maestro PS1-A Debug - Help appreciated!

[jmasciswannabe]

This one is frustrating me. Powers up. Rails appear fine. Passes sound, but no modulation. I've got voltages moving around so it appears the LFO may be working, but I am not sure if the fets are switching. One of the opamps has been replaced, which I thought was strange. I am wondering if they thought it might have been the LFO opamp. I've got an audio probe and meter and am ready to try some suggestions before randomly replacing components. Here are all the voltages! Let me know what you think!

-170mv
-0
-0
-12.3v
-0
1.6mv
170mv
12.7v

-167mv
-0
-0
-12.3v
-0
-0
170mv
12.7v

-.82v (this opamp has been replaced previously)
0
-0
-12.3v
-012mv
-.012mv
158mv
12.7

-80mv - -280mv (lfo opamp)
4.8v
4.8v
-12.36
-2v - 2v
0
-10v - 11v
12.7


Q1, 2, 3, 4, 6
-0
-124mv - -131mv
0

Q5 (only jfet to show voltage on the source)
-12mv
-124mv - -131mv
0

Q7 – didn't bother, pretty sure it is good

Q8 -
-168mv  switched -.8v
-3.89v  switched  -377mv
-.8v switched -.8v

Q9
-12.3
-.4
233MV

Q10
234mv
-9.8 - .83v
119mv - 235mv

[PRR]

The important thing is that the gate voltages move around.

> Q1, 2, 3, 4, 6
-124mv - -131mv

We expect more than a Volt of swing; tenth-Volt is hardly any. Can you get real swing here?

[jmasciswannabe]

Right. Well, I say that with the understanding that they are acting as switches but did not know how much voltage is needed to turn them on or off.  Furthermore, I do not understand what creates that wide swing or what could be prohibiting it. I am assuming the lfo opamp, but am unsure as if the voltages look off at the chip or if it could be between that and the jfets. I know the trims tested fine and showed continuity to all the right parts arounbd it and that there was accurate voltage coming into the fet bias and on the wiper....I think it went up to about 3v. I could double check that electro there, but do things look right voltage wise coming from the lfo opamp?

....and thanks for your help, Paul!

[pinkjimiphoton]

ugggh. makin popcorn. got almost exactly the same symptoms here.

[cjlectronics]

I've worked on many of these... if your LFO is working but the phasing output is weak then most likely the JFETs are not matched or one or more is bad.  These pedals work best if all JFETs are matched.  At one point I bought a large lot of JFETs and spent hours matching sets.  It truly is the key but a pain to get there.

While you're under the hood, replace all electrolytic caps if they haven't been replaced already.

[pinkjimiphoton]

yeah, but match 'em to what? is there a range of operation for these that's actually good?
i mean, what voltages to shoot for?

i see the lfo was working, all the jfets seem to read the same, pot changes lfo speed. all the electros were changed. maybe bypass switch?

makes my widdle bwain hurt.
i've got bags of j113 and 5n547's still... but seems to me, and i am usually wrong, there's gotta be more to just stuffing 4 new semis in there. they gotta be able to work IN  the circuit, so the vgs or whatev needs consideration, right?

[PRR]

> yeah, but match 'em to what?

To each other! All 4 (8, 2) with the same Vto and Idss.

Any JFET can cover the range of resistances needed for standard cap values. Your LFO can be tinkered to deliver any likely gate voltage.

But if the FETs are not matched, it is like your 4-horse wagon with four different horses hitched up. They don't pull the same, some strained some slack.

[jmasciswannabe]

Hey fellas. Following the setup procedure that I found on the schem, it has the oscillator turned totally counter clockwise which eliminates the voltage swing on those jfets. Once I figured that out and turned it to fully clockwise I get a  -2 - +2 swing. Even with that and then slowly going with the fet bias trimmer it never really kicks in. My lfo ic voltages changed and I will post those up. Is there a way to determine via audioprobing whether if any of the fets are working or do they all have to work to hear phase?

I don't feel so bad knowing I am in the same boat as Jimi. This one has been frustrating to the say the least. I wanted to save its vintage integrity, but looks like I might gut the ics and fets and save the "brain pain."

[cjlectronics]

RG Keen has a great article on how to match JFETs.  Go here http://www.geofex.com/ and search for JFET matching for effects.

[pinkjimiphoton]

i think the problem isn't matching the fets, for me, at least, so much as trying to get a quad of them to work in the circuit.
i pretty much suck with anything non fuzz, so,.....
the lfo seems to be wiggling, voltages seem to be right where they should be, can watch the fets wiggle. but no bueno.
that said, i haven't touched it in months.
i can't even tell if the thing is on or off sounds pretty much the same in bypass. whatever that is.
i think i should stick to fuzzes lol

[Yazoo]

I have just built and debugged this. I got the boards fabbed and wondered whether I had made mistake. I started off with a matched set of J113s but no phasing. I then replaced these with J201s, still no phasing. When I probed the LFO signal to gate with no transistors inserted, it was fine, a nice sine wave, but as soon as I put the transistors in, the LFO voltage dropped dramatically. I even tried inserting an extra gain stage to increase the LFO but still no phasing. Finally, I removed the 4u7 capacitor linked to the R2 and R3 trimpots and, oh baby, phasing.

I have no idea why this works but decided to try this after seeing another clone layout where the polarity of the capacitor was reversed. Without the capacitor, the LFO is a triangle wave.

[pinkjimiphoton]

shoot, i may try that... thanks for the tip!