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Author Topic: Fixing a Yamaha G100 112 ii  (Read 18939 times)

dropanchor812

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Fixing a Yamaha G100 112 ii
« on: July 24, 2015, 11:15:22 PM »
Hello all!  New to the forum.  So before I lay out the problem, know that I have some experience with electronics.  I've been building effects pedals for a few years now and I can debug a pedal alright.  If I had a schematic this may be a little easier, but I don't and I can't find one free online.  If anyone has one to share I would be very grateful! 

Okay-  so I got this amp in a trade.  I've heard awesome things about them for years.  It powers on, everything lights up, but there is no sound.  No signal, nothing.  I probed it and it sounds like the input jacks are working, but the signal dies once it gets past the input jack solder joints on the PCB.



The guy I got it from says he doesn't know anything about it, and that he never did anything to it.  In the chassis I found two old power transistors, so it looks like they were changed at some point.  There is a spot on the preamp pcb where it looks like someone tried to fix a busted trace and wired in a jumper to reconnect it. See picture.



I took out the power board the transistors are mounted to, and cleaned up the contacts leading to the second power board (at least thats what I think it is).  They seem to be making good contact.  I've checked the fuses and they're all good. 

Power Board with Trannies:


Power Board up top:


Besides the jumpered trace the only thing I can find that is suspicious is a blue solid core wire that looks like its supposed to be a ground-wire since it is bolted to the chassis (see 4th picture).  It's just hanging out and I'm not absolutely sure where it goes, but I do have a guess:  On the 2nd power pcb there is a blue stranded wire running from the preamp board.  On the underside of the pcb the solder joint where that stranded wire meets the power pcb has a little brown dot in the middle- almost looks to me like something had broken off at some point (it's right on the top edge of the PCB marked with a "-" above it).  I want to alligator clip that solid wire to that joint to see if its just a ground point- but since I don't have a schematic I'm not all that confident.   



Since the signal is dying at the preamp pcb, my guess is that the preamp pcb isn't getting power somehow, or the signal is grounding out somewhere.  My primary request is just for a schematic, but if you have any bright ideas I would love to hear those too.  Thanks everyone! 


galaxiex

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Re: Fixing a Yamaha G100 112 ii
« Reply #1 on: July 25, 2015, 01:39:55 AM »
Here is a Yamaha G100 service manual.

http://bmamps.com/Schematics/Yamaha/Yamaha_G100-III.pdf

If it ain’t broke I’ll fix it until it is.

Enzo

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Re: Fixing a Yamaha G100 112 ii
« Reply #2 on: July 25, 2015, 02:53:17 AM »
The blue wire screwed to the chassis is pretty stiff wire, right?  Notice all the various colored wires running right past it?  You wrap that piece of wire around the ones going somewhere, and it holds them in place.  Essentially it is a wire tie.  it does not connect to anything.

PLEASE.  Do not just conect a clip wire to see if grounding something works.  That is not how to do it, you will likely damage something more often than guessing right.

The guy may be honest about never doing anything to it.  An amp this old has had plenty of chances for other people to have repaired it.

The heat sink module with the six pins that plug into the main board - it is your power amp.  I rarely see anything wrong with them other than blown output transistors or cracked solder on teh female pins on the big board.  That doesn't mean something else can't be a problem, but on average not.

Start at the start.  No sound?  Does the speaker work?  Pull the wires off it and touch a 9v battery briefly to its terminals.  A working speaker will pop or thump when you do that.

Galaxiex posted the series III manual, but the series II manual is on the same web site, and the first file in the zip is the schematics.

Then look for power supply.  The preamp runs on +/-20v supplies, and there are 15v supplies for the preamp as well.  Are they all four present?  The power amp runs on a single supply, I don't know the voltage, but it has its own fuse.  Even if that voltage was way wrong, the amp should still make sound, so what I would look for is that power supply is either present or not present.  If it is there, it is probably OK.  Try switching channels, do the channel A and B LEDs go on and off?

The output of the power amp goes through a cap to the speaker, because it is single supply.  So make sure that cap is not disconnected somehow.

With power on, turn the reverb up some and rock the amp to make the reverb springs crash.  Does that sound out the speaker?  APply a signal to the input, and plug a cord from the REVERB FOOT SWITCH jack to some other amp.  And turn the reverb up some.  We are using that jack as a preamp out for testing.  DO you hear sound over in the other amp now?

DrGonz78

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Re: Fixing a Yamaha G100 112 ii
« Reply #3 on: July 25, 2015, 03:33:37 AM »
Just wanted to post another great site to find tons of free Yamaha schematics. This site is excellent and a great resource.

http://www.owner-manuals.com/-manuals-1-20a.html?alpha_filter_id=71

http://www.owner-manuals.com/G100115II-service-manual-YAMAHA.html
« Last Edit: July 25, 2015, 03:35:52 AM by DrGonz78 »
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dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #4 on: July 25, 2015, 03:49:36 PM »
Thanks for the schematics, and thanks for the feedback Enzo!  I'll work on it this week and see if I can't figure it out and report back.  Thanks a lot for all your help!

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #5 on: August 03, 2015, 01:53:40 AM »
Alrighty-  So I finally got a little time with the amp again tonight.  Like I said in my previous post, the signal is dying after I get past the initial lines to the PA board.  I get a very very faint signal on pin 5 of the first opamp (jrc4558dv).

Here is the component side of the PA board.  The first op amp is down there on the lower left. (Sorry for the huge picture size)


To Enzo's questions:
1. The speaker works fine and I've been doing most of the testing using the direct out jack. 
2. The big cap is also connected. 
3. I checked the voltage of the lines to the PA board and they all measure as they should.  I checked the voltage to the first op-amp and it is getting power, although its only measuring 10.2v on my crappy DMM.  Pins 6 and 7 look kinda chewed up from the component side view, and then there is the issue of the messed up trace that was re-done (see picture in first post, it's pin 7 of the 1st op amp to a 0.0056 cap), but otherwise I can't SEE any other problems.  If I'm reading the schematic correctly I should be getting audio signal from pin 7, but instead all I get is a burst of noise when the probe first touches the pin.
4. I did what you said with the reverb footswitch and I got no signal. 

All the lamps and LEDs work, and they switch on and off properly according to which channel is engaged.  All the electrolytics look intact and I don't see any other traces that need attention. 

Before I start thinking I need to replace that first opamp, is there something else I should be looking at?  Thanks again everyone for your suggestions! 

Enzo

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Re: Fixing a Yamaha G100 112 ii
« Reply #6 on: August 03, 2015, 07:36:58 AM »
As a technician, I want to troubleshoot to determine where the problem lies.  I do not want to just start replacing things, hoping to find the problem that way.  Many a rookie wants to do just that.  However, once I have reason to suspect the first IC, then I won't hesitate to replace it.  I charge my customers a dollar a minute to work on their stuff.  How many minutes should I take wondering if that IC is bad, when the IC costs me 30 cents each?   4558 is about as common an IC as there is.  If you don't have one, a TL072 or something will work fine.   If a new IC fixes it, great, and if not, oh well, at least we can move on.

Signal tracing and injection are classic techniques.   If touchcing your probe to the output pin of an op amp makes noise come out the speaker, that sounds like the amp is working from that point forward.  At least enough to make sound.   If that is pin 7, then touch the probe to pins 5 and 6.  If the op amp is working, then that ought to make noise too, the op amp will amplify the noise.  If you get no sound that way, then replace the IC.

phatt

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Re: Fixing a Yamaha G100 112 ii
« Reply #7 on: August 03, 2015, 08:39:56 AM »
  I charge my customers a dollar a minute to work on their stuff. 

Floppin hang   Enzo,,,  In that case I must owe you a fortune by now. :o :lmao:

jokes aside; Your input is Always greatly appreciated. :dbtu:
Phil.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #8 on: August 03, 2015, 08:52:38 AM »
Thanks for the wisdom Enzo.  I definitely am a rookie when it comes to this stuff. 

A couple of questions:
-I've been attaching my probe (which is just an alligator clip jumper with an old soldering iron tip on one side) to the direct out, thus bypassing the vast majority of the circuit, just trying to see how far the signal is getting into the circuit before it dies.  Is there anything wrong with this method?  What do you mean by "injection"?  I'm thinking you mean doing something like alligator clipping the output of the opamp and seeing if signal passes through as I try to bypass parts of the section before it, just to see if the rest of the circuit works. 
-I have another 4558 on hand, so that won't be difficult to replace- I don't know the technical name for certain regions of the circuit, but could the network of diodes and resistors before the opamp affect the output of it?

I'm getting very faint signal from pin 5, a blast of noise from pin 6, and the same blast of noise on pin 7.  The "blast" silences very quickly, and if I move my probe around on the joint it might blast again.  The blast is much louder than the faint signal I get on pin 5.  So, to say I'm getting silence from pin 7 would be incorrect, but its not exactly a signal.   

Given the evidence of where the signal is going (or not going rather) and that there has been work done on the opamp (I'm thinking it easily could have been fried if too much heat was applied when that jumper was installed.  And with the legs looking all chewed up... its anybody's guess what the opamp has been through).  Thanks again for your help, its much appreciated! 

Enzo

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Re: Fixing a Yamaha G100 112 ii
« Reply #9 on: August 03, 2015, 03:49:38 PM »
Any amp is a process, it has an in and an out, and inside is a signal path through it all.   If you put a signal into its input, then it exists at each stage through to the output.  Signal tracing means we do just that, put a signal at the input, then we use some means to trace it through the system.  That could be a scope or a meter, or even another amp for a listen.  If the signal path is interrupted somewhere, we would detect the input signal up to that point and not after.

Alternatively, we can listen at the output, and inject a signal - music, noise, test tone, hum - at the start of the power amp, and then move the injection point farther and farther back towards the input.  If there is a break in the signal path, we will find a point where going further back no longer comes out the speaker.

I can use a test tone for injection, but when I am faced with a silent amp, even just noise will serve.  Touching a sensitive point with my probe or even a screwdriver will inject some noise or hum, sufficient to see if the amp is amplifying.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #10 on: August 03, 2015, 08:59:07 PM »
Gotcha.  I see what you're saying.  I'll do some more testing and let you all know what I come up with.  Thanks again!

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #11 on: August 07, 2015, 01:10:14 AM »
Alright... now I've really gone and screwed up.  So, in a moment of carelessness while I was probing the board, I decided to touch my probe (which was coming from the input) to a point on the DC board which fried a resistor on the MA board.  To give you a visual: if you look at my first post and see the board captioned "Power Board with Trannies", you'll see two green chiclet caps and a small 1/4w carbon film resistor on the bottom of the picture.  That carbon film resistor is completely fried.  It's a 150 ohm resistor, and I have several 150 ohm 1/2w metal film so I replaced it.  Turned on the amp and it fried the new resistor immediately.... and this time I wasn't probing and I made sure nothing was touching something it shouldn't have been.  That resistor is coming right off the pin 1 connector between the DC and MA board.   

Now, the spot I probed that initiated that fried resistor was a joint on the DC board where a white wire leads to the big cap right before the output jack (the DC board is the picture captioned "Power Board up top" on my first post).  It doesn't look like anything else is damaged.

Doing a little bit of searching around on this site and others, I think the fried resistor is the emitter resistor.  I've read that they fail sometimes because of output transistor failure.  Soooo... I should be testing my transistors to see if they are good, correct?  And if I do find a faulty resistor, that should be the problem, right?  I'm going to run in that direction for now.  If I'm wrong please stop me from blowing something else up.

Also, I replaced that first opamp- but I haven't been able to test it now that its acting crazy.  Interesting note-  when the amp started smoking a loud hum was coming from the speaker... more confirmation the speaker works, yippee! Thanks for any help anyone can provide. 

EDIT- Tested the transistors and sure enough, one of them isn't reading anything between the base and emitter.  So it looks like I'm off to get a new pair of power transistors... ugh.
« Last Edit: August 07, 2015, 02:18:59 AM by dropanchor812 »

DrGonz78

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Re: Fixing a Yamaha G100 112 ii
« Reply #12 on: August 09, 2015, 04:43:18 PM »
So basically, in this instance, a resistor burning up could be working like a fuse. Your not going to search around for bad resistors (typically), but more likely bad transistors or other semi-conductors. This is what you have found so far. Now word of advice, the humming you heard is probably DC voltage reaching your speaker. Not really a good way to confirm the speaker is working. Now you can confirm the speaker is working with a small 9v battery(do google search if not clear) but with a larger DC voltage coming out of the amp it might fry the speaker. So disconnect the speaker and pray that it was not damaged. Also, at this point do not connect the speaker up until you have repaired the amp and confirm that the amp is not putting DC voltage on the speaker output. Note that this amp does not need a load or speaker on the output while you are in the repair process.

Build a Light Bulb Limiter before you power up the amp again. http://www.ssguitar.com/index.php?topic=2093.0 This device will save fuses and resistors from burning up. With blown transistors the amp will pull a ton of current and burn out other parts. Each time you turn on this damaged amp, it can cause more and more parts to become compromised. So, that is why you need the light bulb limiter, read up on that one.

Find all the bad transistors or diodes etc... and replace only the bad parts. Replacing parts at random will only paint you into a corner that you may never get out of... 
« Last Edit: August 09, 2015, 04:44:50 PM by DrGonz78 »
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dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #13 on: August 09, 2015, 10:15:17 PM »
Thanks DrGonz-  I bought the parts I didn't already have lying around for the limiter today and will be putting it together before I try to turn it back on.  I have to order new transistors, mica insulators, and thermal compound too- all probably coming from the slow boat so I'll have a few days, haha.

As an emitter resistor in that position- it doesn't matter what voltage rating it is as long as its not lower than what is specified, correct?  And it being metal film over carbon film shouldn't matter either, right?

And yes- the speaker will be disconnected from now on.  :dbtu:

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #14 on: September 06, 2015, 12:51:52 AM »
Alright folks- 1 month later and here's where I'm at...

-Sorry again for the gigantic pictures-

Light Bulb Limiter - Check.


New output transistors, new insulators - Check.


New resistor and replaced nearby charred caps (far right of pic) - Check.


And I replaced that first jrc4558 I mentioned earlier. 

I fired it all up with the light bulb limiter on and nothing bad happened!!!  I was ecstatic.  But I'm still kinda scared to fire it up full power, and I'd like input on what to check first before I try it on full power again. 

A few observations:
-The light bulb immediately fades out when I turn it on, and from what I've read that means its doing what it is supposed to do.
-I checked the 15v and 20v lines on the preamp board and they are measuring where they should.
-I don't think it was doing this before, but when I turn the "Level" knob on the EQ I actually here a change in the noise coming through.
-When I press the channel switch button there is a small pop that I can hear.
-When I audio probe around the opamp I replaced I'm not hearing anything different.  Everything that was/wasn't happening before the swap is still happening.

So yeah- I'm optimistic and feeling pretty all right about this.  If anyone has any "next steps" I'd love to hear them!  Thanks again everyone!

« Last Edit: September 06, 2015, 03:10:52 PM by dropanchor812 »