HP 201C questions

[George]

I have a HP201C oscillator, which mostly works. But, on the one freq range that
I need (x1), the output dies after a short period. This sometimes takes just
seconds, sometimes a few minutes. Other freq ranges are OK. A couple of
questions:

1. Is there some "all you've gotta do ..." fix for this problem?

2. How do you open the case up?

I have a schematic.

TIA,
George

 

[George]

I have a HP201C oscillator, ...

2. How do you open the case up?

OK. Rmv the screws in the back, slide guts forward.

Opening it doesn't lead to an immediate soln to (1). There is no black part.

George

 

[Peter Orban]

Hi George,

Your oscillation stops, because the loop gain drops off.
Similar HP generators (the 200, that I have) do have a potentiometer
that adjusts the amplification, and hence the loop gain. Your generator
just might need a little bit of adjustment. If you have the manual, it
describes how to adjust that properly.
Another problem that you might have is the aging of the paper and
electrolytic capacitors in the circuits. The fact that the oscillation
stops at the lowest frequency, might indicate this. In this case you
just need to recap the instrument ;-)

Good luck, Peter

 

[George]

I have a HP201C oscillator, which mostly works. But, on the one freq range that
I need (x1), the output dies after a short period. This sometimes takes just
seconds, sometimes a few minutes. Other freq ranges are OK.

It turned out to be a bad tube. The grid on the 6AU6 developed a soft short (<
1M) to the cathode when it got hot. This was enough to kill the loop gain on
the x1 range, where the bridge resistors were 8.3M, but didn't disrupt the other
ranges, where the bridge resistors were smaller.

George