Threshold S/300 optical, turn-on thump & offset

[HyKlas]

I've read through a few threads on the thump issue but figured I'd bring it to the forefront again.

Recently purchased one of these guys and want to get it going tip top again. That said, there's a pretty good thump during turn-on (haven't measured it yet) and my offset is a bit... off... Readings there are 24mV left and 16mV right. While this isn't terrible, it's not great either.

I do plan to recap the few ELs/tantalums and maybe change out the Mallorys. The service history on this unit is fuzzy but apparently there was a single transistor changed out sometime in the 90s. I also plan to track that down and see what got subbed in but from a quick look nothing seems to have been disturbed.

As per the thump, what does one recommend here? Happens on both channels. All rail voltages rise sharply at turn-on. All rail voltages drop sharply at turn-off to about +-0.8V after 3 or 4 seconds. Not sure what I'm supposed to see on this amp so I'm giving all the data I've collected thus far. As stated earlier, no solid measurements as of yet. My bench is full of a 100lb amp at the moment...

Thoughts?


On a side note, are these 29000uf Mepco cans original?

 

[Zen Mod]

output offset in range of 25mV is great

replace tantalums and small elcos , then try

borrow ESR meter and test big cans

 

[HyKlas]

Suppose I'm just used to seeing my amps at 10mV or less. Out of any amp I have owned this has the highest offset.

I need an ESR meter anyway so I'll put a cap list together and order one of those as well. Any harm in bumping the capacitance up here if they need replacing?

 

[Zen Mod]

for PA amps you can tolerate everything up to 100mV

for domestic amps , 50mV is completely sane range

all that - measured in temp. equilibrium , where even something as 650mV of starting (cold, just powered on) offset is nothing to worry about, specifically speaking of amps without servo circuits

regarding caps - you can squeeze even 47mF in , instead of original 29

 

[HyKlas]

I'm a bit of a perfectionist, bordering on anal and OCD, but I understand 50mV is "okay". After a quick check I found these Epcos/TDK units which seem to have pretty good specs and should fit perfectly. Reason not to if necessity deems them replaced? I'm no pro at this so I must ask too many questions sometimes (most of the time).

B41456B9479M EPCOS (TDK) | Capacitors | DigiKey

 

[Zen Mod]

these are good caps , so you have my blessing

also - you can always check other places for computer grade caps

say Apex Jr.

you can e-mail him , if you can't see anything adequate on web page

 

[Zen Mod]

Radial Capacitors

I see few candidates

you can ask for production date in e-mail

I wouldn't hesitate buying up to 15yrs old , of course with some re-forming procedure , prior to putting them in amp