HEATH Digest - 27 Mar 2003 to 28 Mar 2003 (#2003-88)

[Chuck]

Randy

Check the -70 vdc bias supply for ripple. 60 Hertz type. I have had
the bias supply caps go bad and the supply develops about 3 to 6 volts
of hum. This is then rectified by the ALC diodes which are usually switched
out of the circuit during tune-up.  Thus tuneup looks great but you can
scream into the D-104 with amp and Microphone gain full clockwise and barely
get a watt or two.

Chuck WD4HXG
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Date:    Thu, 27 Mar 2003 22:13:24 -0600
From:    "Randall C. Iliff" <riliff at COMMSPEED.NET>
Subject: Still chasing SB-101 problems

Hello again...

I had been chasing a problem with very low LSB/USB output and suspected the
ALC circuit.  To date I have not found any obvious wiring or component
issues.  After doing some resistance checks, I hooked power back up and this
time heard arcing when MIC control was advanced but couldn't tell where
(obviously I shut down immediately) and repeated visual inspections haven't
turned up any clues.

Since then I have the following symptoms:

1) Receive side seems to be working fine.

2) Power output in the TUNE mode looks normal on all bands.

3) Supply voltages check out almost perfectly

4) Voltage at the junction of R311 and R312 shows -50 instead of called for
-70 (measured per manual on receive), but R311 and R312 values are in
tolerance.

5) Voltage at the junction of R308 and R309 shows -77 instead of called for
-50 (measured per manual on receive), but R308 and R309 values are likewise
in tolerance.

6) PTT switch closure in LSB or USB pegs the plate current meter, no RF
output indication.

There must be some fairly obvious cause for this situation, but so far I
haven't been able to decode all of the clues.  Any SB-101 wizards out there
recognize these symptoms?

(Sounds like a bias problem to me, but given that I'd rather not fry the
finals or the PS, I'm hesitant to retry the LSB/USB power-on checks.  I've
read through the alternate neutralization procedure, and was wondering if
pulling the plate and screen voltage might allow me to do fault location
with less equipment risk?)

73's

Randy

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