Electronics > Repair

Wien bridge oscillator help - Eico 377

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Squiddaddy:
I need some help with a couple of wien bridge oscillator repairs. I like to restore vintage equipment. I have two Eico 377 audio oscillators, vacuum tube based on the old HP 200A. I also have a Hickok Teaching Systems solid state oscillator of similar wien bridge technology. They all have a similar problem. I replaced old leaky capacitors and out of spec resistors. I did not touch the solid state one yet. They work great except on the lowest range.
There is an amplitude bounce on the oscilloscope waveform when getting near 60Hz or it's next two harmonics. It seems like it bounces at the difference frequency from 60Hz. For example at 57Hz, it bounces at about 3Hz. It's like the lamp feedback stability is having trouble evening it out. It's odd that they all have the same issue.
I read everything I could about the wien bridge circuits, but I cannot seem to solve it.
I don't believe all those years of HP making these, and others copying them, that this would be common.
They are supposed to be very stable and low distortion.
Anyone have any experience with these and can provide any ideas?
Thanks.

floobydust:
What you describe sounds like the internal power supply has AC hum and ripple heterodyning with the main oscillator.
At this age, electrolytic capacitors are pretty much toast and fail low value. Check the main filter capacitors are OK.

edit: proof is the wobble happens at harmonics of mains frequency, like 60, 120Hz.
It can be seen also if there is a ground loop between a scope and sig gen. but not sure if this gear has a three-prong power cord, earth-ground or not.

Squiddaddy:
I did replace all of the power supply caps.
I suspected 120Hz ripple too, but it measures very low.
I tried adding a 3 prong cord, no change. So I put it back to the original 2 prong. The solid state one has a 3 prong on it.
I have tried both with and without an isolation transformer.
Used both a digital scope (Rigol) and an analog (Tektronix). Same waveform display.
I can't seem to grip on why they all exhibit the same behavior. Like you said, some kind of hetrodyne or beat frequency going on.
Two Eico's are even different vintage. One has octal tubes, and one has miniatures. One has a tube rectifier, one has diodes.
The solid state one uses FET's. (From what I could deduce, no documentation on it.)
I even at one point rewired the filament circuit on one to use a twisted pair and 100 ohm hum resistors, with the 3 prong cord, thinking it was 60Hz AC hum. No change.

Thanks for your thoughts and insight. If you can think of anything else I can try, I'll give it a go. Been monkeying with these for months now. That's why I bought a second Eico last week, to compare.
I know they are vintage, and can't hold a candle to today's stuff, but they should still be pretty good.

Andy Watson:
One thing that can make oscillators (and amplifiers) unstable is unintentional DC or low frequency gain. Usually, the DC path is blocked by capacitors (I'm looking at C7 and C9), if these capacitors are leaking it can give rise to unintended gain at DC or LF. Capacitors like C7 need only leak a few microamps to totally shift the biasing of V2 - and provide a DC feedback loop via the power-supply.

TimFox:
Another possibility for problems around 60 Hz with the vacuum-tube units is a tube with a heater-cathode short or excessive leakage.
Problems around 120 Hz are more likely with power supply capacitors.

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