[Kenwood] TS-820s - Final tubes overheat/toast when heater is on.

[Bill Kuhn]

Subject says it all (mostly).

 

I'm looking at a 820S a friend (new ham) picked up cheap last week.

 

When originally tested, it appears to all work w/exception of the S
meter - no response on it at all no matter how strong a received sig. RF
and ALC settings seemed to display properly.

 

Tested into wattmeter/dummy load it seemed to tune up well (close to
100W on 40M - unknown how accurate the wattmeter is).

 

I brought it home to take a look and see if there was anything obvious
wrong/loose/toast/set that was affecting the S-meter.

 

Obvious issue  - VR2 on the IF board, which is used to calibrate the
S-meter, is broken off (legs are still soldered in - looks like the
insulator at the bottom of the VR is still there across the legs, but
nothing above it.

 

Insidious issue - I turned on the heater switch to let the radio cook
for a while. Within a minute, the lights on the radio suddenly dimmed
and I heard what I think was an arcing sound. Turned it off REAL quick.

 

Keep in mind I had seen this radio tested with heat on for at least a
half hour at the previous location. It seemed to be behaving fine at
that time. I do not see anything loose and nothing fell out when pulled
the covers later.

 

I looked for anything obvious and didn't see/smell anything.

 

Tried the heater again - same result.

 

Opened the final cage assuming it has something to do with the
tubes/heat. Turned on with heater. One final glowed brightly (obviously
not idling glow) and the other didn't glow.

 

Tested finals on my IT-17 tester. The one that didn't glow was dead. The
one that had glowed brightly in the radio tested OK (not real great
though).

 

I replaced the dead tube with  in a NOS (tested GOOD) 6146A.

 

Tried the heater again - both tubes glowed very brightly.

 

Tried checking BIAS on the IP setting (w/o warming it up more than a few
seconds). No reading on the meter (maybe related to the S-Meter prob?).

 

Could this be just neutralization issue? Would that happen with the
heater on and NOT transmitting (still receiving)?

 

I'm hoping not to roast more tubes trying to figure this out.

 

Help?

 

73 DE NO0OO

 

Bill Kuhn

Middle-of-Nowhere, KS