ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

To all you guys who adjust bias pots

:

5/10/1999 5:11 PM
Loudernhel	To all you guys who adjust bias pots Radio Shack sells a set of two "non metallic tv adjusting tools" for 99cents that is perfect for this job. I know, I know: "I've stuck my Stanley screwdriver in there for years and never caught an arc." But hey, it's 99cents. Is there a reason NOT to use them? Regards, David Becker

5/11/1999 8:52 AM

Older amps, like the typical BF/SF fenders, have a full size pot (like the volume control) with a nylon (non-conductive) rotor where you plug in your screwdriver tip. And since it's full size (1/4"), a screwdriver actually fits. And there's plenty of room to work in there, so it's unlikely you'll slip off the target and create sparks.

But more recent amps built on printed circuit boards tend to have miniature trimmer pots for their bias adjust. Some have slots so tiny, compared to the early standard described above, that alignment tools such as you propose are the only thing that fit. Even a small screwdriver may not go in, because of the widening taper of the blade.The all-plastic alignment tools have small straight tips, and are great for probing blindly, which is becoming a necessary skill on some of these amps that are compactly built.

There is a trimmer adjust tool commonly available (although I'm not sure if at Radio Shack) that is a plastic pencil with a tiny metal blade tip at one end, and a similar, but fully recessed blade at the other end. These things probably cost about $3, but they're very useful. (The cost of a beer, but you get to keep it rather than just rent it.) The external blade will fit down in the center of any of the disc type trimmers, and the recessed tip will fit those external screws on the multi-turn pots (like Bourns) so that you can turn the screw w/o the blade slipping off. AS you can probably tell, I like mine a lot.

5/11/1999 9:12 AM

I use a non metallic tweaker even on BF/SF because they beat it into my head at school not to put a metallic tool inside an energized chassis.

I use a tweaker like the one you describe that Raytheon sells to the govt for about $9 so about $3 sounds right. I don't think I've seen one like it at the Rad Shack. I only recomended the Rad Shack because everybody has one (for better or worse).

regards,
David

5/11/1999 6:00 PM

David:

For working on equipment with RF tuners (not most guitar amps except the ones that pick up the local radio station!) it is very important to use non-metallic adjustment tools because the metal could throw off the tuning. But even inside guitar amps, a metal screwdriver could act as an antenna that picks up RF noise and transfers it to the audio circuit. Good advise!

Steve Ahola

P.S. Some people who do a lot of electrical work will wrap their screwdriver blades with electrical tape just so they don't short out against something inside the electrical box. Or you can buy a set that has a plastic coating on the shafts.

5/11/1999 11:40 PM
Bruce	Yeah, if you use a small screwdriver use it with heat shrink tubing on the whole metal shaft right up to about the last 1/4" of the blade. Bruce

5/12/1999 8:50 AM

Another tech I work with was struggling for hours once to do an alignment on an HF radio. He would tune it up to spec. Take it back and reinstall it and the damn thing wouldn't work right. He looked at couplers, antenna, transmission lines etc and finally (correctly) concluded the radio was at fault.

I provided peanut gallery support and commentary for a couple of alignment attempts until it hit me like a ton of lead that he was tweaking on a variable inductor with a thin bladed metal screwdriver. Essentially that adds to the mass of the core, which screws things all up. We tried my tweaker and it worked perfectly.

As a joke I wanted to install it back in the rack with the cover off and the screwdriver sticking out of it (it worked fine that way too!) but I got vetoed....

Sometimes its the little things that kill you.

regards,
David

5/15/1999 1:07 AM

I just have to add that I had a sony dual cassette deck which sat on a shelf for years as it was made w/o a p.t. (as part of a set of components that had to be interconnected) I put a trans in it and used it in my studio but there was no speed control on the panel so I drilled a small hole in the top and shaved a chopstick just right to poke into one of the trim pots on the board for the playback speed ! ..put a fluted knob on it and used it for years that way !

Page 1 of 1