Mairsy doats and doezy doats and Memory Mans eat diodes

[Mark Hammer]

After two months of stab after stab at repairing an old Deluxe Memory Man, I finally triumphed and was able to pack it away.

After I couldn't find a replacement for the 2N6111 used in the power supply (which I simply assumed was shot), some here had suggested simply scrapping the voltage regulation scheme EHX had used and just sticking in a 7915 negative regulator.

I dutifully removed the half-dozen components EHX used to get -15VDC, and installed the regulator, making use of available pads with some jumpers.  The 2N6111 seemed to test fine, so that hadn't been the problem.  However, replacing the more complex subcircuit with a 3-pin regulator (plus 33uf cap on the output) wasn't getting me -15V.  What the...?

Checked and rechecked connections to verify everything was where it should be, and it all checked out fine.  So why no power?  On a whim, I decided to do a simple diode test on the bridge rectifier and, I'll be damned, only two of the diodes out of the 4 tested good.  I unsoldered it, dug around in the parts bin and found an exact replacement, cannibalized from something else long ago.  Installed it, and bingo bango, I now have a DMM that plays and sounds as it should. Adjusted the bias on both BBDs.  Adjusted the balance to make the whine go away, and adjusted the gain on the 2nd BBD to get a better balance of wet and dry.  Finally, replaced the slide switch for chorus/vibrato (which was wobbly and shot), and its cosmetically showing its age, but otherwise as good as new.

Just how the bridge rectifier was damaged, I have no idea, but can now happily return it to its rightful owner, and proceed on to his Micro-Synth.  Truthfully, a burnt bridge rectifier would have likely been one of my last hunches.  Glad I decided to follow up on both whims: testing the rectifier, AND keeping the one I had cannibalized (which I tested before installing, just to be sure).

[anotherjim]

I've found small bridge rectifiers to be somewhat fragile. In fact, since it's so cheap to buy regular single IN4xxx or 1N5xxx, I would build bridges from the singles in my own builds.
Possibly with the small bridge rect's, any heating effects are locked inside a small insulating block, and even silicon junctions can suffer thermal runaway.
OTOH, maybe that little bridge sacrificed itself during a power surge and saved the rest from worse damage?

[ElectricDruid]

Yeah, I've managed to blow up the odd bridge rectifier, even to the point at which the little circular package cracked open to let the magic smoke out. I no longer remember how. I'm sure it was all part of the learning process...
Like Jim, I don't bother with them now and build rectifiers from 1N400x series diodes if I need one.

Anyway, good work getting it running.

T.

[GGBB]

[Rob Strand]

uthfully, a burnt bridge rectifier would have likely been one of my last hunches.  Glad I decided to follow up on both whims: testing the rectifier, AND keeping the one I had cannibalized (which I tested before installing, just to be sure).

In electronics you never know what you will find!

I must say debugging with a CRO certainly shows up a lot of stuff.  Perhaps stuff you didn't plan on looking at!

I don't bother with them now and build rectifiers from 1N400x series diodes if I need one.

Same here.   The only reason I use them is if I see an opportunity to get rid of one on a shitty project I don't care about:   That would be some 1A bridges I bought from Tandy/Radio Shack 30 years or more ago, or, stuff I've pulled out of equipment.

I do use those 25A rectifiers in power amps.