[Kenwood] TS-820 ALC more yet

[Kenneth G. Gordon]

On 11 Mar 2012 at 20:36, Steve wrote:

> At 06:40 PM 3/11/2012, Kenneth G. Gordon wrote:
> >According to my calculations, it will take 26 minutes for the ALC to
> >leak off to zero with no audio input as it now is.
> >
> >Sounds like an open resistor to me...
> 
> In the glorious good ole ancient days of Heathkit, The 
> SB100-SB101-SB102 series transceivers suffered a very similar 
> symptom, except it affected AGC. After transmitting, when the radio
> went back to receive, the S-meter would peg and the sensitivity
> wouldn't come back until the capacitor that was charged by the leaky
> diode had discharged, just as if the RF gain was turned all the way
> down; usually took up to 15 seconds to fully recover. IIRC, it was
> usually a diode in the ALC circuit that had become leaky, I believe
> associated with the final's' screen grid power source. IIRC, the
> condition of the final tubes seemed to contribute, in that new,
> non-leaky tubes would appear to alleviate the problem. But pretty
> soon, the new tubes would "wear in", causing the problem to re-occur
> and gradually become worse.

Yes. That problem was fixed by a late Heathkit service bulletin: what was 
done was to short the final amp screens to ground when the rig switched 
back into the receive mode. It required a minor rewiring of one of the relays.

What was occuring was that a static (steady-state) voltage would build up on 
the final amp screens, sometimes from "grid emission", and would hold the 
AGC level high.

I have all those details up on my website at www.w7ekb.com/glowbugs/

Thanks.

In the case of this TS-820S, I am not sure what is the cause. I am going to 
short the ALC line to ground temporarily to see if that helps.

The problem may have something to do with R-44 on the RF board, or the 
circuit around it.

What has me baffled is that sometimes everything works exactly as it should. 
Then the next time I turn the rig on, it is back to its old tricks, and nothing I 
have yet found will "fix" it, not even temporarily.

vy 73,

Ken W7EKB