Pioneer SA-500 Transistor Questions

[joshg3p0]

I recently acquired a Pioneer SA-500 amplifier. The left channel kept blowing a fuse. I replaced the fuse, and it would blow again. After some internet research it appears that the IC Transistors often fail in these. As a result, I took the first left channel transistor out of the circuit, and then powered it up... the fuse didn't blow and the audio worked. As a result I ordered a full set of replacement transistors for the amp. I'm assuming that the one I pulled out was shorted, but I'll replace all 4 since I'm using NTE crossrefs.

My questions to the forum are... why did the amp work without the transistor in the circuit, was it unsafe to run it with it removed, and should all my previous problems be solved if I swap out the old old Toshiba transistors with 4 new NTE equivalents. Any thoughts or insight would be great. Thanks!

[cbdb]

if you have parralell output tranys, removing the shorted one would "fix" the amp but it wouldnt have the same currrent drive, the 4 transistors are all on one side or is this for stereo? If each side has 4 then there probably in parralel and replacing them should work. If only 2 per side something else might be wrong. Do you have a schematic?

[anatech]

Hi joshg3p0,

After some internet research it appears that the IC Transistors often fail in these.

Not true unless something happened to the amplifier.

As a result I ordered a full set of replacement transistors for the amp. I'm assuming that the one I pulled out was shorted, but I'll replace all 4 since I'm using NTE crossrefs.

NTE replacement transistors are not reliable or suitable for audio amplifiers. You bought junk.

Do you have a soldering iron, desoldering pump and braid and a digital meter? You will need proper thermal compound and you should use new mica insulators. The old grease must be cleaned off the area. Everything must be clean where you mount power transistors.

You only need to work on one channel. Never swap parts between the two channels in an attempt to troubleshoot. You'll end up with two damaged channels.

why did the amp work without the transistor in the circuit

That transistor might be damaged, probably is shorted. You should also measure it to make sure.

was it unsafe to run it with it removed

Very ill advised to run it like that. There is often more wrong than just one output transistor. Many components are directly connected and get stressed when a part fails. It's possible you got lucky.

and should all my previous problems be solved if I swap out the old old Toshiba transistors with 4 new NTE equivalents.

No. How are you going to set the bias current? How can you check the other components for damage? An open resistor can cause excessive bias current. The amp will work for a bit before failing again. There are so many things that can go wrong if you don't check. BTW, those old Toshiba transistors are very good parts. They do need to be replaced because they were stressed beyond their limits. Their characteristics may have changed as a result. You should use new quality parts to replace those original ones.

Get a schematic or manual. Figure out what is going on and test the rest of the parts in the output area. You may simply have been lucky so far.

-Chris