Need help figuring out how to fix a couple of Beeb PSUs

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KarateEd
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Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Sat Jul 07, 2018 1:52 pm

Hi Folks,

Paul and I each have a PSU that's stopped working, reasons unknown.

I thought maybe mine had a bad switch but I've checked continuity on both out to the switch and proved both have continuity up to the switch and the switches are working so we're that far.

On Paul's PSU, it stopped working a couple years ago. Mine was working a month ago but has since stopped working.

What are the most common failures on these PSUs, besides the caps which have been changed on both?

What steps should a person take to troubleshoot why they have stopped working?

I'm hoping for not-too-technical jargon here so I can follow easily and if people can have pics or diagrams, that would be good. We're seriously hoping to learn something as well as get these fixed.

Thanks very much for any help that is offered.

Ed...... :-)
Ed...... :-)

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tricky
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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by tricky » Sat Jul 07, 2018 2:30 pm

I don't wish to discourage anyone from answering.
I was wondering if there were any more general retro or electrical groups near you that you could drop in on and maybe demo them some Acorn stuff in return for sharing their PSU knowledge :D

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Sat Jul 07, 2018 3:08 pm

tricky wrote:
Sat Jul 07, 2018 2:30 pm
I don't wish to discourage anyone from answering.
I was wondering if there were any more general retro or electrical groups near you that you could drop in on and maybe demo them some Acorn stuff in return for sharing their PSU knowledge :D
I don't want to discourage anyone from answering either. I think this could be useful to a lot of members on this group and where I am, it's at least an hours drive before I can talk with someone. I am unaware of any general groups of that nature here. I also know that for the most part, people here are unaware of the Beeb, PSU or machine so they would have their own learning curve. Needless to say, that's a barrier for most here.

I know we have the expertise on *. and I know this could be valuable to many.

So I guess the answer is...... don't know of any groups that are fairly easily accessible to me and I want to learn from the best. Those people exist on this site.

Ed...... :-)
Ed...... :-)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by 1024MAK » Sat Jul 07, 2018 3:43 pm

Not just "general retro or electrical groups", but anyone who repairs or who may work on mains powered switch mode power supplies. The principles are common to all switch mode power supply systems.

Anyway, the important thing, is that these work at mains voltages (110V to 120V or 220V to 240V) and convert the AC mains input to a DC voltage of about 339V DC.

Some of the testing will involve testing voltages on the PSU when live. Are you happy to proceed and are you confident that you know how to work safely when doing this?

If yes, then we can continue.

Mark

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by MartinB » Sat Jul 07, 2018 3:47 pm

Assuming it’s not the fuse (you’ve checked?), when these PSU’s were re-capped, did you also change the start-up capacitor, C9? If it’s not the fuse and C9 is ok, it /might/ then be the main switching transistor, Q2 (a vertically-mounted TO3 case) but this is a little more of a job to test and then to replace if faulty.

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Sat Jul 07, 2018 4:34 pm

1024MAK wrote:
Sat Jul 07, 2018 3:43 pm
Not just "general retro or electrical groups", but anyone who repairs or who may work on mains powered switch mode power supplies. The principles are common to all switch mode power supply systems.

Anyway, the important thing, is that these work at mains voltages (110V to 120V or 220V to 240V) and convert the AC mains input to a DC voltage of about 339V DC.

Some of the testing will involve testing voltages on the PSU when live. Are you happy to proceed and are you confident that you know how to work safely when doing this?

If yes, then we can continue.

Mark
Hi Mark, the answer is yes. Around things like SMPSs I'm very careful. The only thing I'm wondering about, I seem to remember there is a way to make testing safer (isolating the PSU?) but don't recall how to isolate the SMPS for that safety. Am I just blowing bubbles here or is there something I should be doing before switching on to work on it. What I do know is there are no voltages coming out the +5, +12 or -5 so between the output of the switch (120V in Canada) and MB outputs there is a failure (basically somewhere on the MB of the PSU).

Ed...... :-)
Ed...... :-)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by flaxcottage » Sat Jul 07, 2018 4:35 pm

Along with what MartinB says, BITD I had some of these transistors crack the soldered joints under the PCB. Reflowing the solder solved the problem then.
- John
Image

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Sat Jul 07, 2018 4:36 pm

MartinB wrote:
Sat Jul 07, 2018 3:47 pm
Assuming it’s not the fuse (you’ve checked?), when these PSU’s were re-capped, did you also change the start-up capacitor, C9? If it’s not the fuse and C9 is ok, it /might/ then be the main switching transistor, Q2 (a vertically-mounted TO3 case) but this is a little more of a job to test and then to replace if faulty.
Hi Martin,

The fuse has been checked and the C9 on both PSUs has been replaced. Right now I only have my PSU but when Paul and I get together again we'll process his PSU to see what we see using whatever you good folks on here advise me to do to find the issues.

Ed...... :-)
Ed...... :-)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Sat Jul 07, 2018 4:37 pm

flaxcottage wrote:
Sat Jul 07, 2018 4:35 pm
Along with what MartinB says, BITD I had some of these transistors crack the soldered joints under the PCB. Reflowing the solder solved the problem then.
Hi John.....

I will remove the MB and reflow in whatever spots I can..... it's likely something like that..... simplest issues first I guess.

Ed...... :-)
Last edited by KarateEd on Sat Jul 07, 2018 5:17 pm, edited 1 time in total.
Ed...... :-)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by 1024MAK » Sat Jul 07, 2018 10:52 pm

For isolation from the mains supply (which is referenced to neutral, and hence to earth / ground), use a 1:1 ratio "isolating" transformer. The transformer should have a VA rating at least 15% greater than the wattage of the Beebs PSU.

Yes, the next step is to carefully remove the PSU from the Beeb. Then using a known good multimeter (test it first on a known DC voltage), test the voltage across the two large 200V / 250V electrolytic capacitors. Then even if you don't get any significant voltage, discharge them either using a suitable resistor or the metal shaft of a screwdriver with an insulated handle.

Now carefully examine every solder joint. A magnifying glass and good light is needed for this. You are looking for any cracks / rings, or distortions in the solder, or for any that have gone very dull. You may want to clean up the track side of the board with IPA.

Resolder any suspect joints. I prefer to add a bit of solder, then use a solder sucker to remove most of the solder.if needed, clean the pad and the component leg/pin. The resolder with fresh solder.

Then test the PSU again.

If there are still no output voltages, with the PSU powered up, switch your multimeter to the 200V DC range, then carefully test the voltages across the two large 200V / 250V electrolytic capacitors. You should get about 169V across each.

Mark

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by 1024MAK » Sat Jul 07, 2018 11:08 pm

Here is one example of an 1:1 isolation transformer from digikey.

It does not have to be this one, feel free to shop around. But don't buy an auto transformer, as these are not isolating.

The isolation transformer is not essential if you are very careful. But it does mean that when working on a live item (that is powered via an isolation transformer) if you do accidentally touch a live conductor or connection, you will not receive a fatal electric shock. Unless you touch two different points that have a voltage difference.

Mark

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Sat Jul 07, 2018 11:09 pm

1024MAK wrote:
Sat Jul 07, 2018 10:52 pm
For isolation from the mains supply (which is referenced to neutral, and hence to earth / ground), use a 1:1 ratio "isolating" transformer. The transformer should have a VA rating at least 15% greater than the wattage of the Beebs PSU.

Yes, the next step is to carefully remove the PSU from the Beeb. Then using a known good multimeter (test it first on a known DC voltage), test the voltage across the two large 200V / 250V electrolytic capacitors. Then even if you don't get any significant voltage, discharge them either using a suitable resistor or the metal shaft of a screwdriver with an insulated handle.

Now carefully examine every solder joint. A magnifying glass and good light is needed for this. You are looking for any cracks / rings, or distortions in the solder, or for any that have gone very dull. You may want to clean up the track side of the board with IPA.

Resolder any suspect joints. I prefer to add a bit of solder, then use a solder sucker to remove most of the solder.if needed, clean the pad and the component leg/pin. The resolder with fresh solder.

Then test the PSU again.

If there are still no output voltages, with the PSU powered up, switch your multimeter to the 200V DC range, then carefully test the voltages across the two large 200V / 250V electrolytic capacitors. You should get about 169V across each.

Mark
Thanks Mark,

I'm going to have to go to Vancouver to get the isolation T/F.... see... my electronics repair course for PCs wasn't totally useless. I remembered about isolation, just not how to do it. Until I get that, I'll go over the circuit with a fine tooth comb. Have to go out now but will have a chance to do that tomorrow. The board is out of the casing right now but not plugged in because the plug and M/B have gone 2 separate ways for now.

Ed...... :-)
Ed...... :-)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by duikkie » Mon Jul 09, 2018 8:52 pm

there is on this forum how i fixed a bbc psu, someone can find the link for you ? i am very bad in finding things

there are a lot of things that can go bad on a psu , mostly it is the flyback that stops the psu from working

try to find the link.

sometime a transistor looks good but is bad , and so on


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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Fri Jul 13, 2018 8:19 pm

Thanks Dukkie,

Were you able to make a larger/clearer proper drawing of the second, professional looking drawing? It would be useful to have to be able to read it.

Thanks,

Ed...... :-)
Ed...... :-)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by 1024MAK » Sat Jul 14, 2018 6:30 am

This one:Image
:?:

That's something I doodled. In the service manual there is a schematic diagram of the Beeb PSU. But there are some differences to the actual real life PSUs. Hence why I reverse engineered the important primary side. I have started the secondary side. It is very similar to the version in the service manual, but also has some differences. Once I have overcome the distractions of real life and other interesting diversions, I will plonk it on here. Please note that the arrangements for transformers T2 and T3 do not show the direction of the windings correctly or the correct windings for the secondary side.

Independently of me, duikkie also reverse engineered his Beeb PSU. So if you see any differences between his diagrams and mine, well either the PSUs are different or one or both of us has made errors.

Mark
Last edited by 1024MAK on Sat Jul 14, 2018 6:38 am, edited 1 time in total.

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by duikkie » Sat Jul 14, 2018 5:27 pm

i am a old men pc's are to moderne for me :)

i can only work with paper , pencile and so on :shock:

i even have plastic things with symbols #-o to draw :)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by 1024MAK » Sat Jul 14, 2018 6:11 pm

duikkie wrote:
Sat Jul 14, 2018 5:27 pm
i am a old men pc's are to moderne for me :)

i can only work with paper , pencile and so on :shock:

i even have plastic things with symbols #-o to draw :)
Yeah, but I often work with paper and pen or pencil. It's only when I intend to publish or make a PCB board that I go to the effort of using a schematic capture application. For me, it's so much faster to just draw it on paper with a pen :mrgreen:

Mark

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by GadgetUK164 » Sun Jul 15, 2018 8:23 pm

1024MAK wrote:
Sat Jul 14, 2018 6:30 am
This one:Image
:?:

That's something I doodled. In the service manual there is a schematic diagram of the Beeb PSU. But there are some differences to the actual real life PSUs. Hence why I reverse engineered the important primary side. I have started the secondary side. It is very similar to the version in the service manual, but also has some differences. Once I have overcome the distractions of real life and other interesting diversions, I will plonk it on here. Please note that the arrangements for transformers T2 and T3 do not show the direction of the windings correctly or the correct windings for the secondary side.

Independently of me, duikkie also reverse engineered his Beeb PSU. So if you see any differences between his diagrams and mine, well either the PSUs are different or one or both of us has made errors.

Mark
Do you happen to have the secondary side of that diagram? Just for future reference!
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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by 1024MAK » Mon Jul 16, 2018 7:03 am

GadgetUK164 wrote:
Sun Jul 15, 2018 8:23 pm
Do you happen to have the secondary side of that diagram? Just for future reference!
See the text in the post of mine that you quoted! I have not finished it yet.

Mark

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Mon Jul 16, 2018 7:11 am

GadgetUK164 wrote:
Sun Jul 15, 2018 8:23 pm
1024MAK wrote:
Sat Jul 14, 2018 6:30 am
Hence why I reverse engineered the important primary side. I have started the secondary side. It is very similar to the version in the service manual, but also has some differences. Once I have overcome the distractions of real life and other interesting diversions, I will plonk it on here.
Mark
Do you happen to have the secondary side of that diagram? Just for future reference!
I think he's already stated he's working on it GadgetUK164.....

Ed...... :-)
Ed...... :-)

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Re: Need help figuring out how to fix a couple of Beeb PSUs

Post by KarateEd » Mon Jul 16, 2018 7:11 am

Thanks Mark,

Much easier to read.....

Ed...... :-)
Ed...... :-)

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