[Heathkit] Heathkit HW-100 nightmare

[Drew P.]

On annoyances with his HW-100, Dick KF4NS wrote:

[snipped into salient points]

"Trying to trace out the bias is a very difficult route and a
total nightmare. It is like a maze, going everywhere. "

I find it difficult to follow even on the schematic (I have the very similar SB-101).  It looks to me as though adjustable bias for the final grids is also being applied to the frequency shifter in the LMO.  Very strange.  I've been considering redrawing (and freely distributing) a bias system schematic.

"The first thing happened when the HP-23A went into the
self-destruct mode with the rig setting on the bench in RECV mode
while I was out of the room. The breaker in the PS failed to trip and
took out the ICL (current limiter), the diodes in the HV, B+ and LV,
leaving the supply in the shorted condition. (I since replaced the
breaker with a slow blow 3A fuse)."

The SB-600 PS/Speaker for my SB-101 originally had, I believe, a fused line cord plug.  When I got it, there was apparently a replacement cord (non-fused) installed and trace of neither fuse nor circuit breaker. It is mentioned on a number of websites that fused line cords have been determined to be dangerous, but no fuse at all takes it one step further.  First order of business, then, was installation of chassis-mounted fuseholder and 3 wire grounded line cord.  

" Next thing I found was that the BIAS could not be set, adjusting the pot (which tested good) would only give me about 30ma max, then it started pegging the needle."

Could be the pot (intermittent), even though your subsequent tests found it to be good. I've repaired enough audio equipment having fried output transistors or tubes because of intermittent bias pots.  These I like to modify as a voltage divder with a fixed (more reliable) resistor from bias supply to grid feed circuit, and the pot connected as a rheostat from grid feed circuit to ground. With the usual pot failure mode being open, a pot failure sends the bias voltage to maximum and drives the stage to cutoff.  Saves tubes/transistors.  I haven't yet implemented this on the SB-101 for not yet having a complete knowledge of exacly how the complex bias system works.

"Nothing but relay chatter and full drive even with the pot full ccw."

Which pot, Level or Bias? Could still be pot trouble.  I had this problem and it turned out to be instability in the final amp, however. Tighten all screws securing the circuit boards and shields to remedy poor ground connections which develop over time due to oxidation.  Reneutralize the finals using the improved technique mentioned in the service bulletins (no high voltages on final, adjust neutralizing cap for minimum feedthrough).  It has been noted that in some cases, use of 6146B instead of 6146 or 6146A can cause instability.

Check all resistor values, particularly those over about 50k and particularly those having a high impressed voltage.  I found nearly all of the 100k resistors and many other high value resistors in my SB-101 to be grossly out of tolerance (500% and more), intermittent, or open. The service bulletins mention a number of resistors which are susceptible to failure due to underrating.  An IF amp screen resistor which is also part of the s-meter circuit drifts with heat and the s-meter zero adjustment then doesn't hold.  I replaced it with a 2-watt (nice and small these days) metal oxide resistor and the s-meter is now nice and stable.

If the broken coil slug is not too crumbled, yow might try epoxying or supergluing it back together.  Make sure the adhesive is well cured before reinstalling, or the adjustment will be forever permanent.

Drew