Osborne I power supply

[NutmegCT]

Good day all.

My 1983 Osborne I finally decided to retire. Powered it up today, heard a "sizzling" sound and saw a bit of smoke come up through the top vent.

Pulled the plug, then tried a re-start.

Screen comes on, but 9 times out of 10 with garbage text. One time out of ten you can make out a "Insert disk in Drive A" on the screen.

Both drives are spinning. Insert system disk in A, close latch, hit Return, nothing more happens. No read light, etc. Reset button has no effect 9 times out of 10.

Prior to today, the Osborne worked fine. Booted system, ran WordStar, etc.

Transformer on the power board looks slightly blackened and smells very "hot".

Does anyone have a working power supply board to sell?

Thanks.
Tom in Connecticut

 

[Chuck(G)]

Before you toss the old PSU, I'd suggest replacing the electrolytic capacitors on it. Very often these old ones will short, producing the symtoms you describe.

 

[NutmegCT]

Chuck - thanks for the quick reply. Would a bad capacitor give the "strange screen", weird drive action, lack of power-on "beep", etc.?

Tom

 

[Lorne]

Does anyone have a working power supply board to sell?

Try what Chcuk suggested, and see if that works.

There's also the Sams Computerfacts that I scanned, and Erik has posted at:
http://www.vintage-computer.com/vcforum/showthread.php?t=16552

If none of that works, I have a "parts" Osborne but I'm not sure yet what I need, and what I don't.
I do need a couple weeks of slack time to be able to figure that out.

You can PM me later about the power supply if those things don't help or work out.

 

[cosam]

The sizzling sound and especially the smoke would certainly suggest a shorted capacitor. With any luck you'll be able to see which one blew and can replace it. Chances are the rest aren't far behind though, so replacing the lot is a good idea.

Would a bad capacitor give the "strange screen", weird drive action, lack of power-on "beep", etc.?

A bad power supply can definitely cause all kinds of wacky behaviour, the above included.

 

[Chuck(G)]

What they said...

 

[phogren]

While you are looking at your parts Osborne, I need the little black plug that goes over the external video port on an Osborne 1. I found a so-so One that didn't have that one part. There's no rush because I need to replace the capacitors and who knows what else in the power supply. To add to the thread I had a Kaypro with a similar problem and when I pulled the power supply board out the faulty capacitor literally fell out. Made finding the faulty one real easy.

 

[NutmegCT]

Thanks for the quick suggestions, gents. And that link to the SAMS schematics is a god-send.

I'll see if I can figure out how to remove those capacitors for testing this weekend.

Thanks all.
Tom

 

[tezza]

Yes, I concur,

My Osborne PSU blew two filter capacitors...a bang..smoke..smell the works. Easy to fix. However it also had a RAM problem which gave a garbage screen. Also not difficult to fix.

If capacitors are blown on the power supply (highly likely) some AC ripple might be coming through. That might throw the machine into "garbage screen" mode at times.

Terry

 

[NutmegCT]

Well now that was quick. I looked at the PSU and by golly, the same cap as yours has had a "melt down". Looks exactly the same as yours.

Tezza - you mentioned that two had blown. Other than the one in the picture, which other burned also?

Thanks for the pointer!
Tom

Yes, I concur,

My Osborne PSU blew two filter capacitors...a bang..smoke..smell the works. Easy to fix. However it also had a RAM problem which gave a garbage screen. Also not difficult to fix.

If capacitors are blown on the power supply (highly likely) some AC ripple might be coming through. That might throw the machine into "garbage screen" mode at times.

Terry

 

[tezza]

Hi Tom,

I'm glad my repair blog comes in useful to folk occasionally.

The other one was right next to the fuse (see attached pic)

Tez

 

[NutmegCT]

Tezza - thanks for the update.

Here's a question for anyone. When I pulled the PSU I found what sure looks to be an unused connector. See the attached. It's in the center, a white plastic female plug with an orange tag labeled "115 vlts" on its single black wire. That black wire pops up from a spot on the board, just inside the big power transistor heat sink. Far as I can tell, there are no unused male connectors on that board, and the short wire on the plug in question couldn't reach anywhere else.

I run the Osborne on 115v current, so I wonder if that plug was "extra", in case the system was to run on 230v?

Anyone have any advice? Leave it unused? I'm going to replace that burned out capacitor (see it on the bottom left), but would like to find out what that "unused" wire is for, or whether it plugs into somewhere I'm just not seeing.

Thanks all.
Tom
Edit: on the SAMS charts, that mystery wire/plug isn't even shown in the photo of the PSU.

 

[NutmegCT]

Update: put in a new 0.1 mF capacitor (metalized-film, RadioShack # 272-1053), re-assembled the Osborne, powered up, and saw the "normal" boot sequence at last! Altho' still no "beep" at power up.

But the screen is still just a single graphic character repeated all over the screen. A: drive spins and light comes on, no screen change. Reboot - same thing but there's a different graphic character on the screen.

This *is* progress, as the boot sequence wouldn't even occur prior to the capacitor replacement.

Any thoughts? You probably already know that I'd really prefer *not* to de-solder every single capacitor on that PSU to try to test it. But none of the others (electrolytic, ceramic, plastic) appear damaged. The one I replaced was *very* damaged - cracked, bulged out, etc.

Thanks.
Tom

 

[tezza]

Tom,

I'm pretty sure the same mystery connector is present on my PSU too. I remember being puzzelled by it as you are.

Tez

 

[NutmegCT]

Tez - I just posted an update at the same moment you posted.

Tom

 

[Chuck(G)]

I'd start by checking all of the output voltages of the power supply. If those check out, the next thing would be to suspect sockets--resat the socketed chips.

In case it wasn't mentioned, there's a service manual here.

 

[nige the hippy]

The flying (115V) lead is designed to connect one of the AC inputs to the centre of the capacitors, as a nifty voltage doubler. it may well be that the board has the connection hard soldered somewhere.
Those RIFA filter capacitors ALWAYS go bang.
If you need a service manual for the Astec PSU send me a PM, and I'll email it (3.3MB)

 

[NutmegCT]

As always, thanks again gentlemen. And Chuck, I had no idea about that service manual, which I've just now downloaded.

Nige - PM is "in the mail".

Tom

 

[nige the hippy]

I don't know if C1 is the same manufacturer but C1 and C2 should be class X rated and C3 & C4 go from live & neutral to earth so should be class Y (& afaik under current legislation there should be 2 in series!), but as they are ceramics it's unlikely that they'll fail.

Actually, something that us 240V-wallahs should really think about is the probability of filter capacitors failing live to (poor) earth. That Osborne has it's 0V & earth connected. We should always use a good earth leakage circuit breaker in the supply when doing stuff with any vintage kit.

Incidentally Tom/Nutmeg you should post your TV in the off-topic section, I'm sure others here would appreciate!

 

[Chuck(G)]

The flying (115V) lead is designed to connect one of the AC inputs to the centre of the capacitors, as a nifty voltage doubler. it may well be that the board has the connection hard soldered somewhere.
Those RIFA filter capacitors ALWAYS go bang.
If you need a service manual for the Astec PSU send me a PM, and I'll email it (3.3MB)

On the OCC1, input voltage selection is handled off-board. The Astec PSU is a commodity one and is normally NC in the Osborne.