Different BBC Model B PSU?

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map856
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Different BBC Model B PSU?

Post by map856 » Sun Jul 14, 2019 7:09 am

I recently acquired a BBC Model B with Issue 7 board from an eBay seller. One of the first tasks I set for myself was to recap the PSU so had the usual C1, C2 X2 and C9 220uF electrolytic capacitors ready to replace.

However, when I opened the power supply I found a totally different physical circuit board layout to the usual familiar one.

C1 and C2 are no longer the usual RIFA types, their replacements having a rectangular white casing. C2 seems to have been replaced by two .22nF capacitors wired in series!

In fact, all three X2 caps looked in such good condition, no sign of stress at all, that I ended up leaving them as is. However I did replace the 220uF electrolytic for peace of mind.

Anyway, I hope the pictures are clear enough. I'd be interested to hear whether anyone knows anything about this alternative PSU design.

https://photos.app.goo.gl/xoVZpXM9qTr9Xn338
https://photos.app.goo.gl/iTEYSyrku7aUwEGA8
https://photos.app.goo.gl/Ftux51dSXNeyemddA
https://photos.app.goo.gl/uBupcLXmoSYgzCwD9

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danielj
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Re: Different BBC Model B PSU?

Post by danielj » Sun Jul 14, 2019 7:16 am

Check the PCB - it'll probably be a Wong's PSU - they don't have the dodgy RIFAs :)

You might still want to change out the startup cap (should still be C9 I think?) to prevent future problems.

d.

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sweh
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Re: Different BBC Model B PSU?

Post by sweh » Sun Jul 14, 2019 3:50 pm

danielj wrote:
Sun Jul 14, 2019 7:16 am
Check the PCB - it'll probably be a Wong's PSU
Yeah, you can just see that on the second picture.
Rgds
Stephen

map856
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Re: Different BBC Model B PSU?

Post by map856 » Sun Jul 14, 2019 10:17 pm

Yes, it's definitely a Wong. So who made the more common PSUs?
danielj wrote:
Sun Jul 14, 2019 7:16 am

You might still want to change out the startup cap (should still be C9 I think?) to prevent future problems.

d.
I did change the startup cap but on this board it's actually labeled C20. The X2 caps are also numbered differently - they're no longer C1 and C2. I can't remember exactly what they're labeled as now - I should have taken note.

Martin

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1024MAK
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Re: Different BBC Model B PSU?

Post by 1024MAK » Mon Jul 15, 2019 2:46 am

The more usual Beeb PSU is made by ASTEC. They made a lot of switch mode PSUs. They also made the TV UHF modulators.

Mark

map856
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Re: Different BBC Model B PSU?

Post by map856 » Mon Jul 15, 2019 10:34 pm

Thanks Mark. That's interesting and good to know.

Martin

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