[Hallicrafters] Voltage question re SX-71

[RJB]

Yes, the B+ voltages are right on spec, so i think it is somewhere in the
cathode bias/AGC circuitry, but am running out of things to replace :)

The SX-71 has a fairly messy AGC/MGC/grid bias system, no two tubes using
quite the same setup (unlike the NC183D in which all the RF and IF amps use
the same circuit) - a minor problem in grid bias voltages can affect the
whole amplifier chain.

Bob

-----Original Message-----
From: hallicrafters-admin at mailman.qth.net
[mailto:hallicrafters-admin at mailman.qth.net]On Behalf Of Joel R. Hallas
Sent: Thursday, March 25, 2004 7:34 AM
To: Lee Bahr; rjb at lynden.com; hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] Voltage question re SX-71


Well, the line voltage was my first thought too, until he said the B+
voltages were right at the supply.

I think the clue may be the cathode voltages, indicating (and contributing
to) lower or higher tube current. If the tube currents are higher or lower,
that will impact the plate votage significantly. Often times it is a result
of AGC action and/or problems, so I'd start there, especially if the non-AGC
(audio) sections are closer.

Alternately, if you like the performance, just note the "new" readings in
pencil for future reference and go with the flow!

Regards, Joel Hallas, W1ZR
http://mailman.qth.net/mailman/listinfo/hallicrafters