EHX Deluxe Electric Mistress v1 Repair

Mothballs · Post by **Mothballs** » 22 Jun 2019, 07:19

Howdy all.

I'm gonna preface this - I normally build from scratch, not repair vintage effects. I am less then good at troubleshooting beyond "This part is obviously bad". Here's the Schematic: http://www.metzgerralf.de/elekt/stomp/m ... ematic.gif

I found an old, not bad shape looking Deluxe Electric Mistress in a recycling bin. Sorta a jaw dropping moment. Of Course, It wasn't functioning: It screamed when switched on, like if you had a bad AC leak.

The only obvious fault was the 220uf 25V Cap right after the power line-in; Totally dead, doesn't even show up when you probed it with a cap tester. Replaced that, and The noise and screech is gone: Now there's low/no signal when the effect is engaged, but there's a dull thump noise that happens cyclically that's fairly loud, so the LFO is fine. I am probably looking at a dead SAD1024, but Looking at other causes before just buying a new one of those is probably smart; If something other then the dead filter cap killed this pedal I should find that first.

Probing around a little with the pedal unplugged, I can tell for a fact that the Diodes on the power input are not 1N4001 diodes, and either faulty or Zenner diodes: Will putting the 1n4001's in there be better for the pedal, or is the zenners backward flow allowance needed for some reason? Are there any other big trouble spots I should be probing at? The thing was free, so I'm sorta just starting at the power supply and working my way in from there right now.

Scruffie · Post by **Scruffie** » 22 Jun 2019, 08:13

Post a full set of voltages, a lot of times it turns out to be a dead op amp on old EHX so you may well find your SAD is fine.

Mothballs · Post by **Mothballs** » 22 Jun 2019, 19:43

Unfortunately, I'm, ah, how you say, the type who knows just enough to be dangerous; I have no right idea what to be looking for on a spec sheet to figure out correct voltages, and any circuit I've made is a permutation or combination of someone else's work (With the exception of Vacuum tubes)

Full voltages for all ICs I'm assuming? I'll have to get on that, but a cursory probe at JRC4558 gave sub 1v on every leg and a 0 on pin 4 and the 78L15 (Which I had probed before replacing the Filter Cap and was Outputting 15v) is reading 2.7v on the output, which is, uh, not correct. If I'm having some sorta cascading failure as I replace parts this is going to be a painful experience.

Scruffie · Post by **Scruffie** » 23 Jun 2019, 10:38

Well something is acting as a short and pulling that down, considering you found it in the recycling (you lucky...) I'd just replace all the electro and tantalum caps, they're about 40 years old now and it'll save headaches of testing each one.

You could also try taking the 4013 and SAD1024 (while using ESD precautions) out of their sockets one at a time and see what happens to your supply voltage.

Full voltages for all the ICs yes.

Dirk_Hendrik · Post by **Dirk_Hendrik** » 25 Jun 2019, 12:22

Mothballs wrote: I am probably looking at a dead SAD1024,.

What you're doing here is a classic case of assuming upfront that the defect is caused by the most difficult to replace part. Don't. I have repaired loads of BBD based effects, including SAD1024 based ones, and it was only in a very few occasions "the chip".

What Scruff says. Remove the SAD (ESD indeed!!). Make sure that all DC voltages are in place and correct in magnitude. Replace all electolits anyway. Ensure signal shows up at the inputs of the SAS1024 socket. Place the 4013 and ensure (modulating of fixed frequency) clock signals arrive at the 4 clock lines of the SAD. Then, and only then it's time to re-insert that SAD. Probably adjust BBD bias and feedback.

Scruffie · Post by **Scruffie** » 26 Jun 2019, 09:30

Dirk_Hendrik wrote:
Mothballs wrote: I am probably looking at a dead SAD1024,.
What you're doing here is a classic case of assuming upfront that the defect is caused by the most difficult to replace part. Don't. I have repaired loads of BBD based effects, including SAD1024 based ones, and it was only in a very few occasions "the chip".

Another thing to mention here is that even when it does turn out to be the SAD1024 (and I agree that is the exception, definitely not the rule) it's not always a total lost cause, sometimes only half the chip is dead and the standard Electric Mistress only needs half the chip working to function.