DOD Stereo Chorus FX60 - weak effect?

[drummer4gc]

Hi, I've repaired a DOD FX60, but the effect sounds weak - a little lifeless, not much modulation present. I don't use a lot of chorus or even have another to compare it to unfortunately. I know it's a subtle effect, but it just sounds weaker to me than I'd expect, and it sounds weaker than demo videos online. Under 12 on the depth pot sounds almost like it's off, and even cranked it's very minimal. In all fairness, I'm running it mono, not stereo, but I'd still expect more.

A couple observations:
-Depth pot measures properly.
-Clock signal dances faster/slower and more/less while twiddling the knobs and looking at a scope - that's about as scientific as my scope work gets unfortunately.
-Pulling the output plug out halfway to get the "output right" signal gives a stronger effect. I can tell the filtering on that side is different, not exactly sure how that would expect to alter the signal. But, its got more treble, and the effect is more noticeable.
-Parallelling additional capacitance across pins 5 and 7 of the MN3101 increases the effect as expected.


Here's what I've done:
-touched up wiring and repaired a number of broken traces.
-replaced the LM1458
-replaced all electrolytic caps

Here are voltages from the relevant ICs:

U1 (Pins 1-8):
5.5
5.5
5.5
0
5.7~ (wavering a bit)
5.7~
5.7~
8.8

U2 (MN3007):
8.6
4.5
5.4
.58
0
4.4
5.6
5.6

U3 (LFO):
alternating between ~0-7
~3-5
~3-5
0
~3-5
~3-5
~3-5
8

U4:
5.4
5.4
5.4
0
5.4
5.4
5.4
8.8


U5(MN3101):
8.6
4.4
0
4.4
8
0.9
5.3
0.6

Would love your thoughts, thanks!

[drummer4gc]

Bump for good luck...open even to wild ideas to check out!

If it's helpful - I was able to crack open a properly working Danelectro Cool Cat Chorus, another mn3007 based chorus which I cannot find the schematic for. The LFO is putting out a stronger output signal, and it's clear that there is more wobble coming from the delay/modulation circuit. It's also an 18v pedal, so not sure how that might play a factor. But it definitely leads me to believe something isn't right with the DOD!

[willienillie]

Have you tried tweaking the bias trimmer?

[iainpunk]

I'm running it mono, not stereo, but I'd still expect more.

not that it matters anyway, since its a mono chorus effect, but with two outputs, not two chorus effects parallel. the only difference is that one output is darker and the other brighter.

i strongly recommend twisting the bias trimmer for best sound, i ''repaired'' one of those too, the only thing that was wrong is a totally off bias trimmer, set to the minimum voltage, if i remember correctly, i set it to 4.9 V, but the optimal will differ per unit.

cheers, Iain

[drummer4gc]

Thanks, I appreciate the suggestion. I have played with the trimmer - it didn't change the sound perceptibly at any setting. I set it to the point that achieved the most symmetrical signal coming out of the BBD, which I think was just over 5 volts.

[Govmnt_Lacky]

Odd that they only use one of the delayed outputs from the MN3007. Don't most Chorus circuits sum the outputs?

[Fender3D]

Odd that they only use one of the delayed outputs from the MN3007. Don't most Chorus circuits sum the outputs?

I've never seen any DOD stock schematic without errors here and there....

[anotherjim]

Those "R?" marked as 47k feeding dry signal to the output mixers. It may help to pull one end of those out of the board so you can hear the BBD signal better without the dry and find the most optimal bias. Make sure pin1 of U1 is varying as you turn the trimmer. While you're only listening in mono, you only need to pull the relevant 47k out.

[The Crooked Man]

I've got a DOD FX65 that's really weak, too, and tweaking the internal bias trimmer doesn't help at all.  I've searched high and low for suggestions on how to increase the depth, but haven't been able to find anything.  I did come across Austin Hot Mods, who have a set of mods for the FX65 which include "increase(ing) the rate and the depth to allow everything from wide open to super fast" but there are no details beyond that.  It's gotta be a simple resistor swap, right? 

[anotherjim]

Increase the 33k next to C27 should give deeper sweep.
Reduce both 62k after Q3 to 47k to increase wet signal to the output mixers.

[The Crooked Man]

For the FX65, would I increase resistance in R20 next to C27 to, say 56k?   
Do you think R38 and R43 at 62k serve the same purpose in the FX65, since they're both also 62k in the same part of output section, and should be lowered to 47k?

[Fender3D]

You may use one 100k trimmer instead of R19/R52 partitor to set min depth, and one 47K trimmer instead of R20/R21 partitor to set max depth.

[ElectricDruid]

Odd that they only use one of the delayed outputs from the MN3007. Don't most Chorus circuits sum the outputs?

That looks like a deliberate error to screw up copyists to me. It'll massively increase clock noise and anyone with half a brain would avoid doing it like that. So anyone who's stealing your circuit but doesn't know what they're doing will find the result unusable and won't know why.
I'd expect the PCB is as-per the MN3007 datasheet.

[drummer4gc]

The two outputs of the 3007 are tied together, so that's an error on the schematic I posted. The trimmer seems to be functioning and altering the bias voltage, but I can't hear an audible impact. I audio probed the 3007 output while adjusting the trimmer, and at every setting it's just SO mild, it can't be right.

I'm suspecting the LFO - thoughts on how I can test that it's putting out a properly "deep" signal to the 3101? I can't get a good read of its output on my old school scope - I'm guessing I need to adjust settings to capture such a low frequency signal but I'm not sure how.

Thank you all for your input, please keep it coming!

Crooked Man - you can also try replacing C11 with a larger cap, Boss uses a 47pf.

[anotherjim]

You have to use DC coupling setting for the 'scope channel. AC coupling won't keep up with it. Slow time base & use a hood to keep out ambient light. Persistence of vision can help see the trace. Don't have intensity too bright. Use an edge trigger input with another probe on U7 pin1 (should be square wave).

The electro' caps either side of the Depth pot could be bad?

[The Crooked Man]

drummer4gc: I'll definitely give the cap swap a try and I'm 99% sure I have spare 47pf's kicking around somewhere. 
anotherjim: Can you confirm that the resistor swaps you suggested would also apply to the FX65 and not just the FX60?  I don't see why they wouldn't, but I'm more of a "point and click" kinda guy at this stage, so I want to be 100% sure that I'm not making bad assumptions.

[anotherjim]

They seem to be the same in those places, so yes, the same resistors.
The 4.7uF cap on the LFO side of the Depth pot will be rounding off the peaks of the triangle wave so removing that cap will also increase the depth of sweep.
On the 47k/62k wet/dry mix resistors, the design seems to expect some gain in the wet path. However, all the filters and buffers aim for unity gain so the mix may not reach 50:50 with those resistors. The Boss CE-2 is the same kind of thing and it uses equal mix resistors, 47k/47k.

[The Crooked Man]

Just swapped out the R38 and R43 resistors with 47k's and it definitely helped.  Those alone took it from "why the hell do I even have this pedal?" to perfectly useable, but nowhere close to "liquidy."  I'll try the caps next and report back.  Thanks for the tips!

[bloxstompboxes]

Haven't read every word in this thread but I have had a few of these and had a similar issue with them. Best thing to do is change the electrolytic caps first. then make adjustments to the trimmer. These are old pedals and the caps most likely have drifted. Once I have done that, they usually spring back to life and then the trimmer just gets the sweet spot. They sound as good as a CE2 to me.

[drummer4gc]

Wish that was the case with mine, but new caps and a trimmer adjustment haven't helped. Trying to locate another one now to borrow and test out to compare!