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5151: Resistor R514 corroded, monitor not functioning.

G

Gregert Beardrondo

Member
Joined
Jan 30, 2009
Messages
30
Location
Ontario, Canada
Well a few weeks ago when I turned on the good old 5150 to try and ping 127.0.0.1 to test the NIC, instead of the cursor flashing while I held my breath, nothing happened.

Not nothing: The monitor whined quietly, too high for my mother to hear it.

So after a few days I got up the courage to take the back off. Don't worry, I was careful not to touch any capacitor contacts (including the tube itself). There were several things that looked like they might be causing the problem. All the capacitors appear fine, and since I don't have a way to test them I'm going to assume they are fine. But J7 (just a wire soldered to two points) and R514 both are very corroded. This past weekend I saw that Canadian Tire had a multimeter on sale for $9.99 (80% off!). I couldn't pass this up, so my father and I went and bought one on the way to Union Station.

After some poking around with it set to measure resistance, I've found that R514 appears to be corroded to the point of infinite resistance. It is open. It must be replaced.

So I come before you to ask: What is the value of this resistor? Or does anyone have a schematic? Perhaps a link?
 
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R514

R514

Heh, that's in your heater feed line. No wonder it's dark.
It's a 68kOhm , 1/2 watt resistor. If it's not too badly corroded, you should see the colors blue,gray,orange, and then the 4th band probably gold or silver.
It's from the "IBM Technical Reference", I don't have the link handy, but if you google it, you should find the pdf's.
patscc
 
Oh my... I fiddled with the settings on the multimeter and got about 39kOhms.

The colour bands are orange, white, orange, silver. An online resistance calculator says this is 39kOhms. Which means I feel a bit of a dunce, and have to keep looking.

I'm new at this...
 
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schematic

schematic

Are you sure it's R514 ? I've got the schematic in front of me. Anyway, if you can make out the color bands, go by them.
You could be getting a reading due to other elements in the circuit. Unsolder one leg of it, and have at it again.
patscc
 
I am sure it's R514. The colour bands say 39kOhm, and that's what I read, so I think it must be something else. I just don't know what. one or two of the capacitors seem corroded around the base, but I have no way to check them unfortunately.

Any way to check a capacitor with a multimeter? (voltage, amperage, and resistance) Just for kicks I shorted the terminals of one of the capacitors (To be sure I wouldn't damage the meter) and tested it's resistance. In theory it would start at a relatively low resistance and slowly climb up as the meter inadvertently charged it while measuring resistance. This is exactly what happened. So I repeated this and found one capacitor that just remains at a low resistance. Is this proof enough that that capacitor is the problem?
 
pretty good bet

pretty good bet

Have you downloaded the techref yet ?
Yup, dead on on the caps, at least for functionality. Note that if it's a big cap, it might take a whiiileee to charge. (t = R*C)
You're checking the caps out of circuit, right ?
patscc
 
I did download the tech ref, yes. And yes, a very long time to charge. I doubt the battery in the meter would be enough to charge it anyways. And no, I'm checking them in circuit, but not in use. But I'm fairly sure there's no path from one lead to the other other than through the capacitor. I'd like to avoid taking apart what I don't have to, but to see if this cap is really the problem I suppose I'll have to remove it.
 
Well I've removed, tested, and replaced all the capacitors that I couldn't test without removing them, and they all check out. I found a fuse hidden under a black piece of plastic near the large transformer, but that fuse is fine.

I'm at a bit of a loss. It's not a capacitor, and there's nothing else visually suspect. Any ideas?
 
Gregert Beardrondo said:
Well I've removed, tested, and replaced all the capacitors that I couldn't test without removing them, and they all check out. I found a fuse hidden under a black piece of plastic near the large transformer, but that fuse is fine.

I'm at a bit of a loss. It's not a capacitor, and there's nothing else visually suspect. Any ideas?
Click to expand...

Do you have the ability to test it with another video card? and what is the settings of SW1 nr.5 and 6 on the motherboard?
 
R514

R514

I've got tons of ideas.
It would be helpful if you could look at the schematic and try to figure out why you're R514 is 39K, but the one in the schematic is 68k. It's gonna be hard to help if we're looking at different things. At least look in your copy of the schematic and see if the printed value for R514 is 39k or 68k.
Is the heater in the neck of the CRT glowing ?
patscc
 
I don't have the case open to check at the moment, but I can assure you all the switches are correct: It was working fine for three weeks before this happened, and when I was trying to get it work I went over all the switches and made sure they were correct, as per edition 15 of Upgrading and Repairing PCs.

I've checked my downloaded copy of the schematic as well and R514 is labelled as 68KOhm, 1/2 watt as well. Rather a mystery this is. Presumably they ran low on 68K resistors? I don't know if the heater glows: I'll plug it in and see. The only markings on the board are as follows, on each side:

CHASSIS NO:GSA
3002-701-110
94V-0
 
A variety of places, but usually Belleville. It's kind of like Mississauga, but everyone feels sort of ripped off. Oh and it's smaller. Like Kingston but completely boring. 40,000 people. Beside the Trenton Air Base.

I will test the monitor to see if the heater glows tomorrow afternoon. It's 1:00am here.
 
A stupid question, I know--but I have to ask.

Did you verify that the video card is delivering the proper signal to the monitor? Have you tried a different monitor or checked the signal with an oscilloscope? Have you tried driving the monitor from a different (nonochrome) video card?

I just want to make sure that you're not wandering down a false trail. :)
 
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