Re: [Amps] L4 power supply problem?

[R.Measures <r@somis.org>]

On Nov 13, 2005, at 12:40 PM, Jason Buchanan wrote:

> hi,
>
> I have an L4 with its original power supply.  Been working fine for a
> long time.
>
>
> Today, I turn it on and after a few minutes there is a large pop and
> blue spark from near the HV interlock, or located behind the rear tube.
>
> The .82 ohm resistor fried - I replaced it with a new one and carefully
> inspected here and there in the amplifier cabinet.  I couldn't find
> anything that burned near the interlock.

The 0.82Ω resistor is supposed to be a glitch resistor, but it is not 
since it explodes during a glitch.  One possible reason it exploded is 
an arc from an anode to chassis.  Another possibility is an 
intermittent parasite c. 90MHz.  If the R of the parasitic suppressors 
has not increased substantially, the problem is likely the arc.  If the 
R of the suppressors has increased more than 30%, it was likely a 
parasite.
-  If you would like to replace the 0.82Ω resistor with an 
energy-absorbing type resistor, we stock them:  15-ohms, 120J surge - 
10W-continuous, WW,  $1.77 each [pn--81.3]

> I powered it back on in CW low voltage position and the plate voltage
> reads around 1300V.  On SSB it reads around 2000V, maybe a little less.
>
> That was all the experimentation I wished to try.  I opened the L4-PS
> and looked around but can't find anything on the top side that looks
> burned or stressed.
>
>
> The thing I noticed that was peculiar was that when I powered the L4
> off, the plate voltage would begin to decrease as expected, but then
> ROSE about 200 volts and then slowly decreased again as expected.  When
> I took the amplifier cover off there was still a bit of voltage in the
> HV supply, around 150V on the meter.  I used a pair of alligator clips
> to ground the cabinet to the top of the plate choke and then removed 
> the
> clips - what was odd was that the voltage rose up to about 100V - when 
> I
> grounded it the meter read 0 again.  When I removed the clip it rose to
> about 50V.  Finally I put the clip on and left it on and it finally
> stayed at 0 with the clips off.
>
> Has anyone seen this before?  Any idea what is wrong...  and what I can
> do to repair it?
>
>
> 73 Jason N1SU
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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