CMI SG Systems amp - Phaser issue

[glebert]

I got this amp for free because it was deemed not worth fixing. I got the HV power supply working right (open rectifier diode) and, since I didn't have any 8417s I converted it to KT88 (simple mod to bias circuit to get bias voltage down to some more negative voltage value). Amp actually sounding OK, but the phaser circuit isn't working right. I haven't probed it yet, but have cleaned and played with the trim pots to try to get it working right. It seems like the oscillator is working but it isn't a smooth up and down, more of a hard on-off, and there is an uneveness to the on, and an attacked note will sometimes get doubled like a short one-shot delay.

The board is labeled as Oberheim PS-1A. Here is a schematic that is supposed to be the circuit but I have not verified. Mine only has two trimmer pots that I see. /tmp/xfig-fig002979. (wingspreadrecords.com)

Actually, that is the PS-1, this one might be the PS-1a? Ignore the red circles, that was from some other discussion.



Anyone worked on these and have any tips on where to start looking? Should the oscillator waveform be sinusoidal more square?

TIA,
Greg

[glebert]

I was wrong about something. The phaser board isn't labeled as an Oberheim PS-1A. I must have gotten that in my mind from a picture from a post here on MEF. Mine doesn't say Oberheim at all, the only marking is the number 21-110. Google is no help on this. I think the date on this amp is 1976-ish. It seems they were using the PS-1A boards on earlier amps. The components look mostly in line with the PS1A, other than 2 trimmers instead of 3, but the layout is totally different. Odd.

[dmeek]

Can you post photos of the board, both sides?

[glebert]

Can you post photos of the board, both sides?

Apologies for photo quality and the filthiness of the board. I rotated the pictures to be vertically aligned but then they are mirrored left/right.

[glebert]

Started looking at basics. I'm only getting about +/-13.5V coming into the phaser instead of +/-35V per the schematic. I'm getting +/-10.6V at the 1458 op amps.

SG Systems SG-100 Schematic (1).pdf

Interestingly, I've got a friend who had a 1973 SG100 and it has the typical Maestro PS-1a looking phaser board. Mine is 2nd half of 1974-ish, and the layout is way different on the phaser and the preamp boards (he has separate boards for Ch1 and Ch2, mine has one big board). I think they only made these for about a year, I'm really surprised they would have changed board designs as much as they did in that time. Mine is also labeled as an SG212, but the only schematics with that name are much different.

[glebert]

Well, found ANOTHER schematic, and this one doesn't say anything about PS-1, 1a, 1b, or really anything. This one from later in 1974, so it lines up, does not seem to have the DC offset trimmer pot. The voltage supplies also line up better with my amp.

SG100PhaseShift2.pdf

So now the question is how does it work? I get that the FETs and IC2-4 are a loop oscillator, but I don't understand how IC5 is adjusting the oscillation amplitude.

[Mick Bailey]

TR-1 is the amplitude, TR-2 sets the DC offset.

[glebert]

TR-1 is the amplitude, TR-2 sets the DC offset.

Yes, but TR-1 is connected to IC5. I don't understand how that part of the circuit behaves. Is it just DC that comes out of IC5 or is it dynamic?

Looking at the oscillator section, should the waveform be sinusoidal or something else (eg square)? Since this is supposed to give a sound like a Leslie it seems like it should be gentle up and down, not on/off/on/off.

[dmeek]

Pin 1 of IC5 should be a triangle wave, pin 7 is a square wave

It might be just the trimmer settings. TR-1 sets the sweep width, TR-2 the center.

That 1974 schematic is the right one for this board.

[glebert]

Pin 1 of IC5 should be a triangle wave, pin 7 is a square wave

It might be just the trimmer settings. TR-1 sets the sweep width, TR-2 the center.

That 1974 schematic is the right one for this board.

Thanks, that is the kind of info I need. I've played around with the trimmer settings quite a bit and can only get an "even" sound on the fast setting, low and medium have odd artifacts in the sound.

[glebert]

OK, doing some more research, I learned I was confusing the phase shift section with the LFO. This explanation of the similar Phase 90 circuit was helpful in this regard. https://www.electrosmash.com/mxr-phase90

[glebert]

Probed the LFO circuit. On fast speed I had a nice strong triangle wave at pin 1 of IC5. It was about 10V p-p. As I switched the speed lower the triangle wave started to sag, the edge rate slowing down, still rising or falling but at a slower rate. On the slowest speed I lost about 40% of the peak-peak swing. No idea if that could cause my problem, but thought it was noteworthy. I usually think of triangle waves losing amplitude as the signals get too fast, not too slow.

[Mick Bailey]

My initial check on older equipment is to see if the electrolytics are good. An ESR checker helps, as sometimes the value is still OK even if the ESR is very high.

[Helmholtz]

Probed the LFO circuit. On fast speed I had a nice strong triangle wave at pin 1 of IC5. It was about 10V p-p. As I switched the speed lower the triangle wave started to sag, the edge rate slowing down, still rising or falling but at a slower rate. On the slowest speed I lost about 40% of the peak-peak swing. No idea if that could cause my problem, but thought it was noteworthy. I usually think of triangle waves losing amplitude as the signals get too fast, not too slow.

If you're using AC coupling with your scope the displayed signal amplitude will drop below maybe 10Hz.

[glebert]

If you're using AC coupling with your scope the displayed signal amplitude will drop below maybe 10Hz.

Good point, I will try it on DC.

Put in a sine wave input and I can definitely see the uneven modulation at slow and medium speeds. Will try to get some waveform pictures and frequency/period measurements to see if there is some correlation to something going on in the LFO or AC mains or something else. Also bit the bullet and ordered an ESR meter. Didn't really want to have another meter sitting around but figure it might save some time.