HW-101 IF receive oscillation problem after long transmission

[Dick KF4NS]

Charles,
I had the same trouble back when I built my HW100 back in 1970. Got the fix 
from Heath tech. Will look back through the files and send the fix if I can 
find it. By the way, I am still using the rig to this day. Many technical 
upgrades and mods later, I still enjoy using it.
73, Dick KF4NS
St Petersburg, FL 33714 USA
Keep The Glow!


------------------------------Original 
Message-------------------------------

Date:    Mon, 22 Jan 2007 14:30:57 +0000
From:    "charlesmorris at hughes.net" <charlesmorris at HUGHES.NET>
Subject: HW-101 IF receive oscillation problem after long transmission

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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