Weird 128k (toastrack) PSU modification

Geezerdiamond · November 2016

Hi all.

I've just picked up a bunch of machines from ebay and the 128 PSU has this weird modification:

Photo%2008-11-2016%2C%2012%2032%2020.jpg?dl=0

Photo%2008-11-2016%2C%2012%2032%2030.jpg?dl=0

The component in the cage as 4 wires into it from inside the PSU, 2 red, 1 black and 1 yellow.

Has anyone ever seen anything like this?!

Geezerdiamond · November 2016

Hmmm, pictures:
https://www.dropbox.com/s/hetr2zlp17lmm6e/Photo 08-11-2016, 12 32 20.jpg?dl=0
https://www.dropbox.com/s/il4l3bhk2minuc9/Photo 08-11-2016, 12 32 30.jpg?dl=0

GReW · November 2016

My initial thoughts would be that somebody's added an external fan, but it's difficult to tell without seeing the 'component' inside. Those bolts are just crying out to be removed (once the power's been turned off and the capacitors have discharged of course ;) )

Geezerdiamond · November 2016

Aye! I'm going to (carefully) pull it apart later on and see what the heck is in there.

Geezerdiamond · November 2016

I did plug it in and nothing lit up, sparked or made a noise so god knows what it is.

harriusherbartio · November 2016

Maybe someone in the past thought that an additional external heatsink was a good idea . Maybe they thought that being out in the open would cool it down better or something like that. Does it do any harm?

Geezerdiamond · November 2016

Right, I've opened it up and whatever the component is, it's bonded and screwed to the metal part. There's no distinguishing marks to tell what it is.

Here are some pictures of it open:
https://www.dropbox.com/s/i5s1zgna3bf8kss/Photo 08-11-2016, 18 05 27.jpg?dl=0

https://www.dropbox.com/s/23j9x78lhaxp2bo/Photo 08-11-2016, 18 04 46.jpg?dl=0

https://www.dropbox.com/s/jo1uuseapcalknc/Photo 08-11-2016, 18 05 04.jpg?dl=0

The whole thing is definitely not a heatsink, I was wondering if it was a voltage conditioning gizmo???

Geezerdiamond · November 2016

Hmmm, looks like there are markings wedged down one side. This is the best shot I can get of them:
https://www.dropbox.com/s/mowii7z1fbkgxy3/Photo 08-11-2016, 18 22 00.jpg?dl=0

Any ideas?

Geezerdiamond · November 2016

I can't make out a partial part number of KBPC35xxx which looks like it's a rectifier of some kind. I guess they were trying to make the voltage more stable!?

Will Woodvine · November 2016

It's a bridge rectifier bolted to a heatsink and implemented in a very poor fashion. If you don't know how to have it "put right" yourself, either get it repaired or buy one of the new ones off of eBay. Don't use it in that state.

The rectifier may have been used to replace the diodes in the PSU that would have rectified the AC from the transformer to DC, which should then be smoothed by a capacitor and fed to the Spectrum.

ingo · November 2016

It is a simple diode bridge I think. It substitutes the 4 diodes that are normally inside the power supply. There is no reason to cool it as done at your item.
If I would own it I would remove this part and instead use 4 items of 1N5822 Schottky diodes soldered inside the power supply. These diodes have best properties for this kind of power suplies. Also I would use a new electrolytic capacitor with 6800 µF or 10000µF and 25V (minimum).

Greetings Ingo.

Geezerdiamond · November 2016

Yeah, I don't much fancy plugging it in as it stands! I wonder what they were thinking?

1024MAK · November 2016

As said above, a previous owner has removed the rectifier diodes from the PCB inside the unit and instead wired a bridge rectifier to the PCB, but mounted it on a heatsink. The bridge rectifier can replace four rectifier diodes.

Maybe they were running lots of expansions. Or maybe it was just a bodge.

It's hard to say if it safe to use without seeing more.

Mark

Weird 128k (toastrack) PSU modification

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