vintage deluxe memory man no sound

andyfromdenver · Apr 19, 2020

Hi. I juuuust bought this from a friend. It is the 80s first 5 knob version with the two prong power cord. It had a repair in 2006 and that person changed a few of the electrolytics. It was working perfectly last night, but this morning it only gives a tiny tiny signal at the output whether direct out or echo out. I can hear the delay and chorus working it's just a whisper.

I thought perhaps the initial 4558 chip was bad (one is a buffered signal of the input sent to the direct out, blend, and feeds the second op amp before the compander chip), but I carefully placed a socket and tried two chips with same results.

Can you please help me trouble shoot.

I sent a 330hz sine wave 500mv p-p through the in. I pick it up at the tip but after the 100k resistor to .1 capacitor then the (-) in of the first stage it drops to 40mv. Is this a clue?
The 4558 gets -15VDC as it should and the bias to pin 5 is -7DCv. I get -7VDC on pins 1 and 7, is that normal?

I'm stumped. The over load light only flashes on when first powering, I can crank the level knob and can't light the overload LED.

Thank you for your help.

Here is a link to a redrawn clear schematic except mine is the internal power source not the walwart.
http://www.freeinfosociety.com/media/images/2051.gif

Bo Deadly · Apr 19, 2020

The - input of an op amp is virtual ground so it's current only, not voltage. So you won't see your signal there. Look at the output of the first op amp.

Power readings sound correct.

It's probably a bad solder joint. Look at the bottom of the board for cracked joint. Use resistance test to measure ground between various points. Should be near zero.

andyfromdenver · Apr 19, 2020

Thank you so much!
I'll check the signal again at the output, it should be unchanged on the buffered output. I have this hopeful feeling that it is something easy, but literally nothing changed from when it played great last night and flipping it all on today :/
I'll keep continuity checking and checking resistances for bad joints.

I have my DX7 EPiano sound and neeeeeed to run it through the memory man so bad to the deluxe reverb... it's like spiritual, hehe

andyfromdenver · Apr 20, 2020

aww man, this was a satisfying repair.
i opened it up again and was like, “first things first, i’m going to repair the power cord which had a lot of wear right before the strain relief into the enclosure.
when i looked closely, where one of the ac wires was soldered to the switch, there was a blob that must have been half a millimeter away from touching the casing of the power switch tab. i guess between the night before and the day it didn’t work, moving the power cord finally made that 1/2 mm connection somehow.
Therefore the enclosure and ground plane was in contact with one of the power cord wires!!!! i guess cause its a two pronger, it only served to disconnect the ground? or something??
anyway the second i saw how close it was and saw that it was likely contacting, i snipped it and after trying it again it works!

synthiaks · Apr 24, 2020

to me this sound like you should be happy to be alive ;D

andyfromdenver · Apr 24, 2020

synthiaks said:
to me this sound like you should be happy to be alive ;D

right!?!? scary. i feel like this little quarter of a mm accident waiting to happen must have caused it to cease before because he had said it cut out in the past and no one could figure it out. i’m guessing that’s why the guy in 2006 replaced some parts in the power supply and then after boxing it up....low and behold it’s working again

i’m so tempted to swap out the resistors for metal film, but that’s kind of loco. this thing is a prize and sounding perfect.

it doesn’t look like there are many popular “must” mods online. i don’t care for true bypass.
sheer perfection