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VCA problems 2720 - 1
http://www.paia.com/talk/viewtopic.php?f=8&t=565
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Author:  Billb3 [ Tue Jan 07, 2014 1:28 pm ]
Post subject:  VCA problems 2720 - 1

I have 4 VCA's, none of which is working properly.

I get no output at all, steady voltage on base of Q1 no matter what voltages I put on the control resistors.

I accidentaly removed the 18v supply and found that all the vca's will product output with no 18v applied (although) the output level can not be changed.

looks like a problem with q3 but I have pulled them out and tested them - they are fine (or were fine when I tested them).

the input to q3 is a 20mv wave centered at about 8 volts

the output of Q3 is about 15 volts with no control voltage and about 14.7 with 5 volts.

----

with the 18 volt supply removed the emitter voltage of q1 qnd q2 is about 8.5v with no 18v applied it drops to 6.5 and allows signal to flow through

I'm guessing its a bias problem but my understanding is a bit lacking in this area.

any thoughts?

Author:  Billb3 [ Thu Jan 16, 2014 9:59 pm ]
Post subject:  Re: VCA problems 2720 - 1

I have one VCA working now, not sure why though.

I couldnt get any of them to work but I was using a bench p/s and a function generator for the waveform.

I hooked one up to the rest of the synth and it worked.
curious to see how many of the others work.

I'll update y'all when I have a chance to measure the voltages.

any suggestions for substitues for 2N2712 and 2N3391 ?

Author:  ModularMe [ Thu Jan 16, 2014 10:40 pm ]
Post subject:  Re: VCA problems 2720 - 1

2N2712 and 2N3391 are available at Small Bear: http://www.smallbearelec.com

2N2712 - search for SKU: 2001
2n3391 - search for SKU: 2002

A substitution for 2N2712 is NTE 85-5 available at Radio Shack online (not in store)
https://www.radioshack.com/product/index.jsp?productId=12753553

or Radio Shack on Ebay.

Author:  ModularMe [ Thu Jan 16, 2014 10:56 pm ]
Post subject:  Re: VCA problems 2720 - 1

Can you supply more details? For example, did you just receive a vintage PAIA synth and pulled the VCA modules out to test each individually before powering the entire unit up together?

The best audio source to test your VCA is a working PAIA oscillator hooked up to the same power supply as your VCA. Since four of your VCAs are not working, it is possible that they lack a common ground to the audio source when tested with a lab bench and waveform generator.

Author:  toneman [ Fri Jan 17, 2014 12:35 am ]
Post subject:  Re: VCA problems 2720 - 1

Or, it may be that all the transistors (or some) are installed incorrectly.

Scott posted a chart of the pinouts of the 3 most common PAiA trannys.

Use it to check orientation of the E B and C.

Any function gen or VCO will work as a signal source
**IF** the GNDs are wired properly.

8-)

Author:  Billb3 [ Fri Jan 17, 2014 7:50 pm ]
Post subject:  Re: VCA problems 2720 - 1

I purchased a box of modules, mostly defective, many modified to support patchless opperation.

I think I have a 2nd working vca now

Author:  Billb3 [ Sat Jan 25, 2014 11:57 am ]
Post subject:  Re: VCA problems 2720 - 1

I now have 3 working VCA's.

discovered something interesting.
the 2720 VCA will not work unless the input audio is AC coupled.

using my function generator the output followed the envenlope.
also, I had anchored my VCO ouputs to ground with 10k resistors (which made it much easier to set up with my scope)

removing the resistors and pushing the function generator through a cap made it all work as it should.

Author:  toneman [ Sun Jan 26, 2014 12:08 am ]
Post subject:  Re: VCA problems 2720 - 1

Billb3 wrote:
I now have 3 working VCA's.

discovered something interesting.
the 2720 VCA will not work unless the input audio is AC coupled.

removing the resistors and pushing the function generator through a cap made it all work as it should.

Excellent News!!
Must be some DC bias on those outputs.
Capacitors are just the ticket for that.
And those 10K bleedoff resistors are a good idea too.
Keep that PAiA Phlying!
T

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