[sdiy] Guru help needed - very bad Vox Continental oscillator problem

[Antti Pitkämäki]

Hello everybody!
 
I unsubscribed some time ago since I spent summer in a place where there was no Internet connection, but now I'm back, since this list definetely is the best place for getting help with DIY synth stuff and repairing old synths/organs.
 
Now here's the thing: I'm currently restoring a sixties Vox Continental II (UK version). These are quite valauble these days, so I though buying a cheap broken one would be the smartest solution in getting one. Well...
 
I have asked questions here in the past regarding this organ. Previously the problem was the power supply. I got help and the supply got fixed. Thanks! But now I've got another problem.
 
I've managed to fix _almost_ every problem from this organ. Oscillators that didn't oscillate, dividers that didn't divide, filters that didn't filter... I've managed to fix everything without any problem. But there is one ultra weird problem and I've spent hour after hour trying to fix it but it's a difficult one. The thing is, that the depth of vibrato varies between voices. For example, it's very light for all E's, but very strong for all #G's.
 
The organ and it's vibrato work like this: It's a typical 12-oscillators-with-dividers-design. 12 master oscillators (Hartley type I think, but I'm not sure) generate the notes for the top octave, and divider chains generate the rest of the notes for all the octaves and footages. Then there's a whole lotta wires and key contacts, hehe. Each oscillator card has an input for the vibrato signal. It is the same signal from the same vibrato card that is used for all of the oscillator cards. In the oscillator card there are a couple of resistors and a cap that the vibrato signal goes through (I mean a resistor and cap go to one point in the circuit, and from the same input point, a bigger resistor goes to completely different part of the circuit). These are the same value for all the oscillator cards. I replaced these from a couple of oscillator cards but the vibrato depth didn't change, it was still different. And I changed quite a large amount of components, including the oscillator transistor and all the resistors. No difference in vibrato depth. (Note: different pitch for different oscillators are obtained by one cap, which has different value for different voices, and also the tuning coil itself is of different value for different voices).
 
Then I found something interesting. Voices #G and A have very heavy vibrato, and voices #A and H (B for you Americans) have very light vibrato. I noticed that #A and H have additional resistor on the underside of the card. One pin is where the vibrato signal first has passed one of the input resistors, and the other pin is at the collector of the oscillator transistor. I did some experimenting with #G card... And yes, adding this resistor reduces vibrato depth significantly. But it also makes the tone one full tone higher, making it impossible to tune with the tuning coil.

So my theory is:
- capacitance and inductance of the oscillator circuit affect the frequency of the oscillator, but their ratio also affects how sensitive the oscillator is for the vibrato signal. So not only capacitance needs to be different for different voices, but also the inductance (and that is, I mean as I said, the voice cards have a cap that is different for each voice, and I'm quite sure the tuning coil is also of different value for different voices)

- somebody in the past has "fixed" this thing with wrong parts and possibly removed some of those "underside of board"-components. Maybe some of the voice cards are not even original. The repairer in the past maybe wanted a quick fix for some out-of-tune voice, and ruined the vibrato in process (but there several different depths for the vibrato, it's still a little bit weird).
 
So I could fix this thing by trying the correct additional resistor for each card and also then trying different value caps to get each voice to be in tune with (approximately) the same amount of vibrato. Lotsa work! But one thing before I start this mighty task: Is it still possible that the problem originated from the vibrato card? Like if there's something wrong with the amplification of the vibrato signal, resulting in incorrect impedance or something... But I somewhat doubt it would cause different kind of vibrato for different cards when the signal is the same, right? Also the vibrato has different configuration than in the schematics. One big cap and two smaller ones are missing, a trace has been cut and I think there are shorts where some caps should be (I didn't check this very carefully). However, the big missing cap looks like it was never there. But who knows what is original in this organ and what is not. The cards are of "plug-in"-type, and can be replaced easily if you have spares.

So should I proceed with the method I first described, that is, adding the extra resistor and changing caps from the oscillator cards or could the problem come from the vibrato card? I know this was super lengthy message, and the question is maybe not super easy, but if some guru has had the time to read this, and if they happen to know something about multiple oscillators sharing the same "vibrato LFO", and the possible problems there, that is, is it even, in theory, possible for the problem to be in the vibrato card if the vibrato signal is the same for every oscillator card?
 
I would highly appreciate any help!
 
Note: Somebody might suggest some kinda trimmer resistor solution, that is, putting a trimmer at the vibrato input for each card, and changing the depth this way... Unfortunately this also changes the frequency of the oscillator, so other components that affect the tuning would need replacing, which would ruin the idea of the trimmer in the first place.

Another note: When repairing this, I have gained quite a knowledge of the workings of the voice cards. If anybody is fixing one of these, and encounter a problem, feel free to ask me if I happen know something about it!
 
Cheers,
Antti P.

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