[Heathkit] Heathkit Digest, Vol 93, Issue 15

[Drew P.]

Gary Hattaway wrote: 

"I'm working on a SB-102 that uses a 6HS6 as the RF amplifier...after a few
minutes, the AGC begins to misbehave, and the s meter swings below zero.
These are classic signs of bad (leaky) RF or first mixer tubes. I've checked V-10, 11, and even replaced them with known good tubes. I've checked all the resistor and cap values around the RF amp circuitry and found everything within specs. I can substitute a 6AU6 for the 6HS6 and the receiver works pretty well, but then the s-meter tends to rest below zero on 15 meters and above. It's fine on 80-20."

As others have mentioned, these are symptoms of an AGC-controlled tube which has developed grid emission. Another cause of S-meter drift in the SB-100/101/102 and HW-100/101 is thermal drift in the S-meter bridge resistors.  The problem is mainly caused by the 2 100K resistors, in the screen circuit of one of the 6AU6 IF amplifier stages, which also serve as 1 leg of the S-meter bridge.  In my case, I replaced these resistors with 1 watt metal oxide parts and the S-meter is now stable.

Drew