relay from closing with reverse voltage.

Both the MOX switch and reverse power appear the same – the radio will
not power up. If it won’t power up, it can’t be damaged. This may not
be the case in a mobile installation as likely the radio is grounded
to the chassis. Some mobile installations can fail in a spectacular
fashion depending on how the power is routed. A few scenarios can be
prevented by NOT connecting the DC cable negative to the negative
battery terminal.

A properly working FT-757GX will cycle the fan upon power up. The fan
will spin for maybe 1/2 second and then stop. Likely you will only be
aware of this with the radio opened up for service.

The 4 pin power connector uses all the pins. There is the usual 2 pins
allocated for 13 VDC power, but the 2 other pins, when shorted, allow the
final amplifier to function. This power connector was used in other Yaesu
of this vintage such as FT-107, FT-747 and FT-707. Its known as a Cinch
Jones S-304.

17) Pulling the Circuit Boards

The first time I had to do this I thought that I’d never get it back
together in a working state. It was a simple problem – the AGC capacitor
had opened up. All AGC was FAST. I found out that it really wasn’t THAT
bad to do.

Since then I have had the Local Unit out so many times and back in that
I can almost do it blind folded. Its on this board that the memory back
up battery resides. I have seen the Youtube on how to deal with this
without pulling the Local Unit, as if having to do this was something to
dread. Today, I’d pull that board without hesitation and have a new
battery in there quickly. No mercy. Pull the board. Replace the battery
with a CR1220. Do a proper job.

To get at any of the 757 boards you need to at least pull the bottom
cover. That means 8 – 3mm appearance screws, 2 screws for the side feet
and 2 screws for the handle. When I pull a radio apart I have multiple
bins for the hardware. This would all go into its own bin and I’d start
a new bin for the next “layer” of dismantling. I use old pill bottles.

If you just want access to the RF Unit you only need to undo the four
screws at the top cover/ heat sink. Once you do that, you need to be
very careful in handling the radio and heat sink. They need to move
together.

To access the RF Unit and Local Unit you need to loosen the 4 screws on
either side of the front panel such that you can slide the front panel
out about 3/4″. This will give you room to extract the RF Unit Circuit
Board. Before you undo any of the circuit board screws you need to gently
pull all the pin headers and the coax cables. Note the previous caution
on dealing with the coax cables.

To gain access to the Local Unit circuit board you need to unscrew all of
the above, plus loosen the top cover/ heat sink and fold it back. Next,
remove the shield and then uncouple all pin headers and coax connectors.
Remove the 3 pin header from the voltage regulator bolted on the side.
Near that IC on the circuit board are 2 – 5 pin headers. Mark one of
them so they don’t get swapped. Note that some wires were above the
shield. Remove the 4 small knobs at the back and undo the ring around
the KEY jack. Slide the board out by moving it back towards the front
panel and over the side with the regulator. You may need to push in the
two switches to get clearance from the rear panel.

To gain access to the PA unit and the LPF you need to do all the
preceding, fold the top cover back and undo all the screws holding the
shield in place. Its obvious how it is removed, but be VERY careful of
all the wires and coax cables surfacing out from the cut outs. Note
which screws came from where – they are not all the same.

The front panel is removed by removing the 2 screws on each side of the
chassis and removing all the cable headers. The main tuning knob is a
slip on – it just pulls off. Behind the main tuning knob is the knob
drag mechanism.

The keyer sub board on the top cover is just screwed in. Be careful not
to loose the 3 slide knobs.

18) Final Amplifier and the “MARS Mod”

The PA transistors are 2SC2290 and are capable of more output than the
radio’s rated 100 watts. Actually, through miss adjustment the radio is
capable of almost 150 watts. That doesn’t mean you should do this. That
does not mean the transmitter will be clean at this power and it also
means that you have lost any cushion against a mis matched antenna.

The radio is also dead simple to do the “MARS Mod”. Its just a simple
matter of throwing a switch. It completely baffles me why you would want
to do this. EVERY FT-757 I’ve encountered has had the switch thrown for
open transmit. One of them came to me in the mail and when I fired it
up for the first time the display read 27.165.0. Uh huh. It also looked
like it came from an Opium Den and smelled like it too.

Its a 100 watt radio. Keep it that way. Its also a Ham Band transceiver.
Thats where the transmit should be used. Only there.

Get it?

Oh yes. I have a few of these radios. All of them now have the open
transmit disabled. I cannot think of any reason why a Ham would open up
the transmit on a transceiver other than “Angst Against the Machine”.

There is a control on the back of the radio without a knob. Its the “AM
Carrier” control and usually you will find it fully clockwise. Like many
SSB transceivers the FT-757 does AM TX by injecting a carrier on top of
a normal SSB transmit. From the Balanced Modulator to the SO239 the radio
is a linear amplifier for low level generated AM. The absolute maximum
carrier the radio can support is 40 watts and still stay linear. I set
this control to 25 watts carrier.

Part B – an exception.

You can use the FT-757GX as a signal generator.

The 4 Pin power connector uses just 2 pins for power. The other 2 pins
are jumpered and provide 12V to the final transistors. If this jumper is
missing there is no “transmit” but there is output on the LL Out phono
jack on the back of the radio.

You can “make” a signal generator by throwing the “MARS Mod Switch” AND
disconnecting the jumper on the poiwer plug. Better still buy a Cinch
connector and make up your own “RF Generator Power Plug” for your
FT-757GX.

The output from this phono connector should NEVER be applied to the input
of a receiver. Its documented as 100 mV P/P. I would also NOT trust the
frequency accuracy.

18) Shaft Encoder and Parts Availability

A good assumption with an FT-757GX is that there are no factory parts.
Thats not literally true in some cases like the power on relay or the
meter light blue booties, but the real meat and potatoes parts like the
shaft encoder and Local Unit crystals are long gone.

The good news is that yaesu Service Manuals are on line, and in some
cases a newer model uses much the same parts and they ARE available.
I have yet to test it, but my gut feeling is that a shaft encoder
from an FT-840 or FT-890 might work just fine for instance. I haven’t
tested this (yet).

In quite a few cases a later radio uses the same part under a different
part number.

And last – there is always a junker around from which to raid. In the
case of the FT-757GX the production run was quite stable. The shield
over the Local Unit changed over time but that is nothing compared to
the sea of board changes the FT-107 went through. You are pretty safe
buying a “working pull” from just about any FT-757GX. In extreme cases
you might loose or get a factory QSK mod, but thats about it.

Most of the transistors used are “nothing special”.Netty Electronics may
have some in stock. The MOSFET transistors in the RX IF and the 2SK125
in the balanced mixers are available and rarely fail.

The major exception to all this is the 2SA1193. These are NPN Darlington
transistors used in the TR switching and in the power on realy. They
are unobtainium. I’ve had success using MPSA13. NOTE that this is not
a pin for pin replacement. The basings are different. Fortunately these
transistors rarely fail unless the 8V line is shorted to ground.

19) Capacitors

At one point I had 2 working 757, one beater and one in an undetermined
state. I took the last one off the shelf and fired it up. It didn’t play
all that well. To get a comfortable volume I had to advance the control
further than I should have to and the audio that came out the speaker
was kind of muffled and wooly. This “sounds” like capacitors to me, so I
decided to do an experiment. I’d pull the RF Deck and change all the
electrolytic capacitors I could get my hands on and then see what the
results are.

Modern low voltage electrolytic are MUCH smaller than the ones of 30
years ago. In addition Yaesu used a lot of electrolytic rated for 50
and 25 volts. This resulted in them being huge. Most of my replacements
are rated for 16V which is perfectly adequate.

I didn’t bother with the 10 uF around the LPF area but instead
concentrated on “audio”. This included the 470 uF for speaker coupling
and as many of the 22 uF and 1 uF in the audio section around the Speaker
IC and phono jacks as I could get to. I also needed some 100 uF, 4.7 and
10 uF caps to complete the task. In total, I probably changed close to
20 electrolytics on the board.

I put the board back in the radio and applied power. The difference was
dramatic. The audio is bright and crisp and there is plenty of it. So this
is what a new FT-757GX sounds like?

W0W!

I confess that I had already done this to an FT-107 last year and the
results were similar, but not as dramatic. On this particular 757 I
would say the results were “Night and Day”.

20) Cooling Fan Grinding

The cooling fan sits in brackets and they are mounted on one end to the
PA heat sink and to the other – the thin aluminum cover. The tolerance to
the bottom of the heat sink to the fan disk isn’t much, and the fan must
hover over heatsink near perfectly parallel to it. You will know
immediately of trouble in this area upon power up. When the radio is
powered up the fan will rotate for a few seconds. The radio should power
up dead quiet with the volume turned down.

If it doesn’t you’ll need to flip the PA and its heat sink out and remove
the aluminum cover. Its there you will have the fan exposed. One end of
the bracket will be bolted to the heat sink and the other will be in the
air. Spin the fan with a small screw driver or power up the radio. The
fan should turn freely. If it is binding you need to gently bend the
bracket. Inspect the cover and make sure it isn’t dented and that it lies
flat on the table top.

21) Heat Sink Won’t Fit back On the Chassis

You have one or both of the following situations….

You took the PA cover off and you didn’t put the screws back in where
they came from. The top cover has 3 counter sunk 3mm machine screws
and they sit in a row along the back from the RHS. More than likely
you installed a 3mm cap head screw along this line.

The wiring harness isn’t properly tucked away at the back of the shield.
Its either sitting on the shield or you have a wire sticking out and
you’ve jammed the top cover over it.

22) Radio Dead

As previously noted you cannot kill this radio by applying reverse power.
No doubt this is one of the reasons for an FT-757GX surviving all these
years.

Make sure the MOX switch is not engaged when you apply power. This is yet
another smart think on Yaesu’s part. The radio simply will not power up
with the MOX switch engaged. This was mentioned in 17) above.

If you hear the power on relay close check that the cooling fan spins and
then stops. This is a sign of *some* life, at least. Does the meter back
light illuminate? If you back off the RFG does the S Meter advance? This
does not mean that the RX is healthy; it just means its not completely
dead.

Nothing will happen until that power relay closes. It is closed by a
transistor switch and a series voltage steering diode. You need to get this
relay to close before anything happens.

If you get the relay click and the cooling fan to spin but the radio is
still dead…. the next thing to check is the inverter for the display.
Its located on the back side of the front panel. In addition to powering
the display tube, the inverter provides -8V for the radio. You can even
have a working display and a dead radio. Basically the FT-757GX low level
signals are powered by an 8V LM7808 regulator. The AGC amplifier, an
6555 (LM358) is also powered by -8V from the regulator. No -8V, no AGC
and no RX. Non negotiable.

The CPU is a Motorola 68050. Its proprietary to the FT-757GX. If its sour
I’d just go on eBay and get a new LO board.

Last but not least right at the beginning of this note I did tell you to
clean the radio. This also included pulling every connector and running
some DeOxit on the connectors. You DID do that, didn’t you? I have gone
through many FT-757GX and bought a few advertised as “intermittant….
for parts or repair”. In EVERY case the radio was fixed with DeOxit or
the male posts on the solder side has had a soldering failure.

23) RX Alignment Check

The Noise Blanker uses the RX IF strip ahead of the crystal filters for
its signal processing. In addition it has its own AGC independant of the
RX. A simple check on any band is to engage the Marker and NB. Tune in
the Marker. The voltage on G2 of Q13 should measure about .85 volts. It
is negative AGC. If you adjust T2008 out of alignment you will see the G2
voltage increase.

Most of the RX alignment can be done with the Marker and the S Meter. You
can also use the paragraph above and tune G2 for minimum. A trick is to
control the input by mistuning ONE IF transformer near the 47 MHz crystal
filter. Use the S Meter and tune the RX IF chain for max, all the while
using T1005 to keep the S Meter around S6. The last step is to tune T1005
properly using a low level signal.

Needless to say the alignment of the NB IF is critical to a well working
NB, and the one in the FT-757GX is quite effective.

Summary
~~~~~~~

This has been my experience with this vintage Yaesu. The radio STILL
does a good job on HF even 35 years after its release. Perhaps its not
a Killer – if even it was 35 years ago, but its still perfectly usable.
I won’t defend it by saying “there are worse”. Thats not saying very
much – is it?

The attraction to me is that these radios are “easy” to service. There
is some SMD in there, but you have to look for it. Its 99% through hole
assembly. Its built like a tank and the finals, if run at 100 watts are
bullet proof. It has a real S Meter and there are NO accessories to buy.
It came out of the box in 1984 ready to Rock’n Roll as is. Yes, a modern
radio – say some thing as lowly as an IC-718 will run with it as an equal
if not “better”, but between the two the 757 has much more charisma and
charm. Plus…. I can fix it and align it.

This radio will never be sought after or remembered fondly like a TS-830
or an FT-902DM. It will forever be looked upon as “an old HF transceiver”.
You can get a good hint of this by the number of them being cut up and
having the pieces sold on eBay. There is no shortage. This is indicative
of both lack of parts, general failure with age and it being not cost
effective to send out for repair. It also suggests the overall lack of
skills to fix these radios and possibly the desire to do so in the first
place.

While we may be in a minority there are still folks out there in “radio
land” that really are not that attracted to modern radios despite their
obvious functional superiority. A Yaesu FT-891 will blow the doors off
an FT-757GX.

But its not the same.

 

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