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Replacement PSU for Cheetah MS6

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Re: Replacement PSU for Cheetah MS6

Postby ef37a » Sat Dec 29, 2018 1:09 am

James Perrett wrote:I know it is too late but fuses are generally specified at a certain value for a reason - NEVER replace one a higher rated fuse. I'd go along with Folderol's suggestion to fix what you have rather than rebuild. It sounds like a basic regulated supply which won't be hard to fix if you take a logical approach.

Yes but, I thought the transformer was toast? Might be hard to find one that fits.

Dave.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Sat Dec 29, 2018 11:07 am

ef37a wrote:
James Perrett wrote:I know it is too late but fuses are generally specified at a certain value for a reason - NEVER replace one a higher rated fuse. I'd go along with Folderol's suggestion to fix what you have rather than rebuild. It sounds like a basic regulated supply which won't be hard to fix if you take a logical approach.

Yes but, I thought the transformer was toast? Might be hard to find one that fits.

Dave.

James, you are right, of course. When I got the MS6 from recollection it had a 200mA fuse, but I have no idea if that was correct. It was the 200mA’s that were popping so I thought I would take a chance on the next one up. Is there any way to judge, from the circuit components, what the correct rating ought to be?

Dave, you are correct that replacement transformers (same dims and pin-outs) are supposed to be hard to find. Not sure if either/both are dead. I think this is why others have gone for inserting 12V AC before the diode rectifier.

I am wondering about bypassing (and not re-fitting) the AC voltage selector (which Folderol said may be damaged) and then testing the AC voltage on the transformer outputs / diode inputs. Logical next step? I also notice that the only connection to mains earth is to the casing, none to the PCB that I can see. There does appear to be connections for L / N / E on the PCB, but the Earth is not connected to anything.
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Re: Replacement PSU for Cheetah MS6

Postby ef37a » Sat Dec 29, 2018 11:20 am

Just a quick thought? Hook up a gaggle of AAs to get the supplies to make sure you don't have other problems?

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Re: Replacement PSU for Cheetah MS6

Postby BillB » Sat Dec 29, 2018 12:07 pm

I like the idea of testing the main functions in principle, but it sounds like a lot of of AAs! Insert the voltage before or after the regulators?
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Re: Replacement PSU for Cheetah MS6

Postby ef37a » Sat Dec 29, 2018 2:09 pm

BillB wrote:I like the idea of testing the main functions in principle, but it sounds like a lot of of AAs! Insert the voltage before or after the regulators?
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Re: Replacement PSU for Cheetah MS6

Postby ef37a » Sat Dec 29, 2018 2:16 pm

I didn't say nuffin! It just posted that quote!

Pre or post the regulators, whichever is most convenient. Three AAs gives close to 5V when fresh and eight gives just over 12.

You can buy a rake of Carbon Zinc from the Pound shops or pay a bit more for alkalines for a few minutes more testing. Fortunately I bought several AA holders from Maplin before they went TsU.

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Re: Replacement PSU for Cheetah MS6

Postby Folderol » Sat Dec 29, 2018 5:09 pm

Well, we know the 5V Tx at least is working, so it's only the 12V one to check, and if it's dead that one should be relatively easy to get a compatible replacement for.

And of course the the voltage regulators.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Sat Dec 29, 2018 8:05 pm

OK, thanks Folderol. Won’t be able to check until tomorrow, but will report back.
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Re: Replacement PSU for Cheetah MS6

Postby BillB » Wed Jan 02, 2019 12:37 am

OK, finally got back to it. Thanks for the cardboard tip, Folderol, gave me a lot more confidence to run the tests.

Thought the clearest thing would be to mark the voltages on to the PSU Schematic PDF, so new upload here:

https://www.2btech.co.uk/refimages/ms6_power_testVs.pdf

Pretty clear that there is an issue with the -5V line, registering at -8.5V. Presumably the regulator input voltage is OK at 10V?

For information, it is the two positive regulators which have been put on the heatsink - presumably they run hotter.

Also, coming back to James's point about the 500mA input fuse replacing the previous 200mA fuse, is there any way to know what would be an appropriate rating without a specification for it?

thanks

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Re: Replacement PSU for Cheetah MS6

Postby BillB » Wed Jan 02, 2019 1:04 am

Just looking at the PDF within my iPad safari browser and the PDF annotations don’t show, but if I download it to a PDF reader they are present and correct. If anyone can’t see the (pink) annotations, let me know and I’ll upload a screenshot instead.
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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.

H
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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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