A lot of these are starting to appear as people clear their lofts out. Usual issues, people just plug them in, look mighty proud for the first five minutes as they relive their youth, and then make a lunge for the power switch as clouds of acrid smoke escape via the air vents.
The example in question came from a 1983 issue 4 board. Had been plugged in and had gone whuff as expected. Removal of the SMPS from the case was a bit fraught due to shoehorning into the available space but not impossible. This model was a Hong Kong sourced part but it's interesting to note that later units were supplied by Astec although the insides are the same, early badged engineering no doubt, earlier BBCs were supplied with transformers and standard regulators but heat was an issue, hence the change to a basic SMPS model.
Inside, three capacitors to change. The PCB needs to be removed from the frame which means several earth points, the power switch and aux plug need freeing. C2 is a 0.1uf X class capacitor that forms part of the power input filtering and as expected this one had split it's sides and smoked rather than burst into flames, as an ordinary capacitor would. I usually where possible go for an extra third rating on power, using a 400v rated part rather than a 250v part for example but in this case space was at a premium so a 275v rated X2 part fitted the available space. While your in there C1, 0.01uf, is of the same type as C2 and is often prone to going bang as well, so change that one. Finally, C9, 220uf 10v, which is part of the SMPS start up circuit goes dry with age as it sits right next to the BU208 chopper transistor and causes start up issues so change that with a high temp rated replacement. I used a 220uf 16v replacement but it still looked lost in there compared to the capacitor that was taken out.
Final job, plug in and soak test. Voltages are +5v on the red and black leads, -5v on the purple and black leads. Re-assemble is pretty easy but a bit of a squeeze getting some small nuts back into place.
If your servicing the PSU from a BBC Master, same component change but slightly different orientation on the PCB.
Photos, the offending PSU, PCB board removed, one duff X rated cap, all parts back in and re-assembled.
The example in question came from a 1983 issue 4 board. Had been plugged in and had gone whuff as expected. Removal of the SMPS from the case was a bit fraught due to shoehorning into the available space but not impossible. This model was a Hong Kong sourced part but it's interesting to note that later units were supplied by Astec although the insides are the same, early badged engineering no doubt, earlier BBCs were supplied with transformers and standard regulators but heat was an issue, hence the change to a basic SMPS model.
Inside, three capacitors to change. The PCB needs to be removed from the frame which means several earth points, the power switch and aux plug need freeing. C2 is a 0.1uf X class capacitor that forms part of the power input filtering and as expected this one had split it's sides and smoked rather than burst into flames, as an ordinary capacitor would. I usually where possible go for an extra third rating on power, using a 400v rated part rather than a 250v part for example but in this case space was at a premium so a 275v rated X2 part fitted the available space. While your in there C1, 0.01uf, is of the same type as C2 and is often prone to going bang as well, so change that one. Finally, C9, 220uf 10v, which is part of the SMPS start up circuit goes dry with age as it sits right next to the BU208 chopper transistor and causes start up issues so change that with a high temp rated replacement. I used a 220uf 16v replacement but it still looked lost in there compared to the capacitor that was taken out.
Final job, plug in and soak test. Voltages are +5v on the red and black leads, -5v on the purple and black leads. Re-assemble is pretty easy but a bit of a squeeze getting some small nuts back into place.
If your servicing the PSU from a BBC Master, same component change but slightly different orientation on the PCB.
Photos, the offending PSU, PCB board removed, one duff X rated cap, all parts back in and re-assembled.
