HW-101 IF receive oscillation problem after long transmission

[charlesmorris@hughes.net]

I'm new to this list, but have had Heathkits for a long time (and loved 
to build them, probably like most of you :) 

Anyway I've had an HW-101 for a long time as my primary rig...I 
acquired it in the mid-90's and it was built by another ham back in 
1977. There was a gradually increasing "shot noise" problem over the 
last couple of months which turned out to be a noisy 6AU6 at V4 (2nd 
receive IF). I replaced it with another new surplus JAN-6AU6-WC.

But now it has another IF problem, an intermittent oscillation after 
long periods of transmitting, which I discovered this weekend. I 
usually keep my transmissions short but on Saturday morning I had the 
chance to be a bit long-winded. When I let go of the mike button, the 
receiver was completely cut off and the S-meter was nearly pinned. It 
very slowly returned to normal over about 15 seconds. On subsequent 
short transmissions, operation was normal, but on any extended key-down 
it recurred.

I got out the schematic, took the cover off, and quickly discovered 
that something was driving the AVC line several volts negative as soon 
as I released the PTT after long transmissions. Just holding the PTT 
closed with no modulation did not cause the problem. No one Heath 
factory service bulletin was a direct clue but I did come across this:

>SEPTEMBER 21, 1984
>HW-101 Bulletin No:
>SSB Transceiver HW-101-82
>S METER DRIFTS; IF OSCILLATES
>
>Check the brand of 6AU6 tubes at V3 and V4. If a brand other than GE 
is used at these locations, replace them with GE brand tubes. Parts 
replacement will stock only GE brand of 6AU6 tubes [PN 411-11].

Not very helpful - and as noted I do have GE-made JAN-6AU6-WC tubes in 
V3 (the 1st IF stage) and V4 (the 2nd IF, which drives V13, the AVC 
rectifier). I tried swapping some other 6AU6's and it didn't help 
(including a brand-new 6AU6-WC).

Putting the scope on the output of V4 did indeed show the oscillation 
at about 3 MHz (at or near the IF frequency), which slowly died out 
over a period of about 10-20 seconds, while the AVC voltage returned to 
normal. It would only recur with extended talking. Although I could not 
see anything coming out of V3, (old Tek 515 scope,10x probe)  as a test 
I pulled V3 from its socket and the oscillation coming out of V4 
stopped instantly.

Also, V4 is only used during receive and is definitely being cut off 
during transmit (its screen voltage is removed), and V2 (Isolation Amp 
between the balanced modulator and V3 which is also the first transmit 
stage)  is cut off during receive (screen voltage removed and actually 
biased slightly negative). I verified that this was actually happening, 
too. V3 is "on" during both receive and transmit, so I concluded that 
that stage has to be the source of the problem.

What I can't figure out is the underlying mechanism of the 
oscillation. I believe that the stage runs hot during receive and even 
hotter during transmit (the grid bias while transmitting is set from 
the ALC voltage that is less negative than the AGC signal). It has been 
suggested to me on other lists that the problem is grid emission from 
overheating/overdriving. Certainly the slow speed of onset and decay 
also suggests a thermal cause. Does the stage gain vary with internal 
element temperatures (aside from the heater, of course)? Is this 
something that can be fixed by, for example, redoing the screen bypass? 
Or do I need to reduce the gain a bit, or move the operating point?

A couple of ideas come to mind:
Make sure the screen is properly bypassed to ground.
Move the .02 cathode bypass cap to the other side of the 47 ohm 
resistor, thereby introducing a bit of degeneration.
Increase the 22k screen dropping resistor to lower the plate current 
and gain a bit.

I really don't like designs (or repairs) that are dependent on a 
particular brand of tube... any thoughts?
thanks for any help.
-Charles

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