Wien bridge oscillator help - Eico 377 - Page 1
Forum section: electronics / repair

author: Squiddaddy
author_rank: Regular Contributor
timestamp: May 31, 2023, 01:00:35 am
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: May 31, 2023, 02:55:29 am
content: 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.

author: Squiddaddy
author_rank: Regular Contributor
timestamp: May 31, 2023, 12:01:50 pm
content: 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 31, 2023, 01:10:06 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 01:45:06 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2023, 03:28:15 pm
content: The amplitude measureing part of the ciruit tends to be some kind of peak voltage detector. This can be sensitive to hum coupling in there with the amplifide feedback. Shielding of the part can be important.Another point could be just the temperature modulation of the grid and this way getting a modulation of the amplifier gain, if the tubes are a bit tired. This would be an effect around 120 Hz.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 03:35:56 pm
content: For the traditional vacuum-tube Wien-bridge oscillators, the amplitude stabilization is usually done by a small incandescent bulb or similar temperature-sensitive resistor included in the resistive branch of the Wien bridge.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 31, 2023, 03:43:01 pm
content: Tim is on the money. It is stabalised by a filament lamp.Here's the manual if you want to play along at home:https://bama.edebris.com/manuals/eico/377/

author: floobydust
author_rank: Super Contributor
timestamp: May 31, 2023, 05:58:57 pm
content: It doesn't make sense you are having the problem across multiple instruments, so that would make me suspect the common elements - the scope, the test leads, the work bench, the old capacitors, the new capacitors.You can put the scope on Line trigger and see when it hetereodynes with the sig gen. Or put a loudspeaker on the output and have a listen. No offence but I trust Eico over Rigol.Replacing can electrolytics, you have to pay attention to the grounding, the power transformer CT should connect right to the can/new filter cap's ground for the high ripple current to flow there and not around the other circuits. In 1956 Leo Fender knew how to ground point-point very well but I am not sure how Eico did the 377 and then people get in there and move things around. Note the 6X5 socket does have the line cord connect to a spare pin so a bit of cringe, as long as the line cord+connections are far away from everything else as far as inducing hum.Note there is no hum seen on the Lissajous from the 377 in a movie lol so I would not fault the Wien bridge.bama seems to be down, ping gives a "TTL expired in transit"pic taken fromHackaday Eico-signal-generator-gets-a-repair

author: Squiddaddy
author_rank: Regular Contributor
timestamp: May 31, 2023, 06:46:35 pm
content: Thanks for all the responses so far.I have the schematics and all.Even watched a ton of the YouTube videos on repairs.In this one,, he shows the same issue. At about the 7:00 mark. And others mention the issue in the comments.I completely recapped the first Eico to get rid of the electrolytics and paper. Only 2 micas not changed.The first stage capacitor is only 8uf due to the limit of the 6x5 tube. Maybe it is the ripple there or that ground point.I asked in another forum a while back, and everyone went down the rabbit hole that there was interference in my bench power, AC lines, etc.I took the first Eico and the Hickok to a friends house and used his stuff. Same problem. So I don't believe it's my bench. So I bought the second Eico as a comparison.I replaced the metal can capacitors with individual caps. I am willing to entertain the location and connection points of those as a potential culprit.I appreciate everyone's help. It sure has me puzzled, but I can try your suggestions. I like the suggestion of the filter capacitor current loop. Sure seems to be interference of some type.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 07:18:58 pm
content: The input filter capacitor (limited by the 6X5 specs) would give you trouble at 120 Hz.

author: floobydust
author_rank: Super Contributor
timestamp: May 31, 2023, 07:20:46 pm
content: The power transformer center-tap return current is 60Hz.If it shows up on other Eico 377's then I would suspect a design problem such as bad wiring plan. At some point we'll have to narrow down what the problem is.The second issue I see is the filament wiring - it is chassis grounded on one leg and the hot leg daisy chains to all the tubes, so I have to wonder how/where they are grounded and that makes it back to the power transformer. It would put high currents through the chassis and aluminum/steel/tin/nickel is not the greatest with oxidation.It should use a dedicated twisted-pair for the heaters that returns only to the power transformer, #47 as well. This, and dedicated grounds for the filter caps clean up most of the mess.Unless it's powerline RF getting in- adding a 0.02uF ceramic cap across the rails, filament winding, can deal with that.You could poke around with a scope on the chassis ground connections and see if they are truly quiet, where the cathode's connect etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2023, 07:49:54 pm
content: The wien bridge is amplifying a signal close to the oscillation frequency. When far off the 60 Hz mains hum would be quite a bit smaller when when close to the 60 Hz. With the relatively small capacitor the circuit is quite high in impedance in the low frequency range. So it does not need much (e.g. the filamant voltage) to pick up some hum.From the overshoot / settling of the amplitude control it looks like there is some not so good regulation for slow variations in the amplitude (a few Hz of AM). The details of this response could be changed by the adjustment of the amplitude regulation. The current setttling like like a reasonable compromise in most cases.If there is 120 Hz ripple on the supply and similar 120 Hz temperature modulation for the filament this can modulate the amplifier gain and this would effect the amplitude. Here it makes not difference which sign of the signal is amplified more or less. It make however a difference if the peaks or the zeros are amplified. So the 120 Hz ripple can modulate a signal near 60 Hz. This gets a little like a parametric amplfiier. So 120 Hz ripple can effect a 60 Hz signal, not just a 120 Hz signal.One shuld be able to tell these 2 effects appart from the simple additive hum, as the frequencies should be different. Additive hum would be just a simple beat frequency.The gain modulation should give a faster amplitude modulation for the same base frequency.

author: Squiddaddy
author_rank: Regular Contributor
timestamp: June 01, 2023, 05:53:55 pm
content: An update based on suggestions here.I connected the negative of the first 3 filter capacitors to the same point as the center tap of the transformer. That point is connected to the chassis at the 6X5 socket.I moved all ground points of the first stage 6SJ7 to the same ground point at the 6SJ7 socket.Don't notice much of a change yet.I can see the oscillator signal on the power rails. I guess that's normal?I'm beginning to wonder if all of these kits exhibit the same issue, as they were built cheap.I guess if I'm feeling determined that I could take it all apart and rewire it using some better techniques, like the twisted heaters, a buss ground not using the chassis, common stage wiring, etc.I didn't want to take that path until I had a clear idea on where the issue is coming from. And with it being an oscillator, some placement issues may occur.I appreciate the discussion so far.Thanks

author: Squiddaddy
author_rank: Regular Contributor
timestamp: June 13, 2023, 12:01:29 pm
content: So after some digging, this issue is a 60 Hz beat note problem that seems to plague a lot of these designs including the HP200 series. There seems to be some stray 60 Hz pickup that gets into the feedback loop and modulates the oscillator when approaching 60 Hz or it's harmonics. With the Eico, it must be a design issue with the tactics used to create and build the kits. Heathkit mentions the beat note in their documentation as part of the calibration.There is a service note for the HP200CD that describes a similar issue and lists some grounding/shielding tips to reduce or eliminate it.I noticed that when I placed my hand near the tuning cap and touched the chassis, I could reduce the symptom.If anyone has an older Heathkit or HP200 series on their bench, I wouldn't mind seeing a comparison on whether you can see the issue.

author: jdragoset
author_rank: Frequent Contributor
timestamp: June 13, 2023, 12:34:56 pm
content: I have an HP 200 AB more recent (6V6 output tubes) all electrolytic and 0.5, 0.1 and 0.047 uf film caps have been changed.The 60 Hz beat oscillation is very pronounced when the instrument is out of the case but almost imperceptible with the case installed.

author: Squiddaddy
author_rank: Regular Contributor
timestamp: June 13, 2023, 01:53:24 pm
content: Thanks for that info jdragoset.I noticed that too, that it is better in the case.I just need to try some changes like a 3 prong cord, twisted heaters, some shielding, and maybe repositioning of some components to see if I can get it to where it's not as pronounced.It can bounce several volts at about 57 or 63 Hz, slows as you approach 60, and lock right in at 60 Hz.Thank you for comparing for me. That helps!

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 13, 2023, 10:09:16 pm
content: This discussion raises my curiosity about this low-frequency amplitude modulation which occurs when the oscillation frequency is near the AC mains frequency.This low-frequency amplitude modulation (aka “bouncing”) of the signal amplitude occurs at the difference frequency between the audio generator frequency vs. the AC mains frequency. Example: When audio generator is set to 57Hz with AC mains frequency of 60Hz, then the “bounce” frequency is 3Hz. 60-57=3. This is a “heterodyne” effect.But why does this occur? I doubt it is a circuit defect or fault. Rather I think it is an artifact of using the incandescent light bulb to stabilize the oscillator amplitude.Historical note: The original Hewlett-Packard model 200 audio oscillator (1947?) was the very first to employ a small incandescent light bulb to stabilize the amplitude. Afterwards every other manufacturer of audio generators imitated the Hewlett-Packard model 200 circuit design. In fact most of the later solid-state versions contained a (smaller) incandescent lamp for amplitude stabilization too.I own several traditional audio generators, both tube-type and solid-state. I don’t recall observing this low-frequency amplitude modulation when the oscillation frequency is near the AC mains frequency. But I rarely set any of my audio generators to a frequency between 57-63Hz. Most often I use 20Hz, 400Hz, 1kHz, 2kHz, 10kHz, or 20kHz in my workshop.I’ll try this 57-63Hz range and measure the amount of low-frequency amplitude modulation which my generators have.

author: TimFox
author_rank: Super Contributor
timestamp: June 13, 2023, 10:21:29 pm
content: In the original -hp- Wien-bridge oscillators, the small (3 W) incandescent bulb was in the cathode lead of the first tube; a fixed resistor provided positive feedback to the cathode.The non-linear long-thermal-time-constant element appears as one of the two resistors in the resistive side of the Wien bridge; the other side comprises a series R-C and a parallel R-C circuit, joining at the grid of that tube.If 60 Hz is coupled to the cathode, it could induce modulation in the non-linear resistance of the bulb.My theory (which I can't prove, not having a tube Wien-bridge unit in house) is coupling (either capacitive or leakage) from the AC-driven heater of that first tube to the cathode.Many years ago, on an -hp- AC voltmeter, I did see a faulty tube heater-cathode leakage cause a substantial voltage to appear on the meter with the input shorted; replacing that tube cured the fault.

author: Squiddaddy
author_rank: Regular Contributor
timestamp: June 14, 2023, 12:49:51 pm
content: Thanks elecdonia. That's what it appears like. when you set the oscillator to about 56-57Hz, it bounces heavy, never settles, and slows as you reach 60 Hz then stops. Opposite when you pass it towards 63Hz. Similar to what was shown in the video above from another YouTube user.It's not as noticeable at the harmonics.I have a solid state generator also that does not have the lamp, it has diodes and a JFET, and it does it too. Now that one I haven't replaced any caps yet.I'd be interested to see what your devices do in this range.TimFox, I have swapped and tested tubes in the Eico's. They both have 6SJ7's as the first tube, but one has octals for the rest, and one has miniatures. The solid state one does it too.I could try a 6SJ7 to 6AU6 adapter and try a newer tube.I originally asked the questions since HP made the 200 series for over 60 years, and seem to work fine.

author: floobydust
author_rank: Super Contributor
timestamp: June 14, 2023, 06:16:56 pm
content: OP, you could do an experiment. Power the osc. filaments from batteries and see the difference that makes. Don't float it, still need a ground.Octal tubes did come in metal cans as well, the shielding was important in some applications but people put in glass versions. I had 6SC7 jukebox phono preamp and did not know the metal can version was pretty much mandatory to prevent hum pickup. Although the 377 has a shield for the rectifier, choke and power transformer, it has nothing for the chassis underside. Mains and B+ transformer wiring radiates a lot of hum.I think to fix it, consider DC filament power and an extra shield for the HV AC stuff under chassis, as well as looking at the grounding.