-8v not showing up. Voltage regulator IC5 overheating

Testing points

mobo schematic

The following is fixed and was part of the original post, it’s fixed but further down the line I’ve got more problems.
>I don’t really know what to do, theres not a lot of troubleshooting information available.
>I did a pretty shitty soldering job BUT i’ve checked everything over and tried my
>hardest to prevent shortages where possible.

So IC2 was not even in, i soldered it in and put a 9v DC current through it and wasn’t getting my -8v. I went to the store, picked up a 9v AC power supply plugged it in and smelt burning so i unplugged it and felt that IC 5 was very hot. Trying to troubleshoot it I found C9 (the big capacitor) was the wrong way around, so i desoldered it and resoldered it the right way around. started it up to test and i’m still not getting my -8v on any of the pins on JP1, 3 and 5. It feels like i’ve hit a brick wall and i’m starting to think that i’ve broken something which would be a worse case scenario as I’d have to resource like 2 parts and pay shipping on them.

Perhaps post some pictures ? Especially around the mcp1700 (doing the 3.3v: yellow points) and the 7908 (doing the -8V: blue points).

PS: measuring 0V could also mean bad soldering joints, not shortages. You might need to reflow some of the solder … (just heat it up briefly)

Have you got the regulators mixed up?

MCP1700 does the 3.3v (make sure it’s the right version too, some don’t supply 3.3v).

7908 does the -8v.

Get those two mixed up and you won’t get 3.3 or -8v.

Also, you need an AC 9V power supply, not a DC one (just in-case you assumed it was DC like I did when I built mine).

Turns out i didn’t even have one of the regulators in lol.

I’m checking the schematic and it’s like the troubleshooting bible. However, I’m a bit stumped as i’m not getting the -8 volt reading. The only clue being that I broke the Diode D5 (1N5819) I have a general purpose diode laying about BUT i don’t think it’ll do anything as there’s already one in there (D4). I just shorted the leads, it’s a diode after all and it’s just designed to prevent current going backwards. All the capactiors and diodes leading towards the power inlet are all correct and there are no shortages.

8V missing> Are you sure you are using the right power supply?

It says 9v on the unit, the multimeter is showing 11v when I test the actual input connector BUT I tested a battery and the 9v battery was 5v on the battery and 3v on the actual DC input connector

Is that a 9V AC power supply?

Sounds like a AC/DC power supply instead of an AC/AC power supply …

Is this not DC?

Where am I going to get a AC power supply from. D:

Try Jameco if you are in the US.

There are threads about the Ambika power supply on the forum. You can check Jameco as @mmarsh mentioned, or if you’re in Europe, Andre at Tubeohm had some available.

there’s an electronics store in my area i’ll have to swing my them cause their website has a 9v AC on their catalog.

Thanks guys, i really appreciate the help

Be sure to get one with enough amperage to handle the load. 1500 ma should be good, I think.

as 6581 punk said,
AC is like a drunken driver, DC is the straight one.


Even with a 9v AC i’m still not getting that -8v

The voltage regulator that manages the negative voltage (IC5) just gets super hot. I think there’s a short but I can’t find one. I also had C9 on backwards so i’m hoping i haven’t damaged anything.

> think there’s a short but I can’t find one.

Have you used your multimeter to confirm this?

On IC5, 0 and + beep using the beeping function on the multimeter, the I and O do not beep, O to JP beeps and I to the power supplies diodes beep, i couldn’t get any of the diodes to beep but the diodes must be working if it’s getting power to heat up the unit.

Just a reminder and D5 has been shorted with leads cause i broke it

“I think there’s a short but I can’t find one.”

“Just a reminder and D5 has been shorted with leads.”


“I just shorted the leads, it’s a diode after all and it’s just designed to prevent current going backwards”

You know, I don’t have a deal with Diodes Inc to help them get rid of their stuff. If there’s a diode somewhere in the circuit - especially if it’s a fancy one like a Schottky, there’s a good reason for that and you are supposed to use the part I have specified.

D1, D2 is are half-wave rectifiers. Absolutely essential.
D6, D4, D8 protect the regulators when the power is turned off and the bypass capacitors discharge. Nice to have given how much bypass capacitance is distributed on all the voicecards.
D5, D, D9 prevent a regulator latch-up if you plug the synth during the “wrong” half of the mains cycle. Absolutely essential (and they operate as opens in normal operation).