Videokarma.org TV - Video - Vintage Television & Radio Forums

Videokarma.org TV - Video - Vintage Television & Radio Forums (http://www.videokarma.org/index.php)
-   Diagnostic & Test Equipment (http://www.videokarma.org/forumdisplay.php?f=177)
-   -   Replacing Precision Resistors, TT-1A (http://www.videokarma.org/showthread.php?t=260550)

Winky Dink 01-16-2014 12:07 AM

Replacing Precision Resistors, TT-1A
 
I'm reconditioning this Heathkit tube tester, and other than replacing paper and electrolytic caps; diodes; and a few drifted resistors, everything looks pretty good. I'm replacing two or three of the 1% resistors, and I'd like to know what characteristics qualify a "precision resistor." Is the precision resistor less likely to drift? Why shouldn't I replace a 100 ohm 1% resistor with a 5% resistor that measures exactly 100 ohms?

I ordered all the replacement parts last night, but it took me quite a while to find three 1% resistors in the values (or combinations) that I needed.

I said that everything looks good, but I have one concern with the original 1% resistors. There are ten precision resistors, and they're all spot-on except for two that have drifted 5 to 10 percent lower. Both of the drifters are on the same switch, and a third 1% resistor on the same switch hasn't drifted, but looks like it might have been burned. Is this possibly an indication of a wiring error?

I'm using TT-1A restoration guides available online from Kent Nickerson and RDF Products (thanks, guys), and I have an original assembly manual on order.

bob91343 01-16-2014 01:13 AM

I fail to see why a tube tester needs precision resistors, especially a kit type. However, if you need the value to be as close as 1% you must decide what long term stability you need. Today's resistors are a lot better than the ones from the old days. In fact most resistors I have tested are very close to nominal value, while the old ones had more of a spread.

I'd suggest metal film but even deposited carbon may be stable enough. Wirewound are the most stable but aren't usually available in stock values, plus they have inductance that might present problems in some circuits.

Yes, precision resistors are less likely to drift because they are made of different materials that have been found to be more stable.

So my answer to your question at the end of the first paragraph is that it's okay to do that if you are using a modern resistor.

As for the burned resistor, I would suspect instead that the power rating is marginal, and if the tube is left drawing current for a long time it might get too hot. Calculate the dissipation if you can.

I assume you are measuring these resistors with a precision bridge, not just a digital Ohmmeter. For lower values you will need Kelvin connections.

Winky Dink 01-16-2014 02:20 AM

Thanks, Bob. I am using a digital multimeter, so I don't expect 1% accuracy. I didn't change out resistors because they were 2% off spec. At my level I basically try to put it back together the way it was designed. Anyway, I have some 1% metal film resistors on the way.

bob91343 01-16-2014 10:58 AM

One ploy that will improve accuracy a little is to short the leads and subtract the reading you get from your measurements. But the typical DVM may be off by 1% easily. In fact, there are calibration adjustments inside that may need attention.

I have some precision bridges and find them extremely useful; I don't know how I got along without them. My very accurate voltmeters argue with one another about the actual voltage. As the saying goes, a man with a watch knows the time but a man with two watches is never sure.


All times are GMT -5. The time now is 04:15 PM.

Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
©Copyright 2012 VideoKarma.org, All rights reserved.