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Yaesu FT-736R, FT-757GX, FT-817ND mods and repair tips

FT-736R internal SMPSU repair

After some 10..15 years of operation the internal power supply fails to start properly, only a few LEDs on front panel are lit dimly and eventually even  several repetitive pushes on the Power switch
 do not make the radio start up. The most likely cause are C9 220 uF/35 V (defective with ESR of 450 ohms), and C12 1uF/50V (defective, ESR of 60 ohms) dried electrolytic capacitors on the PSU unit PCB.

Unplug AC cord, remove radio's bottom cover, open 3 long M3 screws from the back securing the PSU heat sink bridge plate to rear chassis plate and those 2 screws from the bottom side in the middle holding the PSU base plate on the chassis. Lift out the PSU unit, open the PSU cage screws and the 4 PCB securing screws holding the base metal plate. I suggest you replace at least C8, C9, C12 or preferably all electrolytic capacitors on the PSU PCB with +105 °C versions, but not necessarily the rectifier filter's 470 uF/200 V big caps, if they seem OK and you can measure their value or even better, the ESR too. Re-solder with new solder all the eroded solder connections on the PCB also, which means everything around the transformer. Remove tin drops (and possible solder bridges) with brush carefully. Reinstall the base plate, cage and secure the unit on it's place in the chassis and temporarily connect the AC cable and see if the unit now works. If it does, unplug AC cord and now reassemble the rest. Roll and shake the unit to see for any loose parts inside: the ventilation grill on top of the FT-736R consists of wide cut slots, which easily allow washers up to size M8 to accidentally drop inside the radio and cause shorts and severe damage! I have seen this on two occasions. Remove any loose objects that do not belong there. The SMPSU should now work for for another 10...15 years.
 

FT-736R 50 MHz frequency instability fault repair

Some years after the PSU was fixed, the 50 MHz band signals began to wobble and jump in frequency at power up and when knocking the radio gently.

The fault occurs only on 50 MHz band and is caused by fractured solder joints between 50 MHz "VCO-PLL" card's edge's ground plane connections, that do not have any screws to keep the PCB mounted securely.

Disconnect power cord! Open FT-7376R's bottom side enclosure to access the 50 MHz module. Open the screws holding the module's outer RF enclosure and then those 4 tiny screws on the sides that keep the (lower - since radio is flipped, in this case easier to access) VCO-PLL PCB on the module frame. Lift up, flip it upside down to access the PCB's foil side, loosen the tinned RF lid's small screws (4 or 6 pcs) and lift it off. See and feel the PCB GND to frame (3 on each side, 2 in the back) solder bridges, if the solder is cracked and the PCB is moving loose. Re-solder with some new solder, clean the copper spring flap contacts with slight abrasive and add some contact grease on them to reduce oxidation. Drop some contact grease also the TO-220 cased voltage regulator in the back of the module and slightly pull and push the socket back. Reassemble the PCB frame to the module, press the sandwich, so the copper spring flap makes good contact and tighten the 4 small screws. Test the fault no longer occurs and reassemble the bottom side radio enclosure. Basically that was it, you may have opted to remove the 50 MHz unit by unscrewing it from the heat sink & radio chassis. Makes it a bit easier to handle, but there are more screws to open and bolt back in.
 

TX key-up output power slope delay


Most of the VHF and UHF stations dedicated to weak-signal propagation modes such as MS, EME, FAI, etc, run high output power and use big coaxial relays for T/R switching. Those relays are slowish and take some time (up to 1/4 s) before they have completed switching from receive to transmit, while modern solid state transceivers and power amplifiers are capable to putting out full RF power within tens of milliseconds from keying. This has to be disabled, because hot-switching (with RF power applied) causes arcing of the contacts, causing interference, ruining the coaxial relay and being a potential cause for preamplifier failure. During relay contact transit from RX to TX position, the isolation given in relay's specifications will not be valid and this may yield a surge of RF, that ruins the preamp.

There are numerous ways to solve this problem, some of them involve use of auxiliary coaxial relay contact wired from the tower to the shack to drive a dedicated T/R controller unit. Another more easy way is to use a "blind" (no feedback) fixed delay in the transmitter, which in it's most simplest from, consists of a method of feeding a DC pulse to TX driver stage blocking the RF power output, until all the relays can be assumed to be settled to transmit positions.

This delay circuit was used successfully with Icom IC-211E since the 1980's and now has been applied to Yaesu FT-736R. ALC connection, whether the transceiver has such a connector or not, can not be used, because of it's slow time constants, so some modification inside the transceiver is required.
 

FT-736R

This circuit works on all modes, with PTT and VOX. It injects a short pulse of DC to the 13 MHz low power output FET's source on the TX unit in the FT-736R, blocking the FET in the RF amp chain and momentarily preventing 13 MHz drive power coming out of the TX unit. Incidentally, the same FET is also used for CW keying.

This gives transient free rise of the RF output and much lower initial power level (-50 to -60 dB from full output power) and is not sensitive by any means to DRIVE setting.
 

TX power-up profile of a modified FT-736R. The lower image is a 10-x zoom of the rising slope of RF output. The 12 dBm reference level is equal to 16 W through a 30 dB attenuator.

Between short dropouts to RX mode, the delay will be there, if the pause is longer than about 100 ms. Hopefully during shorter T-R-T trips the delay won't be needed. So the circuit is not perfect, but a lot better than it was before. Would need more work, and/or more components (a capasitor dischage switch across 22 uF driven from +RX)) .

This circuit is of general type and can be also used on most other transceivers. You just have to find +TX DC voltage and understand the TX driver stages and the RF output power adjusting circuit or the keying circuit, to know where to connect the delay pulse.
 
 

INSTALLING

DANGER of ELECTRIC SHOCK:
WARNING: 230 V AC VERY CLOSE to FT-736R's TX unit!!! PLEASE DISCONNECT AC CORD and BE CAREFUL!
YOU HAVE BEEN WARNED! Feed this radio only with 12 V DC cord from DC powersupply when top cover is open for safety.

NOTE:

You do these modifications at your own risk! I will not be liable to any damage you might cause to your equipment!
 

The TX Delay pulse generation circuit. p5 is the output for the TX blocking pulse. The 150 ohm series resistor is there to limit the current to prevent damage to other components. With other than FT-736 transceiver, you could consider using even slightly higher value. The value of the 22 uF capasitor can be increased up to 100 uF if longer delay time is desired. (1 nF= 1000 pF, ceramic or SMD)

FT 736R TX delay unit, near AC input conn.

FT-736R's TX unit: the TX Delay's PCB is installed vertically, the Q22 is in the middle right side.  MS-keying speed modification: replace C82 (4.7 uF) with 1 uF electrolytic capacitor - behind the white coil core just next to where the TX Delay unit's wire (inductor) is hooked.

Install the components of the delay pulse generator on a small piece of PCB, 30 by 30 mm and mount it vertically to the rear between TX and 144 MHz main unit (near connector J10), with a terminal lug soldered to this delay circuit PCB's ground and fasten the lug with the mounting screw at the corner of TX unit near AC input connector. Make sure the unit is firmly attached and can never get in contact with live AC connector terminals (will cause much damage! - and is a safety issue also), or other live parts on the AC input PCB, like the AC fuse!

Connect a short piece of wire from the TX Delay unit's p5 output of the delay circuit to the source of Q22 in the FT-736R's TX unit. This 4-lead transistor (FET) is on the right side of the unit between the shielded RF transformers. If you wish, you can use a small 10 uH inductor as jumper wire, to make sure no stray-RF can get to the Q22's Source and if you remove the TX unit to change the C82, you can use the (correct) vacant hole of TH01, that isn't there, to solder the other end of the inductor.

Connect the TX Delay unit's terminals p1, p2, etc. (only two shown) from the TX9V lines entering each power amplifier modules from their respective RF UNITs. The only way to make this and make it also work on SAT mode, is to wire the TX9V from the small PCBs that are soldered to the feed-through capacitors on the PA's shields. These small boards are not shown in the Technical Supplement, so I can't give you any pin numbers, but they are the leftmost (AC conn. side) of those 5 feed-through capacitors. Inside the cage I can see an orange wire connected to it. This is the correct spot to take the TX9V out, since the other wires leading into this unmarked PCB on 432 MHz PA, are at +9V all the time, when the SAT mode is selected. You may verify the correctness of the pin with a DVM. Thanks to Mike, N1JEZ for the SAT modification and pre-testing.

If you do not need this delay on one of the bands, do not connect the TX9V line from that band unit to this delay circuit. Inputs from only two bands (p1, p2) have been drawn in the circuit diagram above. By adding parallel diodes, all four bands can be included.
 

Adjust the 10 kilo-ohm trimmer to give you an adequate delay. For a HF-400 coaxial relay, a delay of 250 ms (0.25 seconds) seems proper. Raise the value of the 22 uF capacitor or the trimmer on the TX delay unit to make the delay longer, if necessary. An oscilloscope (preferably a digital storage scope), or a time domain sweeping spectrum analyzer connected to the 13 MHz RF-output connector at the rear of TX unit can be used to verify the delay and rise envelope of RF power.

One way to set the delay on 2 m is to verify if keying the TX causes any noise lines to appear on a VHF band 3, (180 MHz) TV screen. Too short delay with hot-switching will cause some noise pulses on the TV screen even with 20 W output. Increase the delay setting and re-test. When the picture is unaffected, add a little more delay and it is set correctly.

If you operate Packet Radio, set the TXDELAY in your TNC, or software long enough to wait, until RF power is up at full power and data may be sent. This reminds me some operators use brick RF amplifiers with RF-VOX relay switching on packet radio and this sometimes causes terrible noise pulses across the whole band. A delay circuit like this, would not be a bad idea to have in those systems too for many reasons.

Naturally, don't expect the Digital Squelch system in FT-736R to work with the delay in use, because the code is sent out, before the TX power rises up enough.

If you did not understand fully what was explained here, it might be better not to open the cover of you radio, but let someone else do the modification for you, just to be on the safe side.
 

HIGH SPEED KEYING AFTER C82 is ghanged to 1 uF





FT-736R high speed CW keying waveform with time sweeping spectrum analyzer, "R" at 1500, 2000 and 2800 LPM after C82 was changed to 1 uF. The upper practical limit depends on what is acceptable and readable by the receiving operator, so no speed claims are in order here, but it might be well over 2000 LPM.


Spectrum and time analysis comparing AF keying to CW keying


Time sweep 2 ms/div (ignore the false label on the picture). Keying waveform at 3100 LPM, FT-736R on CW mode. MS-Soft keying via CW connector.


Time sweep 2 ms/div (ignore the false label on the picture). Keying waveform at 3100 LPM, FT-736R on SSB mode, MS-Soft keying via DTR's 2300 Hz AF keying.


Frequency spectrum. FT-736R w. external DC power suppy, carrier only for comparison.


Frequency spectrum, average power on high speed CW via CW connector.


Frequency spectrum, average power on high speed CW via DRT's AF keying.


Frequency spectrum, peak power on high speed CW via CW connector.


 

Frequency spectrum, peak power on high speed CW via DRT's AF keying.

The AF keying causes less power to be spread to nearby frequencies, but it is a tradeoff with keying waveform. Using a monitoring receiver 20 kHz up at 144.080, the noise was at S3 on both cases, when noise blanker was on. Without NB the noise was at S6 using the CW keying, but there was no rise with AF keying, so the keying click impulses are extremely short in duration.


432 MHz band RF RX filter

Commercial out-of-band transmissions just below 432 MHz may result intermodulation to FT-736R when using two stage RF pre-amplifier. This can be cured by filtering the spectrum that gets to FT-736R's RX input. This is is a design fault, a poor dynamic range on 432 MHz, mostly due to poor mixer choise.

The first filter I used was part of a UHF duplexer (3-cavities) that was tuned to notch out 425 MHz. The filter has about 2..3 dB loss, which was insignificant since the filter is connected after the pre-amplifier(s). Since by nature a notch filter eliminates only a fairly narrow bandwidth, the unwanted signals some MHz below, still tended to cause IMD. To cure this, a commercial 4-stage UHF Helical cavity surplus filter was retuned to 433 MHz. The loss of this filter was about 6 dB, which still was tolerable, specially since 95% of the IMD stray signals disappeared from the receiver.

The frequency response of both filters at wide (left) and at narrow (right) spectrum sweep. The Helical filter covers the mostly used portion of the 70 cm band and attenuates just a little on the upper edge.


432 MHz RF-front-end (mixer stage replacement) modification

If one wishes to improve the dynamic range of FT-736R on 432 MHz band, there are at least two ways to do it: buy a commercially produced 144&432 MHz RX front-end kit (about 400 €) or do some less comprehensive, cheaper and better targeted modifications by your self. The original 144 MHz front-end uses a different and better design in the first RX mixer than what is used on 432 MHz, which is just a simple dual-gate FET stage. The project takes one-to-two evenings to complete and to do it you need to have some experience in constructing simple RF equipment and repairing ham radios. If you do not have these, there may be troubles, so consider whether you are up to it. You do this modification at you own risk.

This modification met the target of getting rid of overloading out-of-band IMD signals received all over the 432 MHz band (see previous chapter) - the little there is left, is not generated inside the FT-736R. How much the dynamic range of the 430 MHz receiver improved, has not been measured, but I expect the maximum input signal level at 1 dB compression to be about -20 dBm (22 mV PD). QST measurement stated 3rd order intercept point to be -44 dBm on an off-the-shelf FT-736R on 432 MHz.

Basically, what should be done, are:
1. not using the RX PIN-diode T/R switching at front of the 432 MHz RX input when ext. pre-amp & T/R switching are used (the 2 cable (RX and TX) system)
2. replacing the FET-mixer with doubly-balanced diode mixer (DBM)

What you loose, is a little bit of sensitivity when you operate FT-736R on 432 MHz without external pre-amplifier, since the proposed passive mixer stage has a higher noise figure (Nf) (7 dB + 4 dB pad  and 3.4 dB of MAV-11 or ERA-5). Due to 1'st RF stage's estimated gain of 20 dB of the Front End unit, the resulting overall cascaded Nf  will result 2.35 dB, if it was 2.0 dB before (theoretical - not measured). On the other hand, if you do something for the PIN diode problem and succeed well on the modification, you may well win than back or make it better. With external pre-amplifier the system's noise figure depends in practice totally on the external pre-amp's noise figure.

I will not describe the modifications in detail, but as a hint: I have added a BNC connector on the back panel for direct RX input, which requires the back panel of FT-736R to be at least partially disassembled and PA unit's cover removed. I have installed a tiny RF relay to disconnect the RX signal path from connector to Front End unit from the PIN diode located inside the PA unit. The relay can be toggled by the pre-amp switch at the front panel to enable the normal use (single T/R cable) without ext. pre-amp input (the PREAMP switch wiring needs modification to do this, but you may never need it, so perhaps skip the switch thing). Accessing the 430 MHz P A unit (where the PIN-diode is and where the BNC is to be installed) at rear panel, is not easy due to excessive wiring to Power Amplifier modules, etc., but can be done, if you have patience and skills to do it. Caution: Unplug the AC-cord before opening radio's top cover for safety!

Modification 2. involves constructing a new separate 430 MHz band mixer unit and re-directing the RF signal from 1'st RF stage's output Helical filter to new mixer unit and feeding the IF from mixer unit's output to FT-736R's IF-unit and bringing the LO signal in to the new mixer unit.

In practice the work includes removing the 430 MHz Front End unit, opening the covers, removing C07, a 5 pF chip, soldering a thin (UT141) PTFE coaxial to ex. C07's Helical coil side terminal and braid to GND, preferably removing T01, which is the FET mixer output transformer to kill DC power to the FET mixer (this may not be necessary since both gates are at 0 V), adding a feed-through capacitor for feeding DC power in to the Front End unit, since the IF-cable at J03 will no longer be doing that job. The IF output connector J03 and LO input connector J02 are unplugged and are to be connected to the new mixer unit.

There are two ways to construct the mixer unit - cheap and little more expensive, but easier way. The easier way requires basically no access to RF measuring equipment (spectrum analyzer), if all goes well. I have not done it that way, but I see no reason why it would not work.

The cheaper way involves using discrete components, of which, a tunable IF coil, may need some tinkering.

Mini-Circuit's SBL-1, the DBM work-horse, costs only some bucks, and offers maximum of +1 dBm RF input level. It needs +7 dBm injection (LO), but works better with +10 dBm. Fortunately, the injection level at J02 connector is -2 dBm when terminated to 50 ohms and Mini-Circuit has a MMIC amplifier MAV-11, which gives 12 dB gain, so the we can get the +10 dBm with it, to the SBL-1. If you use ERA-5 type MMIC, you should get 7 dB more injection with it and may consider using some +13 dBm-level  mixer type such as RMS-1MH.

The mixer's IF matching network consists of an attenuator that fixes the mixer's output's poor match on frequencies other than 48 MHz. The broadband termination is necessary to help SBL-1 to work optimally giving low loss and good isolation from port-to-port, which it does when it sees 50 ohms at every port over the full bandwidth. The series-resonance LC circuit is to pass only the signals at IF of about 47 MHz to be further amplified, and the pi low-pass filter section further attenuates frequencies above 60 MHz,  which consists of numerous mixing products and the LO signal leak to IF port at about 385 MHz. Without these the MAV-11's would get a lot of undesired VHF and UHF signals from the mixer to their input and since they work up to 1 GHz, these unwanted singals would otherwise be also amplified increasing the possibility of a new IMD source to be generated as a result of poor design. The 10 pF capacitor at mixer's IF port was added as a result of experiment, since it raised the mixer output by a few dB.

To overcome the loss of signal from use of passive mixer and the output matching circuit vs. the original active FET mixer with presumably some 10 dB of gain, we use MAV-11s (or ERA-5s), one or two, as IF post-amplifier. The 50-ohm output (and input) impedance of MAV-11 suffers from slight mismatch when connected to FT-736R's 430 MHz band's RX IF input (T19), so the gain we get from the IF post-amplifier is only about 10 dB (2 dB below specs to 50 ohm match). The cause for using two-stage mixer post-amplifier when you are not using an external RF pre-amplifier, is the lag of some 4...5 dB of total system gain with just one MAV-11. If you have an external pre-amp, or two, you likely do not need the extra IF gain from the second cascaded MAV-11. Note: MAV-11 can be replaced with ERA-5 (or ERA-5SM) with higher output power and with 8 dB of more gain. Note: One ERA-5 IF amplifier stage gives almost the same gain as two MAV-11 amplifier stages!

Many of the discrete components used here could be replaced with integrated ones: the attenuator pad, the band-pass filter (Mini-Circuits PIF-50), the low-pass filter (Mini-Circuits PLP-50), but it raises the costs.


 

The diagram is close to what I have constructed, except for the additional second MAV-11 stage at IF. Mechanically, the components are mounted on two-sided glass fiber PCB, with 2.5 mm wide (strip)lines serving as signal paths and where the SMD and other components are mounted. The SBL-1 mixer and the adjustable 0.7 uH coil are mounted through holes on the opposite side of PCB which serves as ground-plane. If you can find miniature female coaxial connectors alike used by Yaesu in the Front End unit, installing them on new mixer (LO and IF ports) unit would make it easier to plug the LO and IF cables to it.
 

The PCB layout should loosely follow the schematic diagram. You find the pin numbers of SBL-1 and MAV-11 (and ERA-5) from the web at Mini-Circuit's site. Drill four ground-feed-through holes for wires around each MAV-11 MMIC's ground leads for good stability. Solder a brass wall around the PCB connecting both side's ground-planes and use a metal lid on the SMD component side. You may use a small pre-tinned metal enclosure made for small RF projects.

Tune-up: re-peak the last Helical coil of CV01 at Front End unit with signal at RX input before installing it's cover. Peak the 47.43 MHz IF series resonance circuit's coil (0.7 uH) located between the attenuator pad and the IF amp. MMIC. Now all should be in working condition. If you do not get noise or signals, check the DC voltages on MAV-11's outputs and inputs and look for shorts or cuts in the wiring and PCB. Check you have connected SBL-1's pins right (1=RF, 3 and 4=IF, 8=LO, all others GND) and MAV-11's with it's "index" at output side, but with ERA-5, the "dot" at input side.

When tested, seal the new mixer unit RF tight and secure it firmly on the right back corner by the 430 MHz RF unit.

Note: the MAV-11's draw about 50 mA DC current each from 8.5 V RX+ line and if you use 3 of them, Q28, the switching transistor on the 430 MHz RF unit has to take the strain to pass about 140 mA of extra current. If it fails, don't blame me.... specially if you caused a short circuit on the +8.5 V line. If it fails, a transistor with higher Icmax could help or add another PNP type switching transistor with its base and emitter in parallel to the existing transistor for delivering current from it's collector the new 432 MHz mixer unit.



 

FT-757GX maintenance

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)!
 



 
 

Modifying Yaesu FT-718 ND to drive two DB6NT Kuhne electronic transverters of different IFs

FT-718 ND has internal keyer, enough bands to drive any transverter and adjustable power in the few-watt-class, making it ideal and the only really practical choice to work (portable) on microwave bands and as a bonus, still have the option to work also HF and 50/144/432 MHz. The two antenna connectors are very useful if you have transverters with different IF bands, typically 144 and 432 MHz or 28 MHz. For instance, the rear antenna connector can be programmed to be used on 144 MHz band and be connected to a 144->2320 MHz transverter, while the front antenna connector can be programmed to be used on 432 MHz to drive a 432->10368 MHz transverter. This way changing the transverter and band is easy -  only change the band (VFO or memory) on FT-817ND. However, as there is no +TX voltage available for transverter's T/R switching in FT-817 antenna connectors (like FT-290 or FT-790 had), you need to make a simple modification, which enables also the T/R switching voltage to be routed automatically via the proper antenna connector to T/R switch only the transverter that is in use. You do this modification at your own risk and it might affect warranty.

The modification is simple, but needs a steady hand and a magnifying glass. On the rear corner of PA UNIT (underside the radio) is a jumper with +5V when transmitting, labeled in schematics as TX5V.
Solder a thin insulated wire (in the image below, the blue wire) on the jumper and route the wire to near the antenna selector relay (also underside the radio) on PA UNIT close to antenna selector relay located by the rear antenna connector (SO-239 type). Next unsolder and lift the front-side end of the second LPF air core coil (L3081 in schematics) near the relay, so you can fit in a 10 nF SMD series capacitor  (orange cross and arrow in images below) between the coil's now free-end and the PCB trace it was soldered to. The capacitor blocks the +TX trasverter control voltage from straying further in to the HF receiver's front-end and TX PA.

FT-817 transverter T/R control voltage modification

 Solder a 22 kohm resistor on the antenna selector relay's side of the series capacitor with resistor's other lead connected to the TX5V wire you just routed there. The +TX voltage should now get connected to either of the two antenna connectors - verify it's there during transmit mode.

The DB6NT transverters need little voltage (>0.7 V) and some tens of micro amperes of current for T/R switching, so there is no need to use any smaller value series resistor. You may also add a decoupling capacitor of 10 nF between ground (the JP3003 coax braid a few mm away) and junction of 22 kohm resistor/TX5V wire. I used SMD caps, you may opt for small ceramic disk caps with leads (but they need to be able to handle the HF TX power).



 

Rev. 2013-06-16
© I. Yrjölä, 1999... 2013