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Old 6th September 2014, 09:03 PM   #1
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Default Issue While Rehabbing an STA-2100, What To Do?

I posted this yesterday on AK but haven't received a serious response, so I thought I try here.

I'm in the process of rehabbing this Realistic I've had for a couple of years. I played it a bit after I bought it but it sounded a bit off so I stowed it away until I was ready to do a full recap. Now that I've started I found one channel must have failed because there were a lot of replacement components. Since none of the components were the same as stock I decided to follow the EW's advice in this thread and go with On Semi replacements.

While doing the amp board I noticed some reconstruction and replacement of the Sanyo 2SD400P (TR506A) that's mounted to the heatsink (left photo). The replacement is not a TO-92 package and there is no stock mounting clip like the other channel (right).

At this point what would you do? Leave the transistor that's there? Order a 2SD400 replacement and DIY a clamp? Are the clamps available anywhere? I couldn't find any.
Attached Images
File Type: jpg STA-2100_1.jpg (158.2 KB, 28 views)
File Type: jpg STA-2100_2.jpg (153.8 KB, 26 views)
File Type: jpg 2SD400.JPG (14.7 KB, 26 views)
File Type: jpg Diagram.JPG (111.3 KB, 26 views)
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Old 6th September 2014, 09:32 PM   #2
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Join Date: Feb 2006
Location: Ashland,Ky U.S
Using a different transistor affects the thermal compensation. The larger to220 has a larger thermal mass and will compensate differently than the to92, adding the Vbe change with temperature, this could be a large difference. Using the device the amp was designed for is the best for ensuring it's continued operation. The clamp would be easy to reproduce if you can't find a new one. VR502 in the schematic should also be carefully inspected or replaced, this was a poor design choice, if the wiper goes open the output transistors will self destruct.

Last edited by jerluwoo; 6th September 2014 at 09:48 PM.
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Old 6th September 2014, 09:55 PM   #3
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Thanks Jerluwoo, I'll plan on changing VR502.

Rather than trying to find a transistor replacement I'm thinking of finding a donor. That transistor seems pretty common.

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