9v battery goes to 4.5v when plugged into a stompbox

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babythomas
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Post by babythomas »

A have a broken guitar pedal that doesn't work. It's not true bypass and the signal on the output is the same as the one on the input, plus the switch does nothing (and the LED stays always off). But my guess is the switching is not working because of the low voltage - the battery is ~9vDC but as soon as I connect it to the pedal it shows ~4.5vDC (half the voltage). It can be measured on the battery itself or on some of the ICs. And also the battery drains fast, say 2vDC after sitting plugged in for ~15-20 minutes...
I'm an amp guy and I don't usually work on pedals, but my guess is - this should be some standard behavior for a typical fault in guitar pedal. Sadly I don't know what may causes the problem. I have a scope, but the problem is probably not in the signal path so I'm not sure it can be helpful.
I checked the jacks, battery and power connectors and the 100uF cap on the DC input and it's not any of them, they're fine. Sadly almost everything else is SMD and also I don't have the correct schematic (I have one for a different revision of the pedal). It's earlier buffered bypass version of EBS Multicomp 2.
Any ideas how to troubleshoot further? Thanks!

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deltafred
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Post by deltafred »

Drawing that much current it might be possible to feel the guilty component getting warm.

Towards the end of this thread there is link to the schematic
viewtopic.php?f=1&t=323&p=224384#p224384
but that is the true bypass version as well but is probably very similar.

The 10v zener seems a prime suspect. The series resistor also burns, and the tracks beneath them but that is probably when you put it on a power supply with a lot more current capacity.

Please keep us informed of your progress.
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Post by plush »

babythomas wrote: 16 Jan 2021, 12:18 I'm an amp guy and I don't usually work on pedals, but my guess is - this should be some standard behavior for a typical fault in guitar pedal. Sadly I don't know what may causes the problem.
The process stays the same for both amps and pedals: you see voltage drop on your power supply - you look for a short or for any unusual behavior that may lead to a future short if the pedal is powered with something other than a battery (external power supply).

Drawing excessive current usually makes the component become hotter than the ambient temperature, which can also help with diagnosing the fault (yep by touching the stuff with parts of your body or using something like pyrometer or a thermal camera). Sometimes you can even pour some IPA over the powered board and look for a spot that is evaporating the liquid (this is quite dangerous since some faulty components may eventually break and produce a spark that might ignite the IPA and it's vapors).


I usually follow this route:

1. visual inspection and measuring the shit out of the components with DMM and scope for ripple:
- always suspect those yellow and black tantallum capacitors (those fuckers tend to die short), always check them with your dmm, along with other power decoupling (filtering) caps
- look for small holes on the top of ICs, a hole usually indicates that there's an internal short and IC must be replaced
- check on-board for power regulators or boost/buck converters (if any)
- look for any discolored components
2. power the pedal using a current limited power supply (i'd start with something around 10-20mA), check the power draw and control voltages (VCC/VDD on ICs and other power rails)
3. desolder suspicious active components one-by-one, checking if the current draw has been fixed and the voltage is okay.
4. hope that no magic smoke appears

I've fixed 2 multicomps, both of them had visible holes in their SMD chips.

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babythomas
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Post by babythomas »

Thanks for the valuable information!!!

After some digging I'm pretty sure the culprit for this battery behavior is that the battery was somewhat discharged. I tried with "fresh" batteries and the power rail seems to be stable - no sag and I can measure the full ~9vDC voltage (well, almost) on every ICs.

Anyway... The effect is still not working and I'm trying to trace the switching part of the circuit. I thought it might be similar to EBS OctaBass (it's not):
octaSwitching.jpg
octaSwitching.jpg (31.78 KiB) Viewed 1396 times
However I have trouble identifying some of the parts (black rectangles with 3 and 4 legs). The ones with 4 legs and marking L30 I figured are probably two diodes in one enclosure. But there are a few more that I can't find what they are: 3Ft 55, JTp 12, 47W 72:
11.jpg
11.jpg (135.81 KiB) Viewed 1396 times
12.jpg
12.jpg (115.84 KiB) Viewed 1396 times
13.jpg

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plush
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Post by plush »

babythomas wrote: 22 Jan 2021, 08:26 But there are a few more that I can't find what they are: 3Ft 55, JTp 12, 47W 72:
L30 are dual switching diodes in sot-143 - "BAV23"
3Ft is npn transistor in sot23 - "BC857"
JTp are dual switching diode in sot-143 - "BAS28"
47W is a schottky diode in sot23 - "BAS40"

Hope this helps

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Post by babythomas »

Definitely helps a lot! I did some more tracing, but it's going very slow, I'm maybe... no more than 20% "ready"... :(

I measured all the pings on the ICs. Both LM833 and 4558 seem to be fine. But the two 064C have different voltages. One of them have 8.2vDC on pin 4, 0vDC on pin 11 and ~4.1vDC on the rest. The other 064C however have the following voltages:
064C.jpg
064C.jpg (13.45 KiB) Viewed 1320 times
Is it normal for them to be different? Does these voltages depend on the surrounding components (like biasing or something)?

There's a third 064C, but it's below one of the pots and I can't measure anything without disassembling the additional PCB for the pots/LED, which I hope I can avoid.

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Post by deltafred »

"Is it normal for them to be different? Does these voltages depend on the surrounding components (like biasing or something)?"

Yes and yes.

An opamp has a very high open loop gain. Negative feedback is applied to bring the gain (and frequency response) down to the desired figure which are both functions of the feedback component values.

The inputs of each amplifier are differential so any standing DC on them is ignored (in most opamps they need to be biased somewhere between the supply rails to prevent clipping). This means that under static conditions the voltage you measure on the + and - inputs will be near as damn it the same.

The output voltage is the difference in potential between the inputs multiplied by the gain, which as stated above is set by the components around it and not determined by the opamp itself until you get to very high gains and/or frequencies (well beyond the scope of repairing a stomp box).

For further info see The Art of Electronics by Horowitz and Hill (or Google).
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Post by bugg »

I know this is an old thread, but just for future reference those 47W SOT-23 packages are most likely going to be PMBFJ113.
https://www.mouser.com/datasheet/2/302/ ... 127556.pdf

Most of the SMD code lists / lookups will say its a BAS40 like Plush mentioned but I'm working on a trace now (different circuit) and just tested one on my DCA Pro, it's definitely a JFET.

Probably not a lot of help a year later, but this thread definitely helped me so thought I'd pass along the info in case anyone else goes down this road.

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