Hello, an a nice WE to everyone !
I'm about to service a Quad FM2 tuner.
On the quadmod page : Quad FM2 tuner service notes by Quad Mods USA
it is specified : "1. Replace all the resistors attached to the HT line with 1 watt carbon."
I don't understand the choice of carbon resistors. Is it only an "objective" choice, to keep the look and perhaps also the original sound of the tuner (in the HT line ?), or is it most seriously because metal film resistors, in a tuner, can bring inductance problems ?
I have already metal film resistors in my box, and I would like to know if I need to find carbon one, or if I can use those I have without problem (1W metal film resistors, 450 V max, HT line on a Quad FM2 is 175 V).
Thank you very much !!
I'm about to service a Quad FM2 tuner.
On the quadmod page : Quad FM2 tuner service notes by Quad Mods USA
it is specified : "1. Replace all the resistors attached to the HT line with 1 watt carbon."
I don't understand the choice of carbon resistors. Is it only an "objective" choice, to keep the look and perhaps also the original sound of the tuner (in the HT line ?), or is it most seriously because metal film resistors, in a tuner, can bring inductance problems ?
I have already metal film resistors in my box, and I would like to know if I need to find carbon one, or if I can use those I have without problem (1W metal film resistors, 450 V max, HT line on a Quad FM2 is 175 V).
Thank you very much !!
Possibly Nostalgia. Capt Grogg has proved me wrong by saying that he has 80M solid carbon. I don't know anywhere that sells solid carbon resistors at that value these days, most of them are carbon film.
Carbon is OK if there is little or negligible current flowing through it. At higher current they are considered noisy.
They are traditionally used as grid stopper resistors in valve amps but there is no audible difference to metal film as replacements.
All I can say is try them and see, as the metal film generally cost but a few pence each.
Carbon is OK if there is little or negligible current flowing through it. At higher current they are considered noisy.
They are traditionally used as grid stopper resistors in valve amps but there is no audible difference to metal film as replacements.
All I can say is try them and see, as the metal film generally cost but a few pence each.
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Because they are quite a bit more inductive than the carbon resistors originally used and may affect circuit operation in unpredictable and irremediable ways (short of using the proper part).
All passives have parasitics some of which are relevant to certain applications and not to others.
Vintage carbons unfortunately had a propensity to shift values drastically over time, more modern types tend to be stable unless overstressed or subjected to a circuit failure.
When I was restoring vintage tube tuners I checked all carbon resistors in the front end and IF, anything still within the nominal tolerance range was not touched - if bad I replaced the part with a comparable NOS part from my inventory.
All those parts are long gone and I no longer do this sort of work except to keep a couple of my old tuners alive.
When I was restoring vintage tube tuners I checked all carbon resistors in the front end and IF, anything still within the nominal tolerance range was not touched - if bad I replaced the part with a comparable NOS part from my inventory.
All those parts are long gone and I no longer do this sort of work except to keep a couple of my old tuners alive.
You're right, but without failing, they can see their value changing a lot with years, that's why quadmods advices to replace them. And their value can change especially in a tube gear, I think carbon resistors don't like to much heat. That's why I wanted to change for metal film. It wasn't a good idea. Finally,as wrote Kevin, changing only the bad is better.
In most cases a change from carbon comp to film resistors should only need a tweak to the alignment. It will depend on exactly what role the resistor plays in the circuit. High value resistors tend to have capacitive reactance rather than inductive.
The best rule with VHF circuits is: if you know what you are doing, try it, but be prepared to undo the change; if not, don't try it. The IF stages will be less critical.
The best rule with VHF circuits is: if you know what you are doing, try it, but be prepared to undo the change; if not, don't try it. The IF stages will be less critical.
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