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Osborne RF capacitor

NutmegCT

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A little surprise when I cranked up the Osborne. After running about 30 minutes without problem, a sudden pop and lots of acrid smoke coming from the areas above the drives. Note the damaged large capacitor:

IMG_0470.JPG

In Tezza's article on Osborne restoration, he shows the same capacitor. He writes: "Further reading showed these were old "rice paper" capacitors for reducing RF frequencies so that the computer didn't interfere with televisions in the near vicinity. Moreover, these computers can run just fine even when these capacitors are damaged! (Nearby televisions are not so lucky though!)"

https://www.classic-computers.org.nz/blog/2008-12-06-osborne-repair-1.htm

As my televisions all use digital input (not RF signals), seems like there's really no need for the RF capacitors on the power supply. The two smaller 0.01mF capacitors show no damage.

So, can I just clip off the two leads to that burned 0.1mF capacitor, and remove it?

If so, do the two feeds to the capacitor need to be shunted for the PSU to work?

Thanks.
Tom M.
 

VERAULT

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Tom the psu should work without that RIFA CAP; but if it was me Id replace it. You should be able to keep troublshooting your KB issues without it.
 

NutmegCT

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Tom the psu should work without that RIFA CAP; but if it was me Id replace it. You should be able to keep troublshooting your KB issues without it.

Thanks for the info. If the cap is removed, do the two connection points on the psu board need to be shunted?

I'm trying to do the keyboard diagnoses too, so waiting for replacement capacitors to arrive would be a week of dead time.

Thanks.
 

Dwight Elvey

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Thanks for the info. If the cap is removed, do the two connection points on the psu board need to be shunted?

I'm trying to do the keyboard diagnoses too, so waiting for replacement capacitors to arrive would be a week of dead time.

Thanks.

You can shunt the wires, only if you want to do a fuse or circuit breaker test.
NO DO NOT SHUNT IT.
Please just cut the leads and leave it unconnected.
It is possible that blowing this damages the input thermal surge resistor. Check the voltage on the leads that went to the capacitor to ensure you still get 110V.
Dwight
 

NutmegCT

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You can shunt the wires, only if you want to do a fuse or circuit breaker test.
NO DO NOT SHUNT IT.
Please just cut the leads and leave it unconnected.
It is possible that blowing this damages the input thermal surge resistor. Check the voltage on the leads that went to the capacitor to ensure you still get 110V.
Dwight

Thanks Dwight. I clipped the leads, put all back together, powered up, but no response from anything. I checked the voltage at that capacitor's connection points, mm set for 250VAC - but got zero volts.

What should be my next step?

Tom M.
 

Dwight Elvey

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There is a chance it blew the fuse or damaged the thermistor. It may not have a thermistor for inrush but I'm sure it has a fuse. You can check things with an ohm meter. Each side of the capacitor that you cut out should have a low ohms path to one of the input terminals, with the switch in the on position. DO NOT HAVE IT CONNECTED TO THE MAINS WHEN DOING THIS TEST.
You could post a picture of the fuse area so we can look at things.

As a side note, I can see the 220v jumper. I should have noticed it before.
Dwight
 

NutmegCT

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There is a chance it blew the fuse or damaged the thermistor. It may not have a thermistor for inrush but I'm sure it has a fuse. You can check things with an ohm meter. Each side of the capacitor that you cut out should have a low ohms path to one of the input terminals, with the switch in the on position. DO NOT HAVE IT CONNECTED TO THE MAINS WHEN DOING THIS TEST.
You could post a picture of the fuse area so we can look at things.

As a side note, I can see the 220v jumper. I should have noticed it before.
Dwight

Fuse F1 is blown (no continuity). It's marked 5MF 125V 2AMP. I hope it's a "standard" fuse I can get at the hardware store.

Here's a photo of the board:

IMG_0472.jpg

Bad fuse is at the top of the photo. That bad capacitor has one lead clipped.

Should I just replace the fuse and report back?

Thanks.
 

Chuck(G)

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Replace the fuse, but also grab a couple of extras in case there's something else wrong.

3AG = AGC3 2A @ 250V should work

It's interesting that the PCB is marked at 0.5A for 220V and 1.0A for 120V.
 

Dwight Elvey

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I would guess the fuse is not original. I wonder how much power an Osborne requires. It might be that it started as a 120v and when it was jumpered for 220v, they got the math wrong and doubled the current rather than halved the current. For this application, I doubt it is much of an issue.
Dwight
 

NutmegCT

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Dwight - powered on, O1 uses .6 amp at 120v. Disk activity raises that to .73 amp, then back to .6 when drive stops.
 

Dwight Elvey

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Oops, I was looking at the original picture and saw the wire on the 220V. I see now that they'd just wired the voltage select to the back panel. Often they don't do this and it is only jumpered on the supply.
I'm sure you are OK.
If it were wired for 220V, it would cause excess current for the regulator output transistor and usually ends in the supply failing. Looking at the schematic one wonders why they even connected anything to the 220V post. It is usually just a disconnected position to keep the jumper wire from shorting to something else.
All the jumper usually does is to connect the 120V position so that the rectifier becomes a voltage doubler for for the regulator DC level. It just surprised me to see a wire connected to the 220V post, since it doesn't usually do anything electrically there except keep if from shorting to someplace it doesn't belong.
Dwight
 
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