Thanks to all for the initial replies.
I may have not been too clear in my troubleshooting outline below as it was
late and I was tired. I pulled the tubes before the testing outlined below,
ruling out the tubes. I then disconnected the B+ (labeled 1200v in the
schematic) lead of the transformer from the diode board. The amp fired up
just fine at that point. I then tested the diodes & caps, which checked out
fine as far as I can tell with the VOM. When I connected the B+ back up, and
fired the amp up the second time, I had the tap switch in the CW position.
The amp powered up fine. I then flipped the switch to SSB, which immediately
caused the shorting effect, blowing the primary fuses.
With the suggestions received so far, my next steps are as follows:
1. Disconnect the HV lead from the feedthru cap into the RF deck, isolating
the HV supply
2. Check the blocking cap (C25 - .002/15kv)
3. Completely isolate the CW/SSB switch and check for a short from the
contacts to its housing/mounting bracket
4. Leave the amp on in CW for a period to "condition" the caps before
switching to the SSB position
Any other suggestions? Any way to test to see if a cap is breaking down at
the higher B+ potential?
I'll report back as to my findings.....
Thanks & 73
Eric
KE2D
> Date: Wed, 10 Mar 2004 00:51:21 -0500
> From: "Eric Dobrowansky" <kk2ed@comcast.net>
> To: <amps@contesting.com>
> Subject: [Amps] Help with TL922A PS trouble
> Message-ID: <01ac01c40663$b6def310$6601a8c0@eric>
> References: <20040308140312.23D17319538@dayton.akorn.net>
> Content-Type: text/plain;
> charset="iso-8859-1"
> MIME-Version: 1.0
> Content-Transfer-Encoding: 7bit
> Precedence: list
> Reply-To: Eric Dobrowansky <kk2ed@comcast.net>
> Message: 7
>
> Hello to the group,
>
> I am repairing a TL-922A amplifier that keeps blowing the fuses. The
first
> thing I found was a shorted 7.5v zener in the cathode/bias circuit. I
> replaced the diode, and checked the usual stuff, but the fuses still blow.
> No visible trouble (burned components, etc). I then pulled the pair of
3-500
> tubes, checked the plate circuit, and surrounding components. I then fired
> the amp up with the HV shutoff switch active, and the filaments and low
> voltage items worked fine. I then dug in deep and pulled the diode board,
> filter cap assembly, and surrounding assemblies for test and inspection.
The
> caps and diodes all checked ok. I did find a cold solder joint on the -HV
> pin of the diode board, but that may have happened during disassembly.
> Everyting looked/checked good. The bias zener survived this time.
>
> I put every thing back together, double checking everything, then fired
the
> amp up again. This time the HV meter came up to 2500v or so and no blown
> fuses! However, this time the CW/SSB switch was in the CW position. As
> soon as I placed the switch into the SSB position, the buzzing started,
and
> the fuses blew again. I then took some resistance measurements, and traced
> the wiring of the switch to the metering board, where everything checked
> good. Even the transformer secondary windings look to be good as far as
the
> resistance change between the CW and SSB taps.
>
> What next? Is it possible that a diode or HV cap is breaking down when
> subjected to the higher HV potential of the SSB tap? Is it possible that
> the transformer is shorting when the SSB tap is used? I am at a dead end
> this evening, and have quit for now. Maybe tomorrow something that I
missed
> tonight will jump out and hit me in the face!
>
>
>
> 73
> Eric
> KE2D
>
>
>
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