acoustic 150 zobel?

[ilyaa]

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?

[Enzo]

Well, look on the amp side of the speaker cap.  This is a single supply amp, schematic says +75v, and it also says about 37v on the amp side of that cap.  No signal, no load, just powered on at idle, what is the DC voltage there.  If it is within a couple volts of half the power supply, then it sits about right.

You measured 20v DC on the speaker itself?  Or at the speaker terminals but no speaker connected?  With no speaker or load, ther is no way for that cap to charge.  In which case, slap something like a 100 ohm resistor across the output just to give the cap a chance to charge.  Does that DC settle down with 100 ohms? (100-200-whatever, value not important)

[gbono]

Did you check the power supply? I have seen the filter capacitor fail on these amplifiers - had one so dried up you could shake the capacitor and hear a rattle. There is a 1000uF/50V? aluminum electrolytic on the output that should be checked.

[ilyaa]

whoops!

20V ripple on the positive rails!

replace filter cap??

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

[Enzo]

I don't see the power supply portion of the schematic.  But assuming 3900uf/75v is what you want, looking at the description of the cap you linked, it appears to have a 1979 date code.  Almost as old as the cap in there now, plus they want $20 for it.  That is a lot for a cap almost 40 years old.

Since it is a 75v supply, I'd like to see at least a few more volts rating on the cap.  The originals used those huge screw terminal caps.  A look at Mouser showed me a 3900uf 75v cap for $48. But looking at 3900uf 100v, I see a screw terminal cap for $19.73.  And that would have a current date code, not one from 1979.  But the reason they used the huge screw terminal caps is that was what was available back them.  But if we look at other forms, there are snap-in style caps in 3900uf at 80v and at 100v either one in stack and available for under $5.  To save $15, I'd find a way to mount the snap-in.

[ilyaa]

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!

[Enzo]

The 66C7917 looks like a date code to me, 17th week of 1979.  I could be mistaken, of course.   But new or old, you can save some dough by using a more common form factor.  The circuit only cares about the specs, not what the cap looks like.

[ilyaa]

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

[J M Fahey]

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

1) earlier you said you found 20VDC on "both sides of the output cap" ...what will it be?

2) *if* you found 20VAC on what you call the 37V rail (which is not a power supply rail but the speaker out) then you must hear a deafening hum through the speaker.
100W RMS of it.

3) *if* it's AC after all, then it's normal finding it on both ends of a *good*  coupling cap, that's their job.

[ilyaa]

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...

...

[Enzo]

C309/310 will not make hum.

You are scoping?  Then look at the driver collector Q302, it drives the primary of the drive transformer.  Is there a large AC signal there?  If so, then of course the output would have it too.  Look at the +40v that powers Q302 at its emitter, and decoupled by C307, is that clean and close to voltage?

Pins C,D of the power amp module have the input signal.  Is there a hum signal there?  What if you grounded the negative end of C301, where those input signals meet, does that kill the hum.  That would tell us hum was before or after C301.  In fact, that small electtrolytic C301 is suspect in my view, even if it isn;t the immediate problem, in my experience, those small lytics from that era are very often flaky.

[ilyaa]

q302 looks fine/voltages clean

hum appears after R310/C308....

sure it cant be c309?

[ilyaa]

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...

[Enzo]

Just use what the schematic calls for.  1000uf, but if it were mine I would bump the voltage from 50 up to 80 just for good practice.  I forget, but if it is a can cap with screw terminals, it is not necessary to get the same type.  A plain old cap with wire leads will be fine, just mount it securely.  Low ESR would be good, but I doubt you will find high ESR caps advertised.

How much ripple is on the main 75v supply?  The hum you are hearing, have you determined if it is 60Hz or 120Hz?  120Hz hum is from power supply ripple, which could be lack of filtration but could also be due to ripple currents sharing ground path with other circuits.  60Hz hum does not come from filter caps.

[ilyaa]

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!