Arp 2600 Ring Modulator Issue

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Arp 2600 Ring Modulator Issue

Postby lloyd » Sat Nov 16, 2013 2:30 pm

Hi all, I am having a problem with my Arp 2600 Ring Modulator which I am really hoping someone can help me with as I’d rather avoid a hefty repair bill.

According to the service manual, there should be no (< 10mv) output from the ring modulator when the VCO1 slider is down and the VCO2 slider is up, however the output is very loud. Raising the VCO1 slider while the VCO2 slider is up does cause some ring modulation but it’s only a very slight change to the VCO2 tone. It’s like the balance between VCO1 and VCO2 is wrong - VCO1 is too quiet and VCO2 is too loud.

The problem is present regardless of the DC/Audio switch position. I have tried adjusting the trimmers as described in the service manual but they do not fix the issue. R33 doesn’t appear to make any difference to the levels. I have checked the circuit boards and can’t find any obviously broken/burnt out components/bad connections/etc.

Does anyone have any idea what might be causing this? Any ideas would be greatly appreciated!

I am no electronics expert but I have a digital multimeter and an oscilloscope and I am reasonably comfortable with taking measurements, replacing components etc so I’m hoping that with a little bit of help I can fix this myself.

Thanks.
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Re: Arp 2600 Ring Modulator Issue

Postby ppg_wavecomputer » Sun Nov 17, 2013 6:31 pm

Have you checked the switching jacks? They can play silly buggers on you when worn and no longer properly working. In the worst case, they can render the entire instrument useless.

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Re: Arp 2600 Ring Modulator Issue

Postby lloyd » Sun Nov 17, 2013 10:21 pm

Hi Stephen,

Thanks for your suggestion. I've just checked them and it's not the jacks. I have checked the signals on the board before/after the jacks and they are fine. Any other ideas?

Thanks!
Lloyd.
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Re: Arp 2600 Ring Modulator Issue

Postby schmidtc » Thu Nov 21, 2013 1:58 am

Check the diodes in the ring.
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Re: Arp 2600 Ring Modulator Issue

Postby Steve Jones » Thu Nov 21, 2013 5:19 am

I would suspect the transistors in the multipliers.
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Re: Arp 2600 Ring Modulator Issue

Postby lloyd » Thu Nov 21, 2013 9:44 am

Thanks a lot for the pointer! I will try to get hold of some replacements.
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Re: Arp 2600 Ring Modulator Issue

Postby lloyd » Thu Dec 19, 2013 11:34 pm

Hi guys,

I've finally managed to fix the issue! :D In case anyone has this issue in future, the problem turned out to be a faulty LM301A opamp in the preamp section of the 4014 module.

Thanks again for your help.
Lloyd.
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