SB220 Question - no plate current

[Mark Graalman WB8JKR]

  Jim,

 If you have continuity from the tube grids to chassis ground,
also make sure that you have continuity from the T-R relay
pin 6 to the junction of R1 and R2. And from the filament
transformer CT to the T-R relay pin 9
 Are you measuring the cutoff/operating bias at the 3-500
cathodes?

Mark  WB8JKR


On Sun, 11 Apr 2004 16:46:32 -0400 Jim Mullen <jmullen at ROCKYS.COM>
writes:
> OK Gang, any help with this would be appreciated.
>
> I picked up a pretty beat SB220 at an auction last year. Had some
> Harbach
> mods done, front panel was hacked up and lots of stuff had been
> removed.
> Missing was the meter switch, relative power circuit and the ALC
> circuit.
> Some weird wiring done to make it switchable between 120/240 but
> never
> completed. It was wired for 240. It was from a silent key, his widow
> said it
> did work up to the time he bought a new Ameritron.
>
> I didn't have 240 available in the shack and it was not possible to
> return
> it to 120 without extensive rewiring.
>
> So, I rebuilt the whole thing. Just finished it up, basically
> returned it to
> stock Heath with the exception of the Harbach mods. Found a lot of
> good
> parts on ebay to replace the missing ones. It had the
> rectifier/metering
> board, capacitor bank and parasitic mods when I got it. I added the
> soft
> start and soft key when I rebuilt it.
>
> Started testing today. HV is OK, but no resting plate current. I've
> traced
> all the bias circuits and they seem to be both wired correctly and
> working
> correctly. With my meter I have about 115VDC applied to the filament
> CT,
> changing to 4.8V when the amp is keyed. There is never any
> indication of
> plate current flowing. I traced the HV from the plate caps back to
> the HV
> cap bank with an ohmmeter (no HV probe in the shack) and there is
> continuity. I have no reason to believe there isn't HV on the plates
> when
> the amp is on. The TR relay is switching correctly and the bias
> voltages
> noted above are there also.
>
> Just in case I was looking at a dead plate current meter I put some
> drive to
> the amp. 100W in, zero output to a dummy load, pretty much as I
> expected.
>
> Before I spend a crapload more time tracing circuits I guess my
> first
> question is: If the tubes are bad will there be zero IP? I've never
> had an
> amp with 3-500's, but it's hard to believe there wouldn't be a hint
> of
> current. Yes, the filaments light up just fine, filament voltage is
> 4.75 on
> my meter.
>
> Second question (assuming the tubes are not the problem). Any
> suggestions
> where else to look? It's not that complex a circuit, I'm puzzled I
> don't see
> the problem.
>
> Thanks in advance for any suggestions.
>
> Jim, KK1W

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