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Pioneer SX-850 - No power, lights. Blown fuse at FU4

esoxthepike

May 9, 2024
6
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May 9, 2024
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6
Model:
Pioneer SX0850 (a really beautiful old receiver!)

Me:
I have basic skills and tools. I can solder and have recapped a smaller unit before, but by no means competent at reading a circuit diagrams although I am always learning. My last project was putting a V8 engine into a 1990 Volvo 240 and rewiring the entire car so I can get it done I suppose. I do have a DMM and diy dim bulb tester. I also have the service manual for this model.

Condition:
I just bought this at auction for $400, knowing that it was a gamble. The unit appears in nice shape physically, only the slightest "warmish" smell from the cabinet. I say warmish as it does not smell burnt, but does have the smell of age I guess.

Initial Diagnostics:
Visual inspection of top and bottom of the boards did not show anything obviously cooked or swollen. The power board is loose from the standoffs. I tested each fuse (off the unit) and found fuse FU-4 failed continuity and visual inspection.

The unit will not power on at all; no lights. One bulb fails continuity test.

There is power at the aux outlets.

Goal:
First get the unit working generally and then plan for updates to typical components to freshen the sound back the 70s.

Might want to use LED lights to brighten it up.

I have watched a few videos of this process, but would like to pick the brains of people who have the experience so I don't make dumb mistakes and also get more skilled.

Thanks to all.
 

Delta Prime

Jul 29, 2020
2,117
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Jul 29, 2020
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2,117
I just bought this at auction for $400, knowing that it was a gamble
Welcome to Maker Pro! :)
$400 is an reasonable price for speaking with us...
Let's start with the drawing below it must be an incandescent bulb not an LED bulb and it will burn brightly give up the most light when there is a direct short in circuit but will not destroy your unit or your household wiring trip a breaker and such. Safety first.
1713429187778_1715287134753.png
 

Bluejets

Oct 5, 2014
7,050
Joined
Oct 5, 2014
Messages
7,050
Best you try to find a circuit diagram and go from there.
People in the know will have some idea on where to trace any fault with the device in front of them however the device is in front of you hence the first hurdle.
 

esoxthepike

May 9, 2024
6
Joined
May 9, 2024
Messages
6
Dim bulb tester hooked up. Was briefly bright when power on amp, then quickly settled to dim as shown in this image:
dbt.png

The one bad fuse was not installed when I tested. I need to find replacement for that, 1.5Amp 125 Volt. If anyone has a good source, that would help. Otherwise, I will source from mouser.

I am attaching the service manual for download and reference.
 

Bluejets

Oct 5, 2014
7,050
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Oct 5, 2014
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7,050
Doesn't look like a 150w incandescent lamp to me..........

However, start by checking all the voltages as shown on page 11 "block diagram". Watch you use the correct voltage reference point.
 

esoxthepike

May 9, 2024
6
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May 9, 2024
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Found a frosted 150w bulb, same result. Will get a clear one next trip to the depot. Will report back on block diagram voltage tests.
 

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esoxthepike

May 9, 2024
6
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May 9, 2024
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Took measurements on the power board and wrote them down. However, I have not replaced the one blown fuse so o am not sure they are valid or not. Hoping someone will inform me.IMG_5791.jpeg
 

esoxthepike

May 9, 2024
6
Joined
May 9, 2024
Messages
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Replaced the bad fuse and put cardboard under the loose power board as I noticed many pins were not cut short. Unit powers on now! Now FM audio (only one I checked). I’m gonna remove and test and replace all the parts in the path of 13-15 on the power board to see if that gets those voltages back in range.
 
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