Best forum for Kenwood TL-922 and 922A ?

Discussion in 'Amateur Radio Amplifiers' started by KM5L, Jan 26, 2021.

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  1. KM5L

    KM5L Platinum Subscriber Platinum Subscriber QRZ Page

    Hi, is the best forum here?

    I have a 922A, for a couple of decades - need to figure out what upgrades are viable for the old amp.

    Thanks and 73,
    Patrick KM5L
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Yes this is the Amplifier section . . .

    A very simple upgrade is to directly ground the Grids of the 3-500s. Use some copper strip if you can find some.

    On the American versions you should never switch between SSB and CW while the Amp is on, as the switch really isn't man enough. Really, there's no point in using it in the CW position at all - this was a mod to enable US Amateurs to run the old legal CW Input Power, by reducing the HT Voltage.

    The Japanese versions just used that switch to reduce the Standing Current on CW, and I have modified my TL-922 to do just that. (it's easy to remove the transformer output wires that go to and from that switch, and link them on the PCB they came from)

    I noticed that the Transformers get very hot when using the Amp in a Contest, so to prevent them dying I fitted a fan behind them. If you remove all the wiring that goes to the Voltage selection strips you can easily fit a fan there. (just wire the mains input straight to the transformer).

    I fitted a pair of new 240V fans, one either side, with a switch on the back panel to connect them in series or parallel, for Fast or Slow operation. Works really well, and looks like it's original !

    You can find other mods online that many people say NOT to do, like altering the Anode suppressor coils.

    I did replace all the main PSU electrolytics in mine, but it made no difference, so wish I hadn't bothered !

    If you want to operate break-in CW, you could replace the original relays . . . but I use a footswitch on CW, so haven't bothered.

    I also turn it on with the mains lead going through a big 20 ohm resistor - to provide a manual Soft Start - it's so simple to do that it doesn't seem worth modifying the Amp with a proper Soft Start circuit.

    Roger G3YRO
  3. W1QJ

    W1QJ Ham Member QRZ Page

    The 922 absolutely needs a soft key. Not many new radios can handle the 120vdc switching voltage. If you use a newer radio this is absolutely necessary. Also grid and plate meter protection diodes should be installed immediately. Simple process to do. This will save you lots of grief! The power supply caps and bleeders resistors are some of the finest stuff made back on the day. I suggest changing them only if there is indications of problems. Some replacement stuff doesn’t live up. As Roger mentioned grounding the grids is a suggested mod. It is a very good amp and one of the slight few that actually works well on 160!!!! But be careful you don’t use crap antennas on 160! If you are a CW operator, there are a couple of QSK mods out there that work well. If you operate only phone or other non CW modes, QSK does not offer much help. A glitch resistor should be added as well. Cooling the tubes can be an issue if the amp is used in contesting. Unfortunately you can’t see the tubes while operating to see if they are being run too hot. Remember that the amp was designed for 1 kw pep input basically which would be about 650 output according to the old power rule. In actuality the amp can easily do close to or at legal limit pep to today’s limit. It’s capable but not recommended. The transformer is protected against over heating with a temperature switch inside the transformer to disconnect the PTT should it overheat. It will come back on when transformer cools down. DON’T go crazy with every little thing you see on these amps. What has been mentioned here is all that is needed.
    W1NJC and KU3X like this.
  4. KD9IQO

    KD9IQO Ham Member QRZ Page

    Last week, on eBay, I watched a (40 year-old) Tl-922A filament transformer sell for $305.00. The transformers are no longer manufactured and Dahl doesn't offer a replacement. So, the first thing is a soft-start system. I have created a booklet on TL-922A upgrades. E-mail for a copy.
  5. W1QJ

    W1QJ Ham Member QRZ Page

    A soft start won’t save a filament transformer. If you perform the bias mod, that will protect it.
  6. KD9IQO

    KD9IQO Ham Member QRZ Page

    High-voltage soft-start is on the primary and the filament soft-start is on the secondary. It's in my booklet.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Neither of which causes any problems with the filament transformer. Most filament transformers go bad because the 80/90 volt winding burns up in a G-F short.
    W1BR likes this.
  8. KD9IQO

    KD9IQO Ham Member QRZ Page

    The bias winding is not used for that purpose any longer. The amplifier now uses self-bias (W7RY board). This leaves the cut-off winding free so that I can use it as a magnetic amplifier (saturable reactor).
  9. W1QJ

    W1QJ Ham Member QRZ Page

  10. G3YRO

    G3YRO Ham Member QRZ Page

    The Transformers get VERY hot under prolonged use . . .

    If you fit a second fan behind them, they no longer get hot !

    It really is a very simple mod, get rid of the voltage selection wiring, and just drill 4 mounting mounting holes.

    After a couple of hours on CW, as much heat comes out of the Transformer Fan as the PA Compartment . . . so it's very desirable if you don't want the transformers to fail.

    Roger G3YRO

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