Replacement power transistors for Yamaha G100-212

[Jimmy74]

I have an old yamaha hundred 212(first series 1977ish) that needs new power transistors, it has 1 x 2sd425 and 1 x mj15015. The replacements would have to TO-3's because the whole output circuit board is encased inside the heatsink. So what would be my best choice for replacements?

thanks
J.

[Enzo]

Neither of those is original type, but the Mj15015 is a reasonable choice, get a pair of them. Mouser has them in stock. or if you already have them, MJ15003 or even MJ15024

[Jimmy74]

Ok thanks Enzo...do you think I'd have to change the driver transistors too?

[Enzo]

No idea. You did not report any particulars. If both outputs are shorted or something, then replace them. The drivers are TO66 are they not? Those might be hard to come by, so test them first. If we need new ones, talk to me, I actually have a number of TO66 types in stock.

In my experience, almost always, a blown Yamaha G series power amp was repaired by replacing teh bad output and little else. Adjust the bias.

Fire the thing up on a light bulb limiter at first just to make sure something else isn;t drawing current.

[Jimmy74]

well I did replace the filter caps because they were really in bad shape. I removed the heatsink and the case to get to the output circuit board. I found big spider crumpled up inside the case, and 1 or 2 burnt resistors on the board.

Yamaha G100 by jimmy6122 | Photobucket

I think I might just pull the whole thing apart to check underneath the board aswell...maybe there are more spiders under there!!

[J M Fahey]

Spiders harmless and no big deal but cockroaches are disgusting and way more dangerous for the bio weapons they carry, and they can infestate your workshop/home in no time.

I sold many amps to Restaurants , I live in a Tourist area and am surrounded by them, now when I get one back for servicing I never get them straight in, but outside I spray them with some Raid inside a large garbage bag and let them sit a few hours so concentrated vapors kill everything, including eggs (the real danger), and then blast them with compressed air, way more powerful than cheesy vacuum cleaners.

Only then they are allowed inside. Ugh !!!!!

[Jimmy74]

Cockroaches aren't much of a threat in central melbourne-Oz...spiders on the other hand can get in anywhere and everywhere. I didn't find any spiders under the board, but there was another spider in the fins of the heatsink. I checked the resistors that look burnt, the 2 watt 0.22ohm ceramic resistor measures ok, the 2 watt 4.7ohm measures fine but the 160ohm resistor measures 70ohms so I think that needs to be replaced. How do I check if the power transistors and driver transistors are shorted? When I turn the amp on, it gives out a loud low frequency hum typical of blown transistors.

thanks

[Enzo]

ANy 160 ohm resistor that measures 70 ohms is almost certainly in a circuit with parallel resistance paths.

WHAT 160 ohm resistor? My schematic shows some 150 ohms, but to get 70 would mean the output transistor was shorted. If you doubt this, unsolder one end of it, lift it from the circuit and now measure it while it is not in the circuit.

How to detect shorted transistors? Measure resistance between pairs of legs (or between legs and case), just like you'd test anything else for being shorted.

[Jimmy74]

Yes the resistors on the schematic are shown as 150 ohms, but the one on the board is 160 (brown\blue\brown\gold). The board in the schematic is LC15975 whereas the actual board is LC15973 so there may be some differences. Here are the measurements between the pins:

MJ15015 middle pin and right pin 134.5 ohms
middle pin and left pin 0.3 ohms
right pin and left pin 134.5 ohms
2SD425 middle pin and right pin 69.9 ohms
middle pin and left pin 0.1 ohms
right pin and left pin 69.9 ohms

what do you think?
thanks
J.

[Enzo]

Are they still in the circuit or removed?

The output transistors are in sockets, aren;t they? Two screws out and the pull out?

We can't do the left right and middle, since who knows which way you are looking at it, and those parts don't have a middle leg, only the circuit board has a solder point near the middle. Pull them out and measure. There are two pins and the body. Your readings look like both are shorted.

[Jimmy74]

yes they are removable from the sockets...the MJ15015 measures 1.5k between the two pins and between the casing of the transistor and the two pins 1.5k's and 0.2 ohms. The SD425 measures 114.4 ohms between the two pins and 114.4 ohms and 0.2 ohms between pins and transistor casing. There's definitely a forced repair job under the board with a wire replacing the actual path but it's solid and does the job. The wire replaces the path from that burnt 160 ohm resistor to one of the 2SC783 driver transistors.

[Jimmy74]

I'd also need to know what diode would be suitable to replace the diode mounted on the heatsink...there's no brand..just an orange dot on the side and the numbers 55. On the schematic it's shown as D1 but I can't find any specs

[Enzo]

So they are both shorted. 0.2 ohms between two terminals is about as shorted as something can get.

Look at your MJ15015, look where the two pins come out. Is there a B and an E stamped into the metal? Heck, the 2SD424 might have them as well. ALL TO3 have the same pin order. Note also that the two pins are closer to one end than the other. If you hold it up in front of you, pins towards you, and so the closer end is at top so the pins are slightly above center. The B pin should be the one on the left, and the E on the right.

The case is the collector, abbreviated C a lot. The B is the base, and the E is the emitter. We should try to get in the habit of calling them that. I am guessing your 0.2 ohms is between collector and emitter.

Now with those still out, test the similar looking but smaller ones up on the board, some of them may be shorted, we need to know.

[Enzo]

D1, look for the collection of notes on the schematic, is there not one that identifies the transistors and diodes? Mine says STV3H. Those are hard to come by. I happen to have a few. But are you sure it is bad? If it were open, you'd be blowing fuses, if it were shorted, the amp would work, but have more crossover distortion than normal. It is a multiple diode inside. Should measure on diode test like it s two or three series diodes.

[Jimmy74]

Ok power transistors do have E and B on the pins so B is left and E is right. The SD doesn't have anything written on it.
Driver transistor readings (they are still soldered on the board):
2SA483 gives 24.3K ohms between B and E.
first 2SA783 gives 3Mohms between B and E
second 2SA783 gives 2.9Mohms between B and E

The diode: no I can't find any references to that diode on my schematics... but then again one of the pages of the zip file service manual doesn't open up so it may be on that page. The problem is that when I popped the power transistors out of the sockets, the whole power board fell off and left the diode screwed on the heatsink...so it may even be working but I can't solder the wires back on to it. So I need another one.