Help diagnosing Kurzweil Mark 5 - powers up, all lights normal, but no sound

[topoisomerase]

Hi there,

I have a Mark 5 that I purchased new >20 years ago, which until recently worked flawlessly. However, now I find that when I turn it on, all appears normal but no sound comes out - either from the speakers or if you plug in headphones. All lights are operating, I can make any selections I want using the buttons, etc. If I turn the volume slider all the way up I can hear the faint white noise I normally did. Otherwise there is no sound when the keyboard is operated, the Demo button is pushed, etc.

I have already tried the reset methods I've found online with no change. With no success with that, I have largely disassembled the unit and performed a variety of other steps:

• Removed the two metal shields and examined all capacitors for signs of bulging or leakage, damaged ICs, etc. - none found
• Confirmed all fuses are intact
• Unplugged, cleaned with contact cleaner, and reseated all plugs/jacks (under shielded areas and under the keyboard itself)
• Cleaned volume slider pots and confirmed resistance changes with movement of the slide

I have a multimeter (how I checked resistance & continuity across fuses), so am happy to do any other checks that would be recommended. I would welcome any assistance in trying to get things back up and running. Again, I've never had any issues with this unit so I remain hopeful it something simple that can be replaced or repaired.

Thank you!

 

[happyrat1]

Take a mono earphone and attach probes to the end.

See if you can trace the signal along the audio path to see if the problem is with the audio section or the logic section.

If the problem is with the CPU logic it may cost more to fix than it's worth.

Gary

 

[happyrat1]

And eyeballing the capacitors is not a sure fire method of determining leakage.

What you need is an ESR meter to check them out in circuit.

They cost about $50 on ebay and amazon.

Gary

 

[topoisomerase]

And eyeballing the capacitors is not a sure fire method of determining leakage.

What you need is an ESR meter to check them out in circuit.

They cost about $50 on ebay and amazon.

Gary

Thanks very much for your replies, Gary!

 

[happyrat1]

Good luck!

 

[topoisomerase]

Update: I managed to acquire a Service Manual and have performed all the diagnostic tests. The result of that is that it seems the analog circuits on the Engine board is likely at issue. I'm happy to have ruled lots of other things out but now need to try to find a schematic. Support won't give me one of course, even though it's a d/c'ed product.

 

[topoisomerase]

Good luck!

Hi Gary,

I thought you'd appreciate my followup to this thread as well as the other one where I asked about schematics. I did end up getting the schematics on eBay, and also bought and read the book you recommended. I then bought an oscilloscope, which was great as I haven't used one since college. I also picked up an ESR meter which proved really helpful.

Outcome? I fixed it! The instrument is working perfectly once again. So what was the root cause? Two things:
1) failed 1F / 5.5V electrolytic capacitor *
2) damaged via that interrupted the -15V rail, which made the DAC and associated components sad. **

Needless to say I'm pretty excited. Thanks again for the tips! A few more details are below. It was easier than it might have been otherwise as the onboard diagnostics seemed to rule out everything but the analog audio circuit. There are still quite a few components there but just having that area to focus on surely saved me buckets of time.

Cheers,
Jason

Details
* The cap wasn't bulged but it had a drop of what looked like electrolyte on top and it had a very high ESR. Replaced. ESR in range with the datasheet.

** This was tricky to find but good old tracing narrowed it down. I probed around the DAC and found the +5V input along with signals from another chip to the DAC as expected. But the -5V input was not present. I traced that back to a voltage regulator and found it wasn't getting -15V input. Tracing that back to the -15V jumper block I found a via that wasn't continuous. It didn't seem clean so I presume electrolyte got into it and degraded the copper. It's right next to the big cap that failed, hence my idea. I repaired the via and that solved it.

 

[Biggles]

Great news and well done.

Feedback is always welcome but alas it does not always occur.