View Full Version : Pioneer SA-900 Amplifier Help.


Spudnik
08-21-2004, 04:14 PM
Hi everyone. I've been lurking on here for a few days, and have been impressed with the wealth of information and the willingness of people to help other people. So I've decided to register.
I recently acquired this SA-900. I've been looking for some details/specs/manuals for this amp to no avail. Can anyone tell me anything about it?
It is in good shape physically, but one channel is out. When I got it the right channel fuse was blown. I replaced it and it immediately blew again. I checked the power amp transistors (2N3773's) and discovered that the ones (2 of them) in the right channel were shorted. I've replaced them - fuse doesn't blow now, but no audio yet. I've swapped the 2N3773's around and the same channel stays out.
This amp has a pre-amp out connection in the back, so I switched to "separated" instead of "intercoupled", connected to another amp from the pre-amp out. Ah-ha! Audio from both channels. This tells me that everything up to and inclucing the pre-amp section is working. I've checked as much as I can on the driver board, all resistors check out so far. I don't have a capacitor or transistor checker so I'm kind of stuck at this point.

So, anyone have any advice? Is this amp worth resurrecting? Anything else I could check with my limited toolset and knowledge?

Thanks.

bully
08-21-2004, 04:29 PM
An oldie, from 1970-72, and 40 wpc.
Hopefully one of our mavens will see your plea and offer some suggestions.

merrylander
08-21-2004, 04:32 PM
My Yamaha CR-1020 used to drop the right channel and fiddling with the switch that coupled the pre-amp and amp would bring it back. I eventually took it down to my shop and put in a new switch.

So, beside connecting to another amp simply try using the interconnects to connect the pre out to the amp in, that will sure prove whether your switch is good or bad.

Rob

Spudnik
08-21-2004, 04:50 PM
Thanks for the suggestion. I just tried that and I get the same result - channel is still out. So I guess one more possibility eliminated.

Thanks.

merrylander
08-21-2004, 04:54 PM
Well it may be that you have some controls or switches between the amp in jacks and the finals. To bad you are so far away or we could simply bring it over here and run my signal tracer through the channel to see where the sound stops.

Rob

Spudnik
08-21-2004, 05:15 PM
I don't think there are any other switches, etc. but I'll trace the wires again just in case. Thanks for the offer Rob, it really is too bad we're so far away - that would be great to have someone with the right equipment and know-how take a look at it. If someone can tell me it's worth spending money on, I may take it to a local tech for repair if I can't figure it out.

Spudnik
08-21-2004, 08:26 PM
In checking a few more components I think I've found a problem. Don't know if it's the only problem, but it's a candidate. There's two varistors, one for each channel. They're STV-3's. I've compared the resistance of them and here's what I get. On the good channel 2k ohms in one polarity, and infinite in the other polarity. On the bad channel 5K ohms and 15K ohms respectively. I suspect that this isn't normal. Could this varistor have been damaged when the shorted power amp transistors blew the fuse? If so, is this a common part, and can I substitute with something else?

Thanks.

Spudnik
08-22-2004, 08:06 PM
Today I asked the person that sold me the SA-900 if they happened to keep the manual. After a few minutes of digging through piles of manuals, it was found. So I have circuit board layouts and component details. This should help.

By the way, here are some of the specs as listed in the manual:

Continous Power: 4 ohms 64 watt + 64 watt (H.D. 0.3%), 8 ohms 50 watt + 50 watt, same H.D.
Harmonic distortion - Less than 0.5% at 1kHz rated output, Less than 0.08% at 1 kHz 45 watt output
Frequency Response 20 Hz to 50kHz (Over all)
Power Bandwidth 20 Hz to 50 kHz
Output Terminals - Speakers 4 to 16 ohms.


Anybody want to know anything else from this manual?