On 4 Mar 2004 at 16:54, Bee Walton wrote:
> Serial number 4017.
>
> Symptom: When A/C power switch is turned on, plate current is 325 MA
> and does not go to zero.
>
> Plate voltage is normal at 3200 volts DC. Grid current meter goes
> negative; pinned to left side of meter. Do not use QSK.
>
> Troubleshooting thus far: Disconnected B+ line at power supply board.
> Power supply is OK. Substituted two other known good 3-500Z tubes;
> tubes are OK.
>
> Measures 5.79 rather than 5.0 filament volts (T1). Caps C24, C25,
> C22, C23 and C28 check OK.
>
> Should D2 the 1N3308 Zener Diode be suspect? This component seems
> difficult to check.
>
> Request help to troubleshoot the above described symptom.
First guess: Grid to Filament short in a tube. Since you've tried
substituting "known good" tubes, we get to...
Second guess: filament to ground short ... a shorted zener shouldn't
result in a high plate current in "standby" because the cathode
(filament) floats up to where the tube is essentially cut off. But
if the cathode is shorted to ground it can't do that, resulting in
high current even in standby.
I found a blob of solder under the filament RF choke in an LK550 that
caused exactly that sort of a malfunction.
It could possibly be a filament winding in the transformer that is
shorted. Here's hoping it's not the transformer!
73,
George T. Daughters, K6GT
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