Alesis Midiverb II Troubleshooting Help Needed

[xeelox]

My 1987 Midiverb II is acting up. Sometimes the LEDs on the front panel for Signal and Overload light up and stay lit, combined with some static, distortion or feedback mixed into the output.  I've carefully and thoroughly cleaned the pots but it made no difference. I've also seen this behavior when testing with no signal passed through and no cables connected so I don't think it's necessarily audio related.
Front Panel Image: http://imgur.com/EcDBt2T

Overall, the unit still functions and I can usually get it to calm down and work with some gentle tweaking of the pots until the noise stops, but I want a longer term solution.

My electronics knowledge is rather spotty, but using what resources I could find online, I took a multimeter and checked all the voltages at the 3 regulators and all "seem" within the bounds of acceptable, but I could easily be mistaken.
The LM7912CT showed -19.76 in, -12.03 out, and .006 for common/gnd.
The 7812 (LM340T12) showed 19.8 in, 11.91 out, and .006 for common/gnd.
The 7805 (LM340T5) showed 9.36 in, 4.97 out, and .006 for common/gnd.
Voltage Regulators Image: http://imgur.com/FTFxON3

All the caps "look" fantastic—no leaks, none are puffed or exploded, etc., but being 30 years old, some/all could be dried up or faulty w/o any visible signs. FWIW, it's been in storage and not used much over the last 25 years.

I measured the resistance across all 3 of the pots and they seemed fine and smooth. All appeared in line with their specs: In/Out are 10k audio/logarithmic, and the Mix pot is 1k linear.

Question 1:  How do I troubleshoot from here? It looks like it is getting adequate power--at least up to and leaving the regulators--and all other basic audio/FX functionality is working despite having to fiddle with the pots.

Question 2:  On a side note, there appears to be a broken solder connection between a small ceramic capacitor and the leg of a metal ring around the 1/4" Defeat jack's opening--probably for grounding/shielding. It looks as though they may have been soldered together at one time and have come apart. There is an identical metal ring around the Right Input's 1/4" opening, and that one IS tied to a similar ceramic cap next to that jack, which is what makes me think my theory is correct. Am I missing something?
Possible Broken Solder Joint Image: http://imgur.com/0wpwPy3


Thanks,
JG

 

[scott2000]

I just read online that the Alesis units were plagued with faulty power supply caps and their associated coupling caps. Those coupling caps look like decent ones but you never know. I've always had bad luck with 330uf electrolytics though.....Not sure why, they just always go bad more than others from my adventures.....Also Always a good idea to repair solder joints . If you have a decent LCR meter you could do a quick check for cap tolerances..... A decent ESR meter could show you if any of them are lossy... But, reading that the caps have been problem areas in the past would make me want to change them out and go from there.... If the risk to reward is worth it.....

Lot of digital in that.....

Maybe chopstick around and see if it's a loose connection or joint somewhere that recreates the problem.....

Hopefully it's not the microprocessors or any of the DAC stuff going hokey...

IDK, have you tested the regulators when the problem is present?

This has a pretty good look at the unit's operation....

http://www.wolzow.com/analog/mvii.htm

 

[xeelox]

Hi Scott,

I'm with you on the electrolytic caps. Might as well bite the bullet and replace them as they are destined to go sooner than later, if not already shot.

I've fiddled around inside looking for loose connections, cold solder joints and the like.  Nothing other than what I mentioned looked suspicious. I may go back down that road at a later date if I can't turn up anything else.

I have not tested the regulators with cables connected and audio signals going through it, but once or twice when I had nothing connected it was all lit up presumably doing the same thing and the voltages were the same.  I suppose I could try it w/ audio to see if there's a difference.

I'm familiar with the http://www.wolzow.com/analog/mvii.htm page. Great info there, but nothing that struck me as related to my issue.

Thanks for the suggestions!

 

[scott2000]

No problem! I'm only experienced in testing for bad components and replacing them and, it's limited further by mostly analog components. I did see a couple of those orange ceramic caps that supposedly like to go short on that one web page. Although I've never had one in my experience be bad, maybe those could be suspect? IDk....

Sounds familiar to a trim resistor problem I had with a TL Audio unit one time.....There were several trim resistors in the circuit and two of them were really messing things up...They'd work then They wouldn't,,,,then they would kinda with a bunch of white noise...etc...... When I tested them they were extremely jumpy and going to extremes ...Replaced them and all was good..... Sometimes jumper cables, especially ribbon types, can get goofy too.....

Good luck