BYOC Optical Compressor- issue?

[Sea Chief]

Hi- bought a used BYOC optical compressor, & afaict doesnt work too well. Its made ok & all the components from the kit in right places. Fairly simple 2 knob level/ sustain circuit. At $75 it shouldnt sound like this surely..

Symptom: knobs at 1/2 not much difference in effect. Then dial sustain up to 2/3rds & a just tad more sustain but when digging into strings or a normal firm strum, the vol level immediately 'lurches' right down then rises back up as notes decay. Pretty annoying. Dialing sustain further vol lurch gets more pronounced, sustain not really happening that much either even maxed (noisy too last 1/10th turn).

So not very useable. This clip of one working properly you can hear a distinct sustain increace right up to RH sustain knob max & no crappy 'vol lurch down' https://www.youtube.com/watch?v=up6fxr-GOjc

Any ideas? Thanks, SC.

[J M Fahey]

Any ideas?

Yes:
1) post the schematic about what you are asking
2) post the video showing the defect, not some other guy's which works as it should.

[Sea Chief]

Yes:
1) post the schematic about what you are asking
2) post the video showing the defect, not some other guy's which works as it should.

Right then. The only way I know how to post the schematic up is this: I direct you to a page on a website > to then click on a section, to download the build sheet > to scroll down to the schematic. (Ive no idea how to pull the schematic section off this to somehow paste onto somewhere else). I have no idea how to post a clip of my pedal at all- thought referring to one thats working was useful. obviously not then.

So, half way down this page is Optical Compressor > V1. Instruction Archive ? Build Your Own Clone
On page 13 is the schematic.

thanks SC

[nickb]

Here you go.

[Mick Bailey]

Well, it's a tried and tested design and if using the BYOC pcb is reasonably foolproof.

First thing I would do is check every polarized component to make sure it's oriented correctly - no backward electrolytics or flipped transistors. The build instructions are helpful for this and contain the schematic, too. Double check that the resistors are correctly located - no switched values.

Next I would make sure the voltages are correct. You have 9v and 4.5v (or thereabouts). Check the opamp has voltage on pin 8 and that 1/2V is present.

There's a fair chance that whoever built it never got it working properly and was dissatisfied with it, so first thing I would suspect is a build error.

The sustain control will get a little noisy at full rotation as you're increasing the gain of the unit and unlike a studio compressor there's no noise control circuitry.

[Sea Chief]

Here you go.

thank you nickb.

[Sea Chief]

Hi Mike cheers for the info.

Ive checked the polarity of caps & transistors. Resistors are annoying to check as colour bands so tricky to see, and often if measured in situ a resistor wont read its proper value, so at the mo I will have to assume these are ok.

Voltages: on the build notes it says when checking V around various spots, to "put black DMM probe on either jack's sleeve terminals" then set DMM to VDC etc etc. But I can only get the 9.9V as Id expect at the most obvious/ 1st port of call point on board 'POS' with the probe on the ring, not the gnd/ sleeve of the input jack, 8.75V at a few point thereon. I dont read a thing from the sleeve of either jack.

Am I reading V from the sleeve, or the input jack ring?

[Sea Chief]

Well, it's a tried and tested design and if using the BYOC pcb is reasonably foolproof.

First thing I would do is check every polarized component to make sure it's oriented correctly - no backward electrolytics or flipped transistors. The build instructions are helpful for this and contain the schematic, too. Double check that the resistors are correctly located - no switched values.

Next I would make sure the voltages are correct. You have 9v and 4.5v (or thereabouts). Check the opamp has voltage on pin 8 and that 1/2V is present.

There's a fair chance that whoever built it never got it working properly and was dissatisfied with it, so first thing I would suspect is a build error.

The sustain control will get a little noisy at full rotation as you're increasing the gain of the unit and unlike a studio compressor there's no noise control circuitry.

Ok I needed to plug the input lead in to read from the sleeve.. got that then.

Now I read 9v where it says (on the build notes checks) and 1/2V where it says to check too.. but at opamp pin 8 (not on the build notes to check) I read full 9V not 1/2V.

Yes either the orig builder didnt ever have or working right, or he sold it once it failed. Its a small circuit, a simple device/ thingy so thought Id have a go 1st tho. Mind you my brain isnt much more complicated at best of times.

[g1]

Checking the resistors in a faulty diy build is essential. You can't assume they are ok. It is a very common source of problems with builds.

[Sea Chief]

I knew you were going to say that g1.

Resisitors all checked, yes properly.

[Mick Bailey]

Pin 8 voltage is correct.

If you don't know the resistors are correct you could waste a lot of time. Metal film types aren't easy to read, but you can usually get a good idea of the value from metering in circuit. If I'm looking at what I think should be a 100k resistor and get 180k, then I know somethings wrong. If it reads 90k then I'd think that was close enough. If a cap is charging let your meter settle to get a reading.

[g1]

The V+ at pin 8 is correct. Is 1/2V (Vref) at pin5 ?

[g1]

Sorry, that's not pin5 that should have Vref. Someone thought it best to not list the actual pin numbers.
How about a list of DC voltages for all pins of IC1.

[Sea Chief]

The V+ at pin 8 is correct. Is 1/2V (Vref) at pin5 ?

Yes 1/2V at pin 5.

A hunch.. the biggest (220uf) capacitor is worn out. Would my symptom tally with this?

[Sea Chief]

Sorry, that's not pin5 that should have Vref. Someone thought it best to not list the actual pin numbers.
How about a list of DC voltages for all pins of IC1.

Pin 1. 4V
Pin 2. 0V
Pin 3. 4V
Pin 4. 0V

Pin 5. 2.7V
Pin 6. 4V
Pin 7. 4V
Pin 8. 8.5V