Precision Acoustix 250

undefinedza · 6th October 2012, 01:13 PM

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

undefinedza · 6th October 2012, 01:22 PM

Close ups of the main PCBs attached.

What kind of design is used? Specifications?

How easy would it be to diagnose and solve this problem?

Thanks guys.

sreten · 02:32 PM

Hi,

Complementary paired Mosfet output stage.
Around 150W or so 8 ohm, 200W or so 4 ohm.

You might be able to fix it by comparing the two channels and swapping components.

rgds, sreten.

undefinedza · 6th October 2012, 02:25 PM

Hi streten,

Thanks for the response!

I'm hoping that the fix is easy.

I think I'm going to pull the trigger on this

Cheers,
Chris

rudi · 6th October 2012, 08:14 PM

they will be easy to fix. I have the circuit diagram for you and can also show you how to mod them to sound much better.

you can buy them with confidence.

undefinedza · 6th October 2012, 08:20 PM

Quote:

Originally Posted by rudi

they will be easy to fix. I have the circuit diagram for you and can also show you how to mod them to sound much better.

you can buy them with confidence.

Thanks Rudi!

Much appreciated

I'll chat to you soon.

Chris

undefinedza · 10th December 2012, 11:21 AM

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

undefinedza · 10th December 2012, 11:48 AM

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

sakis · 10th December 2012, 12:10 PM

Quote:

Originally Posted by undefinedza

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

undefinedza · 10th December 2012, 12:45 PM

Electrolytics first?

6th October 2012, 02:23 PM	#3
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Hi, Complementary paired Mosfet output stage. Around 150W or so 8 ohm, 200W or so 4 ohm. You might be able to fix it by comparing the two channels and swapping components. rgds, sreten. __________________ There is nothing so practical as a really good theory - Ludwig Boltzmann When your only tool is a hammer, every problem looks like a nail - Abraham Maslow Last edited by sreten; 6th October 2012 at 02:32 PM.

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6th October 2012, 02:25 PM	#4
undefinedza diyAudio Member Join Date: Sep 2011	Hi streten, Thanks for the response! I'm hoping that the fix is easy. I think I'm going to pull the trigger on this Cheers, Chris

6th October 2012, 08:14 PM	#5
rudi diyAudio Member Join Date: Jul 2004 Location: .	they will be easy to fix. I have the circuit diagram for you and can also show you how to mod them to sound much better. you can buy them with confidence.

10th December 2012, 11:21 AM	#7
undefinedza diyAudio Member Join Date: Sep 2011	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

10th December 2012, 11:48 AM	#8
undefinedza diyAudio Member Join Date: Sep 2011	Tried using the MOSFETs from the working amp in the broken one... no luck.

10th December 2012, 12:45 PM	#10
undefinedza diyAudio Member Join Date: Sep 2011	Electrolytics first?