Silvertone 7048 audio problem

2/24/2009 12:04:07 PMCraig Roberts

Okie doke, gurus. I've re-done an ex-basket case Silvertone 7048 console, including a complete re-cap (except IF transformer innards), re-wiring, new resistors as needed and a strip and refinish of the cabinet. It's gorgeous, BUT...

When I turn it on, the onrush current produces a low volume squeal from the speaker which quickly dies out but is replaced by an annoying crackle and unnaturally low output volume. The crackle is largely unaffected by the volume control.

I suspect silver migration from the IF transformer caps or -- more likely, I think -- some arcing in the audio output transformer.

Either scenario leaves me baffled as to how to fix it. If the caps are bad, what value silver micas should I replace them with? If the transformer's bad, how should I wire the replacement? (I have a nice Hammond push-pull output xfmr).

Her's a link to the schematic (last page):
http://www.nostalgiaair.org/Resources/630/M0017630.htm

2/25/2009 12:46:21 AMWarren

I think you might try and disconnect the audio input from the output. That way you can tell if the problem is in the output. If you think you have the silver mica problem, google " hot rodding aa5 " there is good information in there about how to repair IF transformers, or replace capacitors if need.

2/25/2009 8:41:39 AMRene

Scroll down a ways and read an excellent posting by Edd on the very topic of IF cans suffering from silver migration. I think the thread was called "Anatomy of an IF transformer" or something like that.

2/25/2009 9:27:52 AMCraig Roberts

:Scroll down a ways and read an excellent posting by Edd on the very topic of IF cans suffering from silver migration. I think the thread was called "Anatomy of an IF transformer" or something like that.

2/25/2009 9:35:06 AMCraig Roberts

Thanks for the IF transformer info, my friends. I may need it soon.

I'm going to try the audio transformer substitution first, though. My Hammond transformer is a muklti-tap device, so I can play with impedances.

One reason I think it may be the transformer is (this is really embarrassing), upon reassembly in the refinished cabinet, I mismatched the speaker/field coil plug and the chassis jack and possibly zapped the transformer with the filed coil's supply voltage. Tjis cannopt be good fot eh transformer windings.

2/25/2009 4:12:07 PMEdd

Could be. . . . that the output transformer was damaged, they also don’t take too lightly to not having the speaker connected
on as a load, and then one not hearing any sound, and then typically responding my upping the volume to max, with a potential inter/arc /
over of the primary winding. Easy enough to do by subbing in the new transformer and a listening test period.

After looking at that sets top chassis it certainly seems that the set is utilizing the large full size I.F. cans. You might just confirm that for us. On those units I would expect the internal construction to be having the two variable cap sections be made on a large square ceramic bisque block at the top side. The manner of construction being alternate interlaced leaves of thin metal with each having a thin connector tab passing thru the ceramic and then sheets of mica placed between each sheet.

A centrally placed pan head slotted screw is passing thru holes in the center of the leaves/mica spacers and
is then compressing / decompressing on the mass for making the capacitive adjustment.

The inductor portions of the transformer is then utilizing two coils with fixed values.

Seems like the trouble came later on in the evolvement of I.F. transformers wherein they started making the transformers
in a smaller footprint and eliminated the variable capacitance aspect and accomplished the variable inductive tuning via two
adjustable slugs within the two formerly fixed values coils.

That then done away with the massive variable capacitance aspect and then they could be replaced with two physically small, fixed
value silver mica units.
With the size then reduced, seems like the automated manufacturing process upped the ante by shifting into making the
capacitors by the taking of a mica sheet and either masking and spraying / silk screening “silver print” onto both sides of
it after having computed the surface area required for that approximate capacitive value.
The final trimming in of the I.F.
to proper frequency, is then being accomplished by the variable inductors.

Seems like the later real fault with that procedure was with them utilizing compressive metallic contact areas to
make interfacing connection into those silver layered mica sheets. The silver, with time, tarnishing and turning BLACK
with silver oxides just like your great aunt Murgatroids silver plate ware. That condition being exacerbated by airborne
sulphuric and nitric elements, carried within humid air conditions, to then react with the silver and further multiply the
problem and the onset of the “creeping” of the enlarging oxidic mass.

I certainly do believe that your unit is of such age, that it ISN’T even using that newer type of construction.
That was why I put the info on those specific larger Zenith units, as most people are familiar with the newer “AUTOMATIC /
K-TRAN families of units that I just described, but they had never actually encountered that type of Zenith units.

If my set to trouble shoot. . . . . after clearing up the output transformer aspect. . .I initially would have the unit doing its
“popping thing” frequently enough. . .be that cold or after a tube and cabinet warmup and then short out the 1st grid of an output tube to ground
to see if the "pops" would occur during a short ~5 min evaluation. If it even pops, then move on to pulling the jump wire
on that output stage tube and moving it to the other output tubes 1st grid. And then the next point would be to ground the
first grid of the ‘Q7 phase inverter stage and listen for pops.
Then, the last point in the audio string, being the grounding of the 1st grid of the “Q7 first audio tube .

Finally, to differentiate the sourcing of noise to be from the RF /detector section versus power supply / audio section, you
have an access point that isolates the front end when you pop that selector switch from its RADIO position to the PHONO-TELEVISION---FM---“SATELLITE”---“I-POD” position and evaluate for noise, either with no signal at all coming in, or with a ½--1 volt level of audio signal being put into its input jack.

Standing by. . . .

73's de Edd

2/25/2009 4:36:58 PMCraig Roberts

Great info, Edd! Many thanks -- and, yes, they're the big, older construction IF cans.

I'm pretty sure my problem's with the audio output xfmr given my punishment of it. The noise is independent of the volume control and is present with the audio selection in the phono (satellite, iPod) position. Due to work commitments, I can't play with the set 'til week after next, but I'll certainly post results when I do.

VY 73,
Craig