Musitronics Bi-Phase LFO troubles. Voltages posted!!!!

[gez]

Also I see a square waveform when the switch is set to square but only on one IC.

And does this crap out after a while?  Which IC?

[gez]

I am going in to take notes on how my scope is set. Exactly which pins give me what when powering on, etc.

Just keep it simple.  All you need to check with the scope (for the moment) is pins 1 & 7 of ICs U1 and U5 on the schematic you posted.

Turn the rate pot when testing each and let us know what happens.

[moogatroid2000]

Ok here is what I have.
When powering on I get a square on pin1 of U1 and a sine on pin7.
I get the same results from U5 exactly.
All of these will drop after about 2-3 seconds tops.

If I short the pot for LFO B (U1) I can get solid waveforms from both pins and they stay!

If I do the same test with LFO A (U5) I only get a very small sinewave on pin 1, I get nothing really from pin 7.

I was thinking Ton was right with the results of LFO B and perhaps that pot is shot? I was hopeing for the same results on LFO A but...

[moogatroid2000]

Oops I didn't turn the rate pots. :oops: One second..

[moogatroid2000]

The rate pots will change the rate (frequency?) of the waveforms for the brief moment that things work. After the waveform is gone the rate pots seem to have no effect on whatever is left after that pretty waveform goes away.

[puretube]

Gez: whaddaya think of C33 & C36 (0.47µ),
and of C39 (2µ2/50V) in the "vintage schem 1"
(http://m.bareille.free.fr/biphase/vintage/mutron_biphase_sch1.gif)

usual suspects? (assumed the pots and the switches (!) are clean and working)...

[gez]

If I short the pot for LFO B (U1) I can get solid waveforms from both pins and they stay!

Then it's likely the problem is with this pot, as I mentioned earlier.  You didn’t answer my previous question, is it PCB mount or wired?  If the later, could be a break in the wire.  Either that or the pot is dodgy and needs replacing (if you’ve tried cleaning it first).

LFO generator 2 (B) is a triangle wave oscillator (despite the ‘sine’ label in the vintage schematic) but you keep saying you’re seeing a sine wave at pin7.  Are you using the AC input selection on your scope?  If so, switch to DC (measure realative to -ve rail), the input cap of the scope might be causing distortion of the waveform.

Ton, can’t find a C39 but there’s a C35 which is 2u2.  It’s labelled as C1 on the schematic Mooga posted.  Even if this were leaky there’s that 10k resistor between it and U1 A’s square output (pin 1) stopping the output being pulled up to V+.  Surely this load isn't small enough to cause oscillation to cease (if it oscillates with only a 180R load when the pot is shorted!), though it might cause problems further up the line?  I think the pot is the likely culprit for this LFO.

LFO generator 1 is an odd one.  I don’t understand how U6 fits into all this…mystified!

[moogatroid2000]

Sorry Gez, PCB mounted, I did try cleaning it so I iwll try replacing it and see what happens.

Again sorry I have been saying sine this whole time and meaning triangle.
I am using the DC input.

I went ahead and replaced C33 and C36 from the original schematic. I think these are C6 and C3 respectively, I think, on the newer schematic.

Still no change. Off to swap that pot!

[moogatroid2000]

Swapped pot for a 20K lin in leiu of a 33K Log!  :shock:
No change though.
I checked for continuity from pot lug to the ends of the traces and that checks out fine...

[moogatroid2000]

I want to ask if any of the trim pots could be causing trouble? They all have yellow paint on them, assuming for position marking or perhaps covering over the value markingss. I had moved these around a bit but was careful to put them back to their original positions when I got no results. Could I have broekn some paint ships loose and fouled one of these up? Just shooting in the dark.
I also got some interesting waveforms stuff on U2, and also some DC movement on U6 when moving LFO A's rate pot up and down.
Sorry I'll stick to answerng when questioned...
I really appreciate all this extra help, the more I work on this thign the more determined I am to get it alive again.

[puretube]

:oops: , yes, gez: C35 rather than 39 (hard to read) - which is C1 in the Bareille (=newer) schem.

[gez]

This is an odd one!  

When you shorted the pot how did you do it?  Try this, connect pin 1 of U1:A to the junction of R6 and P1's wiper in the 'Bareille' version, with the rate pot at max (wiper points towards pin 1).  Make the connection to the resistor rather than the wiper.  Is oscillation constant?  Just want to find out how you shorted the pot when you got it to work.

[moogatroid2000]

The way I shorted it was by turning the pot all the way down, then shorting the wiper to the CW lug. Or think of it as a short across all three lugs.

I just tried the procedure you describe but I get no oscillation or change at all on either pin 1 or 7.
For the sake of testing I just jumped all three lugs and I get the oscillation.

When looking at the short lived waveform during a power up, with nothing shorted, the waveform is about twice the size (amplitude?) as when I short the pot. Don't knwo if this means anything, just something I noticed.

[gez]

When looking at the short lived waveform during a power up, with nothing shorted, the waveform is about twice the size (amplitude?) as when I short the pot.

And this happens on pin 1 of U1:A I take it?  If so, that makes sence as there's only a 180R load once the pot is shorted.  

I’m wondering if the amp is latching up due to the +ve supply 'shortfall'?

[gez]

Try connecting a resistor in parallel with R3 (croc-clip it if possible), the 47k resistor from pin 7 to pin3 of U1:A.  Another 47k perhaps.  If still no joy, make it smaller.  Keep the pot in, see if the LFO oscillates.  This isn't a 'fix', it's only to tell us if it's latching...

[moogatroid2000]

And this happens on pin 1 of U1:A I take it?

Yes and also on pin7.

I’m wondering if the amp is latching up due to the +ve supply 'shortfall'?

I have been wondering about the supply mismatch as well. Wonder if I should do some testing of the trafo or regulator?
Would a bad diode in the rectification ring cause this kind of trouble?

Ok with a 47K in parallel I get a triangle waveform on pin1. On pin7 I get a triangle but it is not a perfect waveform. It has different sized peaks and valleys.... Sorry for my super high tech terminology.

[gez]

Ok with a 47K in parallel I get a triangle waveform on pin1. On pin7 I get a triangle but it is not a perfect waveform. It has different sized peaks and valleys.... Sorry for my super high tech terminology.

Pin 7 should be a triangle and pin1 should be a square.  You did leave the pot in didn't you (we want it in) and didn't short it?  Does pin1 look squarish?

It looks as though both LFOs are latching due to the supply mismatch (at last we're getting somewhere!).

On schematic 3 Ton linked to it says the supply should be +-15V from the regulator.  Is the regulator in your unit the same as in this schematic?  If so take a DMM to pins 3 & 1 and measure the output relative to ground.  If it's a different reg look for the data sheet and figure out which is V+ & V-.

[gez]

I just edited the above as I had the pin Nos wrong, please read it again if you've already seen it.   :roll:

[moogatroid2000]

Pin 7 should be a triangle and pin1 should be a square

Your right. I don't what I did wrong last time but this time I got square on pin` and triangle on 7. The square seemed to dip very negative. I guess this is in relation to the supply?

Regulator is giving me 20V- on pin 1, and 10V+ on pin 3.

[gez]

The square seemed to dip very negative. I guess this is in relation to the supply?.

Yes, it seems so.

Regulator is giving me 20V- on pin 1, and 10V+ on pin 3.

Now here's the question, is this due to loading by something further up the chain drawing too much current or, more likely, is the problem with the regulator/circuitry surrounding it (or something before it in the chain?).

If I recall, you've replace C39 & C40 (the 47u caps), yes?  How about C37 & C38, the 470u caps on schematic 3?