unhooked the PT from the RC filter - and no tubes in the amp:

transformer side of the main filter cap shows short to ground.....boooo

its the can cap, for sure. all the other leads of the can (its 20/20 and 40/40) seem fine. would i be okay to just interject a separate large filter cap where that first section of the can cap is supposed to go?? think im gonna try that - seems a shame to replace the whole can if only of the sections is shorted, unless that means its time to toss it.....

no light bulb short without the rectifier -

with rectifier tube removed:
transformer end of the main filter resistor shows short to ground - other end shows 50 ohms to ground....
plates on tube socket also show ~50 ohms to ground....but they're looking through the PT....

ill detach the filter cap from the PT and see what we get....

looking like a bad filter cap?

this old kay has a short -

blew a fuse -

with all tubes except rectifier 5u4 pulled, lightbulb shows a short -

the rectifier tube lights up bright on one side only and then goes dim as lightbulb starts to glow bright -

tried diff rectifier tube - same light show

all power resistors, etc look and test OK - could main filter cap be shorted?! or PT rectifier coils....??

no DC anywhere except some negative 14 and 3 Volts on the rectifier plates?!

schematic: http://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/kay_720.pdf_1.png

k recapped power amp and hum is gone - still a bit noisy but preamp probably needs a recap, too!

otherwise, amp sounds great!

it was 120hZ hum - and it was originally about 20 Vp-p and went down to about 1-2Vp-p

not sure if a single cap was to blame - i got a bit impatient, i have to say, so i just recapped whole thing (but left original power supply filter cap - that would probably help, too, but for my purposes wasnt necessary) and seemed to do the trick!

thanks, guys!

Replaced output cap and ripple is down!

We're still humming, tho....I'm just gonna recap whole power amp see if that does it

For the output cap, should I be concerned with any other parameters other than voltage? Wondering if it has to be really beefy...

q302 looks fine/voltages clean

hum appears after R310/C308....

sure it cant be c309?

with a scope and the amp unloaded:

there is normal small ripple on the main filter cap as well as on the 75V power rails

there is a big 120hZ 20VAC ripple on the speaker out (37V rails) -

i do measure some DC on the speaker out as well (but that's with no load plugged in - maybe thats the output cap charging through the zobel network?)

there is a loud hum when amp is plugged in, but not 100W loud

maybe C309 or C310 in the power amp...

...

hmhmhm

upon final inspection, i'm seeing 20VAC ripple on the 37V rails/on EITHER side of the OUTPUT cap -

output cap to blame? easier to replace, too, phew

whoa good catch!! wheres the date code?

the power supply is in the file i attached in my first post -

its a 80V cap - i thought a few volts headroom was a good idea, too - ill check the mouser part!

whoops!

20V ripple on the positive rails!

replace filter cap??

http://www.electronicsurplus.com/sprague-36d392g075bb2a-capacitor-electrolytic-3900uf-75vdc - this one look ok?

kay back to this guy -

zobel network replaced -

open thermistor in output transistor bias network replaced -

still a bad hum when amp is on -

measured at speaker terminals, i get about 20VDC - it seems to go up and down when the amp is turned on/off, but definitely DC there -

whats the most likely suspect? the huge output coupling cap?

hmmmm ill do some more snooping -

the zobel cap was only rated for 100VDC, so maybe a mains spike shorted it?

whether or not it was something simple is not, unfortunately, for me to tell - i bought this amp at a swap meet as a fixer-upper -

replaced the zobel -

looks like things are working OK but there is definitely an oscillation - a square wave in the input looks bad at the output - sharp overshoots on the edges. and a sine wave looks wobbly and double-vision.

any strategies for tracking down this kind of problem, aside from trying to isolate whether its pre- or power-amp?

QuoteAs usual, itnbeing dead id more of a symptom than the cause of a problem.

what might be the cause?

QuoteAll the amp does is provide an output voltage, the load then determines teh current draw from Ohm's Law.  The amp doesn't push out power.

doh! i knew that....

am i write to say that R404 and C404 make a zobel network across the output?

what would cause R404 to blow? my guess: amp plugged in without a load puts the output wattage across the puny 1W R404 and blows it up -

but then isnt that resistor just asking to be destroyed? ive seen one in another acoustic 150 that was blown literally in half...