Broken EH Micro Synth, question about the power supply

[slacker]

In a fit of madness I bought a "spares or repairs" Microsynth off Ebay, thinking it would make a nice long term project.
It's in a complete mess, no input or output jacks or footswitch and it had a very dodgy looking transformer hooked up to the mains lead. Heres a couple of pictures.
dodgy transformer
the whole sorry mess

Obviously the first thing I need to do is hook up some power to it to see if anything works. I've got schematics for it that Stephen Giles posted. It's the version with the NE5554 regulator.

In the long term assuming I can get it working I'm either going to have to replace the transformer or if possible convert it to run of an AC wallwart, which I'd prefer because I'm not 100% confident with messing about with mains voltage 

In order to have an initial poke about and see what state the electronics are in is there anyway to power it off batteries? I'm probably wrong but I'm thinking that if I took 4 9volt batteries in series and hooked the positive end up to point A on the schematic, the negative end up to point C and the middle to point B that would give me + and - 18 volts going to the regulator. Or will that not work or even worse damage something?

Thanks in advance for any advice.

[StephenGiles]

The power supply always ran hot on this. On one unit I had I replaced the 5554 regulator which had blown with a 7812 and a 7912 which worked OK. If you are going to use small 9v batteries, they will not last long. Better to use 2 PP9 batteries in a separate case - I think that is the number of the big ones and forget the regulator. The CA3094s must not see more than + & -12v.

[Paul Perry (Frostwave)]

Slacker, I see you in England have the same trouble we in Australia have... the nororious 230v input tiny EH transformer that shorts internally & burns out... and you can never find one small enough to fit back inside... but if you DO manage that, it happens again

But this makes me wonder.. when the primary of a power supply transformer shorts, I guess that means that (at least for an instant) there are fewer turns on the primary & maybe tehre is a high voltage generated by the secondary. Hope there is no unhappiness in that box of trouble....

[slacker]

Hope there is no unhappiness in that box of trouble....

Far from it, due mainly to my complete lack of understanding of AC power supplies and how rectifiers work I've got it working 
Just so you can all laugh at me and hopefully explain it to me here's what happened. My plan was to remove the transformer (there was no way I was plugging that into the mains) and feed about +20 volts DC into point A on the schematic and -20 volts DC into point C on the schematic with point B being ground. My thinking was that the rectifier would simply send the correct voltages to pins I and 4 of the NE5554. I now realise this was my first error.
To do this I thought I'd get a wallwart of about 24 volts AC and then double half wave rectify it to get + and - 24 volts DC. I did wonder if this was wrong and that in fact I'd end up with +-12 volts DC but I figured in that case I could at least temporally bypass the NE5554 and power the rest of the circuit off that.
I went up the road to the nearest Maplins and picked up a suitable power supply. Got it home, hooked it up to a couple of 1N4001s whacked the DMM probes on and got +-12volts DC  . I thought I'd hook it up anyway and see what happened. Much to my surprise, but probably not anyone else's, I got +-33 volts DC on pins 1 and 4 of the NE5554 and +-12 volts out of it. I adjusted the wallwart down to 15volts AC and got +-20 volts DC on the NE5554 which I think is about right.
Flushed with success, if not a little confused, I hooked up some input and output leads and plugged in a guitar to be greeted with lovely squelchy synthy noises    Most things seem to be working except the distortion.

Here's a picture showing what I've done with voltages on it.


I did a quick google and i can only presume that the rectifier is basically working as a voltage doubler.
Hopefully someone can confirm that nothing is going to blow up doing this. The wallwart seems fine, it's not getting hot or anything, the NE5554 is very hot but I think they are known for that.

See sometimes ignorance is bliss

[Paul Perry (Frostwave)]

Congratulations! You are a lucky man
Incidentally, the power supply is actually referred to as "a pair of half wave rectifiers". The term "voltage doubler" is usually kept for different configuraations.

[swt]

i've measured the original unit supply, and it uses +8, -10, i guess this is for the envelope to work as supposed. anyways, i've tried it that way, and also +-9 to +- 12 and it worked. is this the old version or the last one?. there are two version...you can find the old one at mark hammer's site. the one that stephen sent you is the last one. as factory released, the ps ic gets hot. maybe i can help you from here...i don't know. as a start point, simply use a common ps, with +- 9 to 12 volts, and beguin the trouble shooting from there. let me know how it goes...or pm me if in trouble.

[swt]

sorry...i haven't looked at the schem, this is the old version. so +-12volts will just be ok. i'm curious about how this sounds and work. the envelope is simpler than the new one...

[StephenGiles]

I think it sounds exactly the same.

Time to resurrect my idea of a sweep generator mod in the Microsynth which would allow a filter start frequency proportional to the strength of the note played, but a decay portion which is independent of the guitar note decay. Nobody has come up with a solution for this yet. I remember Puretube mentioning a sample and hold would be involved - come in Ton, your time is up!!

I played around with the peak follower in the Space drum, which provides a nice output voltage proportional to input strength, but could not get it to drive the Microsynth sweep generator.

What do you think SWT? This would convert the Microsynth into a superb unit.

[slacker]

sounds interesting, once mine is completely up and running I'll be up for trying out some mods.

[swt]

yeah i remember that idea you are telling about...and also remember a thread where you suggested the space drum envelope part. i think this would be nice. i'd really like to have a simple filter working with this type of env. still couldn't make the royal work the way i wanted. will re-read your suggestions on those threads, and try to get something. maybe we can work together on vero or proto to get to the point....it would be fun...just like playing chess with a real table and pieces...but in argentina and england..!!

[alecacca]

hi diy masters!
i've got a zip file with 2 power supply:
one give +9 and -10
the other one +12 -12
can i use the +12 -12 power supply? the other one ( the original power supply, i think) requires a transistor and a diode that here are hard to find...
   thanks,   alex.

[slacker]

You won't be able to use the original +-12 volt power supply because the NE5554N regulator isn't made anymore.
If you're making a Microsynth clone then the best thing to do is make a +-12 volt power supply using modern 7812 7912 regulators. Have a look at the bipolar power supply article at general guitar gadgets

[alecacca]

thanks. but in the microsynth schematic there are +9v and -10v, it's the same if i use +9 -12? (7809, 7812)