Looking for help with an actual vintage EHX Memory Man

[Rockhorst]

note: pictures of the unit attached.

Someone sent me an early EHX Memory Man hoping I can repair it. I'm a bit apprehensive to start working on it, so I hope someone with experience with these units can help me out a bit. It looks like there have been some modifications over the years, the transformer is dead and there's a loose wire coming from the boost switch. No clue if the delay chips are still good. Still looking for a verified schematic for this particular unit.

On the EHX forum I found that a replacement transformer for the DMM is the F-115X which Mouser carries. I'm going to compare some specs, but I expect it will work with the MM as well. I have a EHX 24V center positive wall wart, was wondering if I can use this to test the unit before buying the transformer.

Any other suggestions for testing the unit are very welcome.

[Scruffie]

You'll be wanting moi.

I can't see any mods in the photos? Although some of that wiring could be new, looks stranded instead of solid core.

Yes you can use a 24V supply, the minimum transformer EHX used was 12-0-12 for 24 volts (ish, AC 'n' diode drops 'n' all) it's regulated through a zener/transistor capacitance multiplier set up I think on this unit but some did use a 78L15.

Schematic attached, it's not 100% accurate, been a while since I worked on this particular version but I recall that being the case, but close enough. I've replaced one SAD1024 in the past year, on EHX stuff, check the Op Amps First.

[Rockhorst]

Thanks for chiming in Scruffie
The reason I mentioned mods is that I saw some components that aren't on the circuit board but rather fly over it. I just assumed they where added later.

Which terminals should I attach the 24V supply to? Referring to picture 03 in my original post, I would say that the top wire is for the ground terminal (outer sleeve of adapter)? Center tap is for the LED, so -24V on the bottom connection in the picture? Power is then very crudely rectified by those 2 diodes, right?

[Scruffie]

The ones on the track side? Very common on old EHX

The top wire is ground/the centre tap as you say and then the other two are both AC which yeah, get half wave rectified by the diodes. Just stick positive to either of those two bottom wires and you get a polarity protection diode just in case.

[Rockhorst]

Only had time for a quick power test. When powering the effect you clearly hear a high pitched whine that starts up almost immediately and persists. This is usually followed a few seconds later with static noise (from a delay line?). No audio probing yet, that's going to be the weekend.

[one thing I noticed is that approaching the -24V probe to the center tap produces a little spark between circuit board and probe. The outer tap does not]

[Scruffie]

Does the whine change with delay time? Sounds like the clock, the filtering on these ain't great so it bleeds out quite a bit.

Getting +15V from the regulator? Voltages for the two 4558 and the 4047 would tell a lot.

Static noise... could be a lot of things depending on the exact sound.

[Rockhorst]

Whine changes with delay pot, goes supersonic when turning delay time down.

4558 closest to the jacks
Pin 1: 5.5 V
Pin 2: 5.5 V
Pin 3: 5.5 V
Pin 4: 0V

Pin 5: 0 V
Pin 6: 10.6 V
Pin 7: 11 V
Pin 8: 11.6 V

4558 middle of the board
Pin 1: 4.2 V
Pin 2: 4.2 V
Pin 3: 4.2 V
Pin 4: 0V

Pin 5: 5.8 V
Pin 6: 4.8 V
Pin 7: 11 V
Pin 8: 11.6 V

So that looks like both the b-sides of the chips are not doing what they should be doing.

[Scruffie]

Could very well both be dead.

But first things first, your supply voltage, assuming you have the transistor regulator, what voltage do you get on its 3 pins.

[lars]

If you haven't tried already, give the entire unit the "chopstick test". Just get a chopstick and start poking around at various components and wires while the unit is on and making the whine. I've always been amazed how you can find dry solder joints or loose wires with this method, or how one component being poked will change the sound you're hearing (that gives you a pretty good idea what to focus on). It saves a lot troubleshooting time.

[Rockhorst]

So without a load the EHX wall wart sits at 34V. Connecting it to the MM it drops to about 14V. The input of the regulator is at 12.6V, middle pin is 12V and the output is 11.5V. I'm guessing those numbers are off

[Scruffie]

Just a tad

So something is pulling the voltage down, kind of tricky as those old EHX boards don't stand up to a lot of re-soldering so pulling things just to test is not an easy task but it's probably a safe bet to replace both those 4558.

But before, just to be sure, test the 4047 pins 10 & 11, they should have half the supply voltage, it's already whining with the delay time so it's probably okay.

[Rockhorst]

pin 10 is at half supply (5.8V), pin 11 is at full supply (11.6V)

[Scruffie]

Oh, in that case, it's worth pulling it out as it's in a socket and seeing what happens to the supply voltage.

[Rockhorst]

Thanks for all the help so far and apologies for taking so long to get back. I really only have a few minutes a day for this kind of thing at the moment...sigh.

Anyway, rechecked: I made a counting mistake. Pin 10 and pin 11 both at half supply after all!

Next baby step?

[Scruffie]

No need to apologise, we all go through busy patches.

I'm looking over your voltages again and Pin 5 of the 4558 closest to the jacks should be your input and have a bias from the 470k/390k resistors next to it so why is it measuring 0V? That Op Amp determines the voltages of the following one so check for internal shorts and make sure those two resistors are connected. But I suspect that one is dead and should be replaced, the one in the middle of the board might be okay.

In the mean time while you're doing that I would replace any power filtering caps, even if they still work they'll be past their best now and it's not going to affect the sound apart from removing noise in its various forms. On the schematic that would be the 100uF on the BC239 collector, the 220uF on its base and then the 100uF on the +15V line (coming off the emitter) and the 10uF connected to the 1N4001 diode.