BBC PSU problems

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
Post Reply
iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

BBC PSU problems

Post by iancr77 » Tue May 14, 2019 10:17 am

Hi,

The PSU of my BBC B (actually an upgraded model A) has recently developed problems:
  • It makes a rapid clicking sound (c. 10 Hz)
  • The multimeter-measured output voltage is c. 1.2V without a load attached
  • The actual output waveform is very spiky (see image... which I can't figure out how to post on the board!)
  • C1, C2 and C9 have all been replaced in the recent past, and appear to be fine (I actually desoldered C9 and checked it with the meter)
  • All other capacitors appear (visually) to be fine
Any pointers would be much appreciated!
Thanks. :)
Last edited by iancr77 on Tue May 14, 2019 10:33 am, edited 2 times in total.

User avatar
1024MAK
Posts: 8904
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: BBC PSU problems

Post by 1024MAK » Tue May 14, 2019 2:07 pm

FAQ - Uploading a photo, picture, image or other file

To attach a photo or other recognised image/picture file, click the attachment tab below the text edit box. Click on the Add files button. Then navigate to the file you want to attach. The details of this bit depends on your device/OS.
After making your selection, the file (if one of the types allowed and recognised by the forum software) details (including file name and initial size) will appear in a list. You can often add one than one item at a time, or add items one after another.

You can also add a comment to the file item. This will be displayed with the file.

By default, files will show up near the bottom of your post. But you can place some item types inline. To do this, position the text cursor in the text edit box where you want the file to appear. Then scroll back to the file list, then click the Place inline button.

Note that large images will be reduced in resolution by the forum software. So if uploading schematics or other similar images where you do not want to loose resolution, either preprocess the image file, or if this is undesirable, put it in a ZIP container file.

Example photo
71BA024F-81B6-4DE2-A657-9F9E3D66A627.jpeg
ZX81 PCB
It is also possible to link to photos, pictures and images on other sites (where that site permits it) by using the image tags (the icon button above the text edit box that looks like a rectangle with a mountain and sun/moon) and adding the url.

Mark
Last edited by 1024MAK on Tue May 21, 2019 3:19 pm, edited 3 times in total.

User avatar
1024MAK
Posts: 8904
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: BBC PSU problems

Post by 1024MAK » Tue May 14, 2019 2:14 pm

iancr77 wrote:
Tue May 14, 2019 10:17 am
The PSU of my BBC B (actually an upgraded model A) has recently developed problems:
  • It makes a rapid clicking sound (c. 10 Hz)
  • The multimeter-measured output voltage is c. 1.2V without a load attached
  • The actual output waveform is very spiky (see image... which I can't figure out how to post on the board!)
  • C1, C2 and C9 have all been replaced in the recent past, and appear to be fine (I actually desoldered C9 and checked it with the meter)
  • All other capacitors appear (visually) to be fine
Hi

I presume that it was working okay before. Did it work after C1, C2 and C9 had been renewed? In other words, was this work recent?

Have you used a good light and a magnifying glass to check for any dry solder joints (dull colour, cracks, distorted, or lumpy solder joints)?
Is C9 fitted with the correct polarity?

After discharging the electrolytic capacitors, using a multimeter on the resistance range, test across each of the outputs to see if there is a short circuit.

Mark

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Tue May 14, 2019 2:19 pm

Thanks, Mark - much appreciated.

Here's the scope trace:
IMG_3131.jpg
1024MAK wrote:
Tue May 14, 2019 2:14 pm
I presume that it was working okay before. Did it work after C1, C2 and C9 had been renewed? In other words, was this work recent?
The caps were replaced within the last year or so, and the PSU was working fine until a day or two ago.
1024MAK wrote:
Tue May 14, 2019 2:14 pm
Have you used a good light and a magnifying glass to check for any dry solder joints (dull colour, cracks, distorted, or lumpy solder joints)?
Is C9 fitted with the correct polarity?
C9 is fitted correctly, but I've not checked the solder yet.
1024MAK wrote:
Tue May 14, 2019 2:14 pm
After discharging the electrolytic capacitors, using a multimeter on the resistance range, test across each of the outputs to see if there is a short circuit.
Will do.

Thanks again!

User avatar
Elminster
Posts: 3556
Joined: Wed Jun 20, 2012 8:09 am
Location: Essex, UK
Contact:

Re: BBC PSU problems

Post by Elminster » Tue May 14, 2019 2:36 pm

The scope says AC, are you using AC coupling on input when measuring the DC output of psu? I.e. are you just showing ripple? Or are you running scope in Digital Volt Metre Mode?

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Tue May 14, 2019 2:49 pm

:oops: ok, turns out I don’t really know how to use the scope. I’ll have a look later today or tomorrow and report back...

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Wed May 15, 2019 10:15 pm

Ok, so here’s a marginally more competent scope screenshot...
img_3133.jpg
Also, I noticed the PSU makes a kind of squeaking/sighing sound when powering off. :-k

User avatar
Elminster
Posts: 3556
Joined: Wed Jun 20, 2012 8:09 am
Location: Essex, UK
Contact:

Re: BBC PSU problems

Post by Elminster » Thu May 16, 2019 8:31 am

Okay so if it now allows DC through and still says 0V with some weird 5-6v spikes every 80ns, that doesn't look good.

I would follow Mark suggestion with a multimeter above to check resistance. Also make sure you scope goes nowhere near the grounded 240V side of the PSU.

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Thu May 16, 2019 9:39 am

Thanks, Elminster. I'll take a look at that.

I'm treating the 240V side of the PSU with extreme caution, don't worry...

It's actually 80ms, not ns, so that matches the 12Hz the scope is measuring, and roughly matches the clicking sound from the PSU.
Last edited by iancr77 on Thu May 16, 2019 9:45 am, edited 1 time in total.

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Thu May 16, 2019 6:47 pm

The resistance between the +5V line and GND is large (Mohms), but the resistance between -5V and GND is small (c. 70 ohms).

So I guess there’s something amiss.

User avatar
Elminster
Posts: 3556
Joined: Wed Jun 20, 2012 8:09 am
Location: Essex, UK
Contact:

Re: BBC PSU problems

Post by Elminster » Thu May 16, 2019 7:04 pm

Weren’t you checking the resistance (after discharging them) across the electrolytic capacitors? The ones that look like barrels and a minus stripe down them?

Also was it just +5v that was gone or have you lost +12v (used by external devices) volts and -5v (Sound) as well?

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Thu May 16, 2019 7:46 pm

ok - I misunderstood the instructions. :oops:

Thanks for your patience - I’ll take a look tomorrow.

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Thu May 16, 2019 8:24 pm

Ok. C6 and C8 both have resistances in the Mohms, and look to be in good condition.

Neither +5V or -5V is working correctly. Not sure about +12V.

All the solder joints on the underside of the PCB look good as well.
Last edited by iancr77 on Thu May 16, 2019 8:27 pm, edited 1 time in total.

User avatar
Elminster
Posts: 3556
Joined: Wed Jun 20, 2012 8:09 am
Location: Essex, UK
Contact:

Re: BBC PSU problems

Post by Elminster » Thu May 16, 2019 8:49 pm

Mark is the power expert, I just use the tools, so hopefully, he will suggest something else.

Might be worth taking some high res photos of top and bottom of PSU unit, and posting them. Maybe someone will spot something odd about the PSU.

User avatar
1024MAK
Posts: 8904
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: BBC PSU problems

Post by 1024MAK » Thu May 16, 2019 10:27 pm

I’m concerned that the peak output voltage is 5.9V.

Do you have an adjustable bench power supply that has an adjustable current limit?

In your last photo showing your ‘scope, can you tell me where the 0V/GND is in relation to the waveform.

Mark
Last edited by 1024MAK on Thu May 16, 2019 10:29 pm, edited 1 time in total.

duikkie
Posts: 2945
Joined: Fri Feb 07, 2014 3:28 pm
Contact:

Re: BBC PSU problems

Post by duikkie » Fri May 17, 2019 5:31 am

somewhere , some time ago i have posted some pictures making a bbc psu. allso how to diagnost a problem . disconnecting the flyback and the second part of the psu . so you can measure the ac sec side, make sure you disconnect the caps on the second side because the ac can get to 50 volt and the caps go to 25 volt ? , there is allso a zener ? that blows if higher than 5 volt. maybe my memory is wrong :)

duikkie
Posts: 2945
Joined: Fri Feb 07, 2014 3:28 pm
Contact:

Re: BBC PSU problems

Post by duikkie » Fri May 17, 2019 5:37 am

i think it is a flyback problem , because there is a spike ???

User avatar
Elminster
Posts: 3556
Joined: Wed Jun 20, 2012 8:09 am
Location: Essex, UK
Contact:

Re: BBC PSU problems

Post by Elminster » Fri May 17, 2019 9:34 am

duikkie wrote:
Fri May 17, 2019 5:37 am
i think it is a flyback problem , because there is a spike ???
The fly back diode going would cause spikes, but surely you wouldn’t be sitting at near 0volts? You would have spiky 5volts. Multiple failures maybe?

Kazzie
Posts: 742
Joined: Sun Oct 15, 2017 7:10 pm
Location: North Wales
Contact:

Re: BBC PSU problems

Post by Kazzie » Fri May 17, 2019 2:36 pm

duikkie wrote:
Fri May 17, 2019 5:31 am
somewhere , some time ago i have posted some pictures making a bbc psu. allso how to diagnost a problem .
Are you thinking of one of these two threads?

viewtopic.php?f=3&t=12385 - how to fix / repair /measure a smps beeb psu
viewtopic.php?f=3&t=11661 - bbc beeb psu by hand
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Sat May 18, 2019 7:44 pm

1024MAK wrote:
Thu May 16, 2019 10:27 pm
I’m concerned that the peak output voltage is 5.9V.

Do you have an adjustable bench power supply that has an adjustable current limit?

In your last photo showing your ‘scope, can you tell me where the 0V/GND is in relation to the waveform.

Mark
Thanks, Mark.

Unfortunately, I don't have a bench PSU. On the scope image, the little arrow on the left-hand edge of the image shows the 0V level.

iancr77
Posts: 13
Joined: Mon Aug 20, 2018 10:16 am
Location: Swansea, UK
Contact:

Re: BBC PSU problems

Post by iancr77 » Sat May 18, 2019 7:46 pm

Kazzie wrote:
Fri May 17, 2019 2:36 pm
duikkie wrote:
Fri May 17, 2019 5:31 am
somewhere , some time ago i have posted some pictures making a bbc psu. allso how to diagnost a problem .
Are you thinking of one of these two threads?

viewtopic.php?f=3&t=12385 - how to fix / repair /measure a smps beeb psu
viewtopic.php?f=3&t=11661 - bbc beeb psu by hand
Thanks for these. Unfortunately, this is really beyond what I feel comfortable with (I've never worked with mains voltage electronics), so I think I will need to pay someone to fix it for me. :( But at least then (hopefully) it will be done and I can move on to other things. :)

User avatar
1024MAK
Posts: 8904
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: BBC PSU problems

Post by 1024MAK » Sun May 19, 2019 4:11 am

In which case, you need Mark at RetroClinic :wink:

Note, other repairers are out there...

Mark

Post Reply