[Hammarlund] HQ-110 Regulator tube acting up

[Kenneth G. Gordon]

On 5 Oct 2012 at 10:31, Al Schichler wrote:

> I have an HQ-110 that has worked fine for close to a year, but when I 
> fired it up the other day, the OB2 tube was glowing and flashing really 
> bright orange. I could see it through the dial scales. Almost looks like 
> a fire is starting inside. I also can hear an oscillation in the speaker 
> when that happens.
> I know it normally glows, but not enough to light up the whole inside of 
> the radio. I was just wondering if this might be a problem anyone else 
> has run into. I haven't taken the radio apart yet, so I don't know if 
> replacing the tube will fix the problem, or if something is shorting out 
> like a capacitor, and causing the tube to go crazy.
> I might have another OB2 in my boxes o' tubes, but I haven't checked 
> yet. If not, I might be in the market for one.

That is normally caused by the VR tube itself going into oscillation. Usually, the VR tube can 
only go into oscillation if there is some capacitance directly in parallel with it somewhere.

The first thing I would check are the filter caps, although since there is a 4K resistor between 
those and the VR tube, unless C-73A has changed value drastically, I would think that would 
not be the problem.

What also could be happening is that one of the decoupling resistors in the bunch of stages 
that the VR tube feeds (like R-28 or R-41 or others) is shorted, connecting one of the .01 mfd 
bypass capacitors directly from the VR tube output to ground.

In any case, it is most probably NOT the tube, although this oscillation may have damaged it.

Ken Gordon W7EKB