Deluxe Electric Mistress trimpots: help!!!!

[Rob Strand]

Your IC1 (4558) voltages look okay to me though and I wouldn't touch any of the trim pots just yet, if anything i'd suspect the 741.

I was thinking since the unit had been modded incorrectly and now put back to a good state, it would be worth re-measuring some of the DC voltages.   Basically we need to start again.

[Scruffie]

Fair point but I was more looking at them relatively, they're how i'd expect a working opamp to show even if the bias may be off now but the 4558 is disconnected in bypass and from the direct output jack and the 741 voltages while okay are slightly off and is always hanging off the input.

Having said that the 741 is biased by the 4558 so yeah, voltages on the 4558 & 741 again would be a good idea.

[Fenderstratocaster0]

From the top here we go.

16.5v from transformer to both brown leads.

Regulator has 14.9v on the OUTPUT pin
Regulator has 22.1v on the INPUT pin.

Transistor has 13.9v on the E
Transistor has 2.5v on the C

SAD1024A voltages

1. 0
2. 4.3
3. 7.3
4. 0
5. 14
6. 6.4
7. 14
8. 7.2
9. 14
10. 7.2
11. 14
12. 6.3
13. 0
14. 7.2
15. 4.3
16. 0

SCL4013BE

1. 7.2
2. 7.2
3. 14.2
4. 0
5. 7.3
6. 0
7. 0
8. 0
9. 0
10. 0
11. 0
12. 0
13. 14.6
14. 14.6

LM311

1. 0
2. 3.5
3. 2.5
4. 0
5. 14.6
6. 14.6
7. 14.2
8. 14.6

RC4558

1. 4.3
2. 4.3
3. 3.9
4. 0
5. 5.8
6. 6
7. 6
8. 14.6

LM324

1. Cycling 0-13
2. 6.4
3. Cycling 4-8
4. 14.6
5. 1.7
6. 1.7
7. 3.6
8. Cycling 4-8
9. 6.4
10. 6.4
11. 0
12. 0
13. 1.7
14. 1.7

UA741

1. 0
2. 6
3. 5.5
4. 0
5. 0
6. 5.9
7. 14.5
8. 0

[Rob Strand]

I'm assuming you can at least bypass the whole effect with the foot switch?  That means the connectors and Jacks are OK.   It doesn't confirm the footswitch is OK.

Can you make sure the footswitch is switching the output jack to the 5uF cap + 47k resistor at the output of the unit?

Focusing on getting the clean signal working first.

When I check the DC (see below) it all adds up and there's no DC reason why there is no clean signal.

It's a weird circuit because the DC biasing starts at the Bias Trimpot,
then propagates through all stages to the output,
then the input opamp picks up its DC from the output opamp (!)

The thing about this design is it only takes one fault and it all collapses.

The DC voltage on the SAD1024 is a little lower than I expected (maybe 5V or 6V) but 4V is not a crazy value.

You could try tweaking the DC.  Take an accurate DC measurement at pin 1 of the 4558 and write it down so you can set it back later.  Then adjust the Bias trimpot to see if it comes back to life.   *However* at this point hold off on that.

Do you have an Audio Probe or an Oscilloscope?   If you do it is very easy to check the audio at the output of each stage and find where it stops.

http://geofex.com/FX_images/audioprb.gif
--------------------------------
DC around the loop starting at Bias Trimmer sets DC
Inputs first then outputs.

RC4558

2. 4.3
3. 3.9

1. 4.3

SAD1024A voltages

2. 4.3
15. 4.3

6. 6.4
12. 6.3

RC4558

5. 5.8
6. 6

7. 6

UA741
2. 6
3. 5.5

6. 5.9
--------------------------------

[Rob Strand]

Regulator has 14.9v on the INPUT pin
Regulator has 22.1v on the OUTPUT pin.

Are you sure you are interpreting the pins correctly?
15V should be on the output.   

Check out the pic I posted before.
Also note you are viewing the PCB from the bottom *and* the pin-out extract from the data sheet is specified as viewed from the bottom of the device.

https://postimg.cc/image/qaj02loln/

[Fenderstratocaster0]

Rob yes you are correct 14.9 output and 22.1v input.

Corrected my post.

[Fenderstratocaster0]

I don't have an oscilloscope nor have I made an audio probe before or used one.

[Rob Strand]

I don't have an oscilloscope nor have I made an audio probe before or used one.

The Audio Probe is a very simple idea.   It's a probe which lets you listen to the signal and various points in a circuit.   For example you can listen to the signal at the output of the first opamp in your circuit to know if the sound is getting through.   Then you listen to the signal on the second opamp etc.

The Audio Probe is just a capacitor with a wire (the probe) and a jack so you can connect the probe to an amplifier.   You listen with the amplifier you connect it to.  It can be you guitar amplifier - just turn it down!    Or preferably some crappy little amplifier like some old PC speakers.     I wouldn't use a HIFI amp and speakers as some faults can take out the tweeters.  The capacitor just stops any DC in the circuit feeding through to the amp.  It also stops the amplifier placing a DC load on the circuit you are testing.   

You should connect the ground side of the probe to your circuit.

Check it out,
http://geofex.com/FX_images/audioprb.gif

For a one-off job you don't need shielding unless the buzz annoys you.   Personally I'd make the probe out of some coax.

For the type of problem you have the audio probe is the absolute ideal tool.   Minutes to make and minutes to debug where the problem is.   It's a good "investment" - seriously!