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Help identifying a resistor please.

A

acollins22

Experienced Member
Joined
Mar 12, 2007
Messages
257
Location
Leicester, UK
Hi Folks,

In this recent thread I mentioned this I'd got a Canadian PDP-8 clone.

I am in the process of restoring it and have checked out the power supplies and given the boards that make up the CPU and clean and visual check.

I have now moved on to the core memory boards and have run across a snag I hope someone can help me with.
IMG_20191102_144851803.jpg
Inspecting on of the two core memory boards I noticed a broken resistor on the board.

In the hope of finding out what value it is I turned to the other identical core memory board and it has the same fault on the same resistor. Talk about identical.

IMG_20191103_121524180-crop.jpg
So here's is my problem. How do I identify this resistor? I have a photo but I can't see a tolerance band so I'm not sure which way to read it. Could it really be a 50 Ohm resistor?

Can someone suggest a way forward?


Thanks,

Andy.
 
It looks like the next door resistor has the same band structure.

If it was me, I would use a multimeter first to measure the resistance of an intact device and see where that takes you. Measure twice, with the multimeter probes one way them the other.

It is either a very high value or a very low one!

Green, brown, brown = 510 Ohms.

Brown, brown, green = 1,100,000 Ohms = 1.1 MOhm.

Dave
 
I completely agree with Dave. It looks like a 510 ohms one, but I'd not take chances and just measure the twin one.
Frank
 
I have never seen green used as a tolerance band on a carbon comp resistor. It looks like a 510 ohm to me but I can't tell if it is a brown or a faded red.
Both boards likely failed because of the thermal stress. It is likely on a 12v rail. That would be a little over 1/4w for a 510 ohm. The resistor is only a 1/4w resistor.
Anyway 510 is a standard value and 522 is not so I'd say within 99%, it is 510 ohms.
I don't recommend replacing it with another carbon composition. They are poor resistors and drift a lot when run near their rated wattage. They do have low inductance compared with carbon film but then your not running at 1 GHz either. If you are an exact type freak, then use a carbon comp.
Dwight
 
"If you are an exact type freak, then use a carbon comp."

That's why I mentioned the fact that it was composition. I suspect that there are still people on this forum looking for vintage Rifa line filter caps as well.

I note that the 68 ohm resistor to the left of the intact 510 ohm is a film resistor, so it's a big shrug. This must be a piece of industrial kit--given the Teflon sleeving and washers on each resistor. You wouldn't find this in consumer electronics.
 
I might still have some. IIRC though it wasn't depicting 0.5%, it had some other significance.

But on no three band resistor should one be tolerance, I think.
 
KC9UDX said:
I might still have some. IIRC though it wasn't depicting 0.5%, it had some other significance.

But on no three band resistor should one be tolerance, I think.
Click to expand...

No, no tolerance band means 20% or 15% as I recall. Silver means 10% and gold means 5%. I don't recall ever seeing a 1% carbon comp let alone a 0.5%.
 
Thank you everyone for your replies.

I read its companion both ways round and it is indeed 510 Ohms (minus a smidge) so I have replaced it with a new one.

IMG_20191103_140133702-crop.jpg
Looking at the size or the original compared to the 1/4w replacement, I wonder if it was 1/8w. anyway, it's gone now!


Thanks again all,

Andy.
 
acollins22 said:
Thank you everyone for your replies.

I read its companion both ways round and it is indeed 510 Ohms (minus a smidge) so I have replaced it with a new one.

View attachment 57059
Looking at the size or the original compared to the 1/4w replacement, I wonder if it was 1/8w. anyway, it's gone now!


Thanks again all,

Andy.
Click to expand...

1/8w carbon comp. Those would be hard to find. I think a 1/4w carbon film will work fine there.
Dwight
 
It's a carry-over from the valve (electron tube) days, where resistors were not very precise, and it didn't matter that much anyway on such equipment. The idea of a tolerance band didn't exist then, so for uniformity 20% was assumed.
 
I stared at the enlarged photo of the intact resistor and will almost swear that there's a faint gold band there, which led me to the 5% opinion.
 
Chuck(G) said:
I stared at the enlarged photo of the intact resistor and will almost swear that there's a faint gold band there, which led me to the 5% opinion.
Click to expand...

Even if it was label as a 5%, if it were used anywhere near its rated value, it is lucky to be 20%. When I was in the service, I replaced a lot of 5% carbon comps in circuits that did care. Yes it was tube circuits but there are a few cases that cathode resistor values are important.
Dwight
 
I recall the old puzzler. A young engineer looks at resistor pricing and figures that if he purchases a 1000 lot of 20% (or 10%) carbon comp resistors and only 25% of them fall in the 5% tolerance range that he's saved a bunch of money by tossing out the ones that fail to meet 5% of marked value. After all, the yield should follow a bell-shaped normal distribution curve, right?

He gets his 1000 20% resistors and finds that none of them are within 5% of marked value.

Why is that? :)
 
Chuck(G) said:
I stared at the enlarged photo of the intact resistor and will almost swear that there's a faint gold band there, which led me to the 5% opinion.
Click to expand...

I've never seen an allen-bradley style resistor with a 5% tolerance. They're usually 20% or less commonly 10% (silver band).

I have a box of thousands of them I got from an old Motorola engineer, none have a 5% tolerance.
 
GiGaBiTe said:
I've never seen an allen-bradley style resistor with a 5% tolerance. They're usually 20% or less commonly 10% (silver band).

I have a box of thousands of them I got from an old Motorola engineer, none have a 5% tolerance.
Click to expand...

The manufacture made several thousand resistors. Those that were within 5% got gold bands. Those that were within 10% got silver bands. Those that were within 20% were not marked with a band. The rest were either marked 750 or 330, assuming he bought 510 ohm resistors. The rest were tossed as they didn't trust them.
Dwight
 
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