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Replacement PSU for Cheetah MS6

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Re: Replacement PSU for Cheetah MS6

Postby James Perrett » Wed Jan 02, 2019 1:16 am

I'd be a bit worried that the volts on the negative side are higher than the positive side for both transformers. Could there be a ground missing somewhere or are the current demands just very unbalanced? Have you checked the voltages on the pins of Reg2?

As far as fuse ratings are concerned, this is normally marked on the case or on the circuit board screen print. If the regulators were working at full capacity you'd be looking at a demand of very roughly 40W from the transformers but they don't look big enough to handle that kind of power to me. 40W from 240V mains would require a current of 166mA so 200mA should work. One thought - were the original 200mA fuses of the anti surge type? Could the problem be that you replaced them with quick blow fuses? Initially charging the smoothing capacitors will require more current than normal working so anti surge fuses are normally used in this kind of application.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Wed Jan 02, 2019 8:43 am

Thanks James. There is a connection point on the PCB marked ‘earth’ next to live and neutral, which looks like it has had a connection removed. The mains earth goes to a nut on the casing only, and I can’t see any path for that to get back to the PCB. So perhaps I should run a wire from the case earth to the PCB earth then re-take the measurements?

Think I may have mentioned that the MS6’s were noted for mains hum, so this may have been a (poor) effort to remove that. As I have a decent transformer isolator for the audio, it would not be a problem to reinstate any earths and deal with any hum issue via the audio connection. When it was working previously, I was using a pseudo-balanced cable which did the trick. I have better (TI) kit now.

Ref fuse, thanks for the indication of correct value. I’m pretty sure I tried anti-surge 200mA fuses but will check again.
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Re: Replacement PSU for Cheetah MS6

Postby Folderol » Wed Jan 02, 2019 1:35 pm

All shows up very clearly here.

I agree with James that it looks like there is a very iffy ground there. Also, it looks like the -5V regulator may either be faulty or has lost it's own ground connection completely. 10 - 8.5 = 1.5, which is just about the minimum difference for these regulators.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Wed Jan 02, 2019 2:31 pm

OK, I'll reinstate the earth connection and also look at continuity from that to the -5V regulator.

Thank you!
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Re: Replacement PSU for Cheetah MS6

Postby James Perrett » Wed Jan 02, 2019 3:49 pm

I'd actually be a little wary of re-instating that earth connection until you've sorted out the power supply problem. When I mentioned earth connections I was talking about connections on the PCB - probably between the transformers' centre taps and other parts of the board. It would be a good idea (with the unit switched off) to check continuity between the PCB earth tag and every component lead in the PSU that is supposed to be connected to earth.

If your meter has a diode check function then check the bridge rectifier diodes too.
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Re: Replacement PSU for Cheetah MS6

Postby Hugh Robjohns » Wed Jan 02, 2019 4:05 pm

Folderol wrote:Everyone has their preferences, but I generally prefer in-situ repairs where possible.

I agree entirely...

If you have a multimeter, the first things to check out are the voltages from the two transformer secondaries. If one or other transformer is knackered an external PSU (or an external transformer box) might well end up being the only viable solution... but if the transformers are okay then it should be pretty straightforward to find and fix the fault in that simple linear PSU.

So start with the AC voltages across the transformer secondaries, then the DC voltages at the output of the rectifier bridges, and then the DC voltages either side of the regulators.... as per Folderol's guidance above.

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Re: Replacement PSU for Cheetah MS6

Postby BillB » Wed Jan 02, 2019 4:12 pm

James Perrett wrote:It would be a good idea (with the unit switched off) to check continuity between the PCB earth tag and every component lead in the PSU that is supposed to be connected to earth. If your meter has a diode check function then check the bridge rectifier diodes too.

OK, will do.

Hugh, thanks also for your comments, noted.

Happy New Year All!

:thumbup:

or perhaps...

:headbang:
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Fri Jan 04, 2019 2:42 am

Just stopping by briefly to say there do seem to be some inconsistencies but it could just be me, mixing up my +/- Regulator pins. Pins 123 +V, 213 -V doesn’t help! I need time to be very systematic about the checks, which I won’t be able to do for a few days.
Also noticed that there is some soldering iron damage to the casings of C2, C5, C8, which are the red caps nearest the regulators in the photo, so may need replacing.
But I’ll come back with clearer results on connections to the Regulator grounds, probably next week.
Thanks all.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Sun Dec 01, 2019 3:20 pm

...probably next week - hah!
I know I have gone very quiet on this, the MS6 has been pushed down the ‘todo’ list. But today I thought I would take advantage of the good winter light and take hi-res photos of both sides of the PCB (PSU area) in order to create a composite image which should give a better idea of what is going on and make it easier to relate the schematic to the circuit board. Anyway, it inspired me to look for other photos of the MS6 which lead me to this:
https://driessen-music.de/2019/01/04/re ... nthesizer/

It’s not absolutely clear to me what the guy has done or where he has done it, but I did find this comment about the problems, as presented to him, interesting: “Last but not least it blows the house circuit breaker.”... familiar.

So when I get my composite image together and start looking again, I will also be referring that article.
Probably not next week, though... :headbang:
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