SB-1000 PROBLEM

[Jeff Goldman]

I had a similar problem with a Heath SB-221.

When I would turn it on, the T-R relay would  cycle on and off, by 
itsefl (there is no standby/operate switch on the 221) , with nothing 
connected to the back panel.  When the relay was keyed, the plate 
current was about 50-60 ma.

I looked at the separate 120VDC supply that ran the keying circuit, and 
it was cycling from about 130-400 volts.  CRAZY, when the transformer 
was only producing 80 VAC on that supply winding.

I finally removed the tubes and did some testing.  With the tubes out, 
all the control circuits seemed OK.  The final problem was that one of 
the grid RF Chokes (to ground the grid DC wise) was open.  This allowed 
the grid of one of the tubes to float, and when not keyed, that tube 
still conducted current (maybe a few ma.  With the cathode not grounded 
(since the amp's T-R relay was not keyed) the current would slowly 
charge up the keying voltage supply, which is also the cut-off bias 
during standby.   When the bias supply/keying relay supply would get up 
to about 400 volts, the relay itself would short internally to ground 
and it would pull in,  This discharged the 400 volts, which was then 
not enough (down to about 130 VDC) to make the relay short out, so it 
would drop out.  Thus the relay cycling every few seconds.

Replacing the bad RF choke, and reinstalling the tubes, there was no 
more problem.  I also replaced the T-R relay for good measure since it 
had been damaged internally.

So, my suggestion is try operating the amp (with nothing connected) 
with the tubes removed and see if the control circuits work in a normal 
fashion.  If they are OK, check all the components around the tube to 
see if they are OK.

AND BE CAREFUL.  HIGH VOLTAGE IS THERE, SO REMOVE THE MAIN AC PLUG 
EVERY TIME YOU ARE WORKING ON THE AMP.

Good luck finding the problem, and let us know what you find.

73,
Jeff, K3DUA

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