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

[Bill Kuhn]

Sorry it's been so long since I originally posted.

The culprit was the bias - or complete lack thereof.

It took me a while to track it down, but it turned out to be a loose
connector (RF3) on the RF board that carried the bias voltage. Plugged
it on securely and bias is on the tubes and they are behaving properly
(new tubes!).

Tomorrow I'll set the correct bias voltage, neutralize the tubes, and go
back to the original problem - no S meter reading,

Thanks to all who responded!

73 DE NO0OO

Bill Kuhn

-----Original Message-----
From: kenwood-bounces at mailman.qth.net
[mailto:kenwood-bounces at mailman.qth.net] On Behalf Of Kenneth G. Gordon
Sent: Sunday, June 26, 2011 2:40 PM
To: kenwood at mailman.qth.net
Subject: Re: [Kenwood] TS-820s - Final tubes overheat/toast when heater
ison.

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
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