Vintage Synth Repair

I recently posted about my nonfunctional Pro One. I'm getting nothing out of the output, except for some noise.

Anyway, the transformer is ok, and the PSU caps are brand spanking new. I also replaced all the regulators, so there's no problem there.

I got it hooked up to an oscilloscope and tested all the oscillators. Good news is I'm getting saw, square and triangle perfectly. All the functions work including the envelope and pulse width. I've narrowed it down to a critical point in the board.

The CEM3320 filter IC is being fed with both OSC A and OSC B through pin 1. When I put my oscilloscope leads on pin 1 of CEM3320 and ground, I get a perfect reading of whatever wave I am pushing through it. I've done all my testing on this pin so far, and the synth appears to be completely functional.

Here's where it gets weird. (I'm not really hugely experienced in bench diagnostics, I only have the o-scope to calibrate the few YuSynth modules I've built, so diagnosing isn't my strong suit, so your help is appreciated).

According to the schematic, pin 10 is supposed to be outputting everything plus the filtered signal (am I wrong?). Here is a pic of where in the schemos I'm at:

http://i483.photobucket.com/albums/rr195/casiotone1331/SequentialCircuitsPro-OneSchematic2of2.jpg

When I probe pin 10 I get an odd wave with a slight curve, nothing short of key presses or cutoff and resonance adjustments change the wave at all.

Does this mean my VCF IC is bad? Hope not! I might try to change the caps or check the resistors. What do you guys think?

Mark

Mark-

yes Pin 10 is the output (well, the final stage of 4)

Here's a block diagram of the P-One filter:

http://www.electricdruid.net/images/cem3320/larger/ProOneCEM3320Filter.gif


Here's some ideas-
First make sure the chip is getting power +15V at pin 14, -15v (about) at pin 13


Put a 1Khz ( 2 octaves above middle C) square wave into the filter, turn resonance to min., filter cutoff to max. Are you seeing a (sort of) square wave with slightly rounded corners at Pin 10? Note that there are phase shifts being applied at each filter stage so the wave isn't exactly shaped as it went into the filter, ( but it would sound the same if not swept by the EG.)

Continue watching the signal at pin 10 and rotate the cutoff knob CCW . Is the signal getting "rounder"? If so the filter is working, and the signal is getting at least that far in the chain.

If not check each filter section (pin 7, pin 6, pin 15) and see if the signal is getting to each stage. If not check for bad solder joints, bad resistors, etc, and the 150p caps at pins 5,4,16, 11. 

Also the 2.2uF DC blocking electrolytic C143 might be bad is the filter is working as expected.

Seeing any signal at pin 7 of U110?


--- On Mon, 5/9/11, Mark <casiotone1331@...> wrote:

From: Mark <casiotone1331@...>
Subject: [vintagesynthrepair] Sequential Circuits Pro One schematic help
To: vintagesynthrepair@yahoogroups.com
Date: Monday, May 9, 2011, 10:43 AM







 



  


    
      
      
      I recently posted about my nonfunctional Pro One. I'm getting nothing out of the output, except for some noise.



Anyway, the transformer is ok, and the PSU caps are brand spanking new. I also replaced all the regulators, so there's no problem there.



I got it hooked up to an oscilloscope and tested all the oscillators. Good news is I'm getting saw, square and triangle perfectly. All the functions work including the envelope and pulse width. I've narrowed it down to a critical point in the board.



The CEM3320 filter IC is being fed with both OSC A and OSC B through pin 1. When I put my oscilloscope leads on pin 1 of CEM3320 and ground, I get a perfect reading of whatever wave I am pushing through it. I've done all my testing on this pin so far, and the synth appears to be completely functional.



Here's where it gets weird. (I'm not really hugely experienced in bench diagnostics, I only have the o-scope to calibrate the few YuSynth modules I've built, so diagnosing isn't my strong suit, so your help is appreciated).



According to the schematic, pin 10 is supposed to be outputting everything plus the filtered signal (am I wrong?). Here is a pic of where in the schemos I'm at:



http://i483.photobucket.com/albums/rr195/casiotone1331/SequentialCircuitsPro-OneSchematic2of2.jpg



When I probe pin 10 I get an odd wave with a slight curve, nothing short of key presses or cutoff and resonance adjustments change the wave at all.



Does this mean my VCF IC is bad? Hope not! I might try to change the caps or check the resistors. What do you guys think?



Mark

--- In vintagesynthrepair@yahoogroups.com, "Mark" <casiotone1331@...> wrote:
>
> I recently posted about my nonfunctional Pro One. I'm getting nothing out of the output, except for some noise.
> 
> Anyway, the transformer is ok, and the PSU caps are brand spanking new. I also replaced all the regulators, so there's no problem there.
> 
> I got it hooked up to an oscilloscope and tested all the oscillators. Good news is I'm getting saw, square and triangle perfectly. All the functions work including the envelope and pulse width. I've narrowed it down to a critical point in the board.
> 
> The CEM3320 filter IC is being fed with both OSC A and OSC B through pin 1. When I put my oscilloscope leads on pin 1 of CEM3320 and ground, I get a perfect reading of whatever wave I am pushing through it. I've done all my testing on this pin so far, and the synth appears to be completely functional.
> 
> Here's where it gets weird. (I'm not really hugely experienced in bench diagnostics, I only have the o-scope to calibrate the few YuSynth modules I've built, so diagnosing isn't my strong suit, so your help is appreciated).
> 
> According to the schematic, pin 10 is supposed to be outputting everything plus the filtered signal (am I wrong?). Here is a pic of where in the schemos I'm at:
> 
> http://i483.photobucket.com/albums/rr195/casiotone1331/SequentialCircuitsPro-OneSchematic2of2.jpg
> 
> When I probe pin 10 I get an odd wave with a slight curve, nothing short of key presses or cutoff and resonance adjustments change the wave at all.
> 
> Does this mean my VCF IC is bad? Hope not! I might try to change the caps or check the resistors. What do you guys think?
> 
> Mark
>
Is R1156 actually connected? Looks like a feedback device. Is that coupling cap 2.2uf still good?
Also check for power supply ripple at the Vcc / Vdd pins

--- In vintagesynthrepair@yahoogroups.com, Tom Russell <russelltc@...> wrote:
>
> The fact that you get intermittent signal when you press on pins sounds like a bad mechanical connection somewhere, either in socketed chips, bad wire connector(s), broken PCB trace(s), bad solder joint(s), things like that. 
> 
> Get a good lighted magnifier and check the board. I'm not intimately familiar with he construction of that particular synth but if there are soketed chips with the "spring leaf" type sockets replace them with collet-pin types which are much more reliable. You can also try removing chips from sockets and see if there is oxidation on the legs and or the socket pins. Clean them with some Deoxit or similar, Unplug multipin connectors and see if the pins are oxidized.
> 
> Remember these things weren't designed and built to still be working 30 years later- only until the "next" model was available!
> 
> --- On Tue, 5/10/11, Mark Milanovich <casiotone1331@...> wrote:
> 
> From: Mark Milanovich <casiotone1331@...>
> Subject: Re: [vintagesynthrepair] Sequential Circuits Pro One schematic help
> To: russelltc@...
> Date: Tuesday, May 10, 2011, 8:48 AM
> 
> Tom,
> 
> Thanks for the insight on the problem.
> 
> I did what you suggested. No signal at all on pin 10. So I started poking around to see if there was a cold joint or something. I ended up finally getting signal on 10, without doing anything! Signal also popped up on pin 7 of the tl082.
> 
> In the meantime, I found out why I wasn't getting any output. The 5532 was bad, after I changed it with a socketed chip I immediately got output. The bad news is the oscillators sound pretty messy. The waves look fine on the oscilloscope, but I noticed the amplitude is very low. Master volume now works but for some reason I feel like the waves should be higher in amplitude. Osc A is extremely quiet. Osc B works ok with squarewave but is quiet on the triangle and sawtooth. Not only that, but when I start detuning the oscillators, I get an audible clicking or thumping that goes away when tuning back to zero beat. What could cause this?
> 
> Back to the filter, though... still not working. I seem to get output on pin 10 at very spotty intervals, and the signal passes through the other pins most of the time. I can't say how often, sometimes when I touch pins 4 or 5 on the 3320 I get a wave and sometimes I don't. Its weird. When it is working the changes in cutoff and resonance do not affect the signal. When I get nothing but a straight line on pin 10, it bounces up and down when a key is pressed.
> 
> I'm starting to get a headache! This synth is a mess!
>
On socketed ICs it's good to CAREFULLY pull them and buff the pins with a autobody type sandpaper ( like 600 grit or finer)
Lay the sandpaper flat on a table and rest one side of the IC on it and buff gently....I've just recently did this to one unit
The other thing to check is power supply ripple on the pwr pins of the ICs. A DC meter will only show the DC component of the supply volts.

I would certainly not rub IC legs with any abrasive let alone something like
wet and dry emery cloth.  The legs are probably plated could be gold flash
or one of the other precious metals so any abrasive will remove it and make
the contact worse.  Use Deoxit on a cotton bud.

 

Regards

Brian G3OYU

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Scott
Sent: 11 May 2011 4:37
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] Re: Sequential Circuits Pro One schematic help

 

  


--- In vintagesynthrepair@yahoogroups.com
<mailto:vintagesynthrepair%40yahoogroups.com> , Tom Russell <russelltc@...>
wrote:
>
> The fact that you get intermittent signal when you press on pins sounds
like a bad mechanical connection somewhere, either in socketed chips, bad
wire connector(s), broken PCB trace(s), bad solder joint(s), things like
that. 
> 
> Get a good lighted magnifier and check the board. I'm not intimately
familiar with he construction of that particular synth but if there are
soketed chips with the "spring leaf" type sockets replace them with
collet-pin types which are much more reliable. You can also try removing
chips from sockets and see if there is oxidation on the legs and or the
socket pins. Clean them with some Deoxit or similar, Unplug multipin
connectors and see if the pins are oxidized.
> 
> Remember these things weren't designed and built to still be working 30
years later- only until the "next" model was available!
> 
> --- On Tue, 5/10/11, Mark Milanovich <casiotone1331@...> wrote:
> 
> From: Mark Milanovich <casiotone1331@...>
> Subject: Re: [vintagesynthrepair] Sequential Circuits Pro One schematic
help
> To: russelltc@...
> Date: Tuesday, May 10, 2011, 8:48 AM
> 
> Tom,
> 
> Thanks for the insight on the problem.
> 
> I did what you suggested. No signal at all on pin 10. So I started poking
around to see if there was a cold joint or something. I ended up finally
getting signal on 10, without doing anything! Signal also popped up on pin 7
of the tl082.
> 
> In the meantime, I found out why I wasn't getting any output. The 5532 was
bad, after I changed it with a socketed chip I immediately got output. The
bad news is the oscillators sound pretty messy. The waves look fine on the
oscilloscope, but I noticed the amplitude is very low. Master volume now
works but for some reason I feel like the waves should be higher in
amplitude. Osc A is extremely quiet. Osc B works ok with squarewave but is
quiet on the triangle and sawtooth. Not only that, but when I start detuning
the oscillators, I get an audible clicking or thumping that goes away when
tuning back to zero beat. What could cause this?
> 
> Back to the filter, though... still not working. I seem to get output on
pin 10 at very spotty intervals, and the signal passes through the other
pins most of the time. I can't say how often, sometimes when I touch pins 4
or 5 on the 3320 I get a wave and sometimes I don't. Its weird. When it is
working the changes in cutoff and resonance do not affect the signal. When I
get nothing but a straight line on pin 10, it bounces up and down when a key
is pressed.
> 
> I'm starting to get a headache! This synth is a mess!
>
On socketed ICs it's good to CAREFULLY pull them and buff the pins with a
autobody type sandpaper ( like 600 grit or finer)
Lay the sandpaper flat on a table and rest one side of the IC on it and buff
gently....I've just recently did this to one unit
The other thing to check is power supply ripple on the pwr pins of the ICs.
A DC meter will only show the DC component of the supply volts.