This radio has been waiting for some attention for a while, it's been tucked away for several years as I knew it had several problems with it. I swapped it for a working Yaesu FT757GX and power supply. What were you thinking you ask, swapping a working radio and matching PSU with a dead boatanchor? Well, an FT757GX is a rig. The FT-902DM is A RIG!
1) CW keydown all the time from power on. This was a bad keyer chip a Curtis 8044 IC. These are unobtanium, so I just unplugged the board and put fitting a replacement homebrew keyer on the list. If you have a junker with the keyer board in, I need it!
2) Quite severe power fade under all modes. Full power for a few seconds then fading to half power. This was due to a bad capacitor in the PA, its on the final valve PCB. Remove valves, unscrew and desolder two wires. It's a red lozenge type 1kV 1000pF, the ones that seem to like dying in these old Yaesus. It's C01 on the diagram. After this I could get a full 100W on 80m.
3) Low power on bands 20m and above. Performed realignment as specified in the service manual. This got back everything except 12m and 10m
4) Replaced C70 200pF 1kV. Lozenge type. No change.
4) 11m CB conversion. It was modified for CB, so I only have 10A and 10B original crystals. I desoldered the CB crystals, luckily the sockets were still there, the wires were poked through the socket into the PCB. Folded over the crystal legs and fitted them back into the sockets, then fixed them with a small dab of glue. I need replacement crystals for 12m, 10C and 10D. 38.9875, 43,4875, 43.9875Mhz. Please help! Luckily, the digital counter was the version that did not need hacking to display the 11m frequencies correctly. However, the VCO unit has been readjusted for the changed bands 12m, 10A, 10B, 10C and 10D. I can't sort this out, or align at 30Mhz until crystals are obtained
5) Low power on 12m and 10m. Something has changed value. The preselectors in the transmit path still had the original glue on. I checked all over, checked the bandswitch, looked for mods, looked for burnt caps. Nothing stood out. Checked bandswitch contacts, wires. I measured the input to the final valves and it was low on 12m and 10m so it had to be something in the driver/trimmer/preselector sections. I really didn't want to but I had to know if the cans were adjusted right. So I marked the cores, removed the glue and tentatively adjusted. This got power back on 10m by adjusting T3 out a fair bit and then T2 out a little. Hmmm, performed realignment again. Got everything back apart from 20m, it needed an extra 22pF cap on the back of the large trimmer. I seem to have realigned around the problem, it remains to be seen if this is stable or will slowly drift off as whatever it is that is the root cause gets worse. T3 is around 4mm further up than the other cores.
6) PLATE tuning control pattern not quite as it should be. This is probably a cap or two that has changed value. To be investigated, but this isn't too bad or a showstopper. Could be related to (5).
10m 95W with alc action!