Javier Albinarrate - LU8AJA

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Yaesu FT-757GX maintenance tips

This article is borrowed from OH5IY, available here: http://www.kolumbus.fi/oh5iy/HW/Yaesu.html
I have copied it into my site for redundancy, because sometimes important articles like this one vanish from the internet, and when that happens it is a great loss.

Yaesu FT-757GX maintenance tips

Repairing the older version of Yaesu FT-757GX (so called mk 1, with rotary mode-switch) involves "some" work but, since the 757 is a good match  with 736R, I  bought a 20-year old  FT-757GX. During the first day I noted 3 serious faults, which were eventually resolved.

The usual things to do first, are cleaning the radio, replacing the blown dial lamps, treating all the switches, potentiometers and connectors with contact cleaner (CRC 2-26) - twisting&clicking them a lot to remove oxidation and then applying protective contact grease (Electrolube 2GX).  After this you can test how the receiver and transmitter works.

I had no problems with TX -  >100 W output on all bands (or so I thought), modulation and keying was OK, but the widely know fault of frequency drift and wobble by bad trimmer capacitor(s) in series with reference (and other) crystals on the LOCAL PCB board (upper) called for the trimmer caps to be replaced (TC06, TC04, TC05, TC01). In spite they shown no obvious contact problems, the frequency at times began to drift and wobble suddenly for no reason! Listen also for worn bearing noise from the PA blower - try to lubricate or replace the motor if noisy or if the fan doesn't do the short spin burst at all when the rig is switched on. If you need to open the PA's inside cover, check the antenna connector's center pin's jumper's soldering joints are OK, not cracked or loose. If the center contact on the Amphenol socket is loose, it may cause the jumper to break when antenna connector is plugged and un-plugged repeatedly, so use some glue on the inner side to secure it (or replace the socket...).

General inspection with a magnifying glass or a microscope of all PCB's solder joints is useful for future reliability, when a unit/PCB is removed for service. Cracked and bad-looking solder joints needs to be resoldered carefully - pay special attention to connectors and do not make solder bridges...

During troubleshooting and generally working on the receiver, to limit damages, I recommend you use a current-limited DC power supply set to about 1 A limiting, just in case an accidental short-circuit occurs.

You don't find the advice elsewhere on the web to fix these not-so-uncommon faults in FT-757GX!:

Everything else, but the FM receiver, worked - it was silent. Listening carefully at full volume I heard something - the squelch potentiometer worked OK - the problem was not actually a squelch problem, but the FM audio was just extremely weak. The reason was there was about +5 volts on the +8 V TX-line when receiving, which was weird. The RF UNIT has plenty of switching diodes which, when shorted, could have done this, since the switching transistor supplying the +8V TX line, was not shorted or leaky. With DMM you can try to find where the leak may originate by measuring exact DC voltages from the transformer coils, but this helped me none since the leaking diode was in the factory-made add-on modification wrapped in tape and located below the RF UNIT's back corner - close to the red +13.8V output RCA connector.

Those added components; two resistors, two capasitors and two diodes, had one diode partially shorted, and it leaked DC to +TX line, which caused FM detector IC (MC3359 pin 15 had 5V!), to be muted - as if you were transmitting. The RF UNIT PCB can be flipped over for accessing the underside "modification", or doing whatever soldering work you need, by removing the keyer jack's nut, top PA-assy's front screws (2), loosening face-panel's side screws and sliding it out some, unplugging the RF UNIT's coax plugs at rear of the PCB and four PCB connectors and those 5 screws fastening the PCB. The glassfiber tape Yaesu had used for isolating the modification's components, had become very brittle which may have caused a short, or it maybe was just a bad diode (1N4148-type)? Anyway, all the original insulation work done with the brittle turned tape needs to be re-done better with something, that is more durable over time! There are a few other places you need to do the same, like the cage over the LOCAL UNIT, which may short the PA fan wire terminals. When reassembling the PCB, make sure you have inserted all unpplugged cables back on their sockets and none are left under the PCB to cause short-circuit.

Measure the receiver's sensitivity on all bands; should be in the 0.2....0.5 uV (12 dB S/N, FM mode) region with RF AMP ON above 1.5 MHz.

If the FT-757GX receiver's sensitivity has worsened somewhat, like I had (about 3 uV for 12 dB S/N (FM)), there is something broken. If it is almost deaf, check first the light bulb (F01) by shorting it out. I discovered the RF-UNIT's BFP filter bank's input-side switching diodes 1SS97 Shottky's (D19, D07, D09, D11, D13, D15, + possibly others) had been damaged and leaked various amounts of RX signal to every BPF filter section. Real nasty to troubleshoot - the receiver was not supposed to have suffered such damage typical for contester's radios. Only trick to find out was to tune to 28 MHz and note the sensitivity, or S-meter reading of a carrier (from sig. generator) and then just cut D19's other lead and note the sensitivity & S-meter reading to improve (D20 is the respective diode on the filter bank's other side). I reconnected D19 and just for curiosity cut all the BPF's antenna-side Shottky diode's legs one-by-one and found all more or less RF leaky. If the 28 MHz BPF also has damaged diodes (MA190), the method might not work as assumed. Diodes are cheap and it's better replace many enough, so you get no second thoughts later. I used BA244 and some 1N4148 if no switching diodes are at hand. The damage was most likely a result of static surge from the antenna, but too much RF-power into the receiver from another transmitter (any band) can also cause such damage to any modern HF tranceiver. In fact this fault actually also reduced the transmitter's drive, since TX drive power is piped via the same BPF- bank and diodes - the Drive- knob had to be turned fully clockwise for 100 W output, but after repair about half-way setting was enough. The SWR metering seems not to be quite OK or is just designed wrong?: it indicates no reflected power (SWR reading = 1/1) when SWR is below about 1/1.7 or so. I do not know if this the case with all of FT-757GXs.

Measured sensitivity on FM for 12 dB S/N ratio, RF amp ON, RF amp OFF:
 0.7 MHz    N.A.           , -105 dBm (1.25 uV)
 1.7 MHz:  -120    dBm, -108.5 dBm
 3.7 MHz:  -119.5 dBm, -109 dBm
 7.2 MHz:  -119.5 dBm, -109 dBm
14.2 MHz: -121    dBm, -111 dBm (0.2 uV, 0.63 uV)
21.2 MHz: -118.5 dBm, -108.5 dBm
28.2 MHz: -114.5 dBm, -105.5 dB (0.41 uV, 1.25 uV)

The receiver LO1 level is +13 dBm, LO2 is -3 dBm from the coax plugs (unplugged, to 50 ohm load).

Last, calibrate the frequencies; the 15 MHz reference, then LSB carrier (TC05), CW carrier (TC04), USB carrier (VR05) and FM carries (VR04).  See service manual downloadable from the Internet for a full description of the procedure. The radio needs to be warmed-up, since it (the 15 MHz reference XO) seems to drift about 200 Hz during the first hour (no TCXO). I did not get the carrier XO frequencies just spot-on, but they are within given tolerances. The receiver's IF transformers may need a bit of tweaking, which gives you higher readings on the S-meter, which then should be calibrated (VR1 for 6 dBuV=S1 and VR11 for 50 dBuV for S9+60dB), verify  S9 = 50 uV = -73 dBm.

This comes from Ken, ZL1PKS: There were some problems in the PA board that were causing no tx/rx switching. The fault was eventually found to be the transistor Q(60)08 a
2SA1012Y. It was not showing the correct voltages as per the voltage chart - changed it for a TIP42C. On PA board 10 uF 25 V tantalum caps
C24, C27 and C32 were destroyed and showed some signs of burning.

Lower TX power on 28-30 MHz: 100 W on all bands 1.8-24 MHz but on 10 m about 60 W. Adjust VR(10)05 for power output on 10 m, VR(10)06 is output power for all other bands.

When everything works, tape, wrap and place all wirings carefully, specially the ones between PA and LOCAL unit, so they will not get pinched when you reassemble the PA and bottom cover. Negligence in this issue will cause damage to components when radio is powered and some wire(s) get shorted to ground (may happen even weeks later)!


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