17 February 2017

Pioneer SX-800 - round 3

The stereo again?
So soon you're wondering!
Well that's how to get a project done.
Don't stagnate too long,
it'll start turning into a shelf.
Plus what else am I to do 
with the family 1000 miles away!!
A quick recap...
the problem was a scratchy electrical noise out of one side.
Switching tubes, cleaning pots and recapping didn't fix anything,
it actually made the scratching more clear.
I found a copy of the original sx-800 manual,
with the master list of resistors and capacitors.
About 111 resistors in this thing!!
The list made it much easier to order,
otherwise most resistors use a colored band/stripe code.
That would have taken forever.
The downside was I bought all the resistors and a few ceramic caps.
Fortunately only $45 or so shipped.
I over-researched the replacement variations.
There are wire wound, carbon and metal film,
with multiple versions of each type.
Online companies like Digikey and Mouser 
stock basically all the variations for $$,
however a site called justradios.com
specializes in tube restoration,
and they carry what works in most instances.
Their ordering menu is quick and easy.
The down side is there selection is more limited.
Instead of a large pipe style wire-wound,
they had the newer ceramic style,
which needed some hot rodding,
more like rat rodding.
JB weld rated their epoxy at 500 continuous degrees.
This was the perfect test piece.
Good thing these have a shield,
what an ugly mess!
It took two short evenings to replace this pile.
By rough calculations this would take 20 evenings to finish!!
Looks like a big blur to most of ya I'm sure.
Underneath I concentrated on the tone control portion front center,
bass and treble,
as there was a weird thump when the low filter switch was switched.
Out of curiousity the old Heathkit multimeter was brought out.
Let's check out the ohms! 
This carbon dogbone style is known to degrade with time, moisture and heat.
Nothin was way out of spec,
as these all have a 20% tolerance!
Interestingly enough,
the resistors in the pics above
don't match their #'s on the sheet.
An 8 degree colder change in temperature changed them by .02...
Guess what?
This thing sounded perfect!
No more annoying scratching noise.
Bummer was not knowing exactly which ones were the culprit.
Those brown circles - ceramic capacitors,
looked questionable but I can't test them.
Now what to do with the rest of the pile!
I'm figuring that the resistance changed with the tube heat,
well beyond their 50 year old specs.
I'll probably swap at least 20-30 more out.
After about 4 hours burn time,
no big pops of smoke so far!!
Just in time too,
the boys flew up here to visit,
and snagged my PDX listening station!
Good times...

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