SB-220 Troubleshooting

[Goss, Ed]

Hi Folks,
     A friend just bought an old SB-220; fairly good physical condition but it's been stored for a number of years.  He brought the unit up on a variac (120 v input); no problems so he switched to 240 v power.  He sent me the following:

I installed a 240 VAC
> receptical yesterday to power the amp. All ok in receive mode, but the
> instant I activated the Antenna Relay Tx mode, a very loud pop sound came
> from the amp. No RF input was applied yet. I immediately switched off the Tx
> mode and main power. When turned back on, the everything seems normal in
> receive standby mode.
> 
> From what I have read about the amp, this could be anything from antenna
> relay problems, a shorted tube (grid to cathode), or parasitic oscillations.
> I wouldn't think the latter is likely since no RF input was applied yet. The
> method used to switch from standby to Tx was a foot switch. 

Any ideas of what to check first?

--Ed--

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