Microbee Disk System power supplies question

CheshireNoir · 06-07-2023, 11:27 PM

Hey folks,
While I wait to get to the shops, I've been working on the Microbee Disk System drives I purchased.
On the Protek Power Supply, C21 seems to be shorted on the underside of all three of my drives, including the working one. Am I correct in assuming this is a "factory mod"? If so, I assume it's OK just to install a wire jumper in its place?

Cheers!

John

Ernest · 07-07-2023, 09:32 AM

Hi John,
Been a very long time since working on one of those power supplies, but I do recall the bypassed capacitor.
From memory, this might modification Grounded the Zero voltage output to chassis ground rather than have it floating.
This made it more prone to RFI emissions but safer. The Microbee was a bad one for a lot of RFI emissions. Many a radio Amateur of the day will vouch.

Ernest

CheshireNoir · 07-07-2023, 11:37 AM

Thanks Ernest,
I had to remove the capacitor on one of the PSUs (RIFA Madness) so that one I'll replace with a link. For the others, I'll just leave the link underneath in place.
Hopefully I can get another Disk System up and running tonight :-)

John

CheshireNoir · 20-07-2023, 06:12 PM

Another question: One of my 3 PSUs is a bit unhappy. I have a MeanWell PSU with +5v and +12v on hand. Does anyone here know what the various rails were rated for on the original? I might be able to do a PSU Swap if it's not too high.
Googling "Microbee Disk System PSU" really doesn't find many helpful links :-)

Cheers!

John

RedskullDC · 20-07-2023, 11:29 PM

(20-07-2023, 06:12 PM)CheshireNoir Wrote: Another question: One of my 3 PSUs is a bit unhappy. I have a MeanWell PSU with +5v and +12v on hand. Does anyone here know what the various rails were rated for on the original? I might be able to do a PSU Swap if it's not too high.
Googling "Microbee Disk System PSU" really doesn't find many helpful links :-)

Cheers!

John

Hi John,

Power Supply is an ASTEC AA12100, as found in many units of the early 80's vintage.

Osborne #1 service manual actually has a schematic on page#265:
http://www.bitsavers.org/pdf/osborne/2F0...n_1983.pdf

Ratings (on page #264) are:
5V @ 2A
12V @ 1A
-12@0.1A

Be careful if you are making up your own drive power cables that you don't short -12V to GND.

Cheers,
Leslie

Ernest · 21-07-2023, 12:17 AM

(20-07-2023, 06:12 PM)CheshireNoir Wrote: Another question: One of my 3 PSUs is a bit unhappy. I have a MeanWell PSU with +5v and +12v on hand. Does anyone here know what the various rails were rated for on the original? I might be able to do a PSU Swap if it's not too high.
Googling "Microbee Disk System PSU" really doesn't find many helpful links :-)

Cheers!

John

Hi John,
That MeanWell should do nicely, on that one you can even wind down the output to 10.8 Volts, so by the time it runs through the diode bridge, it will arive at your main console about 9.6 Volts, right around the sweetspot.

Ernest

CheshireNoir · 21-07-2023, 01:25 PM

Thanks RedSkullDC and Ernest for your speedy AND helpful replies!
(The forums here have got me out of trouble so much!)

RedskullDC, The -12V isn't used at all in the disk units so I'll just ignore that one for now. I only need +12v and +5v. The ratings you gave me are dead on the information I needed. Nice to have the schematic for the other two that are still working though :-)

Ernest, I'm using Altronics purchased buck converters to drop the voltage on the 12v rail down to 10v for the external Microbee supply. They're nice little units, and the 5¼" drives sometime require 12V so I don't want to mess with that too much.

Cheers!

John

Ernest · 21-07-2023, 03:25 PM

Hi John,
Great your moving forward. I’m guessing these units came from JB’s collection up the hill??

100% correct on the 5.25” needing the 12V, they usually use the 12v for the motor drive.
I have yet to come across a 5.25” that didn’t need the 12v. For some reason my brain was thinking 3.5” / GOTEK.

Ernest

CheshireNoir · 21-07-2023, 05:10 PM

Cheers Ernest,
These actually came from a local collector in Malaga, who had purchased them to use with Sinclair Spectrums as dedicated external drives. (Don't ask me how that worked. Must have been a third party Shugart interface. I am sure they existed). He was cleaning out his storage and offered three of them to me for a great price.
My current plan is to add in a Gotek to replace the primary drive and have the 5¼" as the secondary drive, so you can read old data disks or write new disks from it. I've got a nice Gotek frame 3D printed to fit the 5¼" bay using a full sized LCD screen and a rotary dial.
One of the power supplies was fine out of the box. Two needed RIFAs replaced, and that's now complete. Unfortunately one is still showing out of spec so that's the one I'm contemplating replacing the entire PSU with the more modern Mean Well unit.
I'll need some more bits and pieces to get all three finished off. I've actually run out of Gotek drives so I'll need to order more from my usual supplier. I'll also need another two LCD panels, which I'll source locally most likely.
All in all it's been a good little project to work on.

John