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2 x VCOs issue 6 not working
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author 2 x VCOs issue 6 not working
mumblinstevedubya
Evening all.

I finished two VCOs late on friday but after firing up I need some help.
No magic smoke (phew) but I am failing on 1st calibration which is trimming voltage to 10v.
I am only getting 7.1 volts when PSU trim is fully turned one way and 6 when turned the other way.

I am using compact psu as built by Krisp.

What is next?

Steve
Synthbuilder
mumblinstevedubya wrote:
I am only getting 7.1 volts when PSU trim is fully turned one way and 6 when turned the other way.


Assuming the power supply to the VCO is actually putting out +/-15V then this suggests a problem with the VCO's internal power supply.

Check that the values of R22 and R21 are correct.

Also check that R5 is a 2K2 resistor and that C3 is the right way around.

Tony
mumblinstevedubya
Hi Tony,

R21 & R22 are correct values

r5 is correct as is c3

thanks

Steve
Synthbuilder
Not so good then.

What's the voltage with respect to ground (0V) of the following:

U1 pin 6

U1 pin 4

Both should be around 7V.

U1 pin 12 - this should be 15V.

U1 pin 10 - this should be around 10V.

Is U1 getting hot?

If both boards are failing to trim to 10V at TP1 then this suggests a common fault. This would normally rule out a solder short because the chances of shorting out the same points on both boards are very small. It would also rule out a dead component since getting one is rare from new and getting two bad ones would be very unfortunate. It can happen though.

The most likely scenario is the wrong value component being inserted somewhere. That's not to say this is the problem. It's just the most likely.

If U1 isn't getting warm the most likely things that can affect that voltage are those three resistors, that cap and U1. If it's not those, then something else is pulling down the 10V line and that does suggest quite a serious short to ground. It may be worth measuring the resistance to ground from TP1 with the power off and U1 removed. It should be well over 1K, probably 7K.

Tony
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