On Feb 23, 2005, at 9:34 AM, Georgens, Tom wrote:
> I need some sleuthing to remote diagnose a failed amplifier.
>
> This past weekend I had my TL922A fail during the ARRL CW contest. I
> had
> replaced a bad tube in this unit and did a number of mods. Since then
> it
> has performed flawlessly in heavy contest duty. Until this weekend.
>
> The unit is still out of the country as I could not bring it back at
> this
> time but I was looking for some guidance for a potential field repair.
>
> The amp failed while tuning on 20 meters. There was a burning smell
> and the
> circuit breakers (not the fuses) tripped.
>
> Upon opening up the unit. The two vitreous enamel resistors I used as
> plate
> fuses had melted and broke. Also, the large doorknob blocking cap had
> split
> open and was blackened. There was no other obvious damage or signs of
> distress. The plate choke looked unscathed. I had previously direct
> grounded the grids and removed the grid chokes. The underside of the
> tubes
> looked fine. I regrettably did not check the bias zener.
The zener can short without serious trouble. The only thing that gets
affected is ZSAC.
>
> I have my theory but I was looking for advice.
>
> The plate resistors melting was a surprise and the were not obviously
> burned. The were deformed and cracked.
What W and R rating?
> An alternate theory is that they
> wilted from the heat of the tubes and failed. However, I can't see
> that
> causing the subsequent chain reaction.
Agreed
>
> On the other hand, they may have done their job. I clipped out the
> failed
> resistors and turned on the amp. The HV appeared to be intact in both
> the
> CW and SSB positions.
>
> Any suggestions would be much appreciated.
Check both tubes with a high pot for grid to filament leakage at 5kV.
If it's more than 2uA, that tube may short during operation.
>
> 73, Tom W2SC 8P5A
>
>
>
>
>
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>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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