DOD FX-90 troubleshooting

[CheapPedalCollector]

I've been trying to fix this FX-90 that's really throwing me for a loop. I located a schematic (thanks to who gave it to me) and I cleaned it up and made some corrections to it.

When I received the unit it was in very very good condition but the echo was very very low in volume, and still is. It also had a problem with the LED slowly becoming bright again in bypass mode after a few seconds of the unit being powered up.

Currently the unit is working properly except for one thing, I have very low output on pins 10 and 11 on the NE571 (I used a 571 instead of a 570). The echo is present and quite loud and not distorted at the input pins 14 and 15 so I know the MN3005 and NM3101 are working properly (other than the clock having that weird drifting issue which is probably bad ceramics which I've ordered).

I've replaced all the electrolytic caps in the unit as they were the usual bad DOD caps leaking DC and having wild ESR values. I've checked and replaced Q7 as it wasn't working properly, I've replaced the NE570 with a known good chip (in fact 2), and I've replaced the 1458 with a new one. I've tried swapping the 4007 just in case it was the problem but no difference. The switching is now working correctly according to measurements, and the LED is no longer acting strange in bypass mode.

I've also pulled the PN100A at Q8 to check if that was the issue but it's functioning properly, and I temp swapped in a new 2n5088 with no change. R39 is measuring properly at 3M and C31 has been replaced with a film cap, R30 is 9.9k and C34 measures correctly at 120pf on my cap checker. At this point I am stumped as to what could be wrong.

Anyone have any idea what the issue could be? I'm not sure what other pin on the 571 could be suppressing the output other than the RC pin 16.

[CheapPedalCollector]

Sorry for posting this in the wrong section, I searched first and posted it in the same sub as other people troubleshooting and I added the schematic.

[duck_arse]

it's not really a problem, but out here you get many more people looking, better chance of a fix.

[Rob Strand]

Anyone have any idea what the issue could be? I'm not sure what other pin on the 571 could be suppressing the output other than the RC pin 16.

You did well to get where you are.

I have a feeling a problem like this has been posted before - can't rememeber the answer.

First check the signal level at the emitter of Q5.  If the signal is low the problem is before Q5.

Next check the signal level at the wiper of the TP2 (REGEN) trimpot.   A bad or dirty trimpot could reduce signal.

Q8 acts as a mute in bypass mode but if there is something wrong with Q8 it could reduce the signal in effects mode.

(Watch out following DOD schematics.   They often have errors you need to check how the parts connect on the PCB like the schematic.  Also some DOD pedals have many revisions and the schematic may not match you PCB.)

[CheapPedalCollector]

You did well to get where you are.

I have a feeling a problem like this has been posted before - can't rememeber the answer.

First check the signal level at the emitter of Q5.  If the signal is low the problem is before Q5.

Next check the signal level at the wiper of the TP2 (REGEN) trimpot.   A bad or dirty trimpot could reduce signal.

Q8 acts as a mute in bypass mode but if there is something wrong with Q6 it could reduce the signal in effects mode.

(Watch out following DOD schematics.   They often have errors you need to check how the parts connect on the PCB like the schematic.  Also some DOD pedals have many revisions and the schematic may not match you PCB.)

Thank you Rob.

Yeah the schematic is for Rev H, but I have Rev J so I made a few corrections to it (cap values), and I will probably make a few more as I trace it out a bit more. Glaring stuff was GND pin on the NE570 going to RC and through C13 to ground. Also VDD and GND were swapped on the MN3005 and MN3101. Yeah I find almost all their schematics have errors, it's closer to an FX96 and I've been referencing that schematic too.

I have full echo signal at Q5 emitter, and also at RIN and DG of the NE chip. I checked the 2.2k trimmer, it seems to be fine. I have since cleaned the pots and it works better too. The 25pf cap on the clock is still acting bad so I think the unit was in a humid environment and the ceramics need to be replaced too and I've ordered some that are dipped Vishay ones.

I checked Q6 before, but perhaps it's got a problem or current drain issue, which would also explain the LED behavior. Thanks that's my next thing. I will check R41, R56 and R42 as well, and the LED too. I've had bad LEDs in DOD pedals before, seems they don't last forever after all.

[Rob Strand]

I checked Q6 before, but perhaps it's got a problem or current drain issue, which would also explain the LED behavior. Thanks that's my next thing. I will check R41, R56 and R42 as well, and the LED too. I've had bad LEDs in DOD pedals before, seems they don't last forever after all.

Sorry I actually meant Q8 not Q6, that schematic is hard to read.  I already corrected the first instance but not the second.

All your results put a tight box around U2B.   Maybe check the DC voltages on U2B pins when in effects mode.

[CheapPedalCollector]

Q8, Q6 both good, R56, R42, LED all good. Q8 was the first thing I checked along with caps.

I'm suspecting the 120pf caps at this point, chip is known good I have 6 or so new ones, they all do the same thing in the socket and work in another unit. P5 is also good.

I will check R48, R49, C21, Q7 again. D4 is good.

[Rob Strand]

Q8, Q6 both good, R56, R42, LED all good. Q8 was the first thing I checked along with caps.

I'm suspecting the 120pf caps at this point, chip is known good I have 6 or so new ones, they all do the same thing in the socket and work in another unit. P5 is also good.

I will check R48, R49, C21, Q7 again. D4 is good.

See how you go.

It's a tricky one.   Especially with all the part replacements in that area, doesn't leave much.

I think checking the DC voltages is a good idea since a lot of external problems will show up there.   Most of the pins sit at 1.8V.  On that circuit the output (pin 10) should sit at about 6.6V.   Pin 16 probably sits at zero (maybe 0.7).

Taking a more analytical perspective, if pin 15 had a bad track or bad connection the rectifier would stay off and you would get a very low output signal even if pin 14 was passing audio into the chip.

If you have a signal generator you should see the voltage on pin 16 (Cap C31) being around 0V (maybe 0.7) with no signal and the voltage rises when an audio signal is present.   Can't remember what voltage it rises to.  That would mean the rectifier part is working and charging the filter cap.   The rectifier connects to the gain cell internally so checking that voltage is much as you can see from outside. 

[CheapPedalCollector]

I checked traces, all seem good, solder joints too.

I checked voltages in both bypass and on, here's what I got on pins 9-16 left is off, right is on.

9   1.8   1.8
10  6.3   6.3
11  6.3   6.3
12  1.6   1.6
13  8.1   8.1
14  1.8   1.8
15  1.8   1.8
16  0.0   0.7

Everything seems to look right, this is so weird, I always get these weird ones that are hard to figure out. 

[Fender3D]

Sorry if I missed some bit, but what happens if you completely remove Q6 R39 and C31?

[CheapPedalCollector]

Same thing, I have full echo signal there. It's the output where it's quiet and muffled.

[Rob Strand]

Sorry if I missed some bit, but what happens if you completely remove Q6 R39 and C31?

Worth trying at this point.  It's actually Q8, hard to read on the schematic.

I guess one thing that's not so clear is how much signal is being lost?  a few dB or 20dB.

R31 might need to be increased.    When you use things like R31 it's not uncommon to have a similar part on compressor section U2A.  Having only R31 sort of acts like a noise gate.  However if the signal isn't strong enough it might end up causing an overall attenuation.

Another thing that caught my eye is C30 is around the wrong way on the schematic.  Probably not the cause of the problem and I'm assuming the PCB is like the schematic- with DOD that might not be the case.

If I got to this point I'd be putting a signal generator into the unit and plotting out Vin at C30 and Vout at U2B pin 10 over a range of input voltages Vin.  That will quantify what's happening at the Expandor and perhaps give a hint of the cause.

[CheapPedalCollector]

It's closer to 20db attenuation, I can barely hear the echo with the mix pot all the way to the right.

I got a new signal generator today, and I'll have a new scope soonish so I'll do more troubleshooting then. Currently I think the very small 120pf caps, which appear to only be rated at 10 or 12v are shorting out under load. I can't find anything else wrong at this time. Thank you all for your suggestions.

I'll chime back in if I find out what the problem is.

[Rob Strand]

It's closer to 20db attenuation, I can barely hear the echo with the mix pot all the way to the right.

Ever more puzzling.  It's like the rectifier or gain cell isn't working.

[FWIW, you can bypass the rectifier and gain cell by placing a resistor (say 20k) in series with a cap across (say 10uF) across pin 14 (DG) and pin 12 (INV).   Play with the resistor value to see if you can change the gain.  This wires the opamp as an inverter.   Not really a fix at all since it bypasses the gain cell.  All it does is shows the opamp part can pass audio.   We sort of know the opamp is working already since the DC voltages are correct.]

I got a new signal generator today, and I'll have a new scope soonish so I'll do more troubleshooting then.

Good stuff!

Currently I think the very small 120pf caps, which appear to only be rated at 10 or 12v are shorting out under load. I can't find anything else wrong at this time.

I'm not convinced it's the 120pF caps since the DC voltages look OK.  Those caps are only used as an RF filter.  Some circuits don't use them.

Having said that, I've got no good ideas.   I'm at the point where I'd separate that part of the circuit out by lifting input/output caps, get the signal generator out, and run that part of the circuit through it's paces.

[FWIW, you can inject current into the rectifier cap with a largish resistor to +V.  The idea is to see if the gain cell is working.]

[Fender3D]

Since CPC already replaced electros and 570 chip, and assuming nobody modded the circuit, I wouldn't mess too much, randomly swapping parts without knowing what actually happened to it, I'd rather lift C35 and check if signal on pins 10-11 remains dimmed...

[CheapPedalCollector]

I tend not to do hamfisted shotgun repairs on these things, I was going off past experience with DOD pedals always having bad caps in almost every single one I've ever bought. I did check them all with my ESR meter and the first 5 or so I checked were way off spec, so I just replaced them all. I really try to do minimal stuff, this one is just throwing me for a loop for some reason and I've noticed that there are lots of posts around with other odd issues with this particular model. Probably why it had so many revisions, as my board is marked Rev J and it's still the first DOD style logo of the FX series and not the later font change and/or slanted one.

I went for the 4007 which I removed, put a socket and stuck a new one in, no change so I put the original back. The NE570 was bad and distorting horribly so I left it swapped with the new Coolaudio 571, also socketed. I pulled each transistor I suspected and tested them, none were bad so I put them back. I did temporarily swap Q8 with a new 2n5088 with no change so I reinstalled the original. I pulled and socketed the 1458 too as it was reading a little weird on one of the pins, and the new one is not. I suspect the unit was connected to a wrong power supply, or one with too high voltage which would explain a lot of the issues I found so far. I swapped the FET with the odd internal part number for the standard J113 which is in all schematics I can find, still no change.

All voltages seem to read what they should in each mode, and as I mentioned before I have full echo signal entering the unit at RIN and DG.

This has all resulted in the switching working seemingly correctly without the LED slowly fading back on in bypass mode, the echo clock no longer drifts and acts weird, and everything other wise sounds good and works properly except this muting problem which I figured would have been the .47 uf caps like the issues in the 570 flangers your advice helped me fix. But that seems not the case here.

I'll lift C35 and see if it remains muted, I'm not entirely sure what would cause that however, I've checked all those parts too. I suppose C21 could be shorting somehow?

[Rob Strand]

I'll lift C35 and see if it remains muted, I'm not entirely sure what would cause that however, I've checked all those parts too. I suppose C21 could be shorting somehow?

You mean C31, yes?

The DC voltage test shows the voltage across C31 goes to 0.7V when the effect is enabled.  That looks normal so it can't be a C31 short.

The symptoms look like U2B pin 14 or pin 15 isn't getting signal, especially pin 14 since that is the rectifier input.   You should be able to feed an AC signal into the unit and see the voltage across C31 rise above its 0.7V quiescent DC level.   To start try a relatively high level say 1V to 2V at 400Hz, just under the clip point of the BBD. Pin 16 Pin 14 might go to 1V to 2V above 0.7 depending on the signal level - I can't remember the specifics.   That would be *the best* test at this point.  The DC voltage on the cap is represents the level of the rectified AC signal.  If it does not rise that's a clear indication the rectifier isn't working and the circuit will stay muted.

The next step would be to work out why.   With such a sparse circuit the only thing that comes to mind is a broken track or bad solder joint on pin 14.   If possible measure the DC voltage on pin 14 (and pin 15) *on the top of the chip*.   Then with a signal generator input measure the AC voltage getting to those same pins (*on the top of the chip).   Press lightly with the probes.

[CheapPedalCollector]

The next step would be to work out why.   With such a sparse circuit the only thing that comes to mind is a broken track or bad solder joint on pin 14.   If possible measure the DC voltage on pin 14 (and pin 15) *on the top of the chip*.   Then with a signal generator input measure the AC voltage getting to those same pins (*on the top of the chip).   Press lightly with the probes.

I've already done this, I hooked my probe to both pins (they are tied together) from the top of the chip, did my measurements there too, and got the same very loud signal out of them and 1.8v at idle. I'll doublecheck them tomorrow and see what kind of voltage is present with signal from the BBD coming in, I have some other things to finish today. It has to be that or something very weird.

[Rob Strand]

I've already done this, I hooked my probe to both pins (they are tied together) from the top of the chip, did my measurements there too, and got the same very loud signal out of them and 1.8v at idle. I'll doublecheck them tomorrow and see what kind of voltage is present with signal from the BBD coming in, I have some other things to finish today. It has to be that or something very weird.

At least the signal is getting to the pins.

Pin 16  might go to 1V to 2V above 0.7 depending on the signal level - I can't remember the specifics. 

I did a stupid typo here.  It should be pin 16 not pin 14.  Too many 5am days.

As you increase the AC signal on pin 15 the DC voltage on pin 16 (ie. voltage across C31) should rise accordingly.   If you see this happening it means the rectifier part of the chip is working.   That should then control the gain as the gain cell is internally connected to the rectifier output.

[Fender3D]

You mean C31, yes?

No I meant C35 or 36 can't read it... The cap at 571's output pins.
If it leaks DC, it may affect output signal level.
I'm still assuming the pedal worked correctly before, if it has issues even with a new chip it shouldn't be chip's functional blocks fault