Colin,
While I'm certainly no tube amp guru, I have a couple thoughts. Have you
tried running the amp without tubes and keying it? How about unplugging
the amp and manually operating the relays and doing resistance measurements?
73,
-John NI0K
Colin Cooper via Amps <mailto:amps@contesting.com>
Monday, January 14, 2019 2:21 PM
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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