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Chip Amps Amplifiers based on integrated circuits

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Old 10th March 2008, 02:12 PM   #1
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Default LM4702 meets the Pioneer SX-838

The first "stereo" I purchased (as opposed to "built" or "received as a hand-me-down") was this Pioneer SX-838 -- I bought it used at Harvey Electronics on 45th Street and lugged it back to the apartment in Brooklyn Heights -- circa 1976. Still has one of the nicest FM receivers I own -- but the protection circuitry went wonky and the device would blow output devices all the time. If you can't find one of these receivers in the junk yard you can buy one on the Bay for about $30.

Turns out that the printed circuit boards I had burned for the LM4702 fit almost perfectly onto the heatsink for the Pioneer SX-838. I had to put the HS on the mill and grind down one of the fins a bit so that the spacers would fit:

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Here's the original output board and its location:

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The LM4702 Board mounted on HS:

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Old 11th March 2008, 11:52 PM   #2
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Old 12th March 2008, 12:01 AM   #3
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Nest we have to tackle this -- and it is going to be a challenge to access the PCB w/o taking the whole thing apart:

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Old 17th March 2008, 08:07 PM   #4
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Changed out the coupling caps to WIMAs and BC Components, replaced old electrolytics with new Panasonics, increased the local filtering from 47u to 470u.

Even with the tone controls in the THD% is now down around 0.01%

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Old 20th March 2008, 06:47 AM   #5
dfdye is offline dfdye  United States
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I just noticed this project. I love it when old stuff becomes new! That looks like more fun that I would know what to do with!

Have fun.
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Old 20th March 2008, 12:43 PM   #6
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One of the issues which all these devices had is that all the connections are wire-wrapped. These included the ground returns. Well, I brought all the ground connections to a central locus near the filter caps.

The volume pot had to go. No amount of DeOxit would cause it to remain clean through the sweep.

The amplifier electrolytics were 8,000 uF -- these were replaced by 39mF Panasonics which I had on hand. They are only slightly larger than the 1970's vintage filter caps.

One thing leads to another -- I have replaced all the electrolytics on the power supply board after noticing a low level buzzing - the power supply diodes went as well. It is much, much quieter now.

I must say that it sounds much, much better than the original. It is a receiver, after all, and the only function missing is the "Loudness" control (which was killed when the volume pot had to be replaced.)
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