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Kenwood TL-922 Fault

Discussion in 'Amateur Radio Amplifiers' started by G3YRO, Oct 17, 2019.

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  1. G3YRO

    G3YRO Ham Member QRZ Page

    I've had this amplifier for a couple of years and used it regularly without any problems. The only Mods I have done are to directly ground the grids of the 3-500Zs, replaced the main PSU Electrolytics, and also fit a more powerful Fan on the PA compartment, and a similar Fan behind the Transformers.

    I never use it on break-in or VOX . . . use PTT on phone and a Footswitch on CW.

    I was in the middle of an over on CW the other day when suddenly there was an almighty great "Crack" - like a high voltage arc !

    I immediately turned it off and left it til the morning. When I turned it on again, there were a couple of strange things . . . firstly the Relay went from Standby to Operate a couple of times (momentarily) all by itself !

    Also, the Standing Current varies . . . sometimes it's zero until I apply some drive, then it's normal . . . sometimes it's a bit high.

    Other than that, the amp appears to be fine (although I haven't used it properly, until I investigate . . . only turned it on for less than a minute at a time)

    I haven't opened it up yet, but I'm thinking that perhaps the problem could be that the relay contacts need cleaning? (although I will check the resistors either side of the bias zener)

    But what could have caused that very loud arcing noise?

    Roger G3YRO
     
  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    I do not believe anyone can answer your post without pictures of the amplifier inside. You have to address the arc issue and fix it. It should have left a visual mark inside.
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    What resistors on either side of the zener are you referring to? A loud snap is usually a HV arc directly to ground or through some resistance like a bug or something. Even dust build up can cause an arc. The tube drawing current in stand by is usually a short between grid and anode or maybe cathode. Often times HV arcs will leave a trace. Also I have seen the top of the tube area or the parasitic suppressors arc to the top plate if the clearance is tight.
     
  4. G3YRO

    G3YRO Ham Member QRZ Page

    I know the arc is a mystery . . . although may be apparant when I take the Amplifier apart. (as suggested, maybe an insect got inside !)

    Although the Handbook does mention a "Spark Suppressor", which is a spark gap to stop the Tune variable capacitor arcing if ever the Antenna relay drops out - maybe that is what I heard? (has anyone who owns a TL-922 heard this - is it very loud?)

    But I was mainly wondering if anyone had any ideas what might be causing the variation in Standing Current, ever since this happened.

    Presumably one has caused the other . . . but I can't think of any logical connection.

    Anyway, will take the Amp apart when I get some time . . . obviously not going to use it until I've found the problem.

    Roger G3YRO
     
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    Are HF amplifiers expensive in England compared to the USA?
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Assuming the bias system is still working (pretty easy to check), if ZSAC is changing all on its own, sounds like at least one of the tubes is suspect.

    3-500Zs can develop ion arcs internally that cause damage without fatality. A glitch resistor usually limits the energy to non-destructive levels, but no guarantee of that. What Drake did in the L-4 series was to use a sacrificial glitch resistor, a wirewound 0.82 Ohm, 3W resistor which just sacrifices itself and burns up in the event of a tube arc. I don't think Kenwood did that. But even with the Drakes, if the resistor blows up that doesn't necessarily save the tube! It's there mostly to limit save power supply components.:p
     
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    If it was vary loud, Maybe a cap blew. Some can sound like gun fire.

    Good Luck.
     
  8. G3YRO

    G3YRO Ham Member QRZ Page

    I've been trying to get my head round the Spark Gap in this Amplifier . . . and now get what it's for.

    If the Amp's antenna relay dropped out just as it was getting more drive, this could damage the Pi-Tank capacitors . . . as a lack of load on the output would cause very high RF voltages to appear. So to prevent damage, there is a Spark Gap on the output of the Pi-Tank, that will arc if ever this happens, to get rid of the high voltage and prevent damage.

    However . . . although I use a footswitch, so this should never happen, if the relay is slightly faulty - or maybe just dirty contacts - this could cause the Spark Gap to arc - and maybe that's what I heard !

    Given that the bias voltage obviously goes through this relay too, again a faulty or dirty realy contacts could cause the standing current to vary.

    Anyway . . . I'll be taking the Amp apart later . . . and will report on what I find !

    My perception is that secondhand ones are often slightly cheaper . . . I paid £600 for this TL-922 three years ago. (in mint condition)

    Roger G3YRO
     
  9. N8FVJ

    N8FVJ Ham Member QRZ Page

    Ok, same cost as USA. When is England going to increase power to 1.5 KW?
     
  10. G3YRO

    G3YRO Ham Member QRZ Page

    To be honest I doubt if they will ever . . .

    The reality is there are never any station inspections any more . . . so I guess people feel there is no real enforcement, unless you are causing serious interference issues.

    I imagine many experienced Amateurs feel it's a bit like Speed Limits, and agree with Douglas Bader's famous quote :

    "Rules are for the guidance of wise men . . . and the blind obedience of fools " !

    Roger G3YRO
     
    KD2ACO likes this.

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