Uneven voltages on 13700 - is this ok?

[Waski]

Hello guys,

This is my first post so please, be gentle

As you can probably figured out I am building tri-vibe from ROG. For this purpose I used vero layout form https://tagboardeffects.blogspot.com.

First I had an issue with LFO, but I solved it - there was a cold solder on one of the links. I reflowed all of the joints and it worked. But the effect is stil not working properly. I probed the 13700 IC pins and after the 9 lug (which as per schematic is an output one) everything sounds good. The problem starts on the other side of the 1uF (or 220nF depending of your choice) it 'farts' but only with a hard strumming, when I do it lighlty there is no sound. I replaced the cap and still nothing.

So I started to measure the voltages on 13700 and there is a inconsistency between lugs 5,7,8 and 9,10,12.

First three lugs have the following voltages respectively: 4,20-4,64, 4,20-4,64, 6,33-6,35 (they are fluctuating)
The other three are: 1,46, 0,73-0,77, 0,73-0,77

I don't think this is correct. I am not mentioning the "upper' part of the IC as I compared this to voltages from different thread and it seems to be fine.

Please give me a suggestion, as I spent 2 weeks trying to find out what is happening without any success.

Cheers!

[ElectricDruid]

The schematic is here:

http://www.runoffgroove.com/tri-vibe.html

(This always helps)

There's only a few components connected to the pins you're having trouble with. I'd look at the 1n5 (shorted? dead?) or the 10K and 4K7 resistors. Something is dragging that output down towards ground. Is the 13700 chip ok? They're fairly easy to fry with too much LFO, but the 10K resistors on pins 1 and 16 should prevent that.

HTH,
Tom

[Waski]

Thanks Tom.

I went through the schematic several times and cannot figure out where the ptoblem lays.

I don't think the IC is burnt out. The voltage from LFO fluctuates between 2 and 3 V and after these resistors fluctuates from 1,23V to 1,28V which is correct I guess. I will replace this 1n5 cap and then see if this changes anything.

Cheers

[DrAlx]

The 1n5 is not likely to be the problem.  It certainly is not shorted because the voltage on the other side of it (pin8) is OK.
Check if pins 10 and 12 have a short to some other track like Rob says.  I would not rule out a bad second OTA.

If you have not done so already, recheck the resistor values connected to both OTAs including those at the inputs.

Note that there is nothing special about the OTA order, so you can swap the positions of the 22n and 1n5 caps (i.e. use the 1n5 on the first OTA and the 22 on the second).  So maybe try and swap those two caps.  My guess is if you do that, you will find the second OTA still shows a problem.

[Waski]

Thanks for the advise. Yes, the cap is not a problem - I replaced it and still nothing.

I went to the same conclusion that the OTA is an issue here - I will look for any shorts, but I did it few times and found nothing, though I'll give it a shot one more time.

Checked the resistors they are ok, so I believe I will need to replace the IC anyway, but here in Poland they are not cheap and not easy to order - I believe there is only one shop who has it... should have bought two of them not one as I did, a lesson for the future, grrr...

Cheers!

[Waski]

Ok, replaced the IC with the new one, and still nothing... looked for any shorts - nothing... reflowed the solder joints - nothing... This is just hillarious as I did everything like it should be done, aaarrggghh.

I noticed that when I was probing the signal component after component the sound of course vibrated but also have been distorted after pin 9 up to the 220nF (1uF). On the other side of the cap the sound appear only when I am strumming hard and it just "fart" like there was no power to get through.

Anybody have other suggestions?

[ElectricDruid]

Ok, so what are the voltages around the circuit now?

Let's have another look and see if anything has changed. There's got to be something we're missing - we just haven't spotted it yet.

T.

[Waski]

Hi,

The voltages are the same as they were previously:

Lugs 5,7 and 8 respectively: 4,20-4,64V, 4,20-4,64V, 6,33-6,35V (they are fluctuating)
Lugs 9,10 and 12 resectively: 1,46V, 0,73-0,77V, 0,73-0,77V

So this doesn't look like a bad IC, rather something in the circuit - I cannot determine why. The signal from LFO seems to be correct. Have no clue what is happening....

[ElectricDruid]

You've got a darlington emitter voltage higher than the base voltage. I'm not sure how that works. I don't think it does.

T.