Looks like it wasn't the power output portion at all. I'm not sure where the short is. Need to discharge the caps and unwire them and test them.

Then after that the OT??

Yet another update...

Replaced the rest of the diodes by the transformer. Got rid of the 47VDC at the speaker output.

But.... now the light bulb glows quite brightly and without the bulb in the circuit it blows the fuse. Now I have to figure out where this crazy current draw is coming from.

What voltage should be present at the speaker output? I assume something minimal.

I replaced half of the diodes by the transformer output and I'm now down to 13VDC at the speaker output with a 10ohm load.

Did some measurements.

Speaker output is still reading 47VDC. This voltage seems to be everywhere. there's a six pin molex connector that comes from the amp/transformer section that has that voltage on every pin but the ground wires.

All of these voltages are read with no load, i.e. no speaker connected. They don't really like the voltage.

All -# just represent that they are the same transistor, just different location. They are the same model transistors.

The power transistors.

TIP3055-1 30mVDC and 28mVDC
TIP3055-2 Pretty much have nothing reading on that transistor. Like nothing is feeding it any power.

The rest of the transistors.

2N3397-1 37.9VDC 38.7VDC
2N3397-2 47.5VDC 38.3VDC

2N2955    47.5VDC 0.01mVDC

MPSU57-1 30.2mVDC 0.01mVDC
MPSU57-2 0.01mVDC 0.01mVDC

I'm still not sure what the heck is going on with this thing!

p.s. The more I look at that schematic and the more I look at this amp... they don't really go together. Need the Sano 1000R schematic.

The bulb has a steady glow and there's still the audible hum coming from the speaker. But there doesn't appear to be any oscillation. The glow is fairly dim. About that of a night light so it's pulling current somewhere. I'm really getting frustrated with this thing. It sounded so good too.

And we're seeing around 20V at the speaker output. At least that's cut in half :p

Here is a pic of the power board.

Great suggestion! Forgotten all about using lightbulbs as a load. I'll try it in the next day or so.

What would be your suggestion on limitting the current? I always test it with a speaker connected and the volumes turned all the way down. Should I put in a shunt or something somewhere?

Thanks

Popped in the 2n2905a and the voltage dropped to just under 42V...then something popped and the voltage jumped up to 48V and settled back to 43V. I assume the new transistor popped. Everything else reads normal. Can't find any shorted resistors on the power board.... back to the drawing board...

Looks like a 2905a is a replacement. Going to pick some up today.

I believe the 2n3645 is one of those transistors you mentioned... hard to tell... the board and the schematic don't really seem to correlate most of the time. Looking for a 2n3645 replacement now.

Finally got around to poking around in this thing again. I was looking at the schematic...not that it helps much... and thought that maybe one of the transistors in the power section is bridging some of the existing voltages together incorrectly. So, I started pulling transistors until the 43VDC went away. Wound up being a 2n3645 that I pulled that got rid of the nasty DC voltage. Percieved output volume was way under half the normal volume of this amp. Not convinced that the transistor is actually bad. Doesn't read a short anywhere with the MM. There are a couple diodes on the board that appear to be the same but measure quite differently. One measures like a complete open in one direction(good!). The other... not so much...

Finally found my multimeter. Looks like I'm reading 43VDC at the speaker connection! That ain't right...

Correction... it ain't the signal section... Gotta be in the power section somewhere. Without signal to the power section it managed to smoke the speaker. Good thing it was one I didn't use anymore. Electrons are fun!

Well, it ain't the power section. Replaced the power transistors, and no change. Taped off the input from the signal section to the power section and quiet as could be. Now I guess we'll try the ol' half split method!

Broke one of MPSU57s moving the amp around... those things are damn hard to find and expensive! Fortunately I found a few at a local surplus shop!

Gonna keep plugging away at the old thing.

P.S. Just noticed that the boards all say "Sano". This amp appears to be the same thing as the Sano 1000R Combo Amp. No schematics for that guy either.

Anyone have any leads for a modern equivelant to an MMPS U57 transistor? Can't find anything anywhere!