Shotgun Overhauls: OB-X

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HideawayStudio
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Shotgun Overhauls: OB-X

Post by HideawayStudio » Sat Mar 28, 2015 2:59 pm

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This weeks delight/angst has been a very flaky 1979 8-voice OB-X in from the studio but shelved because was resetting/crashing all over the place with intermittent CV channels, dodgy front panel pots and no sync on one voice card.

What with the cost of components these days the notion of "shotgun" component replacement during a major overhaul makes a lot of sense in some instances....

To give you some idea you can buy a hundred CD4051 multiplexor chips from Farnell for less than $20 and $4 will buy you over 50 electrolytic capacitors!

By far the most expensive parts were a full set of replacement keybed bushings but rebuilding the keybed has made a massive difference and well worth the effort.

So I decided to change all of the PSU caps, regulators, (uprated) rectifiers, tantalum capacitors, CMOS multiplexors, tri-state buffers, op amps in the CV and sample/hold circuits (although I deliberately retained all of them in the audio paths bar the final mixers as they were populated with different parts on each channel). Even the original Mostek branded Z80 processor with hellishly tarnished pins was replaced (they are still available to this day).

Almost all of which were the 35 year old originals...

Why so drastic you might ask... because this is very much a workhorse in the studio and its flaky nature had rendered it pretty much a doorstop.

She's a delight to play again - just waiting for the Encore MIDI kit.

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HideawayStudio
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Re: Shotgun Overhauls: OB-X

Post by HideawayStudio » Sat Mar 28, 2015 8:48 pm

...and after several more hours of soak testing another failure!...

This time the autotune appeared to be failing all voice cards when the instrument was hot. At first I thought maybe the output summation amplifier to the autotune circuit had failed but on investigation it turned out the synth was passing all 8 voice cards and that in fact the mute circuit that drives the final VCA was sticking in the disable state. This was traced to a faulty 40174 D-type register that drives the mute circuit (AUTOTUN) and on reading these chips are also prone to failure I replaced all of them on the CPU board.

Now working nicely again.

Incidentally what was very cool was using my newly restored 1960s EICO 147A signal tracer to hear the output of the summation amplifier to the autotune circuit. This is designed to deliberately clip the audio output and it sounds great with some timbres eg. brass sounding like something out of Stairway to Heaven! Also interesting to hear the auto-tune in action - something that is normally muted.

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I have three scopes (including a modern digital scope) but in the short time I've had my signal tracer it has proven to be incredibly useful. I've added a BNC input so I can plug a modern scope probe into it to safely hear all sorts of things such as clock circuits, tracing audio paths, noisy pots, PSU ripple and even bus signals. It has actually left me thinking why on earth scope manufactures don't add such functionality to their products as its saved me a stack of time debugging issues.

For example - when you look at most cyclic signals on a scope it is actually very difficult to determine whether there is any noise or jitter on it as the scope only shows a snapshot of time. When you hear a signal it becomes very obvious whether its glitchy, jittering or noisy.

The same goes for supply rails - you can literally hear them and any ripple or noise issues become immediately apparent and by hearing this noise you may immediately determine what is causing it eg. the ripple caused by multiplexing a whole front panel of LEDs.

Only the other day I managed to check for the presence of all 142 divider channels on a Polymoog 203A without having to move my head once to check a scope display - I managed to locate the failed divider chips in record time this way.

One of the beauties of a tube signal tracer is that its practically indestructible so the fear of hitting a supply rail or a 20 volt pk-pk signal is a non-issue.

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Re: Shotgun Overhauls: OB-X

Post by Dr. Phibes » Sat Mar 28, 2015 11:08 pm

HideawayStudio wrote: I have three scopes (including a modern digital scope) but in the short time I've had my signal tracer it has proven to be incredibly useful. I've added a BNC input so I can plug a modern scope probe into it to safely hear all sorts of things such as clock circuits, tracing audio paths, noisy pots, PSU ripple and even bus signals. It has actually left me thinking why on earth scope manufactures don't add such functionality to their products as its saved me a stack of time debugging issues.
Because they respect a gadget's right to privacy. I mean really, eavesdropping on all their internal going-ons indeed. Image

But actually, that does sound like it would be incredibly helpful. I think I might throw together a diy version to see if it would be of help. As you say sometimes it pretty hard to see what your ears don't like.

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Re: Shotgun Overhauls: OB-X

Post by Blue Monster 65 » Sun Mar 29, 2015 2:09 am

Still looking for a couple of OB-X voice boards, if anyone has any spares.
Muhahahahahahahahaha!

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Re: Shotgun Overhauls: OB-X

Post by chipaudette » Sun Mar 29, 2015 2:36 pm

Old Crow designed and fab'd his own replacement board. He often has bare PCBs for sale, if you're up to stuffing your own boards (or paying someone else to stuff them for you)...

http://www.cs80.com/crowbx/

And the muffwiggler thread following its development...

https://www.muffwiggler.com/forum/viewtopic.php?t=83363

Chip

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