Maestro PS-1A noisy

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soapfoot

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I've got this old Maestro PS-1A that sounds great, but is a bit noisy when the phaser itself is engaged (noise is constant at any speed).

Even in bypass there's a slight bit of 120Hz hum that I'll try to remediate by replacing the ancient 470µF filter capacitors. But the noise I'm attempting to investigate is a quasi-intermittent "egg frying" sound, rather low in level, whenever the phaser itself is engaged.

I've tried looking at the output of each MC1458 on the scope, as well as the drain of each FET, but to be honest I can't discern what I'm hearing visually.

Any ideas for troubleshooting? Is it a bad idea to order some Freeze Spray and hit the FETs and ICs with a blast to see if one will act up? Aggravating them with a chopstick yields on clues.

Thanks in advance!
 

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soapfoot

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There was an obscene amount of flux residue on both sides of the board, so I cleaned it all off to address the possibility that it had absorbed moisture.

This did not resolve my issue, but it was probably good to do anyway.
 

Gus

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I would start at the power supply first.
Replace the two filter caps first like you posted and let it run for some time.
Also check the 270 resistors
The Zeners are harder to check for noise. Set the scope to AC and look at the + and - supply

If you still have the noise you might need to do a controlled swap of parts
The easiest thing to change might be the 1458s because they are not selected and matched(I am assuming) like the phaser JFETs .
 

soapfoot

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I also just noticed that my unit has 12V zener diodes in the power supply as opposed to the 15V zeners I see on all schematics.

They look quite old and don't appear as though they've been replaced. I'll replace them, but should I use what's on the schematic or replace like-for-like what's in the unit?
 

CJ

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was it ever quiet or did you just get it?

it has been a while, but the one i fooled with was quite noisy.

i would buy both 12V and 15V zeners since they are dirt cheap.
maybe up the wattage if digikey or whoever has them,

look for any cap that will not take 15volts before swapping in the 15V zeners, there might be a lytic in there only rated for 12 or something,

looks like a discrete bucket brigade circuit, way cool!

does that thing have big colored foot pedals?
 

soapfoot

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I’ve had it for awhile, it’s always just been barely on the “usable” side of noisy.

so maybe this is normal? But the intermittent nature of the noise makes me feel like it may be a fault.

All electrolytic caps are getting replaced, and all are getting 63V parts since they comfortably fit and will run nice and cool.

I did order both 12v and 15v zeners so both are on hand.

I also ordered some freeze spray, and I might hit those ICs or FETs with it to see if I can induce some interesting behavior.

a light wave of a heat gun over the board (on “low”) seemed to temporarily exacerbate the fault a little bit, but I couldn’t apply heat in a localized-enough way to identify a precise fault (and was reluctant to do too much).

It has the big colored rocker switches, but I wouldn’t say there are any “foot pedals” on the unit exactly. I believe it was intended for table top or stand-mounted use
 

Gus

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I forgot to add I have "repaired" older effects by resoldering all the boards
I would not change the Zeners voltages
 

JohnRoberts

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I've got this old Maestro PS-1A that sounds great, but is a bit noisy when the phaser itself is engaged (noise is constant at any speed).

Even in bypass there's a slight bit of 120Hz hum that I'll try to remediate by replacing the ancient 470µF filter capacitors. But the noise I'm attempting to investigate is a quasi-intermittent "egg frying" sound, rather low in level, whenever the phaser itself is engaged.

I've tried looking at the output of each MC1458 on the scope, as well as the drain of each FET, but to be honest I can't discern what I'm hearing visually.

Any ideas for troubleshooting? Is it a bad idea to order some Freeze Spray and hit the FETs and ICs with a blast to see if one will act up? Aggravating them with a chopstick yields on clues.

Thanks in advance!
You have to love old hand drawn schematics... that input buffer is probably a 2n3638 PNP with only one emitter :unsure:. Not a very quiet input buffer.

Then LM1458 are early dual op amps based on ua741 technology (30nV/rt Hz, 0.5V/uSec slew rate). Modern 2n4303 spec about 4nV/rt Hz so way quieter than those op amps.

Be careful if you clean this up too much you mach change the sound character of the effect... phasing the noise floor may be part of it's mojo, or not.

JR
 

soapfoot

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Yeah I’m trying to be “minimally-invasive.”

if it were a constant noise floor I’d embrace it. The intermittent crackling and frying is a bit more obtrusive
 

oldskool1

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I also have one of those Maestro Phase Shifters. The same one Zep used on "No Quarter" studio version on the fast setting. Mine has always been very noisy, and I've owned it for over 30 years. The sticker on the bottom says "built for Maestro by Oberheim Electronics". I remember reading recently that Tom Oberheim planned to re-released the circuit in Eurorack form, and he made a big deal about achieving the same sound without the noise. I just searched for the product, but I don't see it on the Oberheim site, and I can't find any mention of it anywhere. But if it does exist, maybe get a Eurorack module and look at what parts he used?

Incidentally, mine doesn't have an "intermittent crackling and frying", but more of a modulated by the LFO white noise "whoosh".
 

soapfoot

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mine doesn't have an "intermittent crackling and frying", but more of a modulated by the LFO white noise "whoosh".
This is what I'm looking to achieve, really. Some noise floor is fine (even charming). But the intermittent noise is distracting and less-acceptable to clients and collaborators.
 

soapfoot

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Interesting development.

Freeze spray arrived today, so I fired the unit up and waited for it to warm up, anticipating the fault symptom... and it never came.

For the entire time I've owned this unit, the bacon frying has been a constant feature right from power on. But today I had only typical noise floor. I waited >1 hour, and no fault.

After awhile, I squirted each semiconductor component with the freeze in turn. Nothing I did could induce a symptom. I let it run for another hour before shrugging and turning it off.

Could it be that my flux residue clean-off yesterday mostly worked, but I'd used so much IPA that it had to fully, fully dry?

That's all I can figure at the moment (You really should've seen it... it was like the whole entire board had been dipped in ancient, sticky flux. I went through about 30 cotton swabs and two paper towels, all soaked in brown by the end). I did also notice that there is a little bit of flux residue hanging around even still, so I may give it another pass.

I still have fresh caps on the way, and those will get done, so we'll see.
 
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oldskool1

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Could it be that my flux residue clean-off yesterday mostly worked, but I'd used so much IPA that it had to fully, fully dry?
I doubt it. More likely you physically moved a tenous connection so that it is now properly conducting. I've seen really hard to find examples of that in my time. Anyways congrats on the fix.
 

CJ

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maybe the flux absorbed moisture over time and became a semi conductor.

or you moved a connection.

i would re-solder the whole board like Gus said, they used Ritz Crackers for PC boards back then,
you talk about flakey soldering,

also look for hairline fractures.
 

soapfoot

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maybe the flux absorbed moisture over time and became a semi conductor.

or you moved a connection.

i would re-solder the whole board like Gus said, they used Ritz Crackers for PC boards back then,
you talk about flakey soldering,

also look for hairline fractures.
All seems plausible.

When I re-do the caps I'll melt all other connections as well.

However, I was pretty aggressive with a chopstick on every component, and could not correlate any amount of mechanical stress with the fault. So the "flux residue absorbing moisture" theory at least seems plausible right now.
 

soapfoot

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Several more hours without a fault. I've become convinced that the issue was the sticky PCB.

Still, I replaced all electrolytic caps and the unit seems happy. I'll hold off on replacing any semiconductors unless/until the fault recurs.
 

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