HELP!!!.... Blown Electric Mistress

[bettsaj]

I have made a huge school boy error... i was looking at running my original Electric Mistress pedal for a recording I'm doing and needed to run it off an external power supply. I had an 18v supply to hand which I just needed to connect up to a DC plug so that I could power the pedal. In my haste, I accidentally connected the negative and voltage wires the wrong way round...... connected the pedal, switched on the power and I immediately heard a small pop and the smell of burning. I have blown an original v2 1977 Electric Mistress effects pedal!!

However, i'm sure it's not dead yet, and it can be brought back to life... Hence this post.

For your reference I have attached the schematic below

can anyone advise the best place to start troubleshooting, I'm assuming anything on the ground side of things could have been the first thing to blow, and I'm thinking a transistor.... bearing in mind the 18v supply was connected to the ground, and the ground was connected to the voltage in. Anything can be replaced, the only issue is if the culprit ends up being the SAD1024 chip that's it... the pedal is a gonner. However I very much doubt the voltage got that far in the circuit to be honest (hopefully)

[theehman]

I doubt there's much wrong beyond the voltage regulation circuit.  If I were you, I'd put together a 12v regulator circuit on some perfboard and wire that into the pedal.  Run your wires from the DC jack to the perf, then a wire to ground and another to the trace on the emitter of the regulator transistor.  Cut the transistor leg out of the circuit.

[bettsaj]

Thanks for the reply.... Is it not easier to just replace the components in the voltage regulator circuit??

[theehman]

 I've found that it's just a touchy circuit and easily goes bad.  The 12v regulator is a lot more trustworthy.

[bettsaj]

Sorry to be a pain... can you suggest a schematic for a 12v regulator circuit.... this is new to me

[theehman]

http://geofex.com/article_folders/oldspyder/oldspyder.htm

Just use one of the circuits and replace the 9v regulator with a 12v

[ElectricDruid]

I'd just like to add a big

DON'T PANIC!

because although something like this seems like a terrible disaster at the time, in fact you just replace a few bits and you're good to go again. Electronics aren't like animals, where once they're dead, they're dead. You replace the faulty bits, power it back up and Bingo! It's new again!

Sometimes I wish other things in life were like this, but it's nice that at least one thing is.

[bettsaj]

http://geofex.com/article_folders/oldspyder/oldspyder.htm

Just use one of the circuits and replace the 9v regulator with a 12v

Thanks for your help. I've found another diagram on Ralf Metzgers web site as below

http://www.metzgerralf.de/elekt/stomp/mistress/images/vr_mod.gif

I think I'll try swapping out some of the components first before i add a whole new regulator circuit. i'll start with easy first with the LM741... I think I have one in my stock of IC's if not, they're only pence to buy, and also likely the BC309, and possibly the zener

[ElectricDruid]

If you heard a pop and smelt burning, it should be fairly easy to see which parts look fried. That 10uF electrolytic across is likely to have gone if it was fitted (is that what the dotted box means?) and they make the kind of pop you describe.

[bettsaj]

Update...... I've found the culprit that fried...... The BC309 (in the pedal it's a BC212L, which is different to the schematic)

It's in 2 halves, blown apart!!

[Ben N]

Cool. You don't see that evey day.

[ElectricDruid]

Update...... I've found the culprit that fried...... The BC309 (in the pedal it's a BC212L, which is different to the schematic)

It's in 2 halves, blown apart!!

Wow! That's pretty conclusive! I wonder if the uA741 survived? I bet it did.

[duck_arse]

Update...... I've found the culprit that fried...... The BC309 (in the pedal it's a BC212L, which is different to the schematic)

It's in 2 halves, blown apart!!

that qualifies as blown, comprehensively. when choosing a replacement transistor, CHECK VERY CAREFULLY your datsheet for the pinout! beware the l suffix BC.

[amptramp]

Your mistress is supposed to blow you, not the other way around.

But at least you know what to replace now.

[bettsaj]

Wow! That's pretty conclusive! I wonder if the uA741 survived? I bet it did.
[/quote]

i have a replacement 741 that i intend using anyway.... I have spoken to someone on the Element 14 forum and the general consensus there is that chip wouldn't have survived... I will however try a replacement transistor first and see what happens, if not I'll replace the IC then start looking at the electrolytic caps

[bettsaj]

when choosing a replacement transistor, CHECK VERY CAREFULLY your datsheet for the pinout! beware the l suffix BC.
[/quote]

I try all my transistors in a tester anyway, so i'll know what the pin-out is just from doing that... Thanks for the heads up anyway 

[bettsaj]

Ok chaps, a small update. I've decided to play safe and replace some components as a belt and braces exercise being this pedal is 43 years old. i've also been advised to swap out the LM741 as it's unlikely that has survived being a fragile IC.


i have now snipped the old 741 IC out of the circuit, and removed the legs form the holes. 1 of the solder pads fell off from the substrate when I was heating the solder up but fortunately it's not connected to anything, it's just there to solder 1 of the legs to for stability. All the others were OK, albeit very fragile. the new socket is now soldered in place and the UA741CP  chip is in place.


I've started desoldering some of the electrolytics and have so far replaced C9, C10, C11 and C12. When i tested the old capacitors after removal they tested fine albeit out of range. C9 should be 100uf but tested at 116uf, C10 should be 10uf but measured 14uf, C11 should be 10uf but tested as 14uf and C12 should be 33uf but measured 47uf. A lot of these caps showed evidence of leakage, so it's probably a good thing to replace them.


i'm waiting on a BC309 as I haven't got one in my stock of transistors so can't fire up the pedal just yet to see if these replacements have sorted the issue.

I'm hoping that the news that these electrolytics weren't blown could be a good sign that the 309  took the brunt of the charge........ any thoughts??

[Slowpoke101]

I suggest that if the SAD1024 is in an IC socket, carefully take it out and put it somewhere safe (antistatic foam or bag ) until you have proven your power regulator repair.

With the power regulator you seem to have it under control but my list for replacement would be as follows;

BC309
LM741
10uF capacitor across the 6V8 zener.

When you test it after repair make certain that the voltage on the collector of the BC309 is between 12 - 13.5V DC. If it is, then reinstall the SAD1024. Hopefully everything should work.

Good luck Andy.

Edit: Replacing the electrolytic capacitors is a good idea considering how old your original ones would be. But do this after you repair the effect and confirmed that it works. You don't want to introduce other possible potential problems. One problem at a time is far easier to contend with.

[Ben N]

Wow! That's pretty conclusive! I wonder if the uA741 survived? I bet it did.

They're like %^&*roaches, donchaknow.

[bettsaj]

Replacing the electrolytic capacitors is a good idea considering how old your original ones would be. But do this after you repair the effect and confirmed that it works. You don't want to introduce other possible potential problems. One problem at a time is far easier to contend with.

I've already replaced 4 of them, I do however still have the old ones so if there is an issue i can always replace them back.