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R/E/P => R/E/P Archives => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: River on March 06, 2009, 12:18:54 AM

Title: Trouble with Short on SP2016 logic board
Post by: River on March 06, 2009, 12:18:54 AM

Hopefully someone here can check me on this...

I'm trying to resurrect an original Eventide SP2016.  I found a short on the CPU board (a 74S00) and swapped out the IC.  Not sure just why it shorted, but I think that there might have been someone messing with it at one point.

I've still got a short on the Logic (for lack of a better description) board.  I'm reading about 94 Ohms of resistance between the +5V rail and ground.  That resistance is about 64 Ohms if I reinstall all of the EPROMs.  

I've checked all the Address and Data busses, and they all read somewhere around 800 ohms from both the +5V rail and Ground, with the exception of D2, which reads about 5 Ohms from the +5V rail and about 90 Ohms from Ground.

I know that the short isn't in any of the EPROMs, as I've pulled them all.  

Any suggestions on what I may be looking at?  It seems odd that the data bus would be shorted to both the +5V rail and ground.

What am I missing?

Thanks!

Riv

Title: Re: Trouble with Short on SP2016 logic board
Post by: Andy Peters on March 06, 2009, 01:39:25 AM

River wrote on Thu, 05 March 2009 22:18

Hopefully someone here can check me on this... I'm trying to resurrect an original Eventide SP2016.  I found a short on the CPU board (a 74S00) and swapped out the IC.  Not sure just why it shorted, but I think that there might have been someone messing with it at one point. I've still got a short on the Logic (for lack of a better description) board.  I'm reading about 94 Ohms of resistance between the +5V rail and ground.  That resistance is about 64 Ohms if I reinstall all of the EPROMs.

You might wish to check the large power-supply decoupling caps and make sure they're still good.

-a

Title: Re: Trouble with Short on SP2016 logic board
Post by: Phil Mayor on March 06, 2009, 08:11:30 PM

Hiya, you just need to look at the circuit diagram if you have one and check all the IC's and decoupling caps(if any)on the data line you say is measuring a short.

You can easily check which component is the culprit by lifting one of the legs on the caps and if the logic IC's are soldered in just clip the +5 leg of each IC one by one(with some very sharp micro cutters) until the short disappears. If you do it correctly it's easy to blob a little solder back on the leg of the IC to re-connect it.

A tri-state buffer on the data buss can short the way you describe.

Title: Re: Trouble with Short on SP2016 logic board
Post by: David Kulka on March 07, 2009, 12:07:46 AM

I'm currently working on 5 SP2016's and building a library of program chips. These are all units that I picked up over the last few months and now it's go time!

SP2016's truly can be tough dogs to work on. Much as I love Eventide gear, there are some definitely build quality issues with this unit -- they don't age well at all. Expect multiple problems. But they do sound great when they are working.

Some thoughts about finding your short:

64 or 94 ohms is not much of a short. Are you sure this is a fault? Of course the resistance might be different when the unit is powered up, but if it's not, this would be just 78 or 53 ma, not very high current really. I can measure the 5v current on a normal logic board if you want and report back.

YES, check those tantalums. You probably know this already, but the little bean shaped tantalums, and the little .1 bypass caps, commonly fail.

Sometimes I'll connect a current limited bench supply to the problem rail and gradually increase the voltage while watching current. For a board like that I might go stop at 500 ma or so. That's 2 1/2 watts at 5 volts, less if a lower voltage - fairly safe territory. You can let it run for a while and then feel for hot spots. Or use a DVM in the millivolt range to look for voltage drops in the power foils. The higher drops will point you towards the short.

Feel free to post further questions. I'm deep in SP2016's currently and will be glad to (try to) help.

Oh, also - the Molex connectors and the sliders are often big problems on these units!

Title: Re: Trouble with Short on SP2016 logic board
Post by: oldgearguy on March 12, 2009, 07:10:25 AM

David, I'm the owner of a couple of SP2016's.  One is the newer build with the rev 3 OS and the other looks like it's barely a step above the prototype stage running rev 2.2

I also have a library of program ICs that I've accumulated.  I also have some PDF docs for some of the libraries.  We should pool our info on this beast.  It's an amazing sounding device and I'd love to keep them running for many years.

BTW - I was measuring about 310 ohms between +5 and ground on that main board (labelled PD 44 I think) with no power applied.

Tom

Title: Re: Trouble with Short on SP2016 logic board
Post by: River on March 16, 2009, 01:38:23 AM

Sorry I dropped out of the conversation - I let this go on the back burner while I got the schematics and all.

I have found a TON of shorted out 74X374s of several different varieties.  (C, HC, LS)  

Now I'm seeing about 194 mV doing a diode check across the rails.  Much better territory.

I've also found a shorted 6802 processor.  Got a replacement on the way.

It's getting closer, but I've still got a little way to go here.

Parts arrive Wednesday.  I'll have more details then.

Thanks!

Title: Re: Trouble with Short on SP2016 logic board
Post by: Phil Mayor on March 18, 2009, 07:44:54 AM

As I suspected(from the short you described earlier) a Tri-state device.

Pretty catastrophic failure by the sounds of it from all the IC's you've replaced...

Have you worked out what caused it?

Does it use 6810 ram? or is it implementing the 6802's on board ram?