All Resistors are Bad

[kt88pppamp]

I just got through testing all the resistors in a Fender Bandmaster AB763. All but about three of them are WAY WAY out of tolerance.

Is there anything that cause this or was this a batch of carbon comps that were defective from the start, or is it too hard to say?

[EWBrown]

If the amp is old, from the 1960s, that can be a normal state of affairs, carbon comp resistors can and will drift in value over the decades, as they are affected by humidity, temperature, and especially "less than optimum" long term storage conditions in an attic, damp cellar or outdoor barn or shed. In scientific terms, the carbon resistors are hydroscopic.

A few years ago I bought a large ziplock bag stuffed full of NOS (or at least unused) 1950s vintage carbon comps, from 1/2 to 2 watts sizes, and about 90% of them were significantly out of tolerance. Some were too high, some were too low resistance. THe bag cost me only a buck, and contained several hundred carbon comps, mostly 10% and 5%, some 20% tolerance.

Best thing to do, is to remove all the old ones, and replace them with new ones, and do the same for all of the capacitors. Pretty much of a "strip it down to the chassis, transformers and tube sockets, and then start over again" job. The pots may or not still be good.

Heck, I'd change out the tube sockets, too, unless they look in very good, uncorroded condition, or if they are riveted in place. (though really bad sockets could be replaced by drilling out the rivets and then screwing new ones in.

HTH

/ed B

[kt88pppamp]

I just got done servicing my boss's Plush PS1000 Fender knockoff. A few of the resistors in that baby were bad, but most were fine. I replaced all the caps and rerouted the filament wires so it would hum less.

The jacks were bad so I replaced those as well.

He said it now is back to the state he remembered it in his youth,

[TomMcNally]

I made a simple set of divider resistors the other day to knock down
a stereo signal with a 5600 ohm and 3300 ohm. The channel balance
was waaaaaay off. I measured the 30+ year old carbon comps and
they were 100% off. I tossed them and found some newer resistors
in the parts drawer and they measured within 1% of marked value
and of each other.

[kt88pppamp]

I just finished servicing the AB763 amp, replacing just about every component in it, including all the pots. My main motivation was that someone glued on some the knobs to the shaft, TISK TISK!

Anyway, I just discovered both the 12" speakers in the cabinet are bad. One has an open coil and the other is damaged, judging from the soft distorted sound that comes out of it.

The first speaker is a Fender special design speaker. The second is a Jensen special design.

Jensen speakers are still available but there are many to choose from. Which ones are direct replacements?