Crumar DS2 problem (clock noise?)

[psygoat]

Hi.
I have a Crumar DS2 with what may or may not be a problem with the DCO - I can't say whether this is a fault or simply down to the design, but it certainly sounds wrong to me. I don't have anything to compare it with so if anyone has one and can check there's against my description, that would be great.

I've seen it mentioned by other people on various forums, but no one has ever had a reply.

Symptom - very high pitched whistle that tracks whatever note is being played. This sound stays in the same octave, whatever dco octave is selected. It is really noticeable when you select the triangle waveform, less so when you select any of the others. You can tune it out by lowering the cutoff frequency.

I haven't looked that far into the way the dco waveform is generated. From what I can gather so far without going right into it, there is a variable frequency squarewave which is shaped by a set of analog switches/multiplexers (mc14016) to produce the required sawtooth / triangle/ pwm wave, which is then sent to the VCF.
The waveform produced by the multiplexers are 'stepped' ie a triangle wave on the scope, looks like an ascending then descending staircase - I reckon this is normal.
The multiplexer is clocked by a variable frequency square wave, it is this clock noise which is leaking through to the output. You can see it superimposed on the waveform.
There doesn't appear to be any filtering on this board to remove this noise. Also I'm getting it on both DCO's, if I was getting it on just 1 DCO, I could A/B them against each other and find the problem that way. I would think that there's a problem with the multiplexer, but then it seems strange that both have gone at once.

Unfortunately there is no schematic for this board, afaik.

If anyone can help me get to the bottom of this I would be really grateful.

I wouldn't recommend anyone to buy a DS2 - the construction is terrible, btw.

[rhino]

schematic here: http://manuals.fdiskc.com/flat/Crumar%2 ... Manual.pdf

Is this a new problem, or always been there? Let me study the schem.
1) mux chip failing
2) may need decoupling caps on the power bus' (0.01 uF)

[psygoat]

Hi Rhino.

Thank you for your reply.
I can't tell you if it's a new problem or not, because the thing was totally dead when it was brought to me.
Decoupling the power line seems like a plan - there isn't any smoothing on that board at all. I'll give that a go and let you know how I get on.
Ta

[psygoat]

If anyone has the same problem, try replacing the ic's on the waveshaper pcb.
They're all standard cmos logic ic's, easily available and they are socketed, so it's no problem to change them.

[adhmzaiusz]

If anyone has the same problem, try replacing the ic's on the waveshaper pcb.
They're all standard cmos logic ic's, easily available and they are socketed, so it's no problem to change them.

Sorry to resurrect an old thread, but I just picked up a DS2 with this same high pitch sound exactly as you describe it in the first post.

I'm confused as to the meaning of your last post,are you saying that you replaced the CMOS chips and it fixed the problem?

Also, which one of these is the waveshaper board? The schematics are very poor and incomplete. I watched the sound doctorin circuit overview video and it sounds like it is the vertical board mounted on the back of the unit, labelled P436. Is this correct?

[psygoat]

If anyone has the same problem, try replacing the ic's on the waveshaper pcb.
They're all standard cmos logic ic's, easily available and they are socketed, so it's no problem to change them.

Sorry to resurrect an old thread, but I just picked up a DS2 with this same high pitch sound exactly as you describe it in the first post.

I'm confused as to the meaning of your last post,are you saying that you replaced the CMOS chips and it fixed the problem?

Also, which one of these is the waveshaper board? The schematics are very poor and incomplete. I watched the sound doctorin circuit overview video and it sounds like it is the vertical board mounted on the back of the unit, labelled P436. Is this correct?

Yes it did.
That Crumar had so many things wrong with it, you'd fix one thing and something would go wrong somewhere else.
If I recall, the edge connectors are particularly weak and liable to corrosion.
The board is P436 according to the same illegible service manual I have.

[adhmzaiusz]

Hi Psygoat, thanks for your reply. I tried replacing the CMOS chips-- i think there were 2 4069's, and 2 401(X) multiplexers (replaced those ones except for the number counter, can't remember off the top of my head the part no., but it was somewhat uncommon to synths) with no luck. Maybe I'll also try cleaning the contacts of the connector as you suggest. Maybe put in the two 1458's too.

All I can say is that this synth blows me away, it sounds incredible and really fun to play using the gate input. Super punchy envelopes too!

[adhmzaiusz]

OK! So for the record the CMOS chips to blame are the two 4070 quad exclusive OR gates on board P436. Makes sense, the gates aren't closing all the way when they go bad. I totally missed these the first time around but seems a huge improvement. The clock sound is still there very faintly, have to crank up the gain pretty high to hear it but it's at a very acceptable level of practically nil.

[psygoat]

That's a result.

Must be a fairly common problem with these - even though it's a really unusual fault.

They're great synths but the build quality is awful.

[adhmzaiusz]

That's a result.

Must be a fairly common problem with these - even though it's a really unusual fault.

They're great synths but the build quality is awful.

Not only is it awful (don't get me started on refurbing the keybed) but extremely odd! So many weirdly arranged boards.