Problems with Sweet Honey - help, please!

[ChristopherM]

Hmm … I can’t get a sound out of this, so I’m hoping that someone can give me inspiration about the solution.

Audio probe traces the signal only as far as pin 2 of the OP275 (and it is not full strength even then). There is nothing at pin 1 or anywhere beyond.

DC voltages at OP 275 with pots full up, no input, output unloaded and 9.4 V supply are –
1 – 4.24
2 – 4.34
3 – 4.32
4 – 0.00
5 – 4.25
6 – 4.38
7 – 4.38
8 – 8.54

I checked all component values except OP 275 prior to build (and in situ as far as possible). I’ve checked for shorts between traces and I’ve even checked continuity of traces. I’ve reflowed all solder joints. I’ve checked layout carefully and even cross-checked against the circuit schematic.

Although the DC voltages on the op amp don’t seem too strange, I’m wondering whether I’ve fried the op amp – it’s not socketed, unfortunately. Anybody got any thoughts about this or what else might be the problem?

The only deviation that I am aware of is that I have no indicator LED connected, but I can’t see that would make any difference to the function of the circuit.

Any help gratefully received!

[IvIark]

Well your voltages look good which suggests there isn't a build problem, so if it's not working it strongly suggests a problem with the opamp.  Unfortunately you may have just experienced the main reason I never solder opamps.

[ChristopherM]

Thanks. I'll replace the op amp (in a skt this time) and report back!

[ChristopherM]

Well, I replaced the op amp (this time socketed) and in the course of doing so, I realised what the real problem was. The way I work is to make a cutting and links template in CAD, which is a mirror image of the original artwork, so that it can be laid on the vero and then drilled through directly. One of the verification steps is to look at the entity count in CAD and confirm that it agrees with the count that you helpfully put on the original artwork. In this case it did, but what I didn't realise was that I'd accidentally duplicated a cut - it was in the count, therefore, but not in the right place. So I missed one cut through build and visual inspection and hours of de-bugging. The reason it was hard to detect was that it didn't affect DC conditions, but effectively bled the audio signal to ground prior to the op amp via the 22uF cap near pin 3.

So two quick cuts with a scalpel and everything is working fine! I am an idiot not fit to be let loose with a soldering iron!

I like this unit. I'm using it with a blackface Bassman, and it pushes that into the crunchy/dirty area really well without requiring ear-splitting volume.

The Focus control is rather subtle and I wonder if some changes to the circuit here would help. I'll add that to the list of things to do!

Thanks for your help.