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

[Garey Barrell]

Bill -

Sounds like a failure of the bias supply.  With zero bias on the PA tubes, they will draw as much 
current as the power supply will support until something breaks.  Purely a DC problem, has nothing 
to do with neutralization or other 'signal' presence or absence.

Remove both PA tubes and determine where the bias is being lost.  I don't know anything about the 
820, but there should be ~ -60VDC on the grid pin of each PA socket.  It could be a failure of the 
supply itself, (electrolytic capacitor usually,) or an open connection somewhere.  I think the 820 
has an internal power supply, so no cable / connector to blame!  :-)

Watch out for HV with plate caps loose and waving around!

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>


Bill Kuhn wrote:
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> Subject says it all (mostly).
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> I'm looking at a 820S a friend (new ham) picked up cheap last week.
>
>
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> 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.
>
>
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> Tried the heater again - both tubes glowed very brightly.
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> 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?).
>
>
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> Could this be just neutralization issue? Would that happen with the
> heater on and NOT transmitting (still receiving)?
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>