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Author Topic: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.  (Read 900 times)

phatt

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #15 on: July 17, 2019, 05:17:25 PM »
If you have full rail voltage at +C0 (the output) then it's a fair bet at least one power transistor is dead.
The problem with these circuits is that they are DC coupled and if one Transistor blows it usually takes out (or severely stresses) the other components.

In this case *DC coupled* stands for Direct Coupling of active devices.
There are no interstage capacitors to block the DC voltages so if a resistor or transistor goes out of spec then high currents flow and burn up transistors.
As these Amps are getting old then it's common for things go wrong.

As you can't take voltage readings with the board in place then it might be easier to replace TR20 to TR23 and build yourself a light bulb limiter Shown here; https://www.ssguitar.com/index.php?topic=2093.0

Then if things are still not right you won't blow up brand new components when testing.

Depending on circuit design and what has actually blown, some Power amp circuits will still pass signal even with blown output Transistors.
In this case maybe Tr20 &Tr21 are still working giving the low output volume you noted.

You won't be able to balance the output (with VR2) at C0 to half voltage until the offending devices have been replaced.

And same goes for VR1 which adjusts the idle current for the power transistors.

Better minds here might have easier/better ways to fix this but I'd just replace the right hand side TR20 to TR23 and fire up with the limiter and see if you can centre the C0 point and set idle current.
Then run a signal through it.

Don't worry about VR3 until you get the DC working,,VR3 is just for balancing the AC signal entering the amp.

Phil.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #16 on: July 20, 2019, 07:00:20 AM »
Hi Phatt,

I managed to purchase some crocodile clips and detach the power amp section to a more workable state.

Unfortunately there was a small pop and no there is no guitar signal at all. So something definitely blew.  :'(

I checked the fuses and they are all okay.

I managed to take voltage readings as best I could across the transistors:

Rail 6 voltage = 123V

TR24 = IN 123 / OUT both terminals 119V
TR16 = IN 119V / OUT both terminals 118V (Working)
TR17 = 117V all terminals
TR18 = 117V / 117V / 12V
TR15 = IN 119V / 104V / 97V
TR14 =  IN 117V / 115.5 / 103V
TR13 = All 117V

TR19 = collector 124V
TR20 = collector 124V
TR 21 = collector = 0.59V


I have to deconstruct the MA board even more to get the emitter and base reading for these, so I could only take the metal plate readings, which to my best known are the collectors?

TR22 = 124 in / 115 / 116
TR23 = 116V CV / 0.044V TP / 0.58V

I hope someone can help decipher what the issue is now, and explain it if possible.

Thanks.

 

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