Resistors gone low

[G4_Pete]

Hi,
The following resistors are in series with the mains as surge limiters to the rectifier in a Kolster Brands FB10. I have read these sets are notorious for component changes so was not surprised to find what looked like two 330 Ohms resistors in parallel. The trader sheet (969) shows a 150 Ohm in parallel with a 470 Ohm giving 113 as the listed value for V5 surge limiter.

However I measured 83 Ohms from what looked like two 330 Ohm resistors in parallel, they could be two 220 Ohms but they definatly look more orange than red. Removing them showed one at 123 Ohm with the other at 257 Ohms. This value is constant when measured with a DVM and when derived from the current flowing with 10 Volts across the resistors.

Interesting phenomenon, any comments.

Photo attached.

[Nuvistor]

Over run low value resistors tend to go lower in value, on the other hand high value ones say approx 100k and above tend to go high with age.

Used to get this in various locations, one was cathode bias resistors in frame or audio output stages. Once they start to go low it's a vicious circle, valve draws more current, resistor heats up more and goes lower in value.

Quite a standard event.
Frank

[ms660]

Second that.

Lawrence.

[G4_Pete]

Thanks for the comments not really noticed it before -a new modern componant required I think.

[MrBungle]

That's interesting. Every vintage resistor I've ever seen seems to drift up. Particularly carbon comp ones. Never noticed if there was a difference between high and low value ones.

[Nuvistor]

The low ones seem to go low when overrun, which in many domestic TV, radios of the 50's and 60's was common, or perhaps near their wattage limit.

Some would go O/C after going very low, the heat would crack them in two.

Frank

[MrBungle]

Will keep eyes peeled for that one. Might explain a few things I have had in older 1960s Tektronix kit as well that left me scratching head.

[Maarten]

The colour code seems to indicate a non-standard 230 ohms. That would be quite close to the mentioned 470 and 150 ohms in parallel.

[Craig Sawyers]

Careful what type of resistor you use. Carbon comp ones are very good at handling surge currents, whereas modern resistors tend to be metal film and not handle surges well at all (ie burn out).

An alternative, although non-authentic, would be to use wirewound.

[Nuvistor]

A WW resistor as a surge limiter in that location in the KB is what I would use, not authentic but probably more reliable.

Frank

[John M0GLN]

Originally Posted by G4_Pete

The trader sheet (969) shows a 150 Ohm in parallel with a 470 Ohm giving 113 as the listed value for V5 surge limiter.

Why would they have used that resistor combination? Two 220 Ohm resistors in parallel would give 110 Ohm which must be close enough to 113 Ohm for a surge limiter and both resistors would be passing the same current.

John

[Nuvistor]

Probably whatever they could source in enough quantities.

Frank

[Hybrid tellies]

We used to find high value carbon resistors around line output stages and boost lines on TV's could go low as well as high. General rule of thumb was resistors between 100K and 1M could go low and high whilst resistors greater than 1M would generally go high.

[Maarten]

Originally Posted by John M0GLN

Why would they have used that resistor combination? Two 220 Ohm resistors in parallel would give 110 Ohm which must be close enough to 113 Ohm for a surge limiter and both resistors would be passing the same current.

Maybe the design started out with a 150 ohm resistor which was then found to be a bit too high value and a 470 ohm was patched in. It may then have been found that 115 ohms was better than 110, or the purchase department found a cheap stock of 230 ohm resistors.