Crest 4801 revival ...

[cl06]

Hello everyone,

I am Claude, from France (nobody is perfect..) and I may need your help
Here is the story, quickly:
I was owning a 2x3 channels audio system, passive filtering.
Some months ago I decided to move to Active crossover.
I decided to use old Class A amplifiers I have, "Le Monstre 8W", Hiraga "20W" and an old Crest Audio 4801 bought recently that may have been made in 1991 (according to its S/N).

For the active Crossover I bought a minidsp.
Loudspeakers are Beyma CP21 (High), Audax PR170M0 (Medium) and BEYMA 15" for low freq.

I need some help about the 4801.
When I received it, I decided to open and clean... (I had no means to listen to it at that time ...)
Please find some pics:

some surprises, broken screws (among others):

Some other views of the "beast":

Oooups .. what's that ?

I cleaned everything and changed the Philips capacitors for Cornel Dubilier 380LX183M063A082.

They are a little bit higher than the Philpps, so I made all that was necessary to adapt them, that is to say, upgraded the box, and made spacers to make the cover higher.

I reassembled everything carefully, and is now working fine, like a young boy ...except a very small hum/sizzling on left channel (channel A), that is audible slightly, and absolutely nothing on right channel, complete silence .

I measured this channel "A", and found 4.2mv AC, nothing on the other channel, meaning that something is leaking/broken/touching or ???

This hum is constant, independent of the position of the gain button.
I think it is coming from somewhere on the preamp or amp itself, as I retrieved these 4mv on the preamp.
I mean the power card seems not to be the guilty.
My knowledge in electronic are limited to Ohm's law...but if you gave me tracks I'll make all the necessary efforts..

You know almost everything...
Any idea/suggestion/recommendation would be welcome,

thanks in advance,

Claude

 

[PetruV]

Check caps and small transistors in preamp area,they can go bad

 

[Fast Eddie D]

Did it hum before you worked on it?

I agree with Petru, but often there is a problem with local bypassing components ( a cap) or low voltage power supplies. I have had this problem with two recievers in a row now. I got a Marantz for free because it didn't power up. Some quick testing revealed that the power amplifiers worked and nothing else. I went straight to the power supply and found a 12 volt regulator (7912?) with no voltage output. I replaced it and the reciever works fine. And my favorite Nakamichi had an issue with the FM humming and then suddenly it would work fine. There was a 7912 and bulging cap that I suspected so I replaced them both and now it's back in service until I get ready to recap the whole thing. (Power amp board was recapped a couple of years ago.)

I also wanted to point out that it's not safe to let your furry friend near an open amplifier. Even if it's off the caps can have a reserve. My cat always has to stick her head into anything I'm working on and when I power things up I always have to keep one eye on her.

 

[cl06]

Hello everyone,

Thanks Fast Eddie for the advice about capacitors, and you're right, they must be at least discharged before any handling, and I did it before my little friend went in the amplifier...

To answer your question, "Did it hum before you worked on it?", unfortunately, when I received it, I had no opportunity to test it...I should have insisted to find a way to test before dismantling..(we are always more clever after than before..), therefore I cannot answer such question....it is a pity..

The hum is not so terrible, but there is a problem anyway, as only one channel is making noise, and I would like to fix it.

 

[cl06]

Hello Everyone,

Small status on my activities on this matter:
1) Swapped preamp and amp cards between channels A and B. The noise is following the preamp card.
So the guilty is preamp card from channel "A".

2) I swapped OAP between preamp cards.
Still have the noise, so guilty is not among OAP's

Thanks to "Pierre" for concrete "howto"

Now I must isolate the guilty component on preamp channel "A" card...

Bye

 

[cl06]

Hello Everyone,

Small status on my activities on this matter:
1) Swapped preamp and amp cards between channels A and B. The noise is following the preamp card.
So the guilty is preamp card from channel "A".

2) I swapped OAP between preamp cards.
Still have the noise, so guilty is not among OAP's

Thanks to "Pierre" for concrete "howto"

Now I must isolate the guilty component on preamp channel "A" card...

Bye

Let me update this post:

An new visual inspection, today, revealed that one transistor (2N5551) was broken. It was very difficult to see the very slight crack in the body....

I'll order, replace and update you....