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Author Topic: Fixing a Yamaha G100 112 ii  (Read 18017 times)

DrGonz78

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Re: Fixing a Yamaha G100 112 ii
« Reply #15 on: September 06, 2015, 04:52:17 PM »
Well it sounds to me that the opamp that you pulled was not bad. If the light bulb is going dim, as you mentioned, then yes that is a good sign. Now it shows you that the amp is not drawing any huge amount of current and won't blow your fuses. Ditch the light bulb limiter now and power up without it. With the speaker disconnected you can measure across the speaker outputs to see what the DC offset on the amp measures. The DC offset on the output speaker should be low around 250mv or less.

Refer to page 4 of your service manual... Look up idle current adjustment. Measure the idle current by the voltage drop across that 0.22r 5W resistor. Says it should be 5mv. Keep the amp on for at least ten minutes while setting the idle current as the amp needs to be at operating temperatures. Just remember that idle current is like the idle on your car.
« Last Edit: September 06, 2015, 04:56:55 PM by DrGonz78 »
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dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #16 on: September 07, 2015, 12:04:57 AM »
Thanks DrGonz!  I took the limiter out and nothing bad happened!  I tested the DC offset and I can't get a stable reading, they just jump all over the place.  I also did the idling current adjustment- at first the drop off was 17mV, but I got it 5mV.  I'll do a little more testing this evening, but I wanted to let you know about those things first.  Thanks!

DrGonz78

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Re: Fixing a Yamaha G100 112 ii
« Reply #17 on: September 07, 2015, 04:22:52 AM »
That all sounds like good news to me. Fact is that the amp is now stable and seems to be working. When measuring the DC offset on the amp it will sometimes jump around a bit. If it is jumping around from 56mv to 40mv to 25mv or even going to -25mv, it is very common to see those readings on a healthy amp. Are you having any other issues with amp?
« Last Edit: September 07, 2015, 04:26:41 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

phatt

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Re: Fixing a Yamaha G100 112 ii
« Reply #18 on: September 07, 2015, 07:51:01 AM »
That small DC offset will wander around while passing signal but should stay stable with no signal.
If so it sounds like it's a win. :tu:
Phil.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #19 on: September 07, 2015, 10:08:36 AM »
I only messed around with it for a little bit last night, and it still won't pass a signal.  I hooked it up to a different speaker with the same result.  I guess I'll go back in there and do some more probing around on the preamp board, I think that's where the problem is.  If I can hear the channel switching pop and the EQ level knob turn then the latter part of the preamp must be getting through.  Is there anything else that could be causing the signal to die at the first opamp besides a bad opamp?

Enzo

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Re: Fixing a Yamaha G100 112 ii
« Reply #20 on: September 07, 2015, 12:41:01 PM »
The same things that can happen anywhere else in the circuit.  Open resistor or cap, open trace between parts, failed solder connection, lead broken off part, failed interboard connection, and so on.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #21 on: September 10, 2015, 12:55:39 PM »
Tinkered around on it a little more last night.  New things and observations:

1. I probed all over the preamp board, alligator clipping from the input to different parts of the board so as to skip over certain sections so I could narrow down where the fault may be... I don't know what this is actually called so I'm just going to call it "bridging" from here on out.  I'm able to pass a signal, albeit a very quiet one when I bridge into pin 7 of the opamps, which is the output of the opamp.  I'm getting an even quieter signal from solder points in between the opamps, but the loudest signal comes from the opamps.  This is a big deal because before I was not getting any signal to pass through the speaker at all before changing the output transistors.     
    * When I was doing all this probing I thought I had narrowed down the problem to a 0.047uf cap and a 220pf cap right after the input to the preamp, so I replaced them.  No change.   :duh 
    *I don't know the dB level of the signal I'm passing through the input is, but it is REALLY loud.  I'm playing music from my computer, which is being amplified through a usb mixer, and the headphone out is connected to the input.  Through the headphones the signal is intolerably loud, just FYI.   


2.  Now, this is really weird... when I try to bridge from the input to certain places in the signal path BEFORE the 1st opamp I will get a very quiet signal after holding the probe on there for a few seconds... BUT THEN the signal will continue to be present for a few more seconds after I lift the probe off of the solder point.  Weird, right?  If there is no signal being applied then how in the heck is it still present?  Unless there's another short somewhere that I haven't found yet?

3.  The speaker will jump a little bit when I adjust the level on the para EQ section.  I kinda eluded to this in an earlier post.   

4.  Would any adjustments made on the "center voltage" and "output waveform" possibly affect this problem? I haven't touched those trimmers on the output board, but I wonder if the dude who had it before me played with them possibly knocking it out of wack.  I don't have a scope, is it possible to perform the right calibrations with a DMM or another method?

Again, I appreciate all of your suggestions.  Don't feel like you have to respond, it's good to just have a running log of what I'm doing on this board.  But... suggestions are always helpful.  ;)
« Last Edit: September 10, 2015, 12:59:26 PM by dropanchor812 »

DrGonz78

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Re: Fixing a Yamaha G100 112 ii
« Reply #22 on: September 10, 2015, 06:14:03 PM »
Many amps will have preamp out and power amp in jacks. Those can help by outputing the preamp to a known working amp to see if the fault is in the preamp. You can then output the preamp of a known working amp to the power amp of suspect amp to check for the fault. Use the half-split technique by figuring if the problem is in the preamp or the power amp then divide and conquer. This amp does not have those handy jacks. Inject a signal right at the point where the preamp meets the input of the power amp section. We need to know if the preamp or power amp is the problem here. Keep the signal volume low at first.

This "bridging" idea is pretty interesting, but I am not sure if this bridging idea is gonna get the job done, I could be wrong. Personally I think you need to unplug the speaker and do signal tracing on the preamp section. Listening to the signal independently from the output of the amp. Do a search for signal tracer on this site and you will get the jest of it. Also, signal injection can be the reverse version of the signal tracer. Read up on both of those techniques.
“A person who never made a mistake never tried anything new.” -Albert Einstein

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #23 on: September 13, 2015, 03:25:32 PM »
Just an update- the preamp output to the power amp is pin 1 on the 4th 4558.  Sure enough, when I apply a signal there it amplifies pretty well for about 10 seconds and then it slowly fades down.  A signal is still passing, but it's just my signal driving the speaker without the amp  My voltages to the preamp board are also off now, the -15 is fine, the -20 tap isn't working, the +20 isn't working, and the +15 is reading -8.9.
Gotta take a break for now, nap-time just ended.  :)

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #24 on: September 14, 2015, 11:39:30 PM »
So, I did some more testing and measuring on the power supply board since those voltages going to the preamp board were off.  To be honest guys, I really don't know what's going on with it because I don't know what's normal.  Besides the lines going to the preamp board there's no other voltage readings I am seeing in the service manual.  So here's what I'm observing at the moment:

There's two different windings coming off of the transformer- the red lines are reading around 54v, the brown lines are swinging between +20mv and -40mv.  Is that what it should be doing?  I was expecting a stable voltage.  If the transformer is blown then I'm giving up on this guy.  If that behavior sounds okay though, then I'll press on. 

It should be known that I was getting the proper voltages on those lines running to the preamp board before the big catastrophe that I discussed in my post from August 7th.  So, that event must have done something to something- I just don't know what.  The 4 amp fuse on the power board looks a little worn, but it's still passing those 54v fine. 

-I started writing this post, got curious and did some more testing and....

There's these two big weird metal things on the power board, I looked up their part numbers (S5151 and S5151R) and apparently they are diodes that form a rectifier bridge.  It looks like they are doubling the voltage because one of them is reading 110v when I touch the casing, the other one is completely dead.  So.... I look at what's feeding the dead one and there's 4 diodes, which I'm thinking is another rectifier, with two big ol' 35w caps right after it.  The little diode network is still carrying voltage and all together it looks like it's producing 24vdc.  There's a little jumper on the top side which enables me to read the voltage coming off of the first big blue cap without having to remove the entire board to take measurements on the solder side.  There is absolutely no DC on that jumper... so could that big blue cap be the problem? 

Here's a pic of the schematic, my pen is pointing to the big blue cap in question... and there's a photo of the board after it for reference.  *Sorry again for the huge pics.





Thanks y'all!

Enzo

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Re: Fixing a Yamaha G100 112 ii
« Reply #25 on: September 15, 2015, 12:25:31 AM »
If -15v is working, then -20v MUST be there, because the -15 is MADE from the -20.  Look on the power supply page, page 8 of the manual.  Upper right is the power supply.  If -15 and -20 are present and +15 an +20 are missing, it cannot be the transformer, because the same winding makes both sets.  it cannot work for one and not the other, so if one side only is missing, then the problem is on the DC board.   Likewise a blown fuse cannot kill only one polarity of voltages.

If you are missing +20, then there will not be +15.

Look at these voltages right on the DC board.  If they are not there, they won't get to the preamp either.

phatt

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Re: Fixing a Yamaha G100 112 ii
« Reply #26 on: September 15, 2015, 01:19:02 AM »
Maybe you trying to read DC with meter set to AC? 8|

Set to ACV before the Rectifier and then switch to DCV after Rectifier.
In both cases rectifiers are full wave bridge so the DC voltage will close to 1.4 times the AC voltage that feeds the diode bridges.
Phil.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #27 on: September 15, 2015, 08:32:19 AM »
If you are missing +20, then there will not be +15.

Look at these voltages right on the DC board.  If they are not there, they won't get to the preamp either.

Yeah, I'm pretty sure the problem is on the DC board given everything I'm reading.  Thanks for that confirmation, it helps to have someone confirm those suspicions.  If you're talking about looking at the voltages on the joint right before jumping to the preamp board- I did that and they are the same as I mentioned before.  So yes, the problem most definitely seems to be on the DC board.

Maybe you trying to read DC with meter set to AC? 8|

Set to ACV before the Rectifier and then switch to DCV after Rectifier.
In both cases rectifiers are full wave bridge so the DC voltage will close to 1.4 times the AC voltage that feeds the diode bridges.
Phil.

Oh gosh, I can't believe I made that mistake.  Duh.  I'll check those voltages on ACV and report back. 

I'm still wondering though about where the voltage seems to be dying on that blue cap- any input on that?  I was measuring those voltages on DCV, and they are after the smaller rectifier.  Thanks again for your help, it is most definitely appreciated!

phatt

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Re: Fixing a Yamaha G100 112 ii
« Reply #28 on: September 15, 2015, 09:01:20 AM »
You can measure both DC voltages right at the diodes, easy to insert probe on Positive nodes of D201 and D202, right beside the Electro cap. First check your AC voltage both sides of the fuse and I've just noticed looking at the picture one fuse holder tang is missing and might be the reason for no readings. ??? :o

Bottom left corner of picture, unless it's an optical illusion that fuse holder is broken. 8|
Phil.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #29 on: September 15, 2015, 09:32:18 AM »
That tang is in fact totally broken off- it came to me that way- but it's still passing electricity through the fuse.  Any suggestions on how to make it stronger without having to replace the whole holder?  The other side is still able to apply enough pressure to make a good contact, it's never come loose or anything.  It had been kind of a 3rd tier problem for me at this point.   ;)

I'll check some more of those voltages as you all have directed and report back tonight.  Thanks again!