[KCMike]

  I picked up the above model pioneer receiver at antique mall yesterday.
I powered it up and found the signal strength meter and one position indicator lamp lit up.
Opened it up and found 2-blown bus fuses in the power amp. unit. Replaced them and powered up again,this time it blew the line fuse immediately. Not the 2 new fuses in the main amp .
 Did some more checking and found 3 of the four bridge diodes in the power supply unit are testing bad.
  I could really use some help on how to proceed with this unit. I.E. what I should replace ,check etc.
The board units look like they will be easy to remove to replace parts. Here is schematic.
  Thanks

 [Image: http://i1336.photobucket.com/albums/o654...cd640d.jpg]

[morzh]

After you check all diodes, filter capacitors and such, before you blow another pair of fuses, check the output transistors too. Likely gone. And while you are at it, check the rest of them transistors.
Can't quite read the sch but this is what I would do.

[Ron Ramirez]

Mike

You will really enjoy that SX-1500TD once you get it going! I have one also, and used it as a daily driver in my office at my previous job until one of the stereo channels quit working.

Do you have all of the service info for that receiver? If not, I do...just let me know if you want a PDF copy.

I would start by replacing all four of the power supply diodes, as well as all of the electrolytics on the power supply board. As a precaution, check the transistors on the power supply board and the audio output transistors also to make sure they are OK.

Edit: Well, while I was composing my post and looking at the schematic, Mike (Morzh) beat me to the punch and basically said the same thing I did.

[KCMike]

Thanks Ron and morzh. I will do what you guys recommend.
1 question for now. Where and what would be the correct replacement power diodes #2sd218? I would like to just buy all the parts needed from one resource if possible.

[morzh]

THose are not diodes. Those are output transistors.

here's a discussion about those.
http://www.audiokarma.org/forums/showthr...p?t=152383


use a suitable NPN 100V 7A or better transistor with 10MHz Unity gain frequency.
If an insulation from heatsinks is warranted do not forget to do it.

[KCMike]

  I'll see what Mouser has for the transistors. Maybe get the other parts there also. Guess  I had diode on the brain.

[morzh]

http://www.ebay.com/sch/i.html?_from=R40...8&_sacat=0
eBay sells them.

http://www.mouser.com/ProductDetail/ON-S...i5M5fkc%3d
This is what I would probably buy. Also in TO204 package.

[KCMike]

Thanks morzh. I was trying to put list together and had the mj21194g's on it and they cost 5.87 ea. you saved me $$
I have to go through parts list to see what replacement #'s I will need for other transistors and diodes.

[morzh]

This is unnecessarily hi-voltage and higher current transistor, with Gain-Bandwidth 4MHZ (your original is 10MHZ) although this might not affect the quality much. Could be used.

The other one I listed has still higher current (10A vs 7A) and 80MHz GB, which even if it is too much, it will still behave nicely I think.


BTW where's the main amp sch piece? (I only see the preamp and headphone amp there).

[KCMike]

I have finally down loaded the entire sch. and parts list. Have to move to photobucket and I will put main amp up here.

[KCMike]

[Image: http://i1336.photobucket.com/albums/o654...e54033.jpg]
Here is amp schematic.

[morzh]

Well, looks like your output transistors are gone.
I would check also at least the previous stage, two complementary one (PNP and NPN).

It is strange the diodes are gone, the fuses should've protected them, diodes do not blow that easy, but then I cannot find datasheet on SRIDM-8.

I also wonder why they used Hi-voltage transistors considering the B voltage is 25V (or so I see in the sch). One could use much better lower voltage higher current transistors.

When adjusting the amp, first turn VR3 and VR4 to short, then when done with making sure the amp works, use these to set the minimum quiescent current (to remove the crossover step distortion).
Best done with a scope.

Of course if those were not touched you could try to leave them in place.

Good news is, more often than not the damage of the output amp is limited to the output transistor pair, but you should check all you can to be sure.

[KCMike]

Thanks morzh. If I can trouble you once more. What diodes should I replace the SRIDM-8's with? I tried looking them up and couldn't find any info. on them. I'm really new to stereo receiver repair. If the VR3-VR4 are the adjustments in the amp. they have a paint mark and it looks like never been touched/turned, and carefully studying the chassis I can not see any evidence that anything has ever been replaced. Also no sign of any burnt parts or smells. Maybe fuses saved big damage?

[morzh]

Well, fuses do save things though their primary function is to prevent fires and the rest be d**ed.

As for diodes: are they axial cylindrical ones?
What is your max wattage per channel and the load in ohms?
Take your max wattage, (say, 25W) divide it by your speaker's Ohms (say 4), extract square root (app. 2.4A) and multiply by two (2channels), you get 4.8A max load.
Now because your diodes conduct 1/2 time, 4A is a good rating that in the datasheet will be marked as "Average Forward Current".

Your reverse blocking voltage should be also 2 times the output voltage so if you have 25 V, 50 to 60V is good.

[KCMike]

Yes they are axial cylindrical. Its the four in the bridge circuit of the power supply unit. SRIDM-8. I tried the alldata cross reference with no luck. Also tried NTE cross data site. search found nothing.
I might pull one out and see what # is on the diode. The MAX wattage per channel 8ohm/58watt--4ohm/70watt.