[Amps] TL922 Failure

[R.Measures]

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