PAIA Synthespin Mk II

[Electron Tornado]

A local guitar shop asked me to take a look at one of these for them. It seems to be working to some extent, but I've never heard one to know if it's working correctly. Several knobs, but not really that much in the way of range of control. The speed control is only useful through about the first 1/3 of rotation. Does anyone have any experience with one of these?

Most references on here are 10 years old, and very general in nature. Here are a couple of references with documentation. Click on the attachments.

(OK, correctly this time)

http://www.paia.com/talk/viewtopic.php?f=6&t=211&start=0

http://www.paia.com/talk/viewtopic.php?f=6&t=68&start=0&sid=380900e91bb01e1805b465223af7fff4

[cloudscapes]

Those aren't attachments, you need to upload the files somewhere.

[Electron Tornado]

Those aren't attachments, you need to upload the files somewhere.

Fixed. 

[EATyourGuitar]

I looked at the schematic. Check for a cold solder joint on one of the outside lugs of the pot going into R33. R33 is a 47k to ground so you should be able to measure that from the power ground somwhere else all the way back to the rate pot. I do not see anything parallel with r33 that would fudge the measurement.

[Electron Tornado]

I looked at the schematic. Check for a cold solder joint on one of the outside lugs of the pot going into R33. R33 is a 47k to ground so you should be able to measure that from the power ground somwhere else all the way back to the rate pot. I do not see anything parallel with r33 that would fudge the measurement.

Thanks, I'll have a look there. Considering its age, and that it was a kit, I'm kind of expecting to find some bad wire connections and a bad solder joint or two when I open it up. So, any of the other controls might be having a similar issue. It's such a rare bird, I'm hoping someone who has actually played through one will chime in with any info or tips. 

[Electron Tornado]

I looked at the schematic. Check for a cold solder joint on one of the outside lugs of the pot going into R33. R33 is a 47k to ground so you should be able to measure that from the power ground somwhere else all the way back to the rate pot. I do not see anything parallel with r33 that would fudge the measurement.

I checked that, and it checked OK.

[Electron Tornado]

Looking at just the LFO section, here are a few other things I found:

Speed pot (R38): set to min, center lug reads 1 V. Set to max, center lug reads 14 V

IC1C:  With speed pot set to min:  Pin 4 oscillates between 5.4 - 5.6 V
                                               Pin 3 oscillates between 7.0 - 7.4 V

          With speed pot set to max: Pin 4 stops oscillating and reads 9 V
                                                Pin 3 stops oscillating and reads 0.5 V    

          Pin 2 stays at about 0.5 V all the time with no oscillation

Hmmm....I'm going to check this with a scope. I should get a triangle wave at the output of IC1C (pin4) and a square was at the output of IC1D (pin5).

Here's a schematic:

[Electron Tornado]

Hmmm....I'm going to check this with a scope. I should get a triangle wave at the output of IC1C (pin4) and a square was at the output of IC1D (pin5).

Got the correct waveforms on both of those pins.

The voltage on the "+" side of R31 (at pin 6 of IC1D) is 14 VDC.

Can anyone venture a guess as to why the speed control is only useful for about the first 1/3 of travel?

[PRR]

> IC1C:  Pin 3 oscillates between 7.0 - 7.4 V

That's a Norton Amp, right?

Input pins can NOT go over 0.6V from the negative rail (they are grounded-emitter transistors).

The waveform could be fooling your meter. Best to verify with a 'scope.

If pin 3 really goes over 0.6V, the chip is blown. Replace it.

Though I really can't picture how this affair could work at all if pin 3 input is blown. (Maybe it gets into an emitter breakdown which floods the substrate?)

As long as you have the nippers sharpened, socket that position (GOOD socket). Those Norton Amps are IME only semi-reliable over the decades.

Pin 2 should swing 0.5V to 0.05V. (Vbe pin 2, to Vce(sat) of Q2.)

[Electron Tornado]

Yes, the chip is an LM3900 quad Norton amplifier.

I checked the waveforms: Pin 4 - triangle, Pin 5 - square, anode of D5 - sine. So those seem correct. I think I was probing a wrong pin yesterday  . What I called pin 3 should have been pin 5. So the voltages on pin 5 are 7.2-7.6v (oscillating) with the speed control at min, and 0.5v (steady) with the speed control at max.


Here are the voltages on the input pins:

Pin
1    0.46v
2    0.14-0.19v   (Speed control min-max)
3    0.13-0.16v   (Speed control min-max)
6    0.14v
8    0.54v
11  0.43v
12  0.39v
13  0.40v  

I could re-check continuity in the LFO, but other than that the LFO seems to be working OK when the speed control is set to a low speed. So what could fail and affect the speed of the oscillator? C11, C12, Q2, or the chip. When operating correctly, the speed control should vary from 1 cycle every 3 seconds to 15 cycles per second.

[Electron Tornado]

Taking a look at the waveform at pin 4, it looks like, with the speed set to minimum, that the LFO is producing about a 4Hz signal.

[Mac Walker]

Does the original design call for a linear or log taper on the speed pot, and I wonder if the original kit builder used the right one?

[Mark Hammer]

The PAiA Phlanger, an artifact from the same era, also used a quad Norton chip, as the Synthespin MkII does, with two op-amps used for the modulation section, and the other two used for the audio path.

A short while later, Craig Anderton had an article in Polyphony showing how to replace the two Norton op-amps in the audio path with a "normal" dual op-amp (he used a 4739 but it could be any of a bunch of others, really) for improved audio performance, including lower noise.

I probably have a scan of that.  You might consider doing similar for the Synthespin.

[Electron Tornado]

A short while later, Craig Anderton had an article in Polyphony showing how to replace the two Norton op-amps in the audio path with a "normal" dual op-amp (he used a 4739 but it could be any of a bunch of others, really) for improved audio performance, including lower noise.

I probably have a scan of that.  You might consider doing similar for the Synthespin.

Thanks, Mark, I'd like to take a look at that.

Does the original design call for a linear or log taper on the speed pot, and I wonder if the original kit builder used the right one?

Excellent point! Ya know... it occurred to me that while I checked the date codes on the pots (they were all close together, dating the unit itself at 1974), I never did see their values marked on them. Sooo, I measured them in the circuit. Here's what I got:

[big edit here]

I had some other stuff here earlier but went back and did a better job of checking the pots.

R36 and R37 are both marked and are the correct values. Neither of the other two are marked, but R35 does seem to be about 500k. The speed pot, R38, is still a bit of a mystery. I measured it between the center lug and the lug connected to the 47k resistor. It is listed as a linear pot, but when sweeping it from min to max, it starts at 0 ohms, climbs to about 140k, and then drops back down to 50k.

[Electron Tornado]

How would a CA3401 work in place of the LM3900?

[toneman]

PAIA used the CA3401 and the LM3900 interchangably. 

They are the same type quad norton amp.

http://www.classiccmp.org/rtellason/chipdata/ca3401.pdf

I've built a couple of SynthaSpins and they sound really good.

T

[Electron Tornado]

PAIA used the CA3401 and the LM3900 interchangably. 

They are the same type quad norton amp.

http://www.classiccmp.org/rtellason/chipdata/ca3401.pdf

I've built a couple of SynthaSpins and they sound really good.

T

Did you ever run into the problem that I'm having with the speed control?

[R.G.]

The speed pot, R38, is still a bit of a mystery. I measured it between the center lug and the lug connected to the 47k resistor. It is listed as a linear pot, but when sweeping it from min to max, it starts at 0 ohms, climbs to about 140k, and then drops back down to 50k.

Whatever taper it is, it don't sposed to do that.

With the pot out of circuit: if you're measuring from the CCW lug to the wiper, the resistance should start at near 0, and increase as the shaft is rotated clockwise.  At no point should it drop back - the resistance increase should be monotonic.

For linear, the increase ought to be linear with rotation, for log/audio taper, it should increase slowly to about 10% of the total end-to-end at half rotation, then climb the remaining 90% over the remaining half rotation. For reverse log/audio, the resistance should increase to about 90% at mid-rotation, and the other 10% of resistance for the other half shaft rotation.

[toneman]

Did you ever run into the problem that I'm having with the speed control?

Don't think so.
My clone boards seemed to work fine, IIRC.
But it's been many many years.
I did use a regulator for the power supply.
Not just a transformer/bridge/cap.

Check resistor/capacitor values around the LFO.
Just a note:
Many PAIA kits were built and didn't work.
Then sat around in a drawer for a long time.
It might be that there is a wiring error (or wrong value part) that was never discovered.

Ahh...PAiA....takes me back......

T

[Electron Tornado]

The speed pot, R38, is still a bit of a mystery. I measured it between the center lug and the lug connected to the 47k resistor. It is listed as a linear pot, but when sweeping it from min to max, it starts at 0 ohms, climbs to about 140k, and then drops back down to 50k.

Whatever taper it is, it don't sposed to do that.

I removed the pot, instead of being lazy, and it reads 500k. It's a linear taper, which is what the building instructions call for. All the pots are correct.

I checked every part on the PCB. There were three discrepancies. R21 and R22 were 330k, so I replaced them with each 33k. C8 was 0.05uf, and I replaced it with a 0.1uf. I verified the correct values with the schematic and the parts list. Those are all "downstream" of the LFO, so had no effect on the LFO. (Not much effect on the sound at all, really.)

I will finish checking the actual wiring and recheck continuity between components on the PCB. Apart from that, wouldit come down to one of the caps, C11 or C12, Q2, or the chip itself?

The real question is - what would cause the frequency range of the LFO to be so high? (It should be 1 cycle every 3 seconds to 15 cycles per second.)