Another Fix - FYI - SB Receivers

[Mark V. Johnson VE3DJU]

Hello everyone. I spent another weekend shooting a bug on a piece of Heath
Gear. Thought I'd share my results. Feedback, suggestions, comments always
welcome.

My SB-310 SW receiver (same design and components as the SB-300 and SB-301)
had an intermittent problem with front end overload and very little
response from the RF Gain control.  It was intermittent, changed with warm
up, sometimes I could bring it on with turning on the Xtal calibrator, it
was never consistent.

I put a VTVM on the RF Gain pot and measured the required -1 to -20 vdc
voltage as per the manual on the pot centre tap, but when the problem was
"on" the actual negative VDC to the RF amp and IF amp grids was low. It
would start out at about -12VDC and then drop to -5 VDC which then allowed
the RF and IF amps to be close to wide open.

I pulled the green lead (AGC) off of the RF amp, grid #1 and no change. I
pulled the white lead (AGC) off of the IF amp board, and the VDC went to
-12VDC. I then started to isolate to the 2 IF amp tubes. I found that by
simply pulling the 1st IF amp tube out (6BA6) the voltage went up to
-12VDC.

Long story short....The tube was no good!! Replaced the tube, had -12VDC
bias with the RF gain control at minimum. No more overload!!! RF gain
adjusts perfectly.

I thought for sure I had a leaky capacitor or some such thing...not a bad
tube....I tested the tube on my tube tester, no indication of short and it
had VERY GOOD emission! Who would have thought!!

Mark V. Johnson
VE3DJU

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