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Author Topic: Yamaha G100 112 - Transformer?  (Read 1349 times)

dlbraly

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Yamaha G100 112 - Transformer?
« on: July 14, 2018, 09:38:42 PM »
Hello Y'all,
We picked up a Yamaha G100 112 first series.
Ive been trying to figure out what exactly should be coming out of the transformer.

When I measure across the 2 brown wires I get 29.3 VAC.
if I measure from the black wire to either of the brown I get 14.3 VAC
if I measure across the 2 red wires I get 73.1 VAC

Is that normal output?

Just curious, what is a good replacement for it?

This was my first post here. I have a lot more questions.

Enzo

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Re: Yamaha G100 112 - Transformer?
« Reply #1 on: July 14, 2018, 09:57:44 PM »
Can you post a schematic?


I rarely wonder about transformer voltage unless one is missing or super low.   What leads you to suspect the transformer?  I mean what is wrong we are trying to fix?  Those look like real reasonable voltages to have.

dlbraly

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Re: Yamaha G100 112 - Transformer?
« Reply #2 on: July 14, 2018, 10:38:28 PM »
Hello Enzo
The biggest reason is curiosity, I have no idea what readings I should see.

Problem - the amp is not as loud as you'd expect a 100 watt amp.
having the volume up past 5 was not excessive loud. and having it on 10 was loud, but bearable.
The distortion channel is a lot less.

I have not posted more questions yet, because I found a bad transistor that I want to replace. I removed the part and tested it on one of those LCR-T4 Tester.
2SK30A GR

Im not sure if I should order that part, wait for it and see if that fixes the issue, or start checking parts on the MA board.

Ive been reading your posts Enzo - Thank you so much for being active here and helping people. Im enjoying reading your posts.
https://www.ssguitar.com/index.php?topic=3815.0

I am novice, and want to learn more. This amp is my Dad's he picked up off craigslist. I wanted to fix it for him. I also bought a Fender amp that has issues, that I will be asking about after this one.
« Last Edit: July 14, 2018, 10:49:36 PM by dlbraly »

Enzo

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Re: Yamaha G100 112 - Transformer?
« Reply #3 on: July 15, 2018, 12:33:39 AM »
A 2SK30 is not a bipolar transistor, meaning it does not have emitter, base, and collector.  You can't test it like a bipolar.   It is a JFET - junction field effect transistor.  With an ohm meter, two of the legs will show a lowish resistance, and the third leg will be the gate and act like a diode towards the other two.

dlbraly

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Re: Yamaha G100 112 - Transformer?
« Reply #4 on: July 15, 2018, 09:33:52 AM »
Thanks, I kind of figured that out plugging the components into the LCR-T4 Tester.

That tester is pretty cool.
it tested the other 2SK30 just fine, but this one failed.

Attached the schematics



Jazz P Bass

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Re: Yamaha G100 112 - Transformer?
« Reply #5 on: July 16, 2018, 11:03:13 AM »
If the 2SK30 in question is 1/2 of the input LTP circuit, I would replace both of them.