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Any Osborne I gurus?

Tom,
Some of these old power supplies do not like to run into an open load. Let's wait to hear from the power supply experts before trying anything else.

From the smoke and flash, I'd guess you will be replacing the big electrolytic capacitors. They would have blown sooner or later.

Power supplies can be fixed. Try to hang in there. One day (in the future) you will be laughing as you tell of this adventure.
-Dave
 
Yes, don't give up Tom. My Osborne required me to deal with three separate issues and it sounds like yours is heading that way too.

I doubt if the RAM fix would have contibuted to this latest problem unless you plugged something in incorrectly. I'd also go for the big electrolytic cap theory. One thing with exploding caps...you can always tell which component has failed.

Keep at it. As with me, you'll be learning a lot doing this, knowledge you'll use for other vintage repairs.

Tez
 
Thanks for the encouragement gents.

Maybe a failing PSU was part of the mix all the while. You both mention replacing the electrolytic caps. Altho' I didn't want to have to chase all that down after spending over $150 - and a week - to get the "new" OCC ... note that the part that fried today was a resistor, not a capacitor.

I have replaced that resistor, but still no luck. System still dead.

Is the PSU one of those "serial devices" that, if one part (e.g. the R5 resistor) fails, other fall like dominos?

Thanks.
Tom
 
I would not re-connect the PSU to the main board or CRT until I was sure the power supply is fixed. This probably means hooking up a load box to the PSU. This box will consist of high wattage resistors to present the proper nominal loads to the secondary power (+5V, +12V, etc). With power off, you can also check for opens and shorts of the various components. Hopefully a good power supply troubleshooter on this forum can give you a safe plan of action. Go slow and don't hurt yourself. Do you have a schematic of the PSU?
 
Yep - I already got all those schematics, including the PSU. As I have no way of testing capacitors (other than for continuity) it seems all I can do is pull every single cap and replace it. Oy.

Steve, yes, R1 is a 4 ohm thermistor.

I don't have a clue about constructing a "load box", so I'll have to replace all the capacitors today if I can find them at RadioShack. No other resistors have blown, so I won't touch any more of those.

Tom
PS - in this PSU, are capacitors just "capacitors"? In other words, as RS has a limited parts selection these days, can I replace an electrolytic with a ceramic, ceramic with poly, vice versa, etc., if the Farad rating is equal, if need be?

PPS - is there something in the PSU I might be overlooking? Here's the scenario: (1) system worked fine, but with a failed ram chip. (2) removed logic board, replaced chip, put all back together, powered up - no beep, no drive action or CRT filament light. (3) removed PSU, powered up, resistor exploded. Seems there was something really *basic* that happened to that PSU - and it happened before the resistor exploded (when the PSU had no load).
 
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NutmegCT said:
Yep - I already got all those schematics, including the PSU.
Click to expand...
Thought so - it was more for the benefit of others.

NutmegCT said:
Steve, yes, R1 is a 4 ohm thermistor.
Click to expand...
Ah, sorry - I was looking at the schematic wondering what could make that blow, but looking back it was R5 that went. Don't know where I got R1 from, so ignore that ;-)

NutmegCT said:
PS - in this PSU, are capacitors just "capacitors"? In other words, as RS has a limited parts selection these days, can I replace an electrolytic with a ceramic, ceramic with poly, vice versa, etc., if the Farad rating is equal, if need be?
Click to expand...
Not just the capacitance but the voltage rating must be equal to (or greater than) the original. Depending on the application, polarisation - or indeed lack thereof - may or may not be important, too. Whilst substitute parts of different types would probably work in some cases, you may have trouble finding parts which will physically fit (or even exist at all) if using very different types, especially when it comes to high capacitance/high voltage capacitors. Best to stick with the same types of parts wherever possible. If available values are very limited, using two or more capacitors in series or in parallel will give you some leeway, although it can get messy and you may of course run into space issues.
 
Feeling a bit overwhelmed here. I stopped at RadioShack and found they don't carry any 250v 100mF capacitors; there are two on the PCB. I assume those are the first electrolytic ones I should replace as they're at the high voltage end?

RS does have some of the lower voltage caps I need, but the leads on those are at opposite ends; the PCB uses "vertical mount" caps with leads that are about 1/2" apart on the board, so to use those RS caps, I'd need to have a *long* extended lead from the top looping down to the board.

Of course, their internet connection has been dead all day, so they couldn't check their supplier. As I don't have a way to test the caps, I assume it's safer to replace them but now I guess I need to find another source.

Came back home after no luck at local electronics/radio shops. Decided to test the resistors on the PCB. Per schematic, R2 should be 330K ohms: orange orange yellow gold. Test on digital VOM shows 2M ohms, slowly dropping to 120K. Per schematic, R3 should be 220 ohms (red, red, brown, gold); test shows 179 ohms. However, that resistor is a solid green cylinder - no color bands. Same for several other resistors - are there resistors that are a single solid color?

Man, talk about the blind sailor in a storm, with no compass and just guessing which ropes he's pulling!

I again assume that if the test resistance shows a significant variance from the schematic, that resistor should be replaced.

Getting frazzled by the minute.

Thanks for your patience.
Tom
 
Tom,
I had a look at the power supply schematic. Wow, it looks like a fairly sophisticated switching power supply. I am not the right guy for this. I hope others jump in.

The Sams documentation calls out the power supply parts list. You will need to take this to an electronics components place that handles these types of parts. Can you find out if you have the local equivalent to this kind of place: http://www.orvac.com/index.html and buy direct equivalent parts that fit especially the capacitors.

Remember, as you are measuring the resistors “in-circuit”, there are components in parallel with it which would normally reduce the reading. The reading of a resistor can never be higher than its listed value, but can be lower. More importantly than the value being off a little, if it looks charred or discolored or the circuit board under it looks discolored, you should change the resistor. They usually fail open or short.

From what you say, the R2 resistor look suspicious. R3 on my schematic from cosam says 180 ohms, 1 watt. Is your version different?
-Dave
 
Hi Dave - Maybe you've discovered part of my problem already. I'm using the "Repair Manual for Astec Power Supply 8151" - which is what mine is labeled on the PCB.

The manual shows R3 as 220R (ohm), +/- 5%, 1 watt.

When you say "my schematic from cosam" do you mean from SAM's? Or is this something you got from Steve (cosam) here?

Yikes - should I be using a different schematic?

Tom
 
NutmegCT said:
Hi Dave - Maybe you've discovered part of my problem already. I'm using the "Repair Manual for Astec Power Supply 8151" - which is what mine is labeled on the PCB.

The manual shows R3 as 220R (ohm), +/- 5%, 1 watt.
Click to expand...

There may be slight differences from the schematic I'm looking at that cosam pointed me too from his message earlier today. You replied to him that you also had the Sams Photofacts documentation.

I did not realize the power supply was a commercial unit (Astec). Have you started to look around for a working unit? Are they expensive? Of course as you now have two bad units, if you learn to fix one, you can also fix the other.

Did you check out tez's link to how he fixed his Osborne? http://www.classic-computers.org.nz/blog/2008-12-06-osborne-repair-1.htm
 
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Other than that R3 discrepancy, the two documents seem the same. I just pulled the R2 and replaced it with new. No change at all in boot-up, drive action, etc. Spending an hour and second-guessing myself at every step, just to pull and replace a single resistor, then having no success, is pretty discouraging.

I've been keeping an eye on Astec Osborne PSU's on ebay. Last week there was one for $69 plus $15 shipping, but it's "as is". A couple days ago there was another for $55 described as "working - looks like new". This morning the price got raised to nearly $100. Weird.

I've now spent two weeks learning a lot about electronics, and about myself, and have pretty much concluded that this is way beyond my ability and patience. An electronics shop in Hartford told me they'd be happy to bring the PSU up to spec, but it would be $65/hr, minimum 4 hours.

Not looking good. Really wanted to bring my original OCC back to life; it sure has some good memories from my college days. I started 2 weeks ago with my original OCC which wouldn't boot and showed screen garbage. Diagnosed the PSU failure. Paid over $150 for another OCC - which I actually saw running SuperCalc - and somehow managed to nearly reduce it to rubble. I've got parts and gear all over the place. Man this is a downer.

T.
 
NutmegCT said:
I've been keeping an eye on Astec Osborne PSU's on ebay. Last week there was one for $69 plus $15 shipping, but it's "as is". A couple days ago there was another for $55 described as "working - looks like new". This morning the price got raised to nearly $100. Weird.
Click to expand...

$100 bucks for an unknown power supply seems too much. And $260 to repair is too much. Before you give up, think about taking the power supply and the documentation to the engineering department at your local junior college and asking if they will take the repair on as a lab project if you pay for the parts. There will be no guaranties, but it may be worth a try.

For not being an electronics guy you have showed a lot of moxie for attempting this fix. I really wish there was better results.
 
OK - one last dying gasp ... You recall I had replaced the paper caps in my original PSU. Connected it and powered up. System beep, A drive briefly lighted; this is "normal". But the screen was still garbage (a single graphic character filled all the lines). This is *after* I replaced that ram chip.

If all is well on that PSU except some aspect of video feed, is there a way to tell what part needs replacing now?

At least my first repair - on my original PSU - doesn't result in a totally dead system. It's like only the video is now flakey.

As an aside, trying to describe a sequence of events, and individual details relating to those events, is quite a challenge!

Tom
 
Put in my original PSU with the new caps (that replaced the original fried paper caps). Cabled up, powered on, system beeped, A drive clicked, screen is that single garbage character throughout. No response to System disk and hitting Return.

I then checked the output voltage from both output jacks on the PSU: voltage seems correct. Top pin is -12.5v, next is +12v, next is Common, next is +5v. This seems spec to me - as schematics show -12v, +12v, Common, +5v.

Need to determine why the video display is screwed up. Keeping in mind that the video was *not* faulty with the "new" PSU (when it was working).

Also, it seems it's not just a video problem, as the system doesn't seem to be starting from ROM; it doesn't respond to insertion of System disk and hitting Return.

Tom
 
BOOT! (almost)

I replaced my PSU cable with the PSU cable from the "replacement" PSU. My original was a dual-cable, with two four-pin sockets at PSU running to a single socket plug for the logic board. My replacement PSU had a single four-pin socket running from PSU to logic board (with a "jumpered" logic board socket).

Powered up - beep, drive click, and SCREEN IS CORRECT. Asked for system disk, I inserted and hit Return. BOOT ERROR repeatedly scrolls down screen.

Powered off, put the original dual-plug cable back on PSU, restarted, and screen is back to garbage. Put in the single plug cable, and system seems fine (and screen is perfect) until I hit Return for the disk read; that gives BOOT ERROR.

It's got to be something in the "mesh" between PSU to logic board to drive board.

Back to more monkeying ....

T.
 
BOOT!

I moved that single cable to the 3rd jack on the PSU. That's the one shown as unused on all the schematics, but it outputs the same voltages as the other two.

I have no clue why ... but the system booted properly from the A drive disk, giving me an A> prompt.

I was then able to load dBase 2.41 from the A drive. Even remembered a few of the old dBase dot prompt commands. Senility hasn't totally taken over yet.

Now I'm trouble shooting why I frequently get "bdos error; sector not found" on attempts at disk access on A and B drives. I'm betting it's still a voltage problem to the drives. Maybe that single cable won't serve a dual density drive setup?

T.
PS - I booted from the original OCC1 system disk, with the help files. It ALL LOADED - didn't freeze up. Looks like replacing that ram chip actually worked. Yeehaa!
 
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Not sure what's up with your drives (are the heads clean?) but congratulations on getting the thing running! I'm glad you stuck at it and weren't afraid to tinker. For a moment there, I was afraid it was going to get the better of you!
 
Screwed the boards back to the chassis, and turned it right side up. Didn't change anything else from previous post.

Powered up, and get opening screen "please insert disk" but nothing but BOOT ERROR whether I try booting from A or from B drive.

This has *got* to be a power problem; wonder if the dual density drive system just uses more power? And that single cable isn't providing enough?

T.
 
Tom,
You are probably fighting an intermittent connection. This is hard to check in a tight assembly like the Osborne. Go slow and keep a detailed log of everything you do so you can keep track of things.

I found an Osbourne Field Service Manual on the web. Do you have it?
 
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