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Vintage threads from the first ten years

Stethescope?

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10/20/1999 11:55 PM

Stethescope?

I have an early japanese tube amp "orpheum" that makes more
noises than anyone would think could be possible. It is a
real power house pushing two 6aq5s to their limit ;-)

;-)

I have done all the routine things, change cathode resistors, replace a leaking coupling cap, tightened up the tube sockets etc.

I have heard of "radio men" in the early days using
a stethescope to listen to parts in the circuit of
radios to identify arcing resistors etc. Has anyone
had any luck with this technique? Is so, what sort of
stethescope should I look for?
Thanks,
Rob

10/21/1999 1:19 AM
jb	Ever seen the car machanic with his rubber hose looking for engine noises? I have used my stethescope to find cabinet rattles. It is a cheapo yard sale model and it does work. I wouldn't spend too much. Good luck. jb

10/21/1999 12:54 PM

Rob,

Well, well, another Orpheum - I thought I'd worked on the only one in existence :>. My victim has 6973s though.

Anyhoo, you are looking for electrical noise, not mechanical noise and I doubt that an audio stethoscope would be of much use. But an old tech's trick, that might have been referred to as a "stethoscope" consists of a pair of high impedance headphones (if you can find them) and a DC blocking capacitor- as long as the stage power is sufficient you can trace for noise on a component-by-component level along the signal path. Now, since not all signal stages have sufficient output to drive headphones (and they really must be high impedance) the preferred method is a low powered audio amp again fed through a blocking cap to trace circuit noise - I've got a small SE record player amp/speaker mounted above the bench for this purpose. Oh, tube amps work best (as we all know) 'cuz they better absorb circuit transients.

GW

10/21/1999 2:06 PM
Don Symes	Would heaphones/earphone from a crystal radio do it?

10/21/1999 4:01 PM

Yes. The magnetic headphones for crystal radios are (or should be, to work right with that type radio) the high impedance type, and are usually in the >2000 ohm region. The higher the impedance the better, both in terms of sensitivity and non-loading of the circuit under test. A crystal earphone would probably work, but watch your circuit voltage. Most of these are for transistor radios, where the voltages are relatively low.

GW brought up a good point. As your scrounging techniques and awareness for discarded tube gear become sharpened, you'll undoubtedly accumulate a surplus of amplifiers with 1-3watt capability. You can only build and use so many home made Champs. So take the audio amp portion of one of those old 5-tube AM table radios, or an old record player amp, and make it your signal tracing amp. With the table radio, just cut the signal wire or pcb trace and add your new .01uf/600v cap and probe wire (or mount a banana jack on the cabinet to accept various probe types or alligator clip leads, etc.). Leave all the tubes in there, because the heaters are in a series string. You have a useful piece of test equipment, with a built in speaker and volume/sensitivity control.

Don't spread this around, but I've even taken an old transistor amp from a record player and mounted it inside one of the small "walnut" speaker enclosures to make some use out of it as a signal tracer. Sensitivity is about 100mv for full output, because I used the whole circuit from where the ceramic cartridge was previously connected. It has Volume, Treble & Bass controls!

10/21/1999 7:30 PM
Graywater	Hey Doc, A-yep, jest the kinda re-using I like. But, I do avoid the good old "All American Five" AC/DC radios so I don't have to worry about line/ground problems - oh, did I mention that I'm a sort of "everything grounded" obsessive due to a few really nasty shocks I've had over the years? GW

10/21/1999 5:17 PM
ADM	I have an amp that was left in a storage space that was too humid, and it was really noisy, popping and crackling etc. I just left it on for for a few hours and the heat of operation dried everything out and it has quieted right down. Just a thought, Andrew.

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