Polysix KLM-376 power supply problem

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shawnsick
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Polysix KLM-376 power supply problem

Post by shawnsick » Thu Sep 03, 2015 3:32 pm

Hello, I have recently received a Korg Polysix to repair for a friend, the synth won't turn on, no sound, no LEDS, nothing. After a minor inspection I quickly noticed that R15 on the power supply board (KLM-376A1-PS) had burnt out. After finding the original value of R15 (150ohm according to the schematic I have found) I replaced the fried resistor with a 150ohm 1/2w, the synth still wouldn't power up and the new resistor instantly burnt out. Any advice on where I could start looking ? I have access to an oscilloscope, digital multimeter, Peak DCA-55 Semiconductor Analyser.

Any advice/information would be greatly appreciated !

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Re: Polysix KLM-376 power supply problem

Post by sam » Thu Sep 03, 2015 8:18 pm

Check for shorts on the 15 v and 5 v lines.
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Re: Polysix KLM-376 power supply problem

Post by shawnsick » Fri Sep 04, 2015 12:21 pm

Turns out that Q4 (2SB744A) is fried (shorted out), what could have cause that ? Just wear ? Or something else ?

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Re: Polysix KLM-376 power supply problem

Post by garranimal » Fri Sep 04, 2015 6:12 pm

shawnsick wrote:Turns out that Q4 (2SB744A) is fried (shorted out), what could have cause that ? Just wear ? Or something else ?
You may have found the cause (or another symptom), transistors that short will cause excessive currents that fry resistors. Transistor killing transients may be getting through if the big filtering capacitors are failing. Rather than test these big caps, if original, just go ahead and replace them and sleep better. Also, make darn sure the voltage regulator tabs are not getting shorted on the mounting rail...sometimes vibrations or over-tightening the mounting screws can cause these to come into contact with each other. To check, unplug synth, set your DVM to continuity test and probe the tabs and rail. There should be NO beep, **EDIT** resistance should be infninity with the DVM just showing dashes **END EDIT**.

Another thing to consider. Kiwitechnics.com make a revised replacement power supply design, and can recommend it as I have installed it in mine.
Last edited by garranimal on Sat Sep 05, 2015 10:37 pm, edited 1 time in total.

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Re: Polysix KLM-376 power supply problem

Post by rschnier » Sat Sep 05, 2015 8:39 pm

garranimal wrote:To check, unplug synth, set your DVM to continuity test and probe the tabs and rail. There should be NO beep, resistance reading should be zero.
I presume you meant to say infinite (i.e. no continuity), not zero. :geek:
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Re: Polysix KLM-376 power supply problem

Post by garranimal » Sat Sep 05, 2015 10:36 pm

rschnier wrote:
garranimal wrote:To check, unplug synth, set your DVM to continuity test and probe the tabs and rail. There should be NO beep, resistance reading should be zero.
I presume you meant to say infinite (i.e. no continuity), not zero. :geek:
:oops: Infinity is open circuit, zero resistance is short circuit. I think I meant to say there should be NO resistance reading, just dashes on the DVM.

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Re: Polysix KLM-376 power supply problem

Post by shawnsick » Sat Sep 05, 2015 11:19 pm

garranimal wrote:
shawnsick wrote:Turns out that Q4 (2SB744A) is fried (shorted out), what could have cause that ? Just wear ? Or something else ?
You may have found the cause (or another symptom), transistors that short will cause excessive currents that fry resistors. Transistor killing transients may be getting through if the big filtering capacitors are failing. Rather than test these big caps, if original, just go ahead and replace them and sleep better. Also, make darn sure the voltage regulator tabs are not getting shorted on the mounting rail...sometimes vibrations or over-tightening the mounting screws can cause these to come into contact with each other. To check, unplug synth, set your DVM to continuity test and probe the tabs and rail. There should be NO beep, **EDIT** resistance should be infninity with the DVM just showing dashes **END EDIT**.

Another thing to consider. Kiwitechnics.com make a revised replacement power supply design, and can recommend it as I have installed it in mine.

Yes, I immediately placed the filtering caps on my suspect list as C14 wouldn't hold charge. As for the tabs and the mounting rail, I can clearly see that the tabs aren't touching the rails, but I'll test that tomorrow just to be safe.

I really appreciate the help! Thank you :thumbright:

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Re: Polysix KLM-376 power supply problem

Post by garranimal » Sun Sep 06, 2015 10:25 pm

shawnsick wrote:As for the tabs and the mounting rail, I can clearly see that the tabs aren't touching the rails, but I'll test that tomorrow just to be safe.

I really appreciate the help! Thank you :thumbright:
You're welcome. With the voltage regulator tabs, its important there is a thin piece of non-conductive membrane between the tab and rail. There are dozens of things that can go wrong here. The screw holding the tab might be metal, and if so, it could ground the tab-and-rail unless the membrane forms a little lip all the way around the hole. Ideally the screw would be nylon plastic, but even so, can break very easily. If the tab-and-rail aren't tight enough together, the regulator won't dissipate heat to the rail fast enough....yes the regulators can get hot during normal use. Hope all goes well and be careful.

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Re: Polysix KLM-376 power supply problem

Post by shawnsick » Fri Sep 18, 2015 7:50 pm

After waiting for the new parts to arrive, I finally changed the filtering capacitors and replaced the fried resistor and transistor, nut now it looks like I'm back to step 1 again... Unfortunately I have only just read your advice concerning the transistor tabs... The transistor has once again gone up in smoke, could this be down the the fact that I didn't use a non conductive membrane between the transistor and the heatsink ? I actually used thermal paste... Should I start looking elsewhere for faults or should I use another transistor with a non conductive membrane ?

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