SB-102, the saga continues

[Bob Groh WA2CKY]

Probably some of those more familiar with the SB and HW tube line will contribute a more solid idea, but I would also suspect bad grounds on the boards. It certainly sounds like oscillation and possibly due to RF feedback all the way back to the input of the driver. So, if you haven't already done so, I would carefully loosen and then then tighten all PCB mounting screws. You might also keep a close eye on the connections for any signs of corrosion, etc.  Just a thought.

73
Bob, WA2CKY

----- Original Message ----
From: Emil Dular <WD4SCZ at AOL.COM>
To: HEATH at LISTSERV.TEMPE.GOV
Sent: Sunday, September 23, 2007 1:16:37 PM
Subject: [HEATH-TEMPE] SB-102, the saga continues

Thanks to all,

A couple have pointed out the finals must be self-excited. 

I pulled the the driver and also subbed another 6CL6. With the tube out of
the circuit the idling plate current on all bands goes about 60 milliamps.
With a tube in and the bias adjusted for 50 milliamps on 80 meters it is 50
on all the other bands except 40 meters. A quick check with "original" 6CL6
it will be anywhere from 150 to 250 milliamps and bias adjustment has no
effect. And it is oscillating to beat the band, plenty of something into the
dummy load. The second driver starts out at 60 or so milliamps but with
seconds starts a slow rise over the next 5 seconds to 150 milliamps.
Swapping out drivers was the second thing I tried.  Neutralizing the finals
again was first. What am I not seeing?  Guess the next thing to do is slide it out of the case and see if doing the voltage checks gives me a clue.

Emil WD4SCZ

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