Broken Midiverb II: Troubleshooting Tips?

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Sat Jul 18, 2020 2:20 am

the resistor values sound correct, given the -200mV, but the -200mV is still odd. are the base voltages still at -25mV?
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Sat Jul 18, 2020 2:26 am

also, shut the power off and measure the value of those resistors (if you havent already).
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Sat Jul 18, 2020 3:00 am

All three resistors measure ~2.4k

The base of is still around 30mV (drops while measuring)

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Sat Jul 18, 2020 3:18 am

ok, lets assume that its not the diffamp. im not sure why its reading -200mv, but maybe its due to a low impedance on your mulitimeter or something. another possibility is that the analog switch isnt doing its thing. if you set it to full wet, and short pins 12 and 13 on the analog switch, and listen to teh L/R outputs, do they sound the same? again, without an oscilloscope, its going to be hard to see whats going on, as most of these signals are switching the MHz range.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by Thunderbird » Sat Jul 18, 2020 5:26 am

You may have already seen this but in the trouble shooting it mentions possible causes for distorted audio.
One in particular caught my eye.....

"Faulty trace, particularly between
the DASP 8 ASIC, and the
DAC, or analog switch (U19). "

Broken pcb tracks or dry joints a possibilty.

Here's the full list for distorted sound.

Distorted audio.
Bad power supply rail. Check PS rails, and troubleshoot
if necessary.
Faulty DASP-8 ASIC. Replace and retest.
Faulty trace, particularly between
the DASP 8 ASIC, and the
DAC, or analog switch (U19).
Troubleshoot, and replace if
necessary.
Faulty op-amp. Troubleshoot, and replace if
necessary.
Faulty analog switch (U19). Troubleshoot, and replace if
necessary.
Faulty power supply bypass
capacitor, particularly the cap
at the analog switch (U9).
Troubleshoot, and replace if
necessary.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by Thunderbird » Sat Jul 18, 2020 12:32 pm

Also this warning
"Exercise caution when examining the output of comparator (U21). Accidentally shorting pins 7, and 8 together
will instantly result in a dead ASIC."

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Sat Jul 18, 2020 1:02 pm

Thanks, I did see that troubleshooting section and will check the analog switch next. I think the reason I hadn't before is because I was confused about the designation numbers. It says U19 is the analog switch in that troubleshooting, which is a 74HC32 in the digital section. But elsewhere U6 (CD4053B) is referred to as the analog switch. Unless they're both functioning as analog switches at different stages?

Also to add to the confusion U19 is a different IC in the midiverb 2 and isn't shown whole in the midiverb 3 schematic, but there's only one so it shouldn't be hard to find.

Faulty DASP-8 ASIC would be bad, but according to other Alesis manuals when it goes bad it gets super hot, which it isn't.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Sat Jul 18, 2020 2:08 pm

the 4053 is the analog switch.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Sat Jul 18, 2020 3:11 pm

I shorted pins 12 and 13 on the 4053 and the L and R output sounded the same. I also double checked the continuity between the 4053 and DASP chip connections, and the DASP and DAC connections, and all seemed fine.

The bypass capacitor for the 4053 is a .1uf MLCC. I checked continuity going in and it's good, and it's not reading continuity across, which I'm pretty sure means it hasn't failed short.

I'm trying to see if there are any test modes like the 3. But the buttons or different so it's a guessing game...

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Sat Jul 18, 2020 3:27 pm

if they both sound the same, then the chip might be bad (assuming pinout is the same as the rev3).
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Sat Jul 18, 2020 4:34 pm

OK, I actually had ordered a few of those a few weeks ago when I read the manual and saw that might be the problem, so I can try replacing it.

The pinout does look the same for that chip in the 2 and 3. The sound is feedback-y, as if some connection isn't being switched off that should be. If the input is turned all the way down there's still an audible tone with no input signal and the overload light is lit. The overload light is a digital overflow indicator from the DASP chip I guess.

One thing I noticed, and this could just be a difference between the versions and not a problem, is that the DAC "Analog +5v" input is reading +12v. The only other place that uses analog +5 is pin 16 of the analog switch and that does measure +5v.

I've been reading various threads here and other general research re: oscilloscopes, but is there a minimum MHz value if I'm looking mostly at digital circuits with this clock frequency? 80s-90s digital stuff, and not necessarily higher speed/frequency modern things.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Sat Jul 18, 2020 5:15 pm

the DAC getting +12V shouldnt effect it.

the bad switch wouldnt explain the overload light. that would come from the DAC having to count to a too high/low value when trying to make the comparator match, which shouldnt happen with no input signal. but, that could either be a digital or analog issue. ultimately, there is only so much you can do without an oscilloscope. if there are no problems with the pcb (traces, solder joints, etc), then all you can really do is replace the LM311, the switch, and the diodes. if that doesnt do it, then its a problem with the pcb or the digital section.

as for oscilloscopes, the fastest thing on this board is most likely the 12MHz oscillator (unless there is a built in PLL in the DSP). most lines will move slower than this, as it takes time for the DSP to do a calculation and output values. so probably in the 100kHz-1MHz range.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by EATyourGUITAR » Sat Jul 18, 2020 9:39 pm

I don't think you can buy a scope that has less than 25MHz bandwidth. even if you can, you probably shouldn't.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by Thunderbird » Sun Jul 19, 2020 3:37 am

I got a 50 Mhz old crt oscilloscope for 70 euro. A lot cheaper than the new digital ones. It even hooks up to a computer to do screen grabs if I need.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Sun Jul 19, 2020 12:16 pm

Thunderbird wrote:
Sun Jul 19, 2020 3:37 am
I got a 50 Mhz old crt oscilloscope for 70 euro. A lot cheaper than the new digital ones. It even hooks up to a computer to do screen grabs if I need.
Thanks, unfortunately I don't have the workspace depth available for a CRT scope. I have a little rolling desk with everything attached/plugged in that I wheel in from a storage room. So it'll have to be either an "entry level" digital or USB (BitScope, etc.). I looked locally for used digital ones and 12+ year old digital scopes are listed for only ~$60-80 less than new ones like the Rigol DS1202Z-E, ones with similar or lesser specs, so that doesn't seem worth it.

It is looking like I'm stuck until I'm able to check the digital section with one. Thanks all for the help though, it's been a very valuable learning experience! :tu:

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Sun Jul 19, 2020 2:42 pm

maybe someone you know has one you can borrow?

at any rate, its probably still worth swapping out those last few ICs, just in case thats it.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by Thunderbird » Sun Jul 19, 2020 4:19 pm

Wow I don't have space for mine either so I stand it upright on the floor beside me when i'm using it!
I have the screen facing upwards so I just look down to see the waveforms.
It's about 20 inches long so the screen is just below knee height.
Then I store it under the bench when not in use.
Best of luck with the repair, I hope you let us know if theres any developments.
Cheers.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Sun Jul 19, 2020 7:14 pm

guest wrote:
Sun Jul 19, 2020 2:42 pm
maybe someone you know has one you can borrow?

at any rate, its probably still worth swapping out those last few ICs, just in case thats it.
Unfortunately, I don't think I know anyone nearby who has one!
You've convinced me to just order an LM311 and rule those out vs. scope purchase. I had been weighing the cost-time-benefit between replacing those chips and buying a scope... desoldering and ordering a few LM311s isn't that hard I guess.
Thunderbird wrote:
Sun Jul 19, 2020 4:19 pm
Wow I don't have space for mine either so I stand it upright on the floor beside me when i'm using it!
That's a great idea and might work even with the limited space...

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by Thunderbird » Mon Jul 27, 2020 7:27 am

Well what happened me was I saw one for sale for 50 EURO and postage 20 and I just pounced!
It was only when it arrived I realised I had no space for it.
Necessity is the mother of invention I guess.

It's the tektronix 2211
Link https://www.us-instrument.com/content/i ... 1_550.jpeg

Cheers

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 27, 2020 11:42 am

The two dimensional view does make it look pretty small!
I've been considering a tektronix 2213 for sale located about an hour drive from me. They're asking $145 though, which might be an ok price, but isn't that much less than newer, smaller, more reliable options, I think. The photos of it show a lot of physical wear, too...

On the Midiverb repair, I replaced the LM311 but the problem remains. It does seem like some of the audible distortion is less and the signal is generally more clear, but the digital glitching stuff is still there.

I found some no longer active random forum where someone had a similar problem ("overflow" light always on, glitching sound) and traced it to a faulty RAM chip. 4 of them are socketed so I removed and reseated them, no difference, but the other one (the "fast RAM" I think) isn't socketed.

With an oscilloscope is it possible to find if there's a fault in RAM with just the data sheet for the chip? Or does it require knowing how the midiverb is using it? The midiverb 2 and 3 might be different in how they use the RAM, and the RAM chip might be a different version.

I also found an old Alesis ASIC DASP 16-c for sale if it that does end up being the bad. I think those can be dropped in as-is and don't require programming?

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Mon Jul 27, 2020 2:56 pm

for this project you probably want a digital storage scope. you need to be able to take a snapshot of a bunch of signals and zoom around through them in time. sure, you might be able to get away with something else, but it would definitely make your life a lot easier. you could probably get away with one of the cheap usb logic analyzers.
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 27, 2020 4:15 pm

guest wrote:
Mon Jul 27, 2020 2:56 pm
for this project you probably want a digital storage scope. you need to be able to take a snapshot of a bunch of signals and zoom around through them in time. sure, you might be able to get away with something else, but it would definitely make your life a lot easier. you could probably get away with one of the cheap usb logic analyzers.
Thanks for confirming that. From what I had been reading it seemed like one of those logic analyzers would work, but I wasn't sure if I'd run into needing more detailed/accurate waveform views. They are cheap enough to try and see though, so I'll go for that if replacing the analog switch doesn't work (delayed in the mail a week...)

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by guest » Mon Jul 27, 2020 4:29 pm

yeah, there are only a few points where a logic analyzer wouldnt work on this. unfortunately, those would also be my starting points: 1. check crystal is at the right frequency, 2. check DAC output. if those seemed fine, then i would start hunting around the logic lines to see if any of them were getting stuck.

do shorter or longer delay times change the behaviour any?
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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Mon Jul 27, 2020 5:08 pm

I could be wrong, but I think the chip that runs the user interface uses the same crystal? And that appears to be working correctly.

I don't know how good of an indicator this is, but just from listening, the direct DAC output sounds the same as the output of the LM311. I don't believe the DAC output feeds back to the LM311, but I could be wrong.

I can't hear a pattern between shorter and longer delay times. Just listening with no input source, it varies from a digital-glitchy feedback (shorter delays have a shorter "loop" and longer delays a longer "loop), to just filtered noise. Some shorter delays sound one way or the other, and the same with longer delays. All of those delays I believe are single tap, no feedback.

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Re: Broken Midiverb II: Troubleshooting Tips?

Post by wadesey » Fri Jul 31, 2020 8:25 pm

I now have a logic analyzer and Sigrok software available. I'm trying to figure out where to start, but if I can be nudged in the right direction, I'd be grateful...
(USPS is completely broken and the package with the analog switch has been stuck there for a week, so I haven't tried that yet)

This site (only available archived) has the presumed pinout of the DSP, if it makes sense to look there.
https://web.archive.org/web/20121115043 ... ii-dsp.htm

There's actually one of the DSP chips available for sale right now for not too much, so it's obtainable at least.

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