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repair help: arp 2600 normalled connection not working
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author repair help: arp 2600 normalled connection not working
there's no repair section on this board so i thought maybe this would be the best spot for this... please advise if there's a better place.

anyway, so the ADSR mod to my oscillator 3 is not working when it's normalled. that is, if nothing is inserted into the ADSR input jack of oscillator 3, then turning up the slider has no effect. if i patch the output of the ADSR envelope into this jack, it works fine; any other input also works just fine.

so i'm wondering what the problem might be... is this normalled connection on the board itself or is there a wire going from the ADSR board to the oscillator board that carries the voltage to the slider? i've cracked the 2600 open a couple times but have yet to take the oscillator board off and look into this. looks like an hour process just to do that so i thought i'd get some advice first...

any ideas? help
The jack on the front panel is a switching type, so either the jack is not switching or the wire from the ADSR has broken off on the connector. You can check on Board 2 (the board with the 3 VCO modules) at a pad near the bottom of the board marked T31. That pad connects to connector E31 and one end of slide potentionmeter R102, which is the fader above the ADSR input. With no patch cable plugged in and using an oscilloscope, you should see the ADSR signal at pad T31. If you don't see the ADSR signal and since you can control the oscillator with the ADSR when it's patched in, the connector is probably your problem.

Try resoldering the connections on the connector and see if that resolves your problem. If not, replace the connector.
richard_k: thanks! that gives me a great spot to start at when i crack it open! i don't have a scope tho; any other way to check the ADSR input at T31? also, which board is E31 on? i see it in the service manual drawings but not on the actual board 2 sketch (but i can find T31 just fine there).

edit: i see that by "connector" you meant what i'm referring to as the jack applause

so that's E31 then. meaning i'll have to remove board 2 (i.e undo 21 nuts from all the jacks!) to get a look at the connection. i just looked and the connection in the back coming from the ADSR seems to be ok. still connected at least, i didn't check the level. i'll have to get a better look once i remove the board.

and, i'm assuming i can just use a multimeter to check the level then? dumb question? (i'm new to this, but i have used a soldering iron a handful of times.)
I made the assumption that you had access to an oscilloscope and should have realized not everybody has one. I should have suggested that you use your multimeter first. d'oh!

With the power turned off and the synth unplugged from the AC power source, you can check the resistance between the two switched pins very easily using the RX1 scale or RX10 if your meter doesn't have an RX1 scale. With no cable plugged into the jack, you should see less than 1 ohm of resistance between the two pins. One of the three leads goes to ground and that's not one of the terminals you use. If you see a very high value of resistance, probably infinity, this indicates the connector is not making connection internally and the connector should be replaced. It might be possible that the connector is dirty inside but these connectors are difficult to clean due to their size and have them function properly afterwards. Usually, the easiest course is replacement.
richard_k: thanks for more advice. i'm still trying to amp up to give this a go. i wanna make sure i'm fully prepared before i take all the time to remove the board.

i posted this over on VSE as well and it was mentioned there that there's a "spring" inside the connector which makes the normalled connection. this jack has less of a "click" than most of the others so maybe this spring is indeed worn out. it's been suggested that i can perhaps bend this spring back but that more than likely, replacement will be the best option (i have to desolder the connector and remove its casing to get to the spring anyway, i think).

do you know what the best replacement part is for this?

again, i appreciate all the help! nanners
The best replacement would be a Switchcraft 35PM2A. I checked Mouser and DigiKey and they list the part but don't stock them.

Newark listed it as non-stock but could get one in about 30 days and the price was $4. The link to this part is tid=153099

Other vendors equivalents would work. At least with the Switchcraft part number you can search for a suitable replacement.

And, yes, you're right. Doing work on a 2600 is very challenging. But, it's worthwhile! hihi
I probably wouldn't use the Switchcraft 35PM2A as it is a 3.5 mm series jack. The Arp 2600 uses the 42A or 142 A series tiny jax (.141 inch) which are a slightly larger diameter than the 3.5mm series. The Arp patch cables are also the slightly larger Tinyjax.

Also to mention that Arp had a service bulletin which describes unsoldering the jacks "spring connection" and pushing it down slightly so it makes a better connection - I believe they also suggested replacing with a 42 series rather than the 142 series jack. If you're interested I'll dig up the bulletin.
Davemoog, you're right! After reading your post, I measured the connectors on my 2600 and sure enough, they were .141 diameter. oops I agree with you, the 42 series would be a better choice.
thanks again for all the help guys! i think i'm finally getting around to this now. i actually chickened out and contacted a tech (analogics in cleveland). but never heard back from him (thanks Rage!). so i'm going to go for it myself help

so... is this the part i want then? d-42a.html

looks exactly like what you describe. i'm still looking for someone who has them in stock but doesn't charge $9 to ship two connectors sad banana
Betcha didn't know this smile


* When a check or money order accompanies your order, Digi-Key pays all shipping and insurance (our choice for method of shipping) to all addresses in the USA and Canada. Shipments of excessive weight or size may require additional charges. Digi-Key will notify you prior to shipment if these conditions exist.

found here:

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