Oscillator repair? What kind of oscillator?

[eccerr0r]

I have this broken oscillator, it should be 11Hz to 110KHz, but it doesn't seem to work properly, the output is really "noisy" - Something seems to have gotten open circuit.

Bad capacitor for sure, or perhaps an open circuit somewhere. However this seems to be a homemade unit, so not sure if there's data available anywhere.

So I did a partial circuit extract to figure out how this works, to find out whether or not this should be generating a sine wave generator as that could be useful (though a square wave is fine too). As I started drawing the schematic, I was kind of baffled by it: What kind of oscillator, and why is it built this way?

This looks like a multivibrator oscillator, using a zener diode to keep voltage constant to maintain a bit of frequency stability.



I'm sure I have a lot of extraction errors here, as this was a vectorboard homemade unit and thus a bit tougher ("virtual" multilayer). Any comments about this circuit (It's not complete, but has the oscillating portion. An oscilloscope attached to this portion of the circuit exhibits the same problem as the output, so the output stage looks OK...

Note: It uses 2N3393 transistors, not 2N3904s...

Any ideas? I'm wondering if I should just junk this circuit and replace it with a few op amps...

[stj]

Re: Oscillator repair? What kind of oscillator?

it will take me a while to see that - what the hell is with the colour choice!

go download eaglecad.

[stj]

Re: Oscillator repair? What kind of oscillator?

i can understand that, but there is a lot of feedback going on - any cap problems will probably be very problematic.

[eccerr0r]

Re: Oscillator repair? What kind of oscillator?

Not sure why it didn't do the usual black on white... alas, it's just gschem, part of gEDA. Looks fine on my screen, alas I normally use a black background for most things (leftover from CRT days)

Not sure why the multivibrator, or is it just that - the 5 transistors along the bottom are the oscillator, the one on top just looks like a follower amp. Why it (and others) are hooked up in Darlington mode I don't know. The single transistor on the rigth-bottom I'll need to look for a pullup somewhere, it does look like in common emitter form but nothing pulling it up to vcc...

[Longbow]

Re: Oscillator repair? What kind of oscillator?

I cannot tell from your words if this is your circuit design. But since it looks unique, I'll assume that it is. A fresh schematic might help and a brief description of why you made the choices that you did. I have never seen darlington connected transistors used like this. Was this device meant to be a Wien bridge oscillator? There is no obvious signal output and much unnecessary duplication of parts.

[eccerr0r]

Re: Oscillator repair? What kind of oscillator?

I didn't design this circuit, else I'd have a complete schematic somewhere. Someone tossed it and I suppose I was the third person to get my hands on it. This was just a rough sketch of what I could hand extract from the circuit. Also note: it's not complete. The output stage "power amplifier" is omitted as I suspect if I can get the circuit to oscillate correct at the output of this first emitter follower stage, the rest can be easier to diagnose.

I can't tell if this was supposed to be a Wien bridge or not, the connectivity looks really weird to me. The signal "output" is in the center of the schematic after the darlington emitter follower stage in the upper right of the schematic. I will need to check the select knobs and make sure it's not dual stage where it selects two resistors/capacitors for each detent...then it very well may be some sort of sinusoidal or Wien Bridge.

I'd like to know what was going on in the designer's head too... Just baffling to me.

This is an adjustable oscillator which is even weirder. The adjustable elements, R and C are towards the right/bottom. If the switches isn't actually a split select, then it would make it confusing as to how it could be (a) sinusoidal and (b) 50% duty cycle across all frequencies.

Weeeeird!!!

[Longbow]

Re: Oscillator repair? What kind of oscillator?

I agree that the whole thing is quite weird. There is no good reason to design such a thing with darlington transistors in the first place. If you are designing a Wien bridge, then you must have some form of feedback control which I do not see here. A discreet 2 transistor astable will give you a nice square wave with no more than 8 parts. These days you can have an astable and an output buffer with an inexpensive quad nand gate. Many times you will see a triangle wave basic oscillator (requiring a constant current source to charge a capacitor) followed by a group of hand picked shaping diodes to produce a near sine wave output. That approach is also not present here. I would scrap this thing and move on!

[budm]

Re: Oscillator repair? What kind of oscillator?

The diagram does not look right at all, for example the two transistors on the left side hooked up as shown, the first transistor has Collector connected to the regulated B+ (regulated by the ZENER diode), the Emitter is connected to the BASE of the second transistor without any current limiter resistor for the BASE of the second transistor. When that first transistor is turned on it will wipe out the E-B junction of the second transistor. Same problem on the other two transistors on the right.
Also on the second transistor, there is Collector resistor but nothing else is connected to the Collector, there is no junction dot on the Collector leg.
I think the board should be retraced again for correct connections.

[eccerr0r]

Re: Oscillator repair? What kind of oscillator?

Yes I had noted that the "upper" transistor in each of the two "darlingtons" are connected funny and would short out the supply if turned on. I'll have to go check again and make sure but after double checking it (on that day I drew it) that's really what it looks like.

The collector load on the first "darlington" probably indeed should be connected to that capacitor that feeds back into the base of the second "darlington" or at least I think that was a mistake I did when I tried to beautify the schematic a bit (without disturbing the placement to schematic correlation too much).

Unfortunately indeed I'm sure I still have some extraction errors, but wondering if someone could notice some basic topology here to say whether this was intended to be some sort of sine wave generator versus a square wave...

If it indeed should have been a square wave (and yes, two transistors is sufficient to do this, not 5) yes I should just scrap this and make my own circuit, I can make square wave oscillators in my sleep, but getting a low distortion adjustable sine wave oscillator I can't (I'm no analog guy...) and it might be helpful to have a test instrument as such...

[budm]

Re: Oscillator repair? What kind of oscillator?

Simple filter turning Sq wave to Sine wave.
https://cdn.badcaps-static.com/pdfs/...39c4148b91.pdf
Sine wave oscillator:
https://cdn.badcaps-static.com/pdfs/...ce4db75dc0.pdf
https://cdn.badcaps-static.com/pdfs/...c9eb5c73ea.pdf

You can also look up App notes on OPAMP.
Tone generator using PC. i do not know what the requirement for your generator are.
http://www.audiocheck.net/audiofrequ...r_sinetone.php

[eccerr0r]

Re: Oscillator repair? What kind of oscillator?

There's a problem with those versus the circuit at hand:
1. This is a variable frequency oscillator, so filters past the fundamental in the output stage won't work.
2. This design is all transistor, and it looks like they are common emitter and thus likely leaks a lot of current through the base... Trying to correlate this with op amp designs, well, wondering which is biasing (not necessarily needed for op amps) and which are for oscillation has been a pain. Also as far as I can tell, there's only one RC section. I need to double check this but as far as I can tell, there are only 3 wires going from the adjustable R and C selectors that go to the oscillator, making it hard to make a multiple section filter. The Wien Bridge is still possible however, but I haven't been able to conclude this for sure... I did notice a few wires that I haven't accounted for, but I only seem to see one set of C for each detent in the select switch...

[more info]
Yes this unit indeed fits well within the audio range. I just wanted something that wasn't dependent on the PC and ideally something that I don't have to worry about frying due to bad connections - thus ideally something I understand and can fix. Plus this thing theoretically should be able to generate 100KHz fundamental, and all the better if it can also generate a sine wave.

[budm]

Re: Oscillator repair? What kind of oscillator?

Well, what is the requirement for the variable frequency range, the output level requirement, the output Z, the acceptable distortion level, etc.
'This design is all transistor, and it looks like they are common emitter and thus likely leaks a lot of current through the base..' So it had Emitter follower out put to get low output Z, I am sure you do not want to have direct couple output with no DC Blocking cap. As far as the Base current, you will always have base current to bias on the transistor and if you want low distortion, then will want Class A output.
The Wien Bridge will be a way to go. No matter what you use you still have to the math on the RC.
Looking at your diagram, I really do not see how it can get up to 100KHz range with the RC value.
Looking the 5th transistor at the bottom, it has at least 1M for the Collector and no DC bias for the BASE at all with lytics cap in the BASE circuit.
The Z out of the circuit is also very high. There should also be a filter cap in parallel with Zener diode.
I wonder who ever made never really finish the project. The B+ feeding the circuit is also makes no sense unless that is in very low Ohms range, it basically become Collector resistor for the output circuit.
You should upload the pictures of the board.
BTW, did you measure the frequency out out of the circuit?

[eccerr0r]

Re: Oscillator repair? What kind of oscillator?

Yeah looking at what I drew, that bottom/right transistor's LPF couldn't possibly make it feed back fast enough, that capacitor would kill everything. And since the circuit doesn't seem to work (except for some weak oscillation behavior occasionally that's very touch sensitive) it makes it hard to analyze.

I looked at the selector knobs again... This time, staring at it, it does look like a R+C and R || C configuration so this very well could be a Wien Bridge, however the two C's it selects are different though the two R's are the same (C2 = C1/10 but R2=2R1). This unfortunately would make a very gapped bandpass which would not make a very good oscillator in my opinion, but there's more to learn from this circuit...

That weird transistor on the lower right, after making some (I think...MAJOR) schematic corrections, it might be part of the automatic gain control? Maybe? And is it possible that just the leftmost two transistors are the oscilllator, the middle "darlington pair" is just a gain stage? This is still very weird.

Will upload a new schematic, Unfortunately the best contrast picture I can get out from gschem is encapsulated postscript and I need to convert this to a format that people can read...

Just as a warning to come: The two "45M" resistors are actually "anti-parallel" DIODES and not really resistors. I also think I have a first pass at the "SelectRes" and "SelectCap" network that are selected by the switches drawn... so new schematic coming...

[budm]

Re: Oscillator repair? What kind of oscillator?

I would like to see good clear pictures of the board, without correct diagram it will be hard to know if the circuit will really work. So far it does not.

[keeney123]

Re: Oscillator repair? What kind of oscillator?

design is all transistor, and it looks like they are common emitter and thus likely leaks a lot of current through the base... Trying to correlate this with op amp designs, well, wondering which is biasing (not necessarily needed for op amps) and which are for oscillation has been a pain. Also as far as I can tell, there's only one RC section. I need to double check this but as far as I can tell, there are only 3 wires going from the adjustable R and C selectors that go to the oscillator, making it hard to make a multiple section filter. The Wien Bridge is still possible however, but I haven't been able to conclude this for sure... I did notice a few wires that I haven't accounted for, but I only seem to see one set of C for each detent in the select switch...

The common emitter was used back in the day, 1980, for speed, up to 100 MZ. They require much more power then common collector. They used these for particle physics to capture events. Noise is a big problem with TTL and the faster one goes the greater the problem. I had to purchase power supplies for these circuits. Each supply was rated for 5 Volts at 200 amps. General Electric sold them.