Precision Acoustix 250

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Hi everyone,

I have the opportunity to buy two Precision Acoustix 250 monoblocks.

They were built locally in South Africa, and very, very little information is available about them.

One of the monoblocks distorts slightly (unfortunately the seller is in another town, so I can't listen), and the seller claims that a resistors looks slightly blackened.

The price seems pretty good, but before I commit to buying, I'd like to get some input on fixing it.

I have attached the two pictures he has provided. I have asked for a close up of the main PCBs and will post them once I receive...

Any comments on the design, repair, potential specifications, etc would be much appreciated.

Thanks,
Chris
 

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Hi all,
I'm still struggling with these.

I've replaced all of the small signal transistors (BC546 and BC556) on the left and side of the board, as well as the input stage transistors (is that correct?) which are BF470 and BF469.

No luck. Same distortion. I've looked at one monoblock vs another and they look identical. The only difference is the output MOSFETS, which seem to have been replaced on the new monoblock. The current output MOSFETS test fine.

Any ideas?

Cheers,
Chris
 
Tried using the MOSFETs from the working amp in the broken one... no luck.


obviously .... replacing outputs that already work from one amplifier to another will not change anything ...... transistors is like a car wheel either working or not

keep the transistors as was ...check soldering recap the all unit ....

expect any of the small transistors to have some small leak
but 99% of the cases is some of the small capacitors doing the trick


Kind regards
sakis
 
Disabled Account
Joined 2006
A walk down memory lane.
I built maybe a dozen or more of these when I was still in my teens, they sounded excellent at the time, no need to mod anything, just get them working properly. If I remember correctly its a dual differential input stage driving two seperate differential transimpedance stages. The name Precision Acoustix doesnt tell me anything but as far as I know these modules were only available as kits and were sold by only one electronics parts store, that is back in SA. They were also available at 120w modules and originally used 2sj50/2sk135 mosfets. I still have a pair of these amps in my garage somewhere.
Have you been able to get the schematics and literature on them ?? - I just might have a copy lying around somewhere. Anyway the store that sold them was still in operation 8 years back but had moved from the middle of the city (johannesburg) to the rosenttenville suburb, they might still be around. The store sold the modules to be used with a JLH preamp (also kit form) and these combined sounded excellent.
 
Hi homemodder,

Thanks for the response!

As far as I have gathered, they were originally sold in kit form by RSE Electronics in Johannesburg. They are still in business, and a revised version (125w) of the monoblocks can still be bought today :)

Unfortunately I haven't been able to get the schematic or any literature that is helpful.

I must say however that I am very new to this game. I have no formal training in electronics... I only know what I have picked up from the internet ;)

If you could dig up any literature that may be helpful that would be wonderful.

I'm really eager to get these puppies singing again!

Chris
 
Disabled Account
Joined 2006
I now live in europe, so maybe its best you get the literature from the store. I just checked the address of the store, here is their contact details.

R S Electronics
Company address
51 Rosettenville Rd, Village Main, 2001, Johannesburg
Phone
011 334 4158
Fax
011 334 4341

Try replacing all the caps, use good quality instead of the ordinary ones, if that doesnt help you could take the amp to the store, they had a technician working there just to build and do repairs on all the kits.
 
I will suggest replacing the output relays. Bad relay contacts can result in distorsion.
Try tapping on the relay case, the distorsion may disappear.

Hi tommy1000,

Thanks for the response. Good idea...

I forgot to mention though, DC offset on the unit is between 150 and 250mV. On the board there are two trimmer pots. As far as I understand, the top trimmer pot (with the output transistors at the top of the board) sets the bias, and the bottom trimmer pot sets the DC offset.

I can't get DC offset below 150mV and I can't get the bias above 4mA...


I probably should have started with the above, my apologies.

Hopefully that will help narrow down the problem.

Kind regards,
Chris
 
Disabled Account
Joined 2006
If you dont come right at the store I could try get all info I have in electronic form and email them to you. Replace the trimmers, the distortion will be quite high for only 4ma bias. As long as the output mosfets are good, the rest of the parts are not expensive.
 
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