Hi Tom, I just stick the sub box in the circuit with the knob near the rated value of the removed resistor. I twirl the knob to the desired result, then just measure the sub box resistance with my VTVM. I do have to be careful it is not set too far off, or it could damage the circuit or the box. I never tried 2 boxes in parallel, that is probably a better method. Thanks for the video (and all the other ones you posted this year). Ian
Did the same thing with my Heathkit Resistor Substitution boxes a few years back. I eventually replaced all the resistors in one box with 1% metal film resistors of higher wattage.
I always measure carbon composition resistors before using them, as all of them will drift up. Got some 5R 20% ones in a grab bag once, and none of them were less than 10R. Odd resistors in that the only marking on them was a single green band on an otherwise black body. Only thing is that the drift rate depends on heat, so they will drift up faster when they are run at close to rated power, and irrespective of voltage all of them over around 100k will drift up by 20 % after a decade or two, so most are now sitting at around double the value after a half century or more, and for 1M they can be almost open circuit at DC, but will be fine at 200VDC. Often easier to replace them with 1W carbon or metal film, unless they have more than 300V across them, where you are better off with 2 or 3 in series of the film units for overload ability. Bet the composition value changed when you soldered the new resistor in, and heated up the wax on the one side of the old resistors. Interesting in that you could overload them till they caught on fire, but still were, till they cracked, at least vaguely within the resistance that was on the bands. Saw many that were crispy and white in the middle, but, till you cut the leads, they were still within 80% down to 200% over the rated value. Cut the lead and they crumbled to powder.
What a silly thing to do. Why do double work? Why throw extra wasted effort after bad parts and still have the old drifting resistor to deal with in the future?? Why not just do one step and simply REPLACE the bad carbon composition 2.2k resistor with a new 2.2 k 1% precision film resistor for a few cents and be done with it? And then you'll permanently avoid future carbon-comp drifting problems too.
Hi Tom, That’s really interesting, thank you. But what is the advantage of adding a resistor instead of replacing the faulty one? Surely the price of the added resistor would be similar to that of the replacement one? And if you are soldering, the work seems to be the same. Cheers Mark
I did this trick a couple months ago with a couple resistance boxes I have. I just used a parallel resistance calculator table on Google. Just type in the resistance you have and resistance that you want. Kinda cheating I guess. Most I used 1/4 watt. If it's not off too much you may only need to add up to 100k to a low value or even 1 meg to higher value. Good to share this approach though.
Very good information! Thanks a bunch!
That's a neat trick, Tom. Thanks!
Hey, Happy New Year! Stay safe, kind sir.
Hi Tom,
I just stick the sub box in the circuit with the knob near the rated value of the removed resistor. I twirl the knob to the desired result, then just measure the sub box resistance with my VTVM. I do have to be careful it is not set too far off, or it could damage the circuit or the box. I never tried 2 boxes in parallel, that is probably a better method. Thanks for the video (and all the other ones you posted this year).
Ian
Nice trick...thank you Tom :)
Did the same thing with my Heathkit Resistor Substitution boxes a few years back. I eventually replaced all the resistors in one box with 1% metal film resistors of higher wattage.
I always measure carbon composition resistors before using them, as all of them will drift up. Got some 5R 20% ones in a grab bag once, and none of them were less than 10R. Odd resistors in that the only marking on them was a single green band on an otherwise black body. Only thing is that the drift rate depends on heat, so they will drift up faster when they are run at close to rated power, and irrespective of voltage all of them over around 100k will drift up by 20 % after a decade or two, so most are now sitting at around double the value after a half century or more, and for 1M they can be almost open circuit at DC, but will be fine at 200VDC. Often easier to replace them with 1W carbon or metal film, unless they have more than 300V across them, where you are better off with 2 or 3 in series of the film units for overload ability.
Bet the composition value changed when you soldered the new resistor in, and heated up the wax on the one side of the old resistors. Interesting in that you could overload them till they caught on fire, but still were, till they cracked, at least vaguely within the resistance that was on the bands. Saw many that were crispy and white in the middle, but, till you cut the leads, they were still within 80% down to 200% over the rated value. Cut the lead and they crumbled to powder.
What a silly thing to do. Why do double work? Why throw extra wasted effort after bad parts and still have the old drifting resistor to deal with in the future??
Why not just do one step and simply REPLACE the bad carbon composition 2.2k resistor with a new 2.2 k 1% precision film resistor for a few cents and be done with it?
And then you'll permanently avoid future carbon-comp drifting problems too.
Thanks
Hi Tom,
That’s really interesting, thank you.
But what is the advantage of adding a resistor instead of replacing the faulty one? Surely the price of the added resistor would be similar to that of the replacement one? And if you are soldering, the work seems to be the same.
Cheers
Mark
I did this trick a couple months ago with a couple resistance boxes I have. I just used a parallel resistance calculator table on Google. Just type in the resistance you have and resistance that you want. Kinda cheating I guess. Most I used 1/4 watt. If it's not off too much you may only need to add up to 100k to a low value or even 1 meg to higher value. Good to share this approach though.
Oh don't fix it properly. I think you need to work on your McGuffin.