Cupcake humm (only)

[Megadeth]

Hi! Just completed my cupcake build but now i have serious problems with humm and som other noise that is oscillating i believe.

At first i had som signal from the guitar but after i starter fiddling with the trimmer it went silent and only the loud humm remained. First i tought it was a grounding issue so i rewired the whole thing and swapped out all the cables except the grund wire to shielded. Removed the spdt switch and rewired it without it. I plugged back in and its the same thing. Im gonna supply gut shots tomorrow.

I have two substitution parts. R5 is two Resistors in parallellt giving the value 2,36k so its close. Last its the potwntiometer is changed to a 10k A.

So far i have checked:

Solderings (seem OK, i doubt hits the problem)
Changed power supply.
Checker diode and electro caps for correct orientation.
Changed op-amp to several substitutes.

[pryde]

I would start simple here. If you had signal before turning the trim pot then you should try re-flowing the trim pot and/or look for solder bridges on this component.

Also post some voltages from your transistors. That will help the troubleshooting for the gurus here

[Megadeth]

Thanks for the tip, resoldered the trimmer and it reads anywhere betwen 0 and 10M somethings not right here, im gonna replace it together with the transistors and come back with results. Anyways below is the voltage readings, no guitar input when this was measured but that should not make a difference right?

Q2

G - S 1.25v
D - S 05,94v

Q1

G - S 0,53v
D - S 0v

Opamp (TL082CN)

leg 4 - 8 07,89v
leg 5 - 7 07,33v
leg 6 - 7 0v
leg 3 - 1 0.063v


Diodes where measured from one end to another (Dunno if thats possible)

1n4001 07,85v
1n34a 0.002v

[midwayfair]

Take your voltages from ground to each pin, not from one pin to the other.

Also, your battery is low. You should have a full 9v on pin 8.

[Megadeth]

 Replaced the trimpott to a 20k and now the humm is significantly less after a bit of tuning with it. It his however there and pretty loud, also i get some kind of funky octave down effect as a bonus.

Fired up my trustworthy solidstate dc powersupply and now the voltages reads (from ground to each pin) as requested. 

Q1

Drain: 1.58v
Source: 1.58v
Gate: 0.008v

Q2

Drain: 9.26v
Source: 2.82v
Gate: 1.58v

OPAMP

1: 5.08v
2: 5.07v
3: 4.95v
4: 0v
5: 0v
6: 9.14v
7: 9.16v
8: 9.31v

Diode 4001 9,32 V (cathode) side.

[midwayfair]

6: 9.14v
7: 9.16v

This looks very wrong. Check for solder bridges and shorts using your multimeter. These pins should not be connected to anything except each other. Did you socket the chip? You may need to replace it if there is definitely nothing else wrong.

Also, don't use a 20K for the trimpot. The trimpot is absolutely not causing your problems, and it's going to be horribly difficult to bias this with a 20K. If the trim pot were broken, you would just have a booster instead of a compressor.

If you socketted Q3, pull it out and see if the dry path works. If it doesn't, your problems lie in the audio path, which is just half an op amp and shouldn't be too hard to trace. Get out the audio probe and figure out where your signal is being lost.

[Megadeth]

I think youre on to something, i confirmed the socket itself is not the problem, no voltage on pin 6 and 7 when i removed the op. However there seems to be a leakage around that area because when i plug the op back i get voltage on the pins. This got me thinking i have the wrong substitutes for the jrc5558 but i was pretty damn sure i could place a TL082CN or a NE5532P without any problems.

I got these laying around.

TL072CN
NE5532P
UA741CN
CA3130E

[midwayfair]

I don't think the type of op amp is the problem, but you will have better results with the NE552, which is another BJT-based op-amp (the TL082 is FET-based and also the 072 would have been lower noise).

You not getting any voltage when the chip is pulled is good, that means you probably don't have any connection issues. Could just be the op amp, but if not, you really will get answers a LOT faster with the audio probe.

[Megadeth]

I've been probing for a while now and i did found out that R2 was like 1mohm instead of 82k. Just replaced that and im gonna continue the mission

[Megadeth]

Plugged in my computer into the circuit and been probing. It seem that there is something wrong with the IC or perhaps the socket, the sound is really distorted (Not in the good way) at the first output of the IC.

I dont know if the power of the computer headphones is to weak to probe everything, i have attached a painted schematic with my results.

[midwayfair]

Play with the bias pot. Can you get it to a point where the signal cuts out suddenly? You want to get to just that point then back off the bias. Make sure Q3 is in right.

Double check D3. It it's backwards, the circuit won't compress.

Also, turn the volume down to be sure you aren't overdriving your headphones.

[Megadeth]

Now im really confused What do you mean by Q3 and D3?

Changed opamp and trannies again, most of the distortion after the first lug on the ic disappeared. and i did  have signal for a while on q2 for a while. As of now i believe the trim pot has something to do with the issues together with the potentiometer wich seem glitchy.   

Checked both the trannies and the diodes and they are placed the correct way.

[Megadeth]

Changed pot, ic, trannies and diodes and now i can confirm that the circuit is working as it should, but the humm is very constant and alot louder then the actual audio going through. The humm now starts after R2. However im gonna put back the shielded wires to see if it helps.


Thanks for your help midwayfair! i have learned alot from your tips.

[midwayfair]

Changed pot, ic, trannies and diodes and now i can confirm that the circuit is working as it should, but the humm is very constant and alot louder then the actual audio going through. The humm now starts after R2. However im gonna put back the shielded wires to see if it helps.


Thanks for your help midwayfair! i have learned alot from your tips.

Hrm.

well, the only signal at R2 is ... your guitar. It MUST be a grounding issue at the input jack or earlier, or your audio probe or something else external to the circuit.

[Megadeth]

Finally got a hold of a decent camera. Today i changed every capacitor on the board, resoldered IC socket, changed potentiometer wire and also cleaned up some flux.

Been probing a bit more and im also getting crazy never has a board given me so much problems!

The last stand is to change transistors once again, if that dont work i will toss this.

Pics

http://imgur.com/uT48bhK,CP1Gj6N,d9pmHxa,ESEBoR1#0
http://imgur.com/uT48bhK,CP1Gj6N,d9pmHxa,ESEBoR1#1
http://imgur.com/uT48bhK,CP1Gj6N,d9pmHxa,ESEBoR1#2
http://imgur.com/uT48bhK,CP1Gj6N,d9pmHxa,ESEBoR1#3