BBC micro black linear PSU repair

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steve3000
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BBC micro black linear PSU repair

Post by steve3000 » Fri Feb 09, 2018 11:23 pm

I'm trying to restore a broken "linear" type PSU from an early beeb. I can't find much about these in the service manual, so other than re-soldering the broken bits, I'm not sure where to start with testing this. Has anyone here tackled one before?

See pictures below - first thing to address are probably the four connections to the transformer which are all disconnected, is there any simple method to test the coils while disconnected? Also need to ensure these are wired back in the right place...but should be simple to compare to photos of others (if I can find some).

Is there anything else I should replace while all this is disconnected? Also, it looks like the aluminium case is also the heat sink for this, so expecting it to get hot - has anyone found good paint to restore these (this one is a bit battered)?
20180209_215822.jpg
20180209_215827.jpg
20180209_215847.jpg
20180209_215907.jpg
Cheers,
Steve

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Re: BBC micro black linear PSU repair

Post by IanS » Fri Feb 09, 2018 11:40 pm

Have you seen this - https://martin-jones.com/2013/08/30/the ... er-supply/

Looks like you need to identify the two output windings, with it disconnected from the mains you should be able to identify which wires are for which winding.

I think you then need to ensure you get them phased correctly, you can probably assume the way the windings come out of the toroid is probably correct. The two wires in the middle will be the common, the outer wires going to the diodes.

Does anyone know if Acorn kept the 3 sections of the later issues main bbc boards isolated? If not then you shouldn't use the linear psu with a later board.

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Re: BBC micro black linear PSU repair

Post by steve3000 » Fri Feb 09, 2018 11:54 pm

Thanks Ian! I actually found that page about 30 seconds before I read your post, so yet to read it, but it's definitely got some useful photos :)

This is a pretty battered PSU tbh, it's suffered from a failed attempt to open and repair it in the past (annoyingly these PSUs were riveted together, no screws, and looks like the last owner has fought a long battle with a couple of the rivets!), so I'm concerned with the amount of battering it's taken to ensure that the windings etc. are all OK. I'd like to test as much as possible before plugging into the mains...

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Re: BBC micro black linear PSU repair

Post by IanS » Sat Feb 10, 2018 12:08 am

It's a very simple circuit, very little to go wrong. Plug it in and see if you get an AC output on each of the output windings before going any further. The rest can be eaily replaced, but the transformer would be harder to source a replacement.

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Re: BBC micro black linear PSU repair

Post by 1024MAK » Sat Feb 10, 2018 1:04 am

Conventional PSU Testing
Suggested steps:
  1. Check the mains plug (including the plug fuse), the cord (cable) and the PSU's T400mA fuse (T meaning Time-delay, also known as slow blow or anti-surge).
  2. Using a multimeter, test on resistance across the switch. Then test the whole mains circuit from the Live plug pin to the Neutral plug pin. This proves all the mains wiring and the primary winding of the transformer.
  3. Leave the wires for the secondary windings disconnected. With the PSU disconnected from the mains, use a multimeter on the resistance range to test the secondary windings. Use a temporary label on each wire as you go. You should have a very low resistance for each winding. And no contact between the two windings.
  4. Being careful not to allow the wires for the secondary windings to touch one another, or short out to anything, use a multimeter on the 20V (or equivalent) AC voltage range (meter leads with insulated croc clips are recommended), power up and test the voltage on each secondary winding. The voltages should be between 9V and 11V.
  5. Power off.
  6. Use test leads with croc clips to temporarily join one wire from each secondary winding together. Select an AC voltage range on your multimeter that can measure 24V. Now connect the meter leads to the remaining wires for the secondary winding. Power up. If the voltage is between 18V and 23V, the joined wires are the mid-point, or "tap". Power off. It is these two that should be connected to the common 0V connection / track on the PCB (but not yet). If the voltage you get is lower than 5V, power off and swap the wires of ONE of the secondary windings around. Then test again. Power off.
  7. Before connecting to the PCB, the diodes on the board should be tested. Switch your multimeter to the diode test range. Now test across each of the four diodes. You should do each test twice, the second test with the meter leads the other way around. If you multimeter shows voltage on this range, each diode should have a forward voltage (in one direction only) of about 0.6V to 0.7V. In the other direction, the meter should indicate over-range (for open circuit). Note some multimeters show a resistance reading - please refer to the instructions for the meter.
  8. Carefully examine all four diodes, all the capacitors, the three 7805 voltage regulators and the 7905 voltage regulator. If you see any damage, or signs of distress, it's best renew the suspect parts.
  9. Switch the meter to it's lowest resistance range. Connect the negative lead to the metal heatsink tab of one of the 7805 voltage regulators. Then connect the positive lead to the positive terminal/lead of the 4700uF capacitor. The resistance should start off low, but should keep rising until either it levels off, or the meter indicates over-range (switch up a range and wait). Now quickly switch to the 20V voltage DC range, depending on the meter, you may just see a value of 1V to 2V decaying down towards zero.
  10. Remove the meter leads from the circuit. Now connect the positive meter lead to the metal heatsink tab of one of the 7805 voltage regulators. Connect the negative lead to the negative lead of the 470uF capacitor. Do the same test as in the above point. The time frame will however be shorter during the test, as the capacitor value is much lower.
  11. Now repeat the above two steps, but connect the meter leads to each of the outputs in turn. There should be no visible low resistance reading, as the output capacitor value is low. The meter should NOT show any low resistance values.
  12. Connect the transformer wires.
  13. Ensure the PSU output wires are disconnected from the motherboard and cannot touch anything that would result in a short circuit.
  14. Power on and use a multimeter on the DC voltage range to test each output in turn (three lots of +5V and one -5V). The voltages should all be between 4.75V and 5.25V (depending on the specifications of the voltage regulator chips, most produce more accurate voltages than this).
  15. If you have a 6V light bulb rated between 2.5W and 5W, you can use this as a test load for the three +5V outputs (test each output one at a time). As well as testing the DC voltage, also test on the AC voltage range. The AC voltage on the +5V output under load should be less than 100mV. Note, some of the less expensive multimeters do not display an accurate AC voltage reading when a DC voltage is also present. So if you get a result that is greater than a few volts, it does not necessarily mean there is a problem .
  16. If everything is okay up to this point, connect to a BBC Micro motherboard. Note that the later motherboard issues have ALL three +5V power connection points connected together via the PCB tracks. This design of PSU is NOT suitable for these motherboards.
Good luck ;-)

Mark

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Re: BBC micro black linear PSU repair

Post by 1024MAK » Sat Feb 10, 2018 1:28 am

IanS wrote:Does anyone know if Acorn kept the 3 sections of the later issues main bbc boards isolated? If not then you shouldn't use the linear psu with a later board.
At some stage, on one of the later issue boards, Acorn connected all three +5V power points together via the PCB tracks. This applies to the issue 7 board, but I'm not sure if it also applies to the issue 3 and issue 4 boards.

Mark

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Re: BBC micro black linear PSU repair

Post by 1024MAK » Sat Feb 10, 2018 1:46 am

Steve, the pictures, text and schematic in Martin Jones's article are consistent with there being TWO 4700uF electrolytic capacitors. It looks like there is one on the underside (track) side of the PCB as well as one on the top of the board. But your board does not have a 4700uF electrolytic capacitor on the track side.

It also looks like the larger solder connections (for the transformer wires etc) may have suffered from dry solder joints. I suggest you use a solvent cleaner (IPA) to remove all the old brown flux residue from the PCB. Then resolder any suspect looking solder joints.

Mark

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Re: BBC micro black linear PSU repair

Post by steve3000 » Sat Feb 10, 2018 4:17 am

Wow, thanks for the detailed write up Mark! This is exactly what I need :)

This came from an issue 2 beeb, and will be returning to it once repaired. I'm fairly sure some early issue 3's also had the linear type psu, but I don't recall ever seeing an issue 4 with one. I expect many of these linear PSUs were replaced with the more usual ASTEC switched mode PSU during the early 80's (or was the linear PSU ever a recall item?) - certainly some user manuals for disc drives/etc. mention the need to replace the PSU, if the aux. power connector isn't present (eg. to power the drive).
1024MAK wrote:Steve, the pictures, text and schematic in Martin Jones's article are consistent with there being TWO 4700uF electrolytic capacitors. It looks like there is one on the underside (track) side of the PCB as well as one on the top of the board. But your board does not have a 4700uF electrolytic capacitor on the track side.
Well spotted :shock: =D>

Thinking about it, this actually makes sense as the previous owner had mentioned he thought he'd lost a capacitor...however my inspection of the only other picture I had of the inside of one of these PSUs just showed the top side of the PCB, which looks fine. Now taking a closer look at Martin Jone's page, you can just make out what could be the lead to a capacitor on the underside. I'll order that cap now!

And yes, I was thinking the dry solder joints are likely the cause of this PSU failing, apparently these run very hot. I'll clean up with IPA and resolder all. :)

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Re: BBC micro black linear PSU repair

Post by daveejhitchins » Sat Feb 10, 2018 7:41 am

Has anyone mentioned changing all the Tantalums?

Dave H :D

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Re: BBC micro black linear PSU repair

Post by hoglet » Sat Feb 10, 2018 7:45 am

FYI, there are several different designs of linear power supplies for the Beeb. See this thread for a later one:
viewtopic.php?f=3&t=9536

Dave

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Re: BBC micro black linear PSU repair

Post by 1024MAK » Sat Feb 10, 2018 11:56 am

daveejhitchins wrote:Has anyone mentioned changing all the Tantalums?

Dave H :D
Martin talks about the Tantalums in his article. It is also mentioned in the linked to thread.

If during testing, problems are found, then yes, it may well be worthwhile changing them.

Mark

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Re: BBC micro black linear PSU repair

Post by daveejhitchins » Sat Feb 10, 2018 12:59 pm

1024MAK wrote:If during testing, problems are found, then yes, it may well be worthwhile changing them.
=; [-( Change them - and save heartache later on \:D/

Dave H :D

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Re: BBC micro black linear PSU repair

Post by CMcDougall » Sun Feb 11, 2018 2:56 pm

1024MAK wrote:some stage, on one of the later issue boards, Acorn connected all three +5V power points together via the PCB tracks. This applies to the issue 7 board, but I'm not sure if it also applies to the issue 3 and issue 4 boards]
correct, the Issue3 has them all separate, the Iss4 are all together like iss7 8)
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Re: BBC micro black linear PSU repair

Post by steve3000 » Sat Apr 18, 2020 11:14 am

Repair thread resurrection! Back in Feb 2018, when I was about to embark on this linear PSU repair, I became distracted by a fault with my main beeb and focused on sorting that out, then using the beeb :)...and this PSU was tidied up into storage before I tackled Mark's excellent testing procedure above.

However, last week I came across this thread again and decided it was really time to get this PSU up and running. So I've spent an evening working examining the PSU and working through the test procedures, with the following results:

1. all look good
2. measured open circuit with switch off across live to neutral, and ~46 ohm with switch on (mains winding measured ~45 ohm)
3. measured <1 ohm for each secondary winding
4. 9.66v AC for each winding
5. ok
6. 19.46v AC measured when bridging the two wires emerging from the centre of the coils in photo, but only ~2v AC when one centre wire was bridged to an outer wire.
7. both white diodes read 0.59v, both clear/orange diodes read 0.64v
8. everything looks good, interestingly the 7905 heatsink tab appears to be insulated from the case with a clear plastic film, and there's a heavily damaged green plastic insert where the securing screw goes (see photo) - I assume this will need replacing to maintain the isolation of this?
9. worked as described, with ~1.8v decaying to zero (I also tested capacitance of the 4700uF capacitor, in place, using the multimeter and it read 4500uF; and once the second 4700uF capacitor was added underneath, this increased to 9430uF)
10. also worked as described, with fast decay to zero (with the multimeter, the 470uF capacitor in place, reads 570uF)
11. wasn't quite sure what was meant to do in this step, but I measured resistance from the +5v to 0v lead for each output, and none were low
12. stopped here for the moment, after I noticed that the sleeving on the transformer wires was melted... see photo

Photos:
20200417_225705.jpg
7905 damaged insulation
20200417_215618.jpg
transformer wires damaged/melted sleeving
Looking back at previous photos I'd taken, it is clear the transformer wire sleeving was melted before I received the PSU, so this probably happened when the PSU was last in use within a beeb. I'm slightly worried there may be something seriously wrong with the PSU circuit board if so much heat built up to melt the sleeving like this, however the tests above seem OK... Should I be concerned?

Next steps: (1) sort out the insulation on the 7905, (2) fit new heat shrink insulation over the transformer wires, then (3) continue with step 12.

I have also obtained a 4700uF capacitor to restore the missing capacitor from the underside of the PSU circuit, which I'll do while awaiting new insulation for the 7905.

Edited 25/4/20 to correct resistance measurements for (2) and (3), also to add second capacitor reading for (9).
Last edited by steve3000 on Sat Apr 25, 2020 1:20 am, edited 1 time in total.

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Re: BBC micro black linear PSU repair

Post by IanS » Sat Apr 18, 2020 11:34 am

steve3000 wrote:
Sat Apr 18, 2020 11:14 am
8. everything looks good, interestingly the 7905 heatsink tab appears to be insulated from the case with a clear plastic film, and there's a heavily damaged green plastic insert where the securing screw goes (see photo) - I assume this will need replacing to maintain the isolation of this?
The metal tab on the 7905 is not connected to GND, so needs insulating from the metalwork.
The plastic sheet is probably a mica washer. Something like this - https://www.bitsbox.co.uk/index.php?mai ... ts_id=2418

The Transformer sleeving probably melted when they were soldering and desoldered. I wouldn't be worried about this.

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Re: BBC micro black linear PSU repair

Post by steve3000 » Sat Apr 18, 2020 12:34 pm

IanS wrote:
Sat Apr 18, 2020 11:34 am
The metal tab on the 7905 is not connected to GND, so needs insulating from the metalwork.
The plastic sheet is probably a mica washer. Something like this - https://www.bitsbox.co.uk/index.php?mai ... ts_id=2418

The Transformer sleeving probably melted when they were soldering and desoldered. I wouldn't be worried about this.
Thanks for the link Ian, yes that looks like it.

As for the melted wiring, looking at the PCB soldering, I don't these were desoldered or at least, not very well - looks like they have suffered dry solder joints and been pulled through, but the melting may have occurred when the previous owner tried to fix it... I'll fit some heat shrink insulation and clean up PCB, hopefully that will help.

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Re: BBC micro black linear PSU repair

Post by 1024MAK » Sat Apr 18, 2020 3:11 pm

Step 2 was supposed to be test on resistance across the switch with it off (very high resistance / open circuit) and with the switch in the on position (low resistance, less than 2 ohms).

Including continuity / resistance test the wiring to check for breaks / open circuit faults. You should also test the resistance of the primary (mains side) windings of the transformer.

Step 3, 15 ohm is rather higher than I would have expected for the secondary windings. I would expected less than 5 ohms. But don’t have that type to test.

Step 8. Yes the negative voltage regulators in the 79xx series have a different pin out, the heatsink tab is NOT connected to the common terminal. Hence an insulating kit is needed. Look for TO220 insulation kits or if it only the bush that is damaged, TO220 insulation bushes.

When fitting the voltage regulators back on the heatsink, clean the surfaces which fit together (heatsink and regulator tabs). Then apply a thin smear of heatsink compound / silicone heatsink grease to the metal tabs of the voltage regulators. It is less important on the 7905, as this produces less heat, but with this, a thin smear goes on the metal tab and on the heatsink where the insulation sheet fits (assuming you are using the existing mica insulation).

The insulation on the transformer leads is likely made of flexible PVC and is likely to have melted when the wires were soldered and / or desoldered. This is a common problem with this type of sleeving. Cut off the damaged section and use heat shrink sleeving instead. But DON’T use hot air anywhere near any of the existing sleeving. Myself, I would not even try to shrink down the heat shrink sleeving.

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Re: BBC micro black linear PSU repair

Post by steve3000 » Sat Apr 25, 2020 1:25 am

1024MAK wrote:
Sat Apr 18, 2020 3:11 pm
Step 2 was supposed to be test on resistance across the switch with it off (very high resistance / open circuit) and with the switch in the on position (low resistance, less than 2 ohms).

Including continuity / resistance test the wiring to check for breaks / open circuit faults. You should also test the resistance of the primary (mains side) windings of the transformer.

Step 3, 15 ohm is rather higher than I would have expected for the secondary windings. I would expected less than 5 ohms. But don’t have that type to test.
Thanks Mark - based on your concern about the resistance at step 3, I checked my multimeter and discovered one of its probes was at fault! So I have replaced the probe and repeated the tests above, editing the post where values have changed (secondary windings show <1 ohm now :) ) and tested the mains winding (~45 ohms).

I've also obtained a new TO220 insulation bush today so will wire it all up tomorrow night and test... fingers crossed!

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Re: BBC micro black linear PSU repair

Post by steve3000 » Sat Apr 25, 2020 2:49 pm

PSU powered up just now, but test results not quite as expected :(

The three separate +5v outputs (short, medium and long leads) and the -5v lead, no-load voltages measured as follows:

Long: 5.15v
Medium: 5.18v
Short: 2.11v

-5v (using short ground lead): -5.10v

Something isn't right with the voltage at the short pair of leads.

Also interesting, for the long, medium and -5v leads, when switching off the PSU, the voltage holds steady for a couple of seconds, then drops rapidly to 0v. However, for the short pair, when you switch off PSU, the voltage holds briefly at 2.11v, then jumps to 5.08v for a couple of seconds, before decaying rapidly to about 0.6v, then it drops much more slowly to 0v.

Any thoughts? Do these symptoms suggest a failed 7805, or could it be something else (failed tant capacitor?)?
20200425_142453.jpg
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Re: BBC micro black linear PSU repair

Post by 1024MAK » Sun Apr 26, 2020 7:45 am

Checks / tests...

-> Check the soldering on the relevant 7805.
-> Reflow the joints.
-> Renew the 7805

For load testing of the +5V outputs, you can also use the simple USB load testers (the cheap and cheerful type based on two large power resistors that are often green in colour). Just use the 1A setting.

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Re: BBC micro black linear PSU repair

Post by steve3000 » Sun Apr 26, 2020 10:48 pm

Brief update - I've reflowed the joints for the suspect 7805, with no change in behaviour. So new one ordered...

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Re: BBC micro black linear PSU repair

Post by steve3000 » Sat May 02, 2020 1:08 am

And success!

I replaced the suspect 7805 (which was the easiest of the three to get at, on the edge of the PCB), and the short cable now delivers a nice 5.01v, with the medium and long cables still on 5.18v and 5.15v respectively.

I have yet to test under load, but are the slightly higher values for the other two 7805's any cause for concern?

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Re: BBC micro black linear PSU repair

Post by Kazzie » Sat May 02, 2020 9:28 am

steve3000 wrote:
Sat May 02, 2020 1:08 am
And success!

I replaced the suspect 7805 (which was the easiest of the three to get at, on the edge of the PCB), and the short cable now delivers a nice 5.01v, with the medium and long cables still on 5.18v and 5.15v respectively.

I have yet to test under load, but are the slightly higher values for the other two 7805's any cause for concern?
The voltages will typically be slightly lower when the machine is running, so a higher unloaded voltage is perfectly normal. As long as you're within 5% of the 5V (4.75V-5.25V), you're within spec for the Beeb.
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Re: BBC micro black linear PSU repair

Post by 1024MAK » Sun May 03, 2020 11:03 am

Yeah, 5.15V and 5.18V are well within the tolerance range. Older 78 series voltage regulators typically have a poorer tolerance range compared to current production parts. Modern parts are available with a closer tolerance range.

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Re: BBC micro black linear PSU repair

Post by steve3000 » Wed May 06, 2020 12:35 am

CMcDougall wrote:
Sun Feb 11, 2018 2:56 pm
1024MAK wrote:some stage, on one of the later issue boards, Acorn connected all three +5V power points together via the PCB tracks. This applies to the issue 7 board, but I'm not sure if it also applies to the issue 3 and issue 4 boards]
correct, the Issue3 has them all separate, the Iss4 are all together like iss7 8)
Interesting to an issue 7 beeb PCB wired up to the same linear PSU as this, on ebay: https://www.ebay.co.uk/itm/324156866856

Presumably it worked at some point, but not recommended?

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Re: BBC micro black linear PSU repair

Post by johnkenyon » Wed May 06, 2020 7:25 am

Perhaps the iss7 boards used a single rail PSU - a single 7805 with PNP power transistor to give a single 5V 3A power rail with multiple outputs to avoid high currents via the PCB traces.

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Re: BBC micro black linear PSU repair

Post by 1024MAK » Wed May 06, 2020 9:44 am

Interestingly, there were (at least) two different version power supplies used in BBC Micro Computers before Acorn changed to using the ASTEC switch mode design.

One used three separate 7805 voltage regulator chips to provide three separate independent +5V/VCC outputs.

The other used a simple three transistor and Zener diode stabilisation circuit to provide a single 5V/VCC output (still via three sets of output leads).

See these topics for details:

Nice write-up (with pictures) on Beeb's Linear PSU

Model A Linear PSU

It’s not wise to use the type of PSU that uses three separate 7805 voltage regulator chips on an issue 4 or issue 7 board. As parallel connected 7805 voltage regulators don’t get on well. They don’t share the current equally, instead one will try to supply all or most of the power and get very hot (although there is a fix for this).

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Re: BBC micro black linear PSU repair

Post by steve3000 » Thu May 07, 2020 12:43 am

Issue 2 model A finally working with it's original PSU :)
IMG_0785.jpg
Running, with caps lock LED on!
I bought this as a repair project, so have never had the PSU correctly fitted in place since I received the computer (back in 2018!), but now it's all fitted properly, I'm a little shocked at just how bad the fitment is. See below - the PSU clearly wasn't designed to fit the Beeb case at the back, and the use of chunky (1.5mm) spacer washers underneath is a real quality bodge [-X
IMG_0786.jpg
Not quite designed to fit...

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