[Amps] AMP INT LK500ZC

[R.Measures]

>Serial number 4017.
>
>Symptom:  When A/C power switch is turned on, plate current is 325 MA and
>does not go to zero.
>
Bee - - 
*•  Sounds like maybe a grid/filament shorted (0v bias) tube.  Typically, 
the short only occurs when the filament is hot/expanded.  Thus, an 
ohm-meter check on a cold tube will not detect the problem.

>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.

**  Do the two other tubes draw 300+mA anode/plate current on standby?
>
>Measures 5.79 rather than 5.0 filament volts (T1). 

**  Measured at the socket pins or at the xfmr?

> Caps C24, C25, C22,
>C23 and C28 check OK.
>
>Should D2 the 1N3308 Zener Diode be suspect?  This component seems
>difficult to check.

**  A simple ohm-meter check will reveal if the zener is shorted in both 
directions.
>
>Request help to troubleshoot the above described symptom.
>
>Thanks,
>
>Bee, K4JLD