Stagefright - Bad Hum, no signal, LM13700s are all super hot

[AntKnee]

As the title says, I've got bad hum, my LFO led is not lighting up, and my LM13700s are all really hot. I've done the usual component and solder bridge check and don't see anything wrong.

I'm afraid I'll damage IC 3, 4, and 5 so I haven't left power connected for long enough to measure their voltages. So, I pulled them out, and here's my voltages on IC 1 and 2:
IC1:
1: 2.85   2: 2.85  3: 2.62  4: 6.3  5: 3.14  6: 2.85  7: 2.86
8: 3.4  9: 2.85  10: 2.81  11: 9.1  12: 2.86  13: 2.86  14: 2.85

IC2:
1: 5.7  2: 2.83  3: 5.7  4: 10.2 (seems high?)  5: 6.4  6: 0.9 (should be 0?)  7: 5.6  8: 2.86

[madbean]

Pull out all the ICs. Measure the voltage on the bare socket for pins 2, 4, 5 and 8 of IC2. What do you have?

[AntKnee]

Pull out all the ICs. Measure the voltage on the bare socket for pins 2, 4, 5 and 8 of IC2. What do you have?

Thanks, Brian!

2: 0
4: 0
5: 0
8: 9.6

[madbean]

2: 0
4: 0
5: 0
8: 9.6

Your voltages are not making sense. Are you sure you listed them in the correct order when you had IC2 installed?

IC2:
1: 5.7  2: 2.83  3: 5.7  4: 10.2 (seems high?)  5: 6.4  6: 0.9 (should be 0?)  7: 5.6  8: 2.86

Without IC2, you should be reading about 4.5v on pin2 and pin5, 0v on pin4, about 9v or so on pin8. The voltages listed with the IC installed are all wrong (for instance you said you have 10.2v on pin4 when it should be ground).

So, one of three things is happening here (or a combination thereof). Your original voltages with the IC installed were listed in the incorrect order, your actual IC is bad, or there are more extensive power problems on your board. What you need to do first is verify you have about 4.5v on pin7 of IC1B. This is the VB supply that should be feeding pin2 and 5 of IC2. If not, you need to look for problems with IC1, and that you have installed 100k resistors for both R53 and R54. Next I would try a different part in IC2 if you have one. A plain TL072 is fine. Test the voltages again and see what you get.

[AntKnee]

I rechecked the voltages. These look better. I'm not sure what happened earlier.

IC1:
1. 4.7
2. 4.7
3. 4.3
4. 9.4
5. 4.7
6. 4.7
7. 4.7
8. 8.7
9. 4.3
10. 4.6
11. 0.3
12. 4.7
13. 4.7
14. 4.7

IC2:
1. 8.8
2. 4.7
3. 8.8
4. 0.3
5. 4.7
6. 8.8
7. 1.1
8. 9.5

[AntKnee]

With IC1 left in (is that what you meant for me to do?) I get the voltages you said I should get on IC2.

With IC1 in, and IC2 out, my LFO led comes on. When I insert IC2, the led does not come on. Not sure if that is good or bad.

I tried inserting a TL072 in IC2, and I get the same result.

R53 and R54 are definitely 100k.

[madbean]

Lemme think about it a bit and get back to you tomorrow. Have some things I need to finish up today.

[AntKnee]

Ok. Thanks for the help!
I got my 13700s, 2n3904 and 2n3906 from Tayda. I ordered another set from Mouser, just to swap them out and rule out possible fakes or junk. I will update after that happens.

[AntKnee]

Ok, popped in all new ICs, and still have the same issue.

I replaced all the electro caps, too.

I noticed that sometimes the led will come on when fiddling with IC2, but I cant find a short or solder bridge on it. I have reflowed the joints twice. Its the same with multiple chips. Maybe the socket itself is bad? Regardless of when it is on or off, if it doesn't come on, IC3, 4, and especially 5 get hot. If the led comes on, I get no effect, but I can get signal through the circuit.

[madbean]

Well I'm not really sure what's happening here. Sorry to ask you to do this, but try this -

Take out all the ICs. Put in only IC3. Does it get hot?
If not, put in IC1. Does IC3 get hot now?
If not, put in IC2...IC3 gets hot now, doesn't it? Just determining if IC2 is the current sink here (which I suspect it is).

If you get to step 3 and it turns out how I think, then let's see a pic of your board and wiring. Actually, we should probably see that anyway at this point.

[AntKnee]

I thought you sounded correct, but surprisingly, IC3 got hot really fast when installing IC1.

[AntKnee]

[AntKnee]

R5 is wrong. Replacing it now.

[AntKnee]

R5 is wrong. Replacing it now.

Wait, no, its right. It measures 47k, but it the striping is odd.

[wgc]

hard to say for sure from the pix, but check your solder on R25 near connection to IC3.  Looks like it might be touching the ground plane?  Could be a similar thing at R24 near the pcb edge.