[Hammarlund] SP-600 AVC voltage

Sat, 06 Jul 2002 07:31:09 -0700

on 7/6/02 12:19 AM, [email protected] at [email protected] wrote:

> hello,
> Just dug back into my sp-600-jx17 and am getting some distortion in AVC
> mode (AM) that cleans up well in MAN mode with RF gain cut back. Checking
> my AVC bus voltages it looks like below -8 volts max (strong signal in
> AVC mode) compared to the standard -51 volts in manual mode and RG gain
> set to zero. My question is shouldn't the AVC mode produce more neg
> voltage? At pin 2 of  V14b my voltage is higher than the final AVC bus
> path-- it drops after crossing the first resistor towards the bus (R60?)
> going from pin 2 of V14b. I havent completely re-capped but cant find a
> short to ground which would lower that voltage via a bigger drop. Maybe
> just a leaky cap. I plan to replace all caps but just want to know some
> theory detail about this corner of the rig. Just seems I should get at
> least -28VDC or so.
> thanks, dave austerman , N5WNM

It really depends on a lot of factors. They don't spec the actual voltage
you should see, because it depends on the tubes and the actual values of the
various resistors in the AVC bus. What they say is that the output should
stay constant over a wide range of RF input voltages.

My experience with the JX-17 has been that it is hard to keep it from
overloading. The receiver was designed with the RF stages running at higher
gain than the IF stages, and they run the AVC bus to V6 and V7 to try to
make up some of the lost gain.

Probably the best thing to do is replace all the parts in the AVC line. This
includes R48 (1M), R53 (3.3M), R97 (3.3M), R60 and R61 (1M), R111 (100K),
C140 and C164 (430 pf but 500 pf works as well). I find all those resistors
routinely wildly out of spec. Unfortunately, they can't be measured
in-circuit.

If it is still distorting, you can reduce R97 from 3.3M down to, say, 2M.
That will give the RF gain control more influence when AVC is on and allows
you to turn it down. Sometimes this helps.

It will still overload the RF section on strong AM stations simply because
of the high gain those stages run at. It was designed for a simpler era . .
.

Let us know how it works out.

-- 
www.jamminpower.com
James A. (Andy) Moorer