FWIW Allen-Bradley carbon composition resistors are identical with Ohmite.? For some reason AB marketed through Ohmite. Same with AB and Ohmite pots.They are about the best of the carbon comp resistors but despite this they do drift.? Usually upward and usually higher values drift proportionally to value.? It is not reversible.? While heat may accelerate the drift the resistors reportedly drift on the shelf.??Carbon and metal film resistors are far more stable, essentially neither drift with age or use.? ?Carbon comp resistors have no real advantage over film resistors, there is no good reason to use them.? ?BTW a little OT,? IRC advertised heavily but I almost never see any in old gear.? I don't remember their having a bad reputation and they don't seem to have been more expensive than AB/Ohmite, so a bit of a puzzle.Sent from my Galaxy
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-------- Original message --------From: "Ulf Kylenfall via groups.io" <ulf_r_k@...> Date: 5/20/25 1:27 PM (GMT-08:00) To:
[email protected] Subject: Re: [TekScopes] Tektronix 494P 1st LO and Frequency failure(s) I have repaired a 494 as well as a 2756. Apart from what KE5FX writes in his document there are a few other things to look out for.On several PCB's there are High Ohm carbon mass resistors made by AllenBradley. Several MegaOhms.During my attempts to repair the analyzers, I have found these resistors to age/drift much above and outside their original specification.In some cases more than 50%. And these resistors can be found in places where frequencies are set.When trouble-shooting using the built in procedures I could get the instrumenst to work only to get unlock messages the nexttime one of the instruments was turned on.? Eventually, I replaced all of them with 1% metal film resistors. After havingdone that the problems disappeared.Ulf KylenfallSM6GXV
