Arp 2600 needs (a lot of?) help..

[Sniarf Eggbird]

Hi guys!

We had the luck of receiving a gift from my friend's brother the other day, .. a "blue meanie" arp2600 in a good state cosmetically!
after the first shock of joy and awesomeness we turned it on (110v from my bench supply - 220v from the socket here) and...
nada  :-\ ... no sound at all ..
sliders ultra-hard to move too ...

to make the long story short, i opened the synth, it seems that they have replaced the original filter with a newer '76 4072 filter model and it has the 4027-1 oscillators.
BTW, i read that the "blue meanies" were the first 2600 Arp made.
Moving on i found a version of the Service Manual http://guitarfool.com/ARP2600.html which seems not to be full.
I read it and made my first tests ...

The manual suggests that first of all you should check the power-supply and you should get +15/-15volts ...
My +15 rail is ok but not my -15 ..... the minus rail gives out -3.2v .
In that situation, again the manual suggests to disconnect all molex pins that transfer power to each board one by one, in order to define which board is at fault.
I did that, but nothing really changed, i was disconnecting one board, checking, reconnecting it and moving to the next one.

After that didn't help, i disconnected all molex pins and measured my power supply without any load and voila, i got ..
+15.05 / -14.65

what i did after, was connecting only one board at a time and measure .. this is what i got ..

molex1 [preamp] = +15.03 / -4.33
molex2 [osc1]    =  +15.05 / -9.57
molex3 [osc2]    =  +15.01 / -6.85
molex4 [osc3]    =  +15.05 / -11.77
molex5 [filter]    =  +15.01 / -10->gradually drifting to 7.3 and lower
molex6 [env+amp] = +15.03 / -2.8
molex7 [s+h]      = +15.05 / -3.0
molex8 [Lag]      = +15.04 / -5.0
molex9 [noise]    = +15.03 / -5.67

what do you think?
Does that mean that all of my boards have problem, or is it a power supply issue at first.
i hope for the latter ...

I would be also grateful if someone has a full version of the service manual that would like to share.

below there is a photo of the guts ..

Thanks for any input!

thomas

 

[topcat]

Hi Thomas,

I'd start with the power supply.... if the caps are original they owe you nothing!

PM with your email address and I'll send you what I believe is a complete manual

Cheers

tc

 

[geoff004]

+1 on replacing caps.
I recently worked on a couple different vintage synths - a Moog and an Oberheim.  The difference in stability after rebuilding the PSUs was astounding.
The Oberheim had a couple bad tant caps too - they basically shorted the -15 rail to ground.  On the OB-1 the tants were a distance from the power section - I'd say look around and count up all the tantalum and electrolytic caps, get new ones and replace them.

 

[Sniarf Eggbird]

Thanks for helping out guys!

topcat, i just pmed you. many thanks!

the capacitors look somehow new, surely not the originals.
i'll go the cap-replacement road and see what happens.

thanks again!

thomas

 

[peterc]

Would be good to join the Synth-DIY list, they have some clever guys on there includingJurgen Haible...

 

[barclaycon]

Good luck with your machine. It will be well worth the effort in getting it going.
The original had a few issues with stability, but from what I've seen there are lots of mods and improvements out there.
It's going to be a research project, but you'll end up with something really good.

 

[Blue Jinn]

I don't have the 2600 schematic handy. But had a similar problem with an ARP Avatar. There is a main power supply connector and then further distribution from there so it was easy to disconnect the main PSU. With all boards connected, no +15v. Disconnect and +15v.  The Avatar has filter capacitors on each sub board board for each rail. One had failed shorting the +15v supply to ground, which shutdown the + rail on the PSU. It was easy to find, with a simple ohmeter finding the short.

 

[PRR]

Build a dummy-load to test the PS alone. Two 20 ohm 20 watt resistors on +/-15V will pull 0.75A each, which should be OK for that module complement. (The 2500's PS was 1.5A, IIRC.)

+/-15V 1A supplies are stock items, so don't waste a lot of time on a sick supply of unknown history.

 

[Sniarf Eggbird]

Thank you all for your support and help!

Tomorrow i will try to power everything up from my bench supply.
I will give +15/-15 to the main distribution hub, as Blue Jinn pointed out and see what also happens.
I don't have power resistors laying around, as soon as i get a couple i will try the dummy load test.

Meanwhile i'm gonna spend some days trying to clean those crazy/sticky faders and also double-check all the boards for
hidden tantalums and electrolytics. The fact that the synth is one of the first they built, shows in the way the boards are "interconnected", you should be really careful not to break anything while trying to pull each board out.

it's gonna take a little bit longer than i first thought 

thanks!

thomas

 

[PRR]

> The fact that the synth is one of the first they built, shows in the way the boards are "interconnected", you should be really careful not to break anything while trying to pull each board out.

They had it right in the 2500. I used to pull boards all day long. The 2600 seems to be the same circuits in cheaper construction.