|pardon two not-so-weird novice questions, please
||[Nov. 5th, 2007|11:19 pm]
I'm going to be recapping a Hallicrafters S-120 boatanchor with classic "hot chassis" and hum problems. First, I want to ask if there's any reason besides cost, collector "original" value, and space, to not permanently add an isolation transformer beyond the testing & repair phase. I use this in my music studio as a sound source and as a weird processor (via my RF generator -> antenna connection) and it would be nice to eliminate concerns of nasty voltages while on creative autopilot. The S-120's rear panel is ugly & worn cardboard, so it wouldn't be much to find a way to accommodate a transformer (wrapped in copper/aluminum tape?). Secondly, I'm wondering if this at $36 be appropriate, insufficient, or overkill for the job. Thanks.|
2007-11-06 03:39 pm (UTC)
Hiya. Pleased to see a fellow boatanchor user... I don't see any reason not to isolate. Now, I would suggest you put the isolation transformer off to the side, next to any other power transformer the radio has. Don't put it right next to any tube (other than a rectifier tube). If you notice any new and interesting hums, try moving it, or mounting it elsewhere. As for the whole 'collector's value' thing... if the radio doesn't look absolutely perfect like it just came out of the box, don't worry about it. I believe in actually USING equipment.
It's far beyond agonizing over collector concerns, with the original ferrite AM antenna gone, black marks from tubes overheating (perhaps exploding, even) before it was mine, twice-replaced power cable, etc. It's pretty amazing that it works at all. I'm right there with you on USING (and abusing) things and that's part of why I buy junky old gear in the first place - I won't question myself when it comes to drilling holes in the chassis or making it more functional than it ever was to start with.
It's an AC/DC radio with NO existing transformer - just a (friggin dangerous) cap coupled from one side of the power line to ground. I'm replacing that with a newer safety cap, but I figure the transformer might be worth it for a little extra insurance. I guess the only concern I could think of was what you mentioned, adding a new source/target of interference & hum. At this point I'm just hoping I can locate a cheaper source. I suppose transformers are one area I've neglected to really understand and I should get on top of it.
You know, your name looked familiar to me, and sure enough, you helped out way back
when I'd found my first SW receiver. Thanks again!
If you have concerns about the aesthetics of the radio for collectible value, just use an external isolation transformer under the table. You'll still get the safety of the isolation transformer but without having to add it somewhere in the case.
Isolation is a good idea, as is adding a polarized plug that will ensure that the terminal connected directly to the chassis will always be the neutral. No need to mount the transformer in the case, since in my experience most isolation transformers despite their metal end shields are often a big potential source of induced hum and EMI.