Assuming all your work is correct and there are not shorts in any of the
transformers, I would have to say that the amp is going into oscillation which
would lead me to look closely at the parasitic suppressors.
-----Original Message-----
From: Colin Cooper via Amps <amps@contesting.com>
To: amps <amps@contesting.com>
Sent: Tue, Jan 15, 2019 9:31 am
Subject: [Amps] SB-220 Problem has me stumped, looking for some guidance on
what to look at next
Hello everyone,
I bought a SB-220 as a project at Hamvention in Ohio, I think I paid about 400
dollars for it, It was pretty dirty but complete. I wanted to restore one of
these to compliment my SB200 that I have already restored.
The problem:
When the amp is plugged in and turned on resting at idle, it seems to be fine.
However when you key the amplifier the Plate and the Grid meters are both
PEGGED. The meter lights also dim as to indicate that the amp is using quite a
bit of current.
Things I have changed/updated:
I Installed the Harbaugh electronics boards, so it has a new cap string as well
as a new HV and metering board.
I also installed a new TX/RX relay and the “Soft Key” so that my new radios
could key the amp without damage.
I Directly grounded the grids on the tube sockets, removing the mica capacitors.
Things I have checked:
HV: HV is good, the cw/tune / ssb switch works I have 2400 volts in cw and 3000
in ssb
Tubes: I also own two other tl-922 kenwood amps that use the same tubes, so I
swapped a set of proven good eimac 3-500z’s from one of those amps into this
one - no change
120-vs-220 voltage: it does this regardless of how the input power is configured
The Bias Diode circuit on the harbach board, I triple checked my
assembly/installation and see no problems
The 120v Bias/relay Power supply is working.
The Bias relay: I initially thought this was a bias issue and so I triple
checked the configuration of the bias circuit. I have also reconfigured the
bias circuit to resistive bias and there was no change. The grid and plate
meters still peg when keying the amp.
The center tap of the filament transformer: It does not appear to be shorted to
ground.
The amp acts like something somewhere is shorting to ground but only when the
amp is Keyed up.
I would really really appreciate any help or guidance that the group could
provide on what to look at next.
Thank you and 73
Colin Cooper
K8MDX, Grass lake Michigan
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