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

[Kenneth G. Gordon]

On 26 Jun 2011 at 12:53, Bill Kuhn wrote:

> Subject says it all (mostly).
> 
> I'm looking at a 820S a friend (new ham) picked up cheap last week.
> 
> 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.

As Gary Barrell said, it sounds as though the bias to the finals is 
missing.

Yank the finals, and being EXTRA careful, with the rig turned on, 
measure the grid voltage at pin 5 of the tube sockets. It should be 
somewhere between -47 and -60 VDC.

If that is missing (and I'll bet it is), the first thing I would look 
at is the bias adjust pot, since you say that the "S" meter adjust 
pot has been broken off the circuit board. Perhaps the same thing, or 
something similar, happened with the bias-adjust pot.

Although I have a copy of the TS-820S service manual, I have not had 
time to go through it yet.

Ordinarily, with these types of SSB  transceivers, one of the 
adjustments that is ordinarily made is to adjust that bias-adjust pot 
until the finals draw a certain specified current with no signal 
input from any mode, but with the rig keyed. Usually, one sets the 
mic gain to zero, and in one of the SSB modes, simply keys the rig on 
with the mic. switch, and with the meter switch set to Ip, adjusts 
that pot for the correct current.

Usually for class AB-2 and a pair of 6146s, the "idle current" is 
about 50 mA when the plate voltage is about 820 VDC.

If the plate current is pegged with no signal input, but with the rig 
keyed, then the most likely problem is that the grid bias voltage is 
missing or extremely "low".

If the bias-adjust pot is intact, yet adjusting it does nothing, then 
the next most likely problem is that the final grid-bias supply has 
failed.

None of these problems should be particularly difficult to trace and 
fix.

Of course, the problem MAY be something else, but these are the most 
likely ones.

I see from the schematic that there are parasitic chokes at each 
grid: perhaps one or both of the coils in those have burnt open, or 
otherwise failed. This would also remove the correct bias from the 
finals, but that sort of problem would be most unlikely since the DC 
current in that circuit is very low.

Ken Gordon W7EKB