Author Topic: TOTAL RECALL - very low voltages on all ICs  (Read 339 times)

Imuthant

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TOTAL RECALL - very low voltages on all ICs
« on: September 13, 2018, 03:48:10 PM »
Hey everyone, first time posting. Just started the biasing procedure on my Total Recall build, but couldn't get anything but the clean signal when probing around early on in the circuit. Nothing on the BBD. So I started checking voltages.

 The power jack brings in -23.8v, and R57 (the 240r 1w) has -23.8v and that end, and only -0.8v on the other end.

So of course the regulator gets hardly any voltage, less than a volt, and the same goes for IC1.

I double checked and mouser did send me a 1 watt 240 ohm resistor, and I checked it for resistance to make sure it wasn't dead, and I got 240 ohms on my meter. Any ideas on fixing this issue? Thanks!!

m-Kresol

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #1 on: September 14, 2018, 07:13:03 AM »
the resistor is unlikely to be the culprit. Check the orientation of your diode D1, the caps C42 and C43 and also verify the pinout of your regulator.
If those all check out, you might have a solder bridge or similar somewhere. It's hard to check with the caps in place, so I would recommend desoldering them and checking resistance between your power rail and GND. in one polarity (forward direction of your diode) it should be rather low, but if you reverse polarity of your probes it should be very high resistance..

hope that makes sense.
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Imuthant

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #2 on: September 15, 2018, 02:12:58 PM »
Thanks for the response! (I've been a bit busy at work until now.)

Double checked at the orientations, didn't see any solder bridges, reflowed at the joints in that part of the circuit and cleaned the flux. That didn't solve it, so I've removed those two caps (C42 and C43) but I'm not entirely sure how to go about testing between the power rail and GND in reference to the diode? Where exactly is a good test point for the power rail? Appreciate the help!

Imuthant

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #3 on: September 15, 2018, 02:16:09 PM »
Yikes! I just really realized I used a cheap cap for C42 that is only rated for 25v instead of 35v...maybe that is causing some issues...

**edit - scratch that, I was looking at some spares. It is a 35v cap that I bought from Tayda.
« Last Edit: September 15, 2018, 02:22:36 PM by Imuthant »

m-Kresol

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #4 on: September 16, 2018, 07:47:38 AM »
I would probe from the positive end of the diode to GND
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madbean

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #5 on: September 16, 2018, 09:17:44 AM »
Unload all the ICs and re-check the supply voltage.

Also, does you power supply read that -23.8v when not connected to the PCB? Is it the EHX one?
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Imuthant

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #6 on: September 16, 2018, 09:28:44 AM »
Great, I'll try that next. In testing the positive side of the diode and GND, I get 1.6M ohms, and then 3.06M ohms when reversing the probes.

I only got -23.8 when it was plugged into the circuit. I remember testing it by itself, and it was much higher. Just check again, and it started around 34v then kept crawling up...

m-Kresol

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #7 on: September 16, 2018, 10:42:15 AM »
sounds like your power section is correct after all and something behind the regulator might pull your voltage down.
try without the ICs as Brian suggested. If you get proper voltage insert one after the other until you find which one is pulling your voltage down.
If this is not helping please provide some pictures, we might be able to spot something.
I build pedals to hide my lousy playing.

My projects are labeled Quantum Effects. My shared OSH park projects: https://oshpark.com/profiles/m-Kresol
My build docs
My etching tutorial

Imuthant

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Re: TOTAL RECALL - very low voltages on all ICs
« Reply #8 on: September 27, 2018, 04:05:23 PM »
Sorry for the delay! I've been moving and had to put projects on hold. I made the huge mistake of skipping sockets for every IC but 6, 7, and 8 (I know, I know...I used my sockets up on other projects and didn't want to wait...) so the desildering process was pretty rough, but they are all out now. I tested after removing each one with no change to voltage at the beginning of the power section. After that, I just took the regulator out as well, and that left no voltage on the far side of R57. Any idea what to check next? I might as well keep taking it apart, since I'll need to buy several new ICs and sockets...