# MFOS VCO fault

Sep 26, 2016
463
Hi all. I've ran into a fault with a home built VCO module which I can't seem to locate.

When I plug in it's power supply -12v +12v 3a it shorts across it's rails. I went to buy a new power supply today as I thought it must be that, i checked the supply befor plugging it in and there was no fault, I then plugged it into the MFOS dc converter and there was still no fault. I plugged it in for a few seconds to the module and it shorted straight away, and now the power supply is broken.

When there is no power supply plugged in there is no short accross any of the rails. So I thought it must be a resistance problem, incorrect resistor somewhere. Checking the resistance accross the rails I get these readings -12v to 0v = 43k. 0v to +12v = 6.5k. -12v to +12v = 42 k.

I went around the circuit looking for the 6.5k elsewhere other than on the +12v supply points and found it at R1(1). Disconnecting R1 (1) gve me 10k resistance between 0v and +12v.

I'm not sure if i'm on the right track with trying to find the fault.

I have taken out all Ic's to rule out them. I disconnected the pots to rule out them.

When I first had the circuit made it worked fine, however it gradually broke down. It's first symptom was the triangle wave would only show up as a saw wave, thus the sine wave only had the fall part of it's wave. I continued trying to check and calibrate to no avail. I discovered that the output pins 1,8 and 14 of U4 all went to 0v, after this I was reading shorts across sine, triangle, ramp and square outputs with 0v. Further testing culminated in U6 shorting across pins 1-4 and going up in smoke. Then my first transformer shorted out for good.

I know I've been making some mistakes with power supplies so this latest incident has forced me to buy a proper dual rail bench supply which will make diagnosing/testing safer.

I can give resistance readings off the circuit, I've checked all over the circuit and nothing is going to 0v it shouldn't be. I will have my bench supply on Tuesday when I wish to resume testing. However I though as i'm stumped maybe you folks could point me in the right direction.

Thanks

Darren

below are attached schematics and a link to the VCO build notes from MFOS

http://musicfromouterspace.com/inde...G=VCFJAN2006/VCFJAN2006.html&VPW=1386&VPH=538

#### Attachments

• vco pg 1.pdf
63.6 KB · Views: 70
• vco pg 2.pdf
85.3 KB · Views: 59

#### kellys_eye

Jun 25, 2010
5,508
Did you connect the ground? It's +12V * GROUND * -12V.

Show the board itself - top and bottom please.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Did this fault develop, or was the unit never working?

Sep 26, 2016
463
Did you connect the ground? It's +12V * GROUND * -12V.

Show the board itself - top and bottom please.

Yes the ground is connected, is ground correct rather than 0v?

Here is the board. Not great photo's I will take better ones in the morning if needed

Sep 26, 2016
463
Did this fault develop, or was the unit never working?

It initially worked fine. It was simply sat on my shelf. I then tested it with a VCF module and found it was only effecting the freq cv as a saw wave.

Here is a list of events;

Worked fine, fully calibrated with Oscilloscope.
Plugged in to test with VCF only to hear it controlling the voltage for frequency as a saw wave when it should have been a triangle wave.
Checked this with Oscilloscope and found it was the same with sine wave also
Checked + ground - points across board and everything was correct
Followed the signal path backwards from Sine to Triangle only to find I was reading ground on this signal path also.
I checked ramp and square and it wasn't effecting them.
I was now beginning to read to read a short accross ground and +.
When turning the PWM pot I would only get a signal if the pot was set at 12 0 clock, either side offered a large pop sound through my speakers which i am guessing was a voltage leak
The coarse pot failed also, it needed to be repeatedly turned to high frequency so to get it to sweep to low frequencies, it eventually stopped being able to do this.
Then the IC went up in smoke and testing stopped with power applied

This was in the space of a couple of hours

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Ok, if it was working and calibrated, then assume the components are correct.

Something must have happened between them and now (unless you power supply or the design is badly at fault).

Which ic went up in smoke?

Sep 26, 2016
463
the insulation on this terminal block is fine. I just removed the connectors i've circled in white below. It has changed the resistance from R1(1) to ground from 6.4k to 9.5k.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Is it possible you connected the power around the wrong way at any time?

Sep 26, 2016
463
Is it possible you connected the power around the wrong way at any time?
It isn't no as the connector only allows it to be connected one way

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
When you say the pots failed, were there any that had to be replaced, or did they just not perform the adjustment they should have?

Where was the pulse adjust width control when U6 failed? Was it at one end? Is it possible that R39 was miswired?

#### 73's de Edd

Aug 21, 2015
3,477
Sir darren adcock . . . . .

Looking at all of the outflowing power points that could load down either the +-12VDC supplies, looks like one needs to check the C7 and C10 filter caps that are directly across them as the power enters. VERY probable as being O.K. wih no problems with them.
Then that U6 IC is next suspect of having broken down internally and loading down either the + or - supply . . .or both..
A positive supply lowering or its loss, would be suspect, if the units circuitry is not being able to create a triangular waveform and instead, it gave you the negative swinging saws instead..

R39 pot has itself is too resistively isolated / current limited to be able to burn its element at either of its almost extreme ranges.

* * * * * U6 changing time. * * * * *

Give info on the/ your units power supply design / technique also. Was it Rays accompanying circuit that he used ? or just left it up to you scrounge up.
Very little power is being consumed by both circuits.

73's de Edd

Last edited:

Sep 26, 2016
463
When you say the pots failed, were there any that had to be replaced, or did they just not perform the adjustment they should have?

Where was the pulse adjust width control when U6 failed? Was it at one end? Is it possible that R39 was miswired?

The coarse pot didn't perform as it should. It would only manage low frequencies when repeatedly turned to clockwise and then tentatively turning it back to get the lower frequencies

The PWM pot also miss behaved, It would let me hear sound in a very narrow part of the pot adjust at 12'oclock. Anti clockwise there'd be a wooshing sound and a loud pop, clockwise would result in a less loud pop.

The PWM pot has remained wired correctly and goes to the correct designators on the PCB

Last edited:

Sep 26, 2016
463
Sir darren adcock . . . . .

Looking at all of the outflowing power points that could load down either the +-12VDC supplies, looks like one needs to check the C7 and C10 filter caps that are directly across them as the power enters. VERY probable as being O.K. wih no problems with them.
Then that U6 IC is next suspect of having broken down internally and loading down either the + or - supply . . .or both..
A positive supply lowering or its loss, would be suspect, if the units circuitry is not being able to create a triangular waveform and instead, it gave you the negative swinging saws instead..

R39 pot has itself is too resistively isolated / current limited to be able to burn its element at either of its almost extreme ranges.

* * * * * U6 changing time. * * * * *

Give info on the/ your units power supply design / technique also. Was it Rays accompanying circuit that he used ? or just left it up to you scrounge up.
Very little power is being consumed by both circuits.

73's de Edd

The caps check out on my meter reading as 10uf.

The units power supply.

I used this transformer. https://www.maplin.co.uk/p/3a-acac-fixed-voltage-12v-power-supply-n57dl

And the MFOS wall wart power supply.

#### Attachments

• mfos power supply.pdf
16.5 KB · Views: 42

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Place some 100 ohm resistors in series with your +12V and -12V supply. They may affect the circuit somewhat, but they'll protect your power supply, and possibly most of the circuit.

(while you're testing)

#### 73's de Edd

Aug 21, 2015
3,477
Sir darren adcock . . . . .

Straight forward on the design . . . with GOBS of prefiltering.

PLENTY of transformer unless its also supplying a power audio amplifier.

Do you get +12VDC and -12VDC at the regulators output +-'s ?
If so, keep a probe set across each set at a time and drop a ~10-15 ohm (minimum 1 watt or MORE) as a load . . . . . just long enough for the meter to ramp up to present a reading . . . . to see if there is a serious voltage pull down under that presented ~ 1A load..
If doing so, back up to the DC inputs to the regulators and test in the same manner.

73's de Edd

Sep 26, 2016
463
Place some 100 ohm resistors in series with your +12V and -12V supply. They may affect the circuit somewhat, but they'll protect your power supply, and possibly most of the circuit.

(while you're testing)
The bench power supply I have ordered has short circuit protection and a variable current limiter. Would your advice of the series resistors still apply?
I will have to wait until i get my new power supply on weds to begin testing as the transformer I have is dead now.

Sep 26, 2016
463
Sir darren adcock . . . . .

Straight forward on the design . . . with GOBS of prefiltering.

PLENTY of transformer unless its also supplying a power audio amplifier.

Do you get +12VDC and -12VDC at the regulators output +-'s ?
If so, keep a probe set across each set at a time and drop a ~10-15 ohm (minimum 1 watt or MORE) as a load . . . . . just long enough for the meter to ramp up to present a reading . . . . to see if there is a serious voltage pull down under that presented ~ 1A load..
If doing so, back up to the DC inputs to the regulators and test in the same manner.

73's de Edd
Yeah I read the prefiltering was over the top, but was being indulgent getting to use such giant caps

It's not supplying an audio amp aswell

Yes I get (got) +12vdc -12vdc on the regulators output.

I'm unsure what you mean by 'each set'. Does the 10-15 ohm resistor provide the 1a load?

#### Alec_t

Jul 7, 2015
3,331
The caps check out on my meter reading as 10uf.
They may well read 10uF, but did you check if they were leaky?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
The bench power supply I have ordered has short circuit protection and a variable current limiter. Would your advice of the series resistors still apply?

Possibly not.

Start with your current limit set to some nice small amount like 20mA and go from there.

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