An old Yaesu FT-901DM (Output problem)

Discussion in '"Boat Anchor" & Classic Equipment' started by HS0ZED, Jun 15, 2018.

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  1. HS0ZED

    HS0ZED Ham Member QRZ Page

    I have on the bench at the moment an old Yaesu FT-901DM. Given that it has sat, covers off, for more than a few years in a corner of the shack just gathering dust I was amazed how well it has survived. It powers up and works rather well on receive so far as I have checked and sensitivity is within specification. On transmit it has issues though. Very low output.

    Though there is no spec that I have seen in the manual the transverter drive output that comes from the driver valve seems far too low. The matching transverter, FTV-901, states it needs about 3v rms to produce its rated 10w output, I can get about 1.5v rms with the carrier control at max. It peaks and the coil pack is peaked also the trimmers and the driver voltages seem okay, if a bit high, 320v on the anode, 230v on the screen grid, -30v rising to around 0v when keyed on the control grid (yes heaters are lit!).

    Does anyone have a typical rms or p-p voltage for the driver output with carrier at maximum? Loaded or unloaded, so long as I know. Just trying to understand how big of a low drive problem I have or just a PA issue. Well I'm sure there is a PA issue as well but that's a question for another time.

    Martin, HS0ZED
  2. W5INC

    W5INC XML Subscriber QRZ Page

    Martin, I would download Will Penders, PA0PGA's 901 survival guide here. Dr. Will is well versed in both the 901 and 902 rigs. The survival guide should have the information you are looking for. Good Luck on your project, let us know how it turns out for you. ;) Web Page.htm
  3. G3YRO

    G3YRO Ham Member QRZ Page

    lf you have peaked all the RF Circuit Trimmers on each band, I would try a new Driver valve (you will need to re-adjust the trimmers again)

    Roger G3YRO
  4. HS0ZED

    HS0ZED Ham Member QRZ Page

    Thanks chaps. I have the PA0PGA doc, very informative and a good addition to the manuals. Thanks Roger, I have 3 or 4 12BY7A valves, and I just found another in an old homemade TV transmitter that found its way here with a pile of other stuff. Driving a QQVO3-10 of all things. It is possible all the valves I have are dead but I would have thought I'd get a bit more life out of at least one. Well noted on the re-tweaking.

    What should a good one produce flat out? I'm thinking I should see a good 5-6 volts rms across 50 ohms. SOund reasonable?

    Martin, HS0ZED
  5. G3YRO

    G3YRO Ham Member QRZ Page

    You cannot feed the output of the 12BY7A Driver into 50 ohms on these rigs, as it is a High Impedance output. (connecting it to a low impedance would drastically reduce the signal)

    This output was made available on the back socket MAINLY to drive Valve Transverters for 6m or 2m. I also made an external low-power valve PA for Field Day use on an FT101.

    And although I worked on hundreds of Hybrid rigs when I worked for Yaesu, I never actually measured the RF voltage produced.

    Roger G3YRO
  6. N8YX

    N8YX Ham Member QRZ Page

    Big Question:

    Has someone modified the rig to work with a lower drive transverter?

    I'd go through the schematics in the service manual and carefully compare the data to the radio itself.
  7. G3YRO

    G3YRO Ham Member QRZ Page

    Also bear in mind that - because it was mainly intended to provide a 28MHz output - there is only a 10pF capacitor feeding the Drive socket . . . so depending on the impedance of your measuring device, the RF voltage may look quite low on the lower bands.

    Roger G3YRO
  8. HS0ZED

    HS0ZED Ham Member QRZ Page

    Good points, I'd need to look at the transverter circuit to see what they intended it to drive into. Was doing all this on 28MHz so hopefully not too far off the intended.

    Everything "looks" stock, though there has clearly been some hacking in the PA. The cathode meter shunt has been replaced with a couple of 3 ohm 1/4 watt resistors :( Hmm!

    Maybe I need to start again with this. Still, knowing that the driver is producing rated output will then let me isolate any further issues to the PA itself.

    Martin, HS0ZED
  9. HS0ZED

    HS0ZED Ham Member QRZ Page

    Well I haven't been able to get back to the 901 recently. Too much DX about and time spent hunting down the KH1 plus outside antenna work on my 40/30/17 open sleeve dipole. Hanging upside down at 60 feet is always interesting.

    Anyway I spied an old FT101E on the floor of the shack, it's been there for years along with a basket case vy early 101, so early it has no 160m but it does have 11m. Anyway I thought I would have a look and see what shape the driver was in. After removing the hand painted covers it doesn't look too bad apart from the fact that the vfo gearbox appears to have been taken care of by a car mechanic, so full of grease and oil it hardly turns. Stripped and rebuilt it so it smooooooth and free. Then I got to seeing if it worked. remarkably well it seems, Other than a weak output, 70 watts max on 40m, less higher up. But the driver, aha, same circuit in the driver as the 901 but instead of a couple of volts I get about 30v rms on 10m. Seriously different so my next step is to look at the input of the driver and see what that looks like. It seems I have either a driver problem or something before that amiss. In the meantime I think I might just spend a bit more resurrecting this 101E. Need to go hunting in the shops downtown for some 6JS6C's.
    W5INC likes this.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    Well in my experience the original FT101 series is MUCH more reliable than the later rigs, like the FT101Z and FT901 !

    It's why I'd never get rid of my FT101E , which has never had ANY fault from new, despite being used almost every day! (and recently bought a spare one very cheap)

    Roger G3YRO

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