[chbot] Transistor for Audio Amplifier Repair

[Mark Atherton]

Just found a couple of new BU406s - they may do the job.

Will bring them to Robotics Monday eve.

-Mark

On 2/19/2023 3:02 PM, Spencer Travers wrote:
> I am repairing someone's much loved audio amp/receiver from the 1990s 
> and need to find a replacement transistor. It is a 2SD1762DEF TO-220 NPN 
> bipolar power transistor. Some datasheets say it's 50V, 3A rated. There 
> are some ebay listings for this part.
> 
> Does anyone have something local and suitable or some pointers? I do 
> have a service manual with schematics. If anyone's interested, here is 
> a  link: 
> https://drive.google.com/file/d/1DhsyuLpafuNbXBOhnIgTRilbyw64tCRr 
> <https://drive.google.com/file/d/1DhsyuLpafuNbXBOhnIgTRilbyw64tCRr>
> 
> Q701 is the dead transistor, located at grid 32B in the schematics.
> 
> This transistor is used to create +15 V from the +40 V rail. It looks 
> like the audio input selector IC401 shorted +15 V to -15 V and took out 
> the transistor along with a couple of traces on the PCB. R405 & R406 and 
> surrounding traces were destroyed. The good news is that injecting +15 
> volts from a lab PSU to where Q701's emitter was connected seems to make 
> the amp work OK.
> 
> I had thought of using a 7815 voltage regulator but I think the input 
> voltage would be too high. I do see some devices on ebay and may have to 
> resort to that.
> 
> I guess another option would be to add a separate transformer, 
> rectifier, and voltage regulator to get 15 V, but I would rather avoid 
> that. Unfortunately there are no suitable lower voltage taps on the 
> transformer.
> 
> Spencer
> 
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