[Boatanchors] Backwards Microphonics?

[Barry Hauser]

Course correction ... 

I think the sound _is_ coming from the output transformer, as best as I
can tell with my screwdriver "stethoscope".  I have come across this with
open frame transformers where the cure is to wedge a toothpick or
something between the coil and frame.  Can't do that with these -- is it
overdue for an oil change?

Flying blind without a manual.  There are a pair of 600 ohm speaker
terminals on the back of the chassis, but also two sets on the on the
side -- one for 600 and one for 20K.  The audio transformer is dual
output, and there's a jumper across the "600" pair, to set it up for that
output impedance, I guess.  There are also three trimmer pots - two on
the left side, one on the right -- unlabeled except for part numbers. 
Anybody know what those are for?

Barry



On Wed, 25 Dec 2002 17:09:19 -0500 Barry Hauser <barry_hauser@juno.com>
writes:
> Hi Gang
> 
> I've come across microphonic tubes in my time -- like one in an 
> HQ180
> where it was picking up my drumming fingers on the table and even 
> my
> voice if I yelled at the front panel. That was a 6GH8, as I recall.
> 
> But the microphonic in this E.H. Scott SLR-F goes the other way. 
> It's
> more of a "speakerphonic". I was checking it out using headphones.  
> Took
> them off and I could still hear something.  Unplugged the phones and 
> it
> was still there.  (Yes, with no speaker connected and, yup -- the 
> sound
> stops when I switch it off, just in case there are any jokers in 
> the
> woodwork -- besides me, that is. ;-).
> 
> Can a tube do that?  I would suspect the output transformer, but 
> the
> sound is coming from the opposite side of the chassis.  Besides, 
> the
> audio tranny is an oil-filled Thorardson so I doubt if the windings 
> could
> be loose from the core or something.
> 
> Aside from the rectifier, all the tubes are metal types.
> 
> This is one of those pre-war/WWII receivers which was over-built 
> with
> extra shielding for use by the Navy to minimize RF radiation and,
> thereby, detection by the enemy.  I don't know about AF radiation -- 
> just
> a sec ... Hey you!!!  Loose lips sink ships!!!.  Nah, I don't think
> that'll work.
> 
> Are these like the AR88's audio setup?  The word is on AR88's that 
> the
> output transformer can get wrecked if operated for more than a short 
> time
> on headphones alone, without some load on the speaker terminals.
> 
> Any clues will be appreciated.
> 
> BTW -- Merry Xmas to All
> 
> Barry
> 
> ________________________________________________________________
> Sign Up for Juno Platinum Internet Access Today
> Only $9.95 per month!
> Visit www.juno.com
> _______________________________________________
> Boatanchors mailing list
> Boatanchors@mailman.qth.net
> listinfo/boatanchors
> 
> 

________________________________________________________________
Sign Up for Juno Platinum Internet Access Today
Only $9.95 per month!
Visit www.juno.com