There's a result then ...

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Mery Christmas!

Agreed. Merry Christmas to all the regulars here and it looks like we have nearly made it through another year of this. As frustrating as it can be, we wouldn't be doing it if we didn 't love it.

Mark Z.

These capacitors are in fact the little mischievous elves that normally assist in the distribution of power thoughout the product, but at Christmas particulary they sit around doing nothing after a year or so of either being exhausted, drinking too much, or just sitting there cross-legged and idle.

It's the cross-legged stance that causes the short circuit. At least you don't have the mess of the ones that drank too much, got bloated and threw up electrolyte all over the board ...

--
Adrian C

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well done! it's great when that happens. thanks also for posting up the solution here - might help someone else in future. maybe post a link to this solution from the original thread, for reference.

-B

Arfa,

Glad you found it, and sorry I missed your original post, as I've likely got that manual packed away somewhere.

I would also like to echo Mark's wishes to all for w great holiday season and New Year.

Tim Schwartz Bristol Electronics

Glad to hear that you solved your problem.

Merry Christmas and happy whatever to all.

Arfa, and others, you might want to check out some of the tech forums where hundreds of techs share resources and tips. Two of the best are TechAssist and Tech-Data. Both have subscription fees in the $50-60 range, but there are hundreds of gigabytes of manuals between the two, thousands of tips, and hundreds of techs with access and experience on most brands. You also won't find the spam and nonsense that are so pervasive on Usenet. Only qualified techs in the repair business are allowed.

Leonard

LOL ! Have a good 'un everyone, and catch y'all in the new year ! d;~}

Arfa

Good deal. I was always thrust into a position of a so called emergency repair without a Sams manual or the luxury of the internet (which Al Gore hadn't invented yet) back in the 70's-80's.

However, the real tough dogs being intermittent faults can stifle even the most persistent, experienced tech. Like this Singer-Kearfott resolver driver/processor I'm working on. Nothing on the net to give me service errata and an intermittent error displayed on the op terminal as a PG error relates to the resolver, Funny thing is you can get the error anywhere from immediately to several days down the road. And the error can shut things down immediately several times in a row then work for a week.

As much experience as I have it doesn't substitute for a diagram showing operation values for voltages, wave forms and digital pulses.

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Just going by the general appearance/style/era of that cnc board, I would suspect a via somewhere, from dealing with similar in the past, re-solder both sides of all vias

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Yeah I was thinking of that. Some of the topside connections aren't the best soldered I've ever seen. One or the two 8 bit multiplying D/A converters has been replaced as have a couple of the quad I/O gated flip-floppers.

I have enough generic info and understanding now to take it to the second level, dynamic tests. I have the specs on the resolvers and what they need as far as an input from the resolver amp. But I must confess I'm not the sharpest knife in the drawer when it comes to old TTL and A/D - D/A stuff being more of an audio and RF tech in the days. There are no LSI devices in this thing besides the 8086 CPUs on the logic and servo CPU boards. Also some embedded software on EEPROMS. Not being formally trained on TTL has hampered my troubleshooting but I am finally starting to 'get it'. It's hard to adapt to a primitive TTL aggregation that was replaced with LSI less than a decade later. I was just getting into digital control, PLL, etc.. when LSI started being used more and more making it much easier to verify the operation of the circuit by data I/O on the pins of an LSI rather than to understand the mathematics of data flow between dozens of seperate TTL devices.

If one clock only then a trick I used once for tracing a digital fault in similar circumstances. I managed to change the clock crystal to 100KHz or maybe was a glass cased

10KHz one, can't remember, long ago. Clock worked and slowed everything down so much that it highlighted, by little happening, on a suspect ,actually dead TTL device.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Merry Christmas, you old buzzard! ;-)

Merry Christmas to everyone. :)

--
Offworld checks no longer accepted!

The amp board is dual function. It provides the resolvers with an AC current at 10khz at a specific voltage. The resolvers then send reference back to the amp board where the phase angles of the resolver feedback are analyzed, compared and then converted to digital going thru a serial to parallel data path and off to the servo cpu to interface with the logic cpu and off to the servo packs to control the pulse-width modulate 48kz control fed to an SCR pack that ends up as power for the servomotor.

This allows each axis of motion to be precisely controllable down to the 10,000th of an inch. Also allows for a precision infinite control of the motor speed.

One thing remains the same in that a resolver does the same thing no matter who built the electronics. Eventually I will be able to find the proper points for measurements, there are three trimmer pots on the board I can rock back and forth and see what areas on the board are influenced signal-wise.

Here is a page that describes resolver transducers if you're interested.

That is not luck.

Given the problem -- no recording and no playback -- I assume you started with something common to both processes, the power supply, and found that the voltages were more or less correct.

Where to from there? It's easier to check playback than recording, so you inserted a test tape and found the signal stopped at the Dolby chip. That pointed to either the chip or the surrounding components. The latter are the easiest to check, so...

No, Arfa, it was not luck. It was good troubleshooting techniques all the way.

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You're confusing luck in the problem being due to a readily replaceable part, and luck in troubleshooting it. "Yes" to the former, "no" to the latter.

Weeelll, I guess that's sort of true, but sometimes with problems like this and no service info, you can go round in circles for ever. Such situations are doubly difficult when you have your arse in a sling over timing (remember it was needed in a hurry) and when not just one, but two components are faulty, as was the (extremely rare) case with this one. The problem could have been down to the Dolby processor chip itself, or at least tests without a schematic might have made it look so, in which case I would have been stuffed without a replacement to try. Likewise, it could have been a resistor which had gone high, or a little ceramic decoupler that had gone leaky, both of which would have been difficult to locate by in circuit resistance checks, given that I would not have been able to find such problems by voltage checks, without service info for the unit, or the correct data sheet for the chip. In the timescale I had, and given the value of the unit, it would not really have been commercially viable to start pulling and refitting or replacing components willy-nilly, in the fond hope of finding the bad one.

I guess that experience and years of solid fault-finding work, probably

*did* point at it being a little blue electro at fault, and yes, good fault-finding practice did lead me to the Dolby chip in exactly the way you surmise, but for all that, it still *felt* lucky to have dropped on a short circuit cap (and its mate next door) that was measurable in circuit !

Arfa

it still *felt* lucky to have dropped on a

Well, whichever way you look at it, that brings us right back around to the start. Sometimes you gets lucky. I got lucky ... :-)

Arfa

test