Ebay DIY delay pedal mod to fix volume drop problem

[buildAndPlay]

How sure are you that the 47pF is correct?
You could try pulling that out like you did with the 5pF.

I was actually thinking about doing that exact thing this morning.  I will try that tonight.

You could also try moving the connection of EC3 (1uF) from U1 pin 1 to U1 pin 7; for example by lifting one leg an wiring it through.   This way you are only listening to the first stage. If you still hear the volume drop then the problem might be around U1b.   If not then the problem is likely to be around U1a.

If you have a multimeter measure the DC voltages on all pins of U1.

Yeah, I think I'm going to just pull the delay chip out again and concentrate on the op amp stages.  I like the idea of running one at a time.  I'll try bypassing each and seeing what happens.

[buildAndPlay]

Tonight I lifted one leg of the 47pf capacitor that parallels the second stage feedback resistor.  That didn't do any good.  Then I started taking voltage measurements just for the heck of it.  When I checked from pin 4 to 8 on the op amp I got nothing.  Pin 4 goes to the PCB ground.  I noticed the same thing on the PT chip (pin 4 to 1).  Then I realized that the negative lead from the DC input doesn't connect to ground.  There is no continuity between the two.   I'm assuming this is intentional but it seems weird to me. 

[Rob Strand]

When I checked from pin 4 to 8 on the op amp I got nothing.  Pin 4 goes to the PCB ground.  I noticed the same thing on the PT chip (pin 4 to 1).

That is very weird.  I'm can't see how it can work at all.  Maybe the test set-up see below.   Find a 0V/ground and 9V then progress through.

Then I realized that the negative lead from the DC input doesn't connect to ground.  There is no continuity between the two.   I'm assuming this is intentional but it seems weird to me. 

I can see the grounds connecting together on the PCB.
It is normal to have switching on the input or output jacks and sometimes the DC jack.  You need to have everything plugged in to check the connectivity.

[buildAndPlay]

Ok - that makes sense.  I didn't have the cables plugged into the input/output jacks.

[Rob Strand]

OK cool.  (sorry for the typo's in my post - bizarre).

[buildAndPlay]

I just disconnected the output capacitor lead from the 2nd stage of the op amp and ran a wire from it back to the output of the first stage (see red line below).  So that should have effectively bypassed the 2nd stage.  I'm getting unity gain that way, although with my earlier resistor changes I was expecting a slight boost.   I'd like to try the opposite so I'm assuming I should run a wire like the pink line below...

[buildAndPlay]

I bypassed the first stage like the pink line in my previous post.  I lifted the cap leg and soldered a wire to it and then soldered the other end where shown without lifting the resistor.  That didn't work at all as I had no signal when I engaged the delay.  I think I would need to lift that resistor too to prevent the signal from going places I don't want but I think I'm done with that.  Every time I remove/replace a component I compromise the board a little bit more.  My desoldering skills aren't all that great.  I got some new resistors the other day so I put a 270k in where the original 510k was at the input of pin 6 and now the volume seems to be dead even between delay off and on.  I'm not noticing any clipping but I've only tried my Strat and an acoustic.  It might be a problem with a higher output guitar but this will go into my acoustic pedalboard so I should be OK.

I'll put the thing back together tomorrow and try to do a video this weekend. 

My son is doing a compressor pedal from the same vendor.  This will be his first project and the parts count is quite a bit lower so hopefully we won't run into any issues.  Thanks again for all the help.

Jim

[Rob Strand]

I'm getting unity gain that way, although with my earlier resistor changes I was expecting a slight boost.

Hmmm,  If you *haven't* got R1=510k and R3=510k and the gain is unity them that seems like the problem area already.

For the pink line mod you should lift the cap C1 *and* the resistor R10 (side with pink line) and connect them together.  However, because R24 is a low value it will load the pick-up and in fact cause a volume drop and dark sound!   This effect is not present when R10 connects to pin 7 of the opamp.  So that experiment will great a false result.

[Quackzed]

Every time I remove/replace a component I compromise the board a little bit more.

I got some new resistors the other day so I put a 270k in where the original 510k was at the input of pin 6 and now the volume seems to be dead even between delay off and on.

yeah, though i'm curious, all the digging and you either kill the board or at best get a 2 second aha! moment with the same end result really.and these type boards arent really robust so the fact that its still working says you got skills enough... hey, you fixed it man, thats all that really matters. and its got mystery mojo !!!