DRIFTING HW-101

[Kenneth G. Gordon]

On 21 Jun 2007 at 13:59, Michael Waldrop wrote:

> I doubt the heterodyne oscillator and/or the carrier generator,
> CW/TUNE/USB/LSB, oscillator(s) are the cause of the drift.

Me too, actually. However, I supposed it WAS possible...

> Sometimes, not always, changing V20 "6AU6" VFO tube help
> will reduce the drift.

Although that certainly could be the problem, that tube is actually only a 
buffer amp, not the oscillator. The oscillator is an FET.

However, despite that being only a buffer amp, I recently worked on a 
friend's HW-101 which had a REALLY WEIRD RIT setup which did 
something to the buffer tube to make the VFO shift frequency. It was 
not connected to the oscillator section at all, but only had a wire 
connected to one of the pins of the buffer tube by being wrapped 
partially around a pin of the tube, then the tube was plugged back into 
the socket.

When the operator turned the "RIT" control, it varied the VFO frequency 
in ONE direction, but not the other, and then over only part of the range 
of the pot.

It was also mounted on a large circuit board and had a really big relay 
associated with it.

I suggested to my friend that he disconnect the stupid thing and remove 
it.

> However, it may take one or more tubes
> to find one that reduces the drift.

Yes. 

> Check the voltage on pin 5 and 6 of V20, the VFO tube. The voltage on
> the rear VFO B+ pin should be 150VDC. Pins 5 and 6 of V20 should be
> 149VDC.  If the voltage varies then check V18, an OA2 voltage
> regulator tube.
> 
> V18 regulates the VFO's 150VDC B+, obtained through the voltage drop
> across R304 and R305 (1K 7 watt and 2.5K 7 watt) resistors from the
> 300 VDC B+ on pin 3 of the rear power connector.
> 
> Q942, a 2N3393 zener, may also need replacing. Q942 regulates the
> VFO's V20's negative cathode voltage.

No. What is happening there is that the FET oscillator gets its operating 
voltage, about 8 VDC, from the cathode circuit of the buffer. The 
2N3393 transistor gate-to-emitter connection acts as an 8 VDC Zener 
of a sort to regulate that 8VDC. The collector of that transistor is not 
connected to anything.

Personally, I think that method is kinda dumb since if the current drawn 
by the 6AU6 buffer tube were excessive or varied too much, the 
"regulated" voltage to the FET oscillator could also vary, causing 
severe drift.

Possibly this is why our friend's HW-101 is drifting. If it is, choosing a 
good 6AU6 WOULD stop it, or at least reduce it.

However, it is an extremely simply project to build a 12 VDC power 
supply using the filament voltage as an AC source and regulate the DC 
output, then feed THAT voltage to another LM-317 or something like it 
to provide the twice-regulated 8VDC to run the FET with.

I use a half-wave rectifier circuit from the filament line, exactly as 
Heathkit does to provide power to the solid-state LMO in the SB-102.

Doing this would eliminate any effects on the oscillator caused by the 
buffer tube going bad on you.

Ken W7EKB

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