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Old 10th December 2012, 01:51 PM   #11
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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.
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Old 10th December 2012, 01:54 PM   #12
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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
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Old 10th December 2012, 02:02 PM   #13
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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.
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Old 10th December 2012, 02:06 PM   #14
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Thank you very much.
Unfortunately I live in Cape Town, so I won't be able to visit them, however I will phone and ask for a schematic.

Would you think the problem is an electrolytic cap?

Thanks again!
Chris
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Old 10th December 2012, 02:20 PM   #15
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I will suggest replacing the output relays. Bad relay contacts can result in distortion.
Try tapping on the relay case, the distortion may disappear.

Last edited by tommy1000; 10th December 2012 at 02:23 PM.
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Old 10th December 2012, 02:24 PM   #16
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Quote:
Originally Posted by tommy1000 View Post
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
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Old 10th December 2012, 02:37 PM   #17
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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.

Last edited by homemodder; 10th December 2012 at 02:40 PM.
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Old 10th December 2012, 02:57 PM   #18
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Thanks homemodder. I'll give them a call tomorrow!

In the mean time I've taken out the DC offset pot and measured as per: http://i.imgur.com/aRPtQ.png

Does that look correct?
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Old 10th December 2012, 03:09 PM   #19
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As per http://i.imgur.com/aRPtQ.png, the bias trimmer pot measures 440R between points B & C.
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Old 10th December 2012, 03:21 PM   #20
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This depends on the value of the trimpots, whats written on their side 470 for the bias one and 200 for the offset one. Also where is the dc offset one turned max clockwise ?
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