Broken Midiverb II: Troubleshooting Tips?

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Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 13, 2020 5:22 pm

I have a malfunctioning Midiverb II that I actually bought broken for a little amateur hobby repairing, but it's getting a little rocky with minimal knowledge and only a DMM to test... Also the service manual for the 2 is unfindable so I'm going off the 3 schematic, which is mostly similar enough..
http://schems.com/bmampscom/alesis/ales ... %203_1.pdf

The symptoms are: overload light always on, dry output normal, but wet signal output is super noisy/distorted with no audible reverb effect.

I first tested the power rails. +12v good, +5v good, -12v only reading around -6.8v. So that's one problem.
I tested the voltage throughout and everything expecting +12 and +5 appears to be getting it, and everything expecting -12v is measuring around -6.8v. Both the +12 and -12 are receiving around 18v at the input of the regulators. And I'm using an Alesis 9vac 830ma power supply.

I replaced the two 330 uf capacitors before the voltage regulator and the -12v voltage regulator since I had the parts, but that didn't solve it.

When a power rail is off like that, and replacing the regulator and power supply capacitors before doesn't fix it, what's the best way to find the problem? Every -12v test point tests off, so there isn't a way to isolate the bad part that way, right? The one chip that seems the likely problem (again with my limited knowledge) is the CD4053BE. Would that malfunctioning cause the -12v rail to drop like that somehow?

I did some searches trying to learn how to sniff these things out, but couldn't find much other than ATX power supply troubleshooting, so any tips would be very welcome!

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Mon Jul 13, 2020 6:06 pm

is the powersupply seperate from the rest of the circuit? is it easy to disconnect the -12V rail? either the regulator is blown, or there is a component on the board that is shorted out. are any components getting hot?
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 13, 2020 6:50 pm

I replaced the -12v regulator, but stupidly forgot to cut the output pin on the old one to test it before I removed it. But it read the same voltage with the old and new, so I'm presuming that isn't the problem?

Nothing seems to be getting unusually hot. I left it on for a while to see and the DAC is pretty warm and the CPU barely warm.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Mon Jul 13, 2020 8:05 pm

power it off, and measure the resistance between -12v and ground. measure twice, once with the black probe on ground, and once with the red probe on ground. turn it on and measure the voltage at the input pin of the voltage regulator (if you hadnt dont this already). are any of the ICs socketted?
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 13, 2020 8:50 pm

-12v with red probe, ground with black probe = starts around 20k and increases
-12v with black probe, ground with black probe = starts around 20k and increases
But right after powering off red probe on -12v and black on ground starts around 5M and decreases, red on ground black on -12v starts at 10k and increases
I tested this at the -12v points on the regulator, DAC and one of the op amps with the same result

A few are socketed, but the DAC is the only socketed chip that uses -12v. I removed that at one point and tested and -12v was still off.

Input at the -12v regulator is 18v

(thanks for the help btw!)

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Mon Jul 13, 2020 8:59 pm

is the regulator getting hot?
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by Graham Hinton » Mon Jul 13, 2020 9:17 pm

wadesey wrote:
Mon Jul 13, 2020 5:22 pm
II replaced the two 330 uf capacitors before the voltage regulator and the -12v voltage regulator since I had the parts, but that didn't solve it.

When a power rail is off like that, and replacing the regulator and power supply capacitors before doesn't fix it, what's the best way to find the problem?
The MIDIVerb II has an external 9Vac power supply. This goes into voltage doubling circuits to get higher positive and negative voltages to regulate. If the capacitors are OK then it is likely that you have a diode gone the way of all flesh. Measure the input voltage to the negative regulator, it's probably only about 9-10Vdc.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 13, 2020 9:50 pm

The input to the -12v regulator measures 18v (that's black probe ground pin, red probe input pin). It measured the same before and after replacing the 300 uf capacitors of the doubling circuit.

There are four 4.7 uf filter capacitors after the -12v regulator. The -12v goes to the op amps before the ADC, the output op amps, the CD4053 (which does switching stuff into the ADC and out of the DAC I think), and the DAC (PCM54HP). I was planning on blindly replacing those filter caps hoping for a miracle, but I'd rather be a little more focused... There is one area before the LM311 that goes to the ADC where the -12v goes through a diode pair and transistor. I think that's where the "stereo input" gets summed to mono?

The -12v regulator isn't getting very hot, it's barely warm. Way less hot than the +5v and +12v which are the usual hot, I think, not blazing but you can feel some heat radiating off of them.

The digital stuff seems to work. The display is normal, keypad works, it changes between presets, etc.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Mon Jul 13, 2020 10:24 pm

are you sure you replaced the -12v regulator with the right component and orientation? if its getting -18V in, and putting -6V out, and not getting hot, then there is something wrong with the regulator. measure the resistance from the ground pin of the regulator to some other ground point in the circuit, and make sure that connection is good.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 13, 2020 11:46 pm

It's a 7912 regulator and it's folded against the board and hard to orient wrong, so I'm pretty confident that's correct. The voltage measured the same amount off with the old regulator, too. Between 5.8-6.8v

The resistance from the -12v regulator ground is super high, like 16M, which then starts to drop. That's measuring to the other regulators ground and a few other points on the board.

Measuring black/com lead to middle ground pin, and red lead to input pin should show -18v or +18v on the input to that regulator? Because that shows +18v. Unless I should be reversing the test leads...

I double checked the capacitors and diodes before the regulator and they are oriented correctly (I didn't change any of those, other than the voltage doubling capacitors)

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Tue Jul 14, 2020 1:17 am

if you put your black lead on ground some where else in the circuit, and the red lead on the input of the 7912, it should read -18V. is this what it reads? and putting the red lead at the output reads -6V?

if the ground on the 7912 has 16M to the ground on the 7812, then the ground is not connected, and there is a broken trace somewhere.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Tue Jul 14, 2020 1:53 am

Ah, OK.

Measuring from the ground of the +5 and +12 regulators to the input of 7912, both measure almost nothing on the input of the 7912. -30mV or so. The +12 on the 7812 reads normal.

Measuring from a ground elsewhere on the board to the the output of the 7912 does show -12v

Measuring from the ground pin of 7912 to output pin of 7912 reads around +7v.

Measuring from the ground pin of the 7812 to the -12v input on the op amps shows -12v, but measuring from the 7912 ground to the -12v op amp input shows roughly +7v

I measured the resistance between the ground of the 7912 and 7812 (after unplugging) and it was around 18k and kept rising, checked a minute later and it was 6M

So it's sounding like the 7912 isn't connected to ground? Or could something else further along the -12v rail not connecting to ground cause that too?

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Tue Jul 14, 2020 2:40 am

ok, sounsd like a broken trace on ground. what you can do, is put your one meter probe on the ground of the 7912, and start probing around with the other to see where it connects and where it doesnt. otherwise you can just solder a wire between the the 7912 ground and the 7812 ground and be done with it.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Tue Jul 14, 2020 2:54 am

also, just want to remind that the pinout of the 7912 and 7812 are different, so the ground pins will be in different locations for them.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Tue Jul 14, 2020 1:28 pm

Well, that's really embarrassing... I totally forgot and was measuring from the input and not ground... but again, thank you for taking the time to walk through this! I'm learning a lot.

I redid all the tests, from the actual ground of the 7912... and the ground appears to be connected and the resistance is low and equal to the other regulators, the input is -18v and the output is -11.6v
I checked a few -12v points in the circuit and they all measured around -11.6v

So I appear to have been on the wrong trail...

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Tue Jul 14, 2020 1:47 pm

ok, thats progress! the next thing to do is to check the audio signal path. do you have anything that you can either see or hear the signal with? like an oscillscope or headphone amplifier with probe attached?
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by EATyourGUITAR » Tue Jul 14, 2020 2:16 pm

Just as important to check the clock, check the clock to the sram ADC DAC if they are external. Assuming there is a microcontroller there. Check for signs of life. 20MHz or less.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Tue Jul 14, 2020 2:17 pm

also, does any of the digital side work? can you change effects, etc?
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Tue Jul 14, 2020 2:48 pm

I don't have an oscilloscope or other way to test the signal, unfortunately, but I'd be down to build or buy some kind of thing that can track this down. A professional level oscilloscope isn't really possible right now, but a simple thing that can do at least some level of debugging is.

The digital side appears to be working. The LCD works, changing programs works, all the buttons work. I don't think I can check the clock without a scope though?...

Changing to some banks, the "gated" bank for sure, which I think uses an analog gate circuit, makes the overload light go off. But the output when the gate turns off is still just as noisy, almost an oscillating tone quality to it. With some of the input signal present, but garbled in the noise. But I think that at least indicates that it's switching between programs.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by EATyourGUITAR » Tue Jul 14, 2020 3:02 pm

Opamps
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Tue Jul 14, 2020 3:16 pm

Thanks, I'll set up a way to listen to the audio in and out of the op amps and see if I can find which one(s) might be dust, or at the least rule those out. Unfortunately they aren't socketed so I can't just pop them out and try a new one.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Tue Jul 14, 2020 3:25 pm

i agree with eatyourguitar, as one of those opamps being a likely culprit. do you have anything that can act as a headphone amplifier? if not, you can build a small circuit for listening to the opamps with headphones directly. you put a 2.2k resistor in series with a 10uF cap, and then connect that to your headphones tip. the ring goes to ground. you can then probe around with the resistor and touch the output of the opamps and give them a listen.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by EATyourGUITAR » Tue Jul 14, 2020 5:57 pm

a 10uF cap connected to an audio cable connected to an eyeglasses screw driver, an alligator clip to sleeve. that is my probe. a 555 oscilator, that is my 1KHz tone generator. total budget $1.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Tue Jul 14, 2020 8:49 pm

Awesome, thanks!

I don't have any 10uf caps on hand that aren't polarized- would a polarized electrolytic work if it's oriented a certain way, or should it be non-polarized?

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Tue Jul 14, 2020 8:59 pm

polarized is fine, you can orient either way. also, the values i gave are just ballpark, you can use bigger or smaller depending upon what you have.
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