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

[jimilee]

R5 is wrong. Replacing it now.

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

Can't measure it while it's still in the circuit.


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[AntKnee]

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.

Thanks for taking a look!
I rechecked those and don't see anything suspect. I cleaned those up and reflowed them anyway.

[AntKnee]

With IC1 in place, here is the voltages at the sockets, (without any other ICs installed):

IC2:
1. 3.8
2. 4.7
3. 3.8
4. 0
5. 4.7
6. 3.8
7. 3.8
8. 9.4

IC3:
1. 4.75
2. 0
3. 4.75
4. 4.6
5. 0
6. 0
7. 0
8. 0
9. 0
10. 0
11. 9.4
12. 0
13. 4.4
14. 4.75
15. 0
16. 4.75

[AntKnee]

R5 is wrong. Replacing it now.

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

Can't measure it while it's still in the circuit.


Sent from my iPhone using Tapatalk

I didn't, I pulled the package of matching resistors and measured them. However, I am assuming the one installed is not defective. I did not remove it.

[madbean]

Those new voltages seem spot on, for the most part. This is a stumper. If this were me, the next step I would take is looking at the traces around the LM13700s with a magnifying glass. What you are looking for is a manufacture error like a micro trace under the solder mask connecting it from a pad to the ground plane...or just something that doesn't look right. Not the pads that are grounded, though - those each have 4 little bridges that connect from the pad to the ground place. I'm talking something smaller that looks out of place. Can't be sure it is a board error but with different ICs from different sources and some good voltages the possibilities start narrowing.

I've looked at your trace side pic and I don't see anything so far, but it will easier for you to get a better look.

[AntKnee]

Looked it all over with a magnifying glass, didn't see anything that looked out of place. My eye is not trained to spot this sort of thing, though. I checked for continuity to ground on every solder pad and marked them wherever I got a tone with my tester. If anything looks odd, or if something is not marked that should be, let me know and I will double check it.

(The two x marks on IC4 are a mistake by me, they should not be marked.)

[somnif]

It looks like you feed quite a bit of solder on to pads, as a lot of the joins are a bit blobby on the component side of the board.

Any chance there is contact between things on that side of the board? Its not easy to tell in the picture as the round shiny surfaces throw some odd reflections. 

Edit/addition:

For example, I think this is just a bit of reflection, but maybe worth a look?

[AntKnee]

It looks like you feed quite a bit of solder on to pads, as a lot of the joins are a bit blobby on the component side of the board.

Any chance there is contact between things on that side of the board? Its not easy to tell in the picture as the round shiny surfaces throw some odd reflections. 

Edit/addition:

For example, I think this is just a bit of reflection, but maybe worth a look?

Thanks for taking a look for me. Its a reflection, though. I have seached both sides of the board multiple times with a magnifier trying to find possible bridges and haven't found a culprit yet.

I also went ahead and removed and replaced my IC1 socket, just in case some debris or something was shorting underneath it.

[madbean]

It appears you may have used 2Ms (or 2 Ohm) in place of 20ks. The fourth band should be red. I can't quite make out what color is on that band in your pic (is it yellow or maybe silver?)

edit - Your 62k also appears incorrect. Do you have more from those batches you can check with a DMM?

[AntKnee]

Shiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiite!!!!!!!!!!!!

I know what happened.
I placed an order for 300k, 200k, 62k, 47k, 39k, 30k, and 20k.
BUT, I ordered ohms, not Kohms. So all of those resistors are wrong. When checking them, I just overlooked that they weren't kohms and just looked at the number value.
Dammit.
Sorry everyone. I'll swap all those out and update. Let this be a lesson to everyone!

[madbean]

Well, I'm sorry I didn't check the resistors sooner. It would have saved us some time. But, I believe it will solve your problem. If you need some resistors, I can send you some to save you yet another order.

[AntKnee]

Well, I'm sorry I didn't check the resistors sooner. It would have saved us some time. But, I believe it will solve your problem. If you need some resistors, I can send you some to save you yet another order.

Brian, I really appreciate that offer, but I'll just order the right stuff. Thank you so much for your personal time and help on this.

Now, I have to pull 20 resistors without causing damage! No worries, I have cool tools. Cheers!

[somnif]

I almost made that same mistake last night. Wanted a few resistors with thicker leads than Tayda's so chucked a bunch of singles and doubles on my Mouser order last night. Thankfully 2.2 ohm 1/4 watt resistors were on 2 week back order (and I noticed that fact on the checkout page) or I would have been just as confused.

[AntKnee]

Its up and running. 
I had to replace a total of 23 resistors, which was a pain, but well worth it.
Sounds great!
Thanks again, B!

[madbean]

Cool. Man what a pain that was I can imagine!