VRG not working. Please help

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PatchMonger
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VRG not working. Please help

Post by PatchMonger » Sun Apr 12, 2020 3:47 pm

Hi all,

I have had a VRG for quite some time which I built and had working ok. I have not played the modular for some time and had been slowly getting it back into shape. However the VRG is not working. I have the LED flashing and responding to the UP potentiometer (the down potentiometer does not seem to affect the LED rate). LED flashes both in LFO and one shot mode. However the Output is not working. I am getting about 13V solid output.
I have swapped cables connecting boards with other cables from different modules to see if that was the problem but still no joy. Any idea where can I start looking at it?
Thank you so much in advance
Kind Regards,

Eduardo

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Synthbuilder
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Re: VRG not working. Please help

Post by Synthbuilder » Mon Apr 13, 2020 3:42 am

Usually when a newly built module stops working after it has been fine for some time the fault is usually due to a cold solder joint on the board. It'll be worth gently pushing down on the board in several places to see if that gets the unit working again. If you have a scope monitor the output signal while the board is being flexed - you'll see if the output goes back to how it should be the moment you press down on the board. Voltmeters will tend to lag so you don't get a fast enough response to see any dips in the output voltage.

It is rare for components to die these days but failures do happen particularly if the devices has been zapped by a static discharge at some point. But there is also another problem that can happen if you have used a certain brand of LT1013 for U3 and U6 - I can't remember which one but it might be Texas Instruments. I have found that some LT1013s do not like to have one of the power rails removed while the other is present - this can happen if the module is plugged into the power supply when the power is on, or if some other fault has taken down one of the power rails. Rather than just not working they die and get very hot. Given this, it may be prudent to check the temperature of U3 and see if it's running warm. Be careful it may be very hot. If this is the case replace U3 and U6 and test the board again. You can use any dual op-amp in there, AD712 or LF412 would do, but the LT1013 does give good results (when they work).

Tony

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PatchMonger
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Re: VRG not working. Please help

Post by PatchMonger » Mon Apr 13, 2020 9:37 am

Thank you do much Tony. U6 was fried! Luckily I had another LT103 in the VCO so I could do a quick swap to see which one was bad. Now I have to order a new one. Working as expected
Thanks again
Eduardo

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