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Two PSUs down - can somebody help
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Two PSUs down - can somebody help
mumblinstevedubya
moved my modular over the weekend

now compact power supply doesn't work as it should.

I have a back up PSU V1 but this is not working either.

Both light up but the meter readings are way off.

I know this doesn't tell much but I am sure I can get these working with some guidance.

Thanks in advance

Steve
Synthbuilder
mumblinstevedubya wrote:
Both light up but the meter readings are way off.

Is this the +/-15V LEDs? If so there is a good chance that the supplies are working and you may be measuring the voltage incorrectly. What sort of readings are you getting?

The first thing to do is to make sure nothing is plugged into the outputs of the PSU. ie, remove any Dizzy boards or modules plugged into the CPSU. Check again the wiring of the power lines going into the PSU module. If your ground wire is off then you will get some very odd results.

Now switch it back on and check again the voltages. Measure at the output terminal block (or one of the headers on the CPSU) and measure between the +15V and 0V and then at the -15V and 0V. What are you getting? Are they stable or bouncing around? If stable and correct then fit the Dizzy to the PSU and remeasure. If unstable then check the fuses and all wiring to the PSU module.

Tony
mumblinstevedubya
Hi Tony,

CPSU shows +15v & -15v with nothing plugged in.
same readings again with the dizzy plugged in.

As soon as I start adding modules to the dizzy board the + rail dips down to 7- 8v. The negative rail stays at -15v.

PSU V1 - (This was the first time over the weekend that I have fired this one up)
+9.6v & -15.6v with nothing plugged in.
same readings again with the dizzy plugged in.
plug in the vrg and I get +7v & -22.8v?!
Synthbuilder
mumblinstevedubya wrote:
CPSU shows +15v & -15v with nothing plugged in. same readings again with the dizzy plugged in.

OK we have no shorts there then. The wiring is probably OK.

mumblinstevedubya wrote:
As soon as I start adding modules to the dizzy board the + rail dips down to 7- 8v. The negative rail stays at -15v.

How many modules? Just the one? If so then it could be a fault either with that module, or the power supply is duff.

If it's fine with one module keep adding modules and see which one pulls the power supply down. Remove it and try another. If that goes well stick another one in and keep going. If adding one particular module keeps bringing the supply down the chances are there's a fault with that one.

mumblinstevedubya wrote:
PSU V1 - (This was the first time over the weekend that I have fired this one up)
+9.6v & -15.6v with nothing plugged in.

This one is stuffed. With those readings I can't say for sure what's wrong. Possibly a dead NPN power transistor. Before you plug anything in again the PSU should be giving out +/-15V.

Actually, thinking about it some more. You may either have two separate faults (as given above) or just one fault related to the mains transformer. The fact that both PSUs are misbehaving is perhaps too much of a coincidence. I wonder whether the transformer is not working properly or not wired up right.

OK photo time. If you can, upload a photo of the power supply board and its input wiring.

Tony
mumblinstevedubya
Hi Tony,

Thanks for your reply.

I too think it may be the supply but it should be pretty robust and has basically sat on my studio floor for the past year!

I have tried all my modules (10) individually with the CPSU & they all give the same result as yesterday.
When I unplug them the plus rail slowly crawls back up to 15v.

Still getting wacky readings with the PSU so didnt bother plugging any modules in. What I did notice though was when I powered the up the PSU and took a voltage reading it was initially at around 22v slowly decreasing to +9.6 & -15v

Please see pics below.




Synthbuilder
Thanks for the pictures - nothing looks wrong there.

Right, the next thing is to measure the DC voltage across the smoothing caps. Both with the modules plugged in and without.

If you can't manage to get the board turned around (the PSU would be difficult) try to find two accesible points on the board to measure the DC voltages. For the PSU the most obvious one is the cathode of D6 and the 0V output terminal for the positive rail and the anode of D9 and 0V for the negative rail.

For the CPSU it would be cathode of D10 and 0V for the positive, and anode of D11 and 0V for the negative.

All voltages should be between 18V and 25V.

Tony

Edited: got cathode and anode the wrong way around. d'oh!
mumblinstevedubya
Hi Tony,

Thanks again for your reply.

readings for smoothing caps on CPSU with nothing plugged in
C10 - 16.4v & C11 - 35.5V
with modules
Mididac - C10 - 6.9V & C11 - 44.4V
VCO - C10 - 12.5V & C11 - 38.7V

D10 to 0V - 11.7V
D11 to 0V - 39.3V

Will post PSU tomorrow or does it still look like PA20?

Steve
Synthbuilder
mumblinstevedubya wrote:
C10 - 16.4v & C11 - 35.5V


Shit. That's not good. Beats me what's going wrong there. Those figures are very strange. Really both should be around 22V or so.

Have you definitely got the three pin power inlet socket wired up right? Middle pin is 0V right?

Just one quick DC voltage measurement on the C-PSU...

Cathode of D10 (the striped end) and the anode of D11 (the unstriped end).

Tony
Synthbuilder
I just noticed in my previous but one post I got the anode and cathode terminology reversed. I've corrected it now. Sorry for any confusion.

However, the voltage readings across the caps are the important ones and they are most certainly not as expected.

Tony
mumblinstevedubya
Hi Tony,

thanks again for your reply

I have measured across the diodes as per your instruction and am getting
52.5V!

seriously, i just don't get it
mumblinstevedubya
Hi Tony,

I have also taken the same measurement across the PSUv1 as well.

This time D8 + D9 getting 51.9V

smoothing caps were C10 40.7V & C9 11.1V

wiring from pa20 should be good as this was bought from Krisp1 and I believe is standard.
Synthbuilder
Thanks for those readings. I think I'm getting an idea about what's wrong. I think the connection from the centre tap of the PA-20's transformer is not connected to your PSU board. So either there is either a break in the wire coming from the PA-20 to its own plug or there is a break in the wire leading from the socket to the board. Or maybe the socket or plug has failed.

Switch the PA-20 off. Remove the plug from the mains socket and detach it from the synth. Measure with an ohmmeter the resistance between the middle hole of the little three way output plugt and each of the holes on either side. I don't know what it will be but both readings should be the same and pretty low. Perhaps 10 ohms or so. If these are fine then the fault is probably with the wiring to the PSU board itself. It will be worth checking the continuity between the middle pin on the socket of the synth and the 0V pins on the power supply.

If either reading is really high (over 1000 ohms) or showing open circuit the cable or the wiring in the little plug has failed. You may be able to open the plug up and fix it.

Tony
mumblinstevedubya
Hi Tony,

I have unplugged supply & measured resistance of the pa20 itself.
nothing between pin 2&3 - nothing between 2&1
2 ohms between 3&1

I have checked continuity between all wires that run from the small lead between pa20 and CPSU and these are all ok
Synthbuilder
mumblinstevedubya wrote:
nothing between pin 2&3 - nothing between 2&1


When you say nothing I presume you mean an open circuit and not 0 ohms? If so then you have indeed lost the connection between the centre tap of the transformer and the little plug. The first thing to do is open that little plug up. You need to remove the sticker off the plug and it will reveal a small screw. Remove the screw and perhaps also the cable clamp screws. And you may find the middle pin has lost it's connection. If so, solder it back on. If not the break is probably somewhere in the cable - which normally means right at the point where it enters the line lump which is really difficult to fix.

Tony
mumblinstevedubya
Hi Tony,

Thanks for your reply again.

Meter is set to 200 ohms and displays 1 as default (with probes not touching anything).

When I say nothing I mean reading stays at 1.
Synthbuilder
mumblinstevedubya wrote:
When I say nothing I mean reading stays at 1.

Yeah, I thought so. Unfortunately, this does mean that your PA-20 is not working correctly. Hopefully, you'll be able to get to the inside of the little plug and fix the broken connection there.

Tony
mumblinstevedubya
Hi Tony,

Thanks again for your reply.

Alas, no broken connection inside the little plug so I am calling it a day.

What I will replace it with will be the maplin N58AT.

Apart from the half wave mods, Is there anything I have to know before going ahead with this as a replacement?

Has anybody else had any problems using this Maplins power supply?

Tony, thanks again for all of your help! applause

Steve
mumblinstevedubya
Hi Tony,

I am buying the maplin N58AT power supply tomorrow.

Apart from the half wave mods, Is there anything I have to know before going ahead with this as a replacement?

Anyone else use this power supply?

thanks

Steve
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