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 paia 2720 cap replacement 
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Joined: Sat Jun 16, 2012 3:00 pm
Posts: 15
Post paia 2720 cap replacement
Replacing caps of a 2720 -2a, all electrolytics Please advise voltage ratings. Hoping to get VCO's to tune across two octaves.


Fri Apr 25, 2014 11:18 pm
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Joined: Wed Jan 01, 2014 1:36 pm
Posts: 61
Location: Toronto
Post Re: paia 2720 cap replacement
the power supply is +/- 9v so the p/s caps should be higher than that. I would use 16v or higher.
the output caps I would probably use the same.

using higher voltage caps than needed will not have any effect on the circuit. they are just bigger (usually)

I selected my caps mostly based on the spacing of the pads on the pcb.

note: the electrolytics will not effect the tuning or linearity of the oscillator.
I had to change C1 and R8 to get my oscillators to be sort of ok.
( tried 3 or 4 .1 caps untill it was working ok )


Billb


Sat Apr 26, 2014 10:01 am
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Joined: Sat Jun 16, 2012 3:00 pm
Posts: 15
Post Re: paia 2720 cap replacement
Thanks for the advice Billb

I'll skip replacing the other caps and concentrate on R8 and C1, then.

Why did you go through so many .1 caps? Are some better than others?

Looks like C1 should be a mylar .1 mfd but R8 is not listed. What is the value of R8?

The owners manual states "R8 should be matched to the unijunction of transistor U2", how do I find that out?

The resistor in place is an 8.2K @ 5%.


Sat Apr 26, 2014 12:16 pm
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Joined: Wed Jan 01, 2014 1:36 pm
Posts: 61
Location: Toronto
Post Re: paia 2720 cap replacement
because of tolerance all caps differ slightly - I just needed to find one that had the "right" value.

R8 is valued to work with the transistor but after 20 years it has probably changed its resistance and the transistor has likely changed as well

Bill


Sat Apr 26, 2014 2:10 pm
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Joined: Thu Mar 07, 2013 1:15 am
Posts: 36
Post Re: paia 2720 cap replacement
Occasionally with these 2720-2A oscillators the pulse modulation pot stops affecting the pulse waveform, or only partially works. I've found that changing the resistance of R8 makes everything work again.

You can always try a pot for R8, like a 10k, rather than a resistor. When you find the "right" resistance, you can leave the pot in or measure it and replace it with the closest value resistor.


Sat Apr 26, 2014 10:30 pm
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Joined: Sat Jun 16, 2012 3:00 pm
Posts: 15
Post Re: paia 2720 cap replacement
Thank you for all the suggestions.

I ordered most the parts we've talked about from Mouser today.

I'll try to 10K pot in place of R8, I likely have one around.

Did I get you right to mean the pulse pot should always work?

ie Should always affect the pulse signal?

I ask because mine only seems to make a difference about halfway up, then the change is very obvious.


Thu May 01, 2014 4:52 pm
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Joined: Thu Mar 07, 2013 1:15 am
Posts: 36
Post Re: paia 2720 cap replacement
On my 2720-2A oscillators, I can hear an audible change across the entire range of the PWM pot. From the manual:

The pulse output produces a waveform variable between a short pulse and a square wave.

The control varies the duty factor of the pulse output. At the "min" setting the output is just a spike and at the "max" setting the duty factor is better than 50%.

Set the bias control to approximately 1 volt and turn the pulse duration control fully counter-clockwise. Adjust trimmer R23 until the buzz of the pulse can barely be heard.


So it appears that the PWM control should be functioning across the entire range of the PWM pot.

With a 10k pot installed for R8, you may drop the resistance so much that the pulse output is lost. And you may lose the pulse output if the resistance too high. And at some settings of the 10k pot, the PWM will only partly work. Somewhere in-between the extremes is the right value.


Fri May 02, 2014 6:21 pm
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