Pioneer A757 MKII - Power caps replacement?

[Jinxid]

Hello! first time I'm writing here.. Been lurking and actually fixed my NAD c350 with hints from here, basically swapped all the electrolytic caps out, as many had been drooling their fluid, and the powercaps were so dry they rattled when i got them out. - So thanks forum, for the posts!

Anyhow, after that success I dug up my old love, a Pioneer A757 mkII. It's been standing around for around 10 years because the input switches and volume pot was making the sound crackle and dissapear. - I fixed that now with simple contact cleaner and a lot of massage.

When I hook it up now, it sounds a bit muddy and imprecise as soon as I increase the volume above "normal". Thats not how I remember this amp.
- It's a powerhouse, but it is now 27 years old, and almost I'm sure it's suffering from the same failing caps as the NAD (although not at all visible).

The caps in the power section are some wierd special made "Shielded capacitors" - and looking from underneath it has 6 pins, and a ground wire going to them. So, they're impossible to find on the net, discontinued, and the replacement also discontinued.

This is someone elses pictures, they're much better than what I can take

Is it possible that a normal unshielded cap of the same size would work? - Can I make a normal unshielded cap, shielded? - I have some of that mesh-alu conductive tape that is for shielding guitars and cables and what not... and some very big heatshrink from other hobbies.. but I'm just not sure if it's something I should worry about?

Sorry for the long ramble, thanks in advance!

Best regards
Katrine

 

[mbz]

Maybe post a pix of the pins.


At the end of the day it's just an aluminium electrolytic cap, after 27 years it's probably
approaching end of life, depending on environment. Typically the extra pins are just for
mounting/stability. Problem with finding replacement caps is fitting in the available
space and pin style, then audio qualities. Consider something like United Chemicon

KMH or SMH 22000/80V, subject to pin out.

 

[huggygood]

I often work on old high-end Pioneer and I can tell you that I have rarely seen a power capacitor out of the spec.
However, if your amp has been dormant for a long time, it may be formed a layer of oxide inside your electrolitics, it is often enough to run a few hours so that it fits in the order .
on the other hand, what you can do (rather than spend a fortune in large capacitors) is to buy a capacimeter

 

[Jinxid]

Hi, thanks for the replies!

- the pins are definately not for mounting, the caps are held by a copper bracket in a clamping fasion and screwed down to the chassis.. The legs are completely in open air, all of them. i'll try take a good picture and add it here when i get back to opening the amp.

I will try let it play a couple of hours first as suggested, since it's assembled currently.

I will borrow an ESR meter as soon as possible and see if I can measure the power caps, I'm unsure how i guarantee they are empty before measuring - as I understand, the ESR meter will blow up if it has charge on it.

Thanks for the suggestions

 

[Mooly]

I think the pins will be for mechanical stability tbh. Although multiple section caps (several caps in one can) and with perhaps 5 pins do exist they were for specialised applications and the wrapper would identify them as such.

Yours are just 22,000uF caps. The band is for even more mechanical security when mounting.

I to would doubt they are out of spec in any way.

 

[PRR]

The A757 does a lot of "trick" things of dubious merit. The hex-stamped panels always amused me.

It's a beast. I don't think the power caps come into play until ear-bleeding levels.

I too say: just play it a while. It may be getting older, but I think there's some exceptional detailing among the frills.

 

[Jinxid]

Hi, finally got pictures of the bottom side of things... As you all said, its clearly just "normal" 2 poles, if there is shielding somewhere underneath, is impossible to tell.

I'll try play to it a bit before i run out and spend money. - I found some screw-terminal caps that fit the physical size, and the capacity/voltage. I cannot find any 80v caps that will fit inside.

Funny enough, I like the hex-stamped plates, they are much much stiffer than a normal flat plate. The amp is 20kg so i like that it does not flex as you turn it to dismantle somthing.

Thanks for the advice, i'll power it up and run it today on my speakers and see if it clears up!

 

[Jinxid]

She lives on

I've been running the amplifier with my speakers for some time now, and honestly it seems like it's working great. As far as my novice ear is concerned, the pair play really well and clear well beyond any sort of volume i'd ever want to listen to. - I think the speakers will pop before the amp runs out of breath.

SO! thank you for "holding my horses", I will go on to spend my time on trying to find some diy speakers for the tv (because why not! ), which i think I will let my restored NADc350 run.

Best regards, and many thanks
Katrine