In the summer of 2005 I finally made my dream a reality when I began to hunt down the C-91 and M-91 components on Ebay. However, the M-91 Amp that i really wanted was snipped out in the last 30 seconds of my bid( I had my bid all the way up to 700 dollars when the current price was like 600) Anyway, much to my disappointment I didn’t win it.
So, about 2 weeks later another M-91 came up for sale. Only problem, this one had issues. The owner claimed it worked, but the left channel would cut out, but generally when it worked it screamed. I took my chance that it might need a “little” service. Now to let you know, I may have an Electrical Engineering degree, but I never claimed to be an expert at fixing this stuff. I ended up getting the piece for $550.
Upon delivery, I found out right away that the left channel didn’t work if connected to speaker A terminal but worked fine on Speaker B terminal. I figured that maybe the output speaker selection relays were stuck. Well, it was more then stuck, it was melted like solder on the left channel. I mean melted completely away and dripping copper/metal on the B side contact. Now if you do a little math and ohms law, that relay was rated at 7 amps. To melt completely and still not fry the output transistor, means that amp had to be pushing out well over 500 watts into a dead short somewhere. I have no idea what this guy did, since he didn’t fess up. But in any event I replaced them with some parts from Pioneer. Cost 76 bucks.
Day two, I fired it back up, and still had an intermittent protection fault on the amp at power on. Couldn’t tell at first because I discovered that the protection indicator and power on light were bad. Cost from Pioneer 10 more dollars.
Day three, Checked the output leakage voltage. Found it to be randomly floating all over the place from .02 volts to 70 volts, trips the protection at like .05 volts. Checked output transistors but they seemed ok, but ya never know. Decided to replace the output finals on the left channel with new equivalent NTE replacements from a local electronics supplier. Now I’ve read a lot of horror stories about NTE replacement transistors and fake imitations from China, so I absolutely refused to purchase any crap off the internet. Total cost for 4 PNP and 4 NPN transistors, 63 dollars
1 Week later, I received the new replacement, installed them. Still flakey, and now I’m beginning to debate my ability to fix this thing. Decided to replace a MOSFET at the front stage of the amplifiers front end, because each time a tapped on it or sprayed cold freeze on it it made the DC leakage voltage swing all over the place. However there were many other areas that exhibited the same behavior all in the same general location.
Week 1 Day 2, After some thought, I decided to really carefully look at the solder connections on the amp board, and bingo, I found a ton of issues. Seems like the solder connections had a thermal breakdown so to speak. I figure the hot cold cycling over 10+ years of service did a number on many of the biasing resistors. I resoldered nearly the entire board. Upon testing, she worked great. No fluctuations of the DC Leakage Voltage. I was ecstatic about the whole thing. Output on my scope from a Frequency Generator showed a good output signal from both the left and right channels.
After all that, I made a decision that if I replaced the left channel with new transistors, I might as well do the right channel. Especially since it was only another 63 dollars and some crazy looks from the guy at the Electronics supplier. So I ordered another set of transistors.
2 Weeks later, I received the new transistors. Got them installed and proceeded to resolder the same connections on the Right channel just to be sure. After getting it partially assembled I fired it up and ran some test. With my Freq generator and scope both channels were flat from 20 hz to 20khz. She started to loose a few DB above 25khz but continued to work to at least 30khz. Not that anyone could ever in there right mind hear that, but it was interesting to see that it could do it.
So in the end and about 200 dollars later, I have a really nice amplifier. I was lucky that the amp I got was cosmetically in mint condition, not a scratch on it. Though the work I did wasn’t done by a professional, I think the quality of the work was fine. In fact, the amp runs cooler then before, but I attribute that to the fact the replacements dissipate 130 Watts each versus the 115 Watts of the original transistors.
Below are a few details of the parts and photos of the amps internals. Oh for those wondering whats near the amp, its my model railroad project. They’re be more on that in another section.
2SA1516 PNP , replaced with NTE2329 180 volt, 130 Watt, 12 A 25Mhz Power Output transistor
2SC3907 NPN , replaced with NTE2328 180 volt, 130 Watt, 12 A 25Mhz Power Output transistor