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Arp 2600 osc scaling.

Posted: Sun Jul 12, 2015 4:15 pm
by Europanaut
Hello!

I've been working on my 2600 (recapped and calibrated PSU, replacing sliders, key bushings, etc), and am closing in on the biggest issue. Osc 2's scaling is impossible to calibrate. So I assume that something on the osc board needs to be replaced.

Any suggestions on where I should focus first? Any opinions that would help me diagnose this problem?


Thanks

Re: Arp 2600 osc scaling.

Posted: Sun Jul 19, 2015 8:24 am
by Broadwave
Hi Europanaut - It could be a number of things...

1. Check the 1.87k Tempco resistor on the VCO module (in my experience, it's the first thing to go)
2. Check the Exp Converter transistor pair (TZ581/81 - you can replace these with 2N3906/04. No need to match them, but the pin-out is different)
3. On the main support PCB, check the 25k CV input trimmer and the 84.5k following it (these two set the 1V/Oct scaling)

That's all I can suggest at the moment... Hope it helps.

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Re: Arp 2600 osc scaling.

Posted: Wed Jul 22, 2015 6:38 am
by Europanaut
Hey thanks Andy! What a thorough reply! I have to place a parts order for some PSU caps next week, so I'll add some 2600 bits to the package.

This is the LAST thing to fix on this synth, and I'm pretty stoked.

Re: Arp 2600 osc scaling.

Posted: Sun Aug 02, 2015 8:03 pm
by Europanaut
Update: Swapped out the tempco, but no change in scaling behavior.

Re: Arp 2600 osc scaling.

Posted: Mon Aug 03, 2015 1:58 am
by Reginator
Hi, are you sure it's the 4027-1 and not something on VCO 2's main section? Can you try swapping VCO 2's 4027 with a known good one from VCO 1 or 3?

Also if you replaced any VCO 2 FM control sliders you should check to make sure none of them are bleeding any DC offset or modulation to the 4027.

One last thing, check for boards for cleanliness, excess flux or solder bridges.

How far off is the frequency when you press low C and high C and adjust the trimmer? Do you get anywhere near 1600Hz or is it way out?

Re: Arp 2600 osc scaling.

Posted: Mon Aug 03, 2015 6:38 am
by Europanaut
Thanks Reginator.

I may try that swap. That would certainly narrow it down. I sure wish they had plugs though, rather than the soldered posts.

As for the sliders, osc 2 had this issue before the slider replacement.

Board is very clean. Like, spotless. Pretty amazing, compared with some of the synths I've bought.

The scaling is WAY out. Not close at all. I can get about one octave, but above that it goes crazy.

Re: Arp 2600 osc scaling.

Posted: Tue Aug 04, 2015 2:51 am
by Reginator
The good thing about the ARP 2600's design is that all three 4027's are exactly the same. After you're able to plug in one of the other 4027's you'll know for sure if it's the 4027 that has an issue or something else in VCO 2.

You can check now for +/-15vdc and ground to the 4027 on pins 6, 7 and 8. This way you'll know if it's receiving good power before you swap them.

Re: Arp 2600 osc scaling.

Posted: Mon Aug 17, 2015 5:38 am
by Reginator
Hi Europanaut, any success with VCO 2? Just curious.

Re: Arp 2600 osc scaling.

Posted: Mon Aug 17, 2015 6:57 am
by Europanaut
I haven't revisited it yet. I've been distracted by a major Solina restoration, installation of the Vecoven JX10 upgrade, and recapping/battery replacement on our Source. It just never ends!

Re: Arp 2600 osc scaling.

Posted: Mon Aug 17, 2015 4:54 pm
by Reginator
Ok cool.