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Author Topic: Fixing a Yamaha G100 112 ii  (Read 18551 times)

g1

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Re: Fixing a Yamaha G100 112 ii
« Reply #30 on: September 15, 2015, 12:03:59 PM »
  It may pass voltage when you are pushing on it with your meter probe, but it may not pass proper current when you aren't touching it.
Try wrapping a small wire around it to hold it tight to the tab, or even solder it if you are careful and quick.  If you take too long with the solder iron you'll melt the solder in the fuse.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #31 on: September 15, 2015, 01:05:08 PM »
  It may pass voltage when you are pushing on it with your meter probe, but it may not pass proper current when you aren't touching it.
Try wrapping a small wire around it to hold it tight to the tab, or even solder it if you are careful and quick.  If you take too long with the solder iron you'll melt the solder in the fuse.

I have some bus wire that I can use to wrap it up.  It's funny- watch this thing that I didn't think was a big deal be the problem.   :duh

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #32 on: September 15, 2015, 11:38:41 PM »
Alright, so the voltages were showing up as they should in some of those places when I used the appropriate DMM settings.  That was humbling.  :duh

Still wasn't getting any readings on the odd-looking silver diode apparatus (S5151R), so I tried measuring the parts on the under side of the shield. I touched my DMM probe to one of the poles on it, saw a spark, and then the amp shut off completely.  The primary fuse blew, the others on the DC board are fine.  I don't have a 250v 3a fuse at the moment so that's all for tonight- any idea why I couldn't read voltages anywhere on that diode and then when I touch the underside it sparks?  It did read over 100v when the spark hit, but then everything went out. 

Here's a pic of the diode thing for reference:

Enzo

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Re: Fixing a Yamaha G100 112 ii
« Reply #33 on: September 16, 2015, 12:38:09 AM »
I had hoped we were working on restoring the 15v supplies.  Have we moved on to other things?  Those dual diodes are shown in the schematic as part of the main supply to the power amp.  Not part of the 15v circuits. 

The dual diodes are each half of a bridge rectifier, D207 and D208.  The dual diode 5151R has common anodes, and if you look at the schematic, that common of that pair is connected to ground.  The frame of it is the common terminal.  The two pins underneath connect to the two red HV AC wires from the transsformer.  So ther would be AC between them, and a meaningless measurement to ground from each.  You probably saw 100v because that is probably roughly what the supply voltage it is making might be.

Please go back to fixing the 15v supplies unless they have already been corrected.

DrGonz78

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Re: Fixing a Yamaha G100 112 ii
« Reply #34 on: September 16, 2015, 01:14:30 AM »
Alright, so the voltages were showing up as they should in some of those places when I used the appropriate DMM settings.  That was humbling.  :duh

I am with Enzo, inform us what we are fixing. You said "the voltages were showing up as they should...", but it is still unclear. Report to us as if we are not there in the room and have no eyes. You are our eyes... What are the reported voltages? Does this mean the 15v supplies have been restored? Are you having any troubles with reading the schematics?
“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 #35 on: September 16, 2015, 08:23:42 AM »

Yep too much fiddling and now you blew the fuse,, not to worry you need to slow down and fix one problem at a time, hopefully you did not blow the main bridge.
Yes replace the fuse, buy a few. if it blows again straight away you will likely need a lamp limiter before you go any further.

http://www.ssguitar.com/index.php?topic=2093.0

NUMBER 1;
Replace the fuse holder or solder it together then check it,, as mentioned before too much heat can melt the internal connection leaving you with a dead fuse.  better to replace the clip.

You have 2 supplies, F3 feeds ACV to the main Amp rectifier.
The 2 metal faces are Pos/Neg (neg is also common/ground)
If you short those 2 faces expect sparks as there is likely over 100 VDC between them.

The rectified DCV is then filtered by a 2,200uF / 160Volt cap so the working voltage is obviously over 100VDC.
And BTY that's enough to kill you if you are foolish so I'd take the advice given in the last few postings.

Don't work with bare feet and take off any Jewelry before you go probing around inside,, no necklace, watch or rings. You can't fix anything if you are dead.

OK the other supply feed from F1 and F2 is the low voltage supply and it's a bit different, Split supply in this case.
So you have 3 poles, Zero is Common (and linked to the main rectifier) so Pos/Zero/Neg.

Learn to work in *Nodes* see my edit of your Schematic for clues to the idea. Colour coded in this case but you can use numbers. The Red node joins at least 5 components together so at any point on that node you should read the same voltage. So the voltage at the Positive end of D201 will be the same voltage at the Collector of TR201,, same node.
With F3 removed you should still be able to work on the low voltage section and fix that fuse holder.
Phil.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #36 on: September 16, 2015, 09:49:11 AM »
Sorry guys for not being clearer about my progress last night-  I acknowledge that you guys are being very gracious with your time and patience and I'm trying not to bog you down with details, and only mention things that seems to me as problems.  That being said, I appreciate it a lot and I'll address some of the things you have mentioned...

Those dual diodes are shown in the schematic as part of the main supply to the power amp.  Not part of the 15v circuits. 

Great to know!  And yes, the 15v supplies are my priority, but I was not understanding why I wasn't getting a reading on the s5151r which led me to believe that it may be a faulty part.  According to Phatt's annotated schematic and what you're saying- because the frame/case of the s5151r and s5151 are carrying different voltage supplies they will not look the same.  That makes a whole lot of sense! 

Are you having any troubles with reading the schematics?

I'll admit that I do have trouble with schematics, especially ones this complicated.  I'm used to working on pedals which, to me, feel a lot simpler.  I can usually trace an audio signal pretty well, but power supply circuitry is definitely something I lack knowledge in.  So yeah, that's definitely been a barrier and I try to understand things about these individual parts and loops before I post on here so as to not waste your time- but its difficult to clear answers on such specific questions through the internet.

Yes replace the fuse, buy a few. if it blows again straight away you will likely need a lamp limiter before you go any further.

NUMBER 1;
Replace the fuse holder or solder it together then check it,, as mentioned before too much heat can melt the internal connection leaving you with a dead fuse.  better to replace the clip.

Learn to work in *Nodes* see my edit of your Schematic for clues to the idea. Colour coded in this case but you can use numbers. The Red node joins at least 5 components together so at any point on that node you should read the same voltage. So the voltage at the Positive end of D201 will be the same voltage at the Collector of TR201,, same node.
With F3 removed you should still be able to work on the low voltage section and fix that fuse holder.
Phil.

I have built a lamp limiter and will be sure to use it after replacing the fuse.  I also did solder the fuse with the broken clip.  It's making good contact.  Your explanation of working in nodes is very helpful, as is the diagram you provided.  Rather than doing the work for me, is there a resource you can provide (or point me in the right direction of one) that would help me map out the whole DC board like that?  When I'm looking at the schematic I see the parts and connections and have a less than basic understanding of what they do together.  So seeing those networks and their function is very helpful. 

Thanks a lot, once I get that fuse I will post those specific voltages later. 

phatt

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Re: Fixing a Yamaha G100 112 ii
« Reply #37 on: September 17, 2015, 05:28:19 AM »

Teemu's Book might be worth reading, :dbtu:

http://www.ssguitar.com/index.php?topic=711.0

Yes power supply is quite tricky and will stump you. :duh
This helped me out years back.
Print it out and keep it as it's a good quick reference chart.
Phil.

Enzo

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Re: Fixing a Yamaha G100 112 ii
« Reply #38 on: September 17, 2015, 08:46:05 AM »
Teemu's book is ALWAYS worth reading.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #39 on: September 17, 2015, 08:46:32 AM »
Thanks a lot phatt!!! 

BTW-  Got some fuses yesterday, popped it in with my limiter and everything worked.  Powered the amp on without the limiter and everything still worked.  I didn't have time to check through voltages, but I'll try to get some of that done tonight.  Thanks!

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #40 on: September 20, 2015, 12:28:58 AM »
Alright guys, finally got around to doing some testing this evening.  Here's what I've found:

There's no easy way to say this: I am a royal moron. I was doing my readings of those supplies using the 20vdc setting on my DMM and when I clicked it over to 200vdc I got these ratings:
-20 supply = -21.5
-15 supply = -15.7
+20 supply= +21.5
+15 supply= -8.3

So I started looking at that voltage on the +15 supply and did some measuring on the two NPN power transistors feeding that supply (if I'm reading this schematic right), which are TR201 and TR203.  Here are my measurements on those:
TR201 - E: 17.3 vdc
           C: 22.1 vdc
           B: 17.3 vdc
TR203 - E: 16.2 vdc
           C: 16.05 vdc
           B: 16.5 vdc

I took voltages of the other two corresponding power transistors (TR202 and TR204) for the -15v supply and this is what I found:
TR202 - E: -17.2 vdc
           C: -21.9 vdc
           B: -16.6 vdc
TR204 - E: -15.6 vdc
           C: -22 vdc
           B: -16.3 vdc

If you look on the schematic how the transistors correspond to each other then you'll notice that the Collector voltage on TR203 isn't where it should be.  That deduction has no basis in any electronics know-how, I just noticed the pattern.  So should I be looking at that transistor (TR203) perhaps?

I also did a little messing about with a signal, trying to see if the power amp was functioning properly.  The 3rd pin on the board that connects the output circuit board and the DC board is the beginning of the output section.  Applying a signal there amplifies the signal very nicely and it doesn't fade out like it did before.  This leads me to believe the power amp is working just fine.   
I also tried applying the signal to the output of the 4th opamp, which is where it is supposed to then go to the power amp.  The signal came through, although much quieter than when I applied it to the 3rd pin mentioned in the previous paragraph.  The fading thing that I mentioned in a prior post did not happen.  So, I applied the same signal on the output of the second opamp- I got the signal again.  It was a little cloudier, and maybe a tad lower, but pretty much the same as the other opamp signal.  Then I applied it to the output of the first opamp, the pesky one I thought was the problem all along, and there was no sound.  I turned the signal volume up to its maximum on my little usb mixer and I could very faintly hear the signal.... very faintly, and very grainy too.

I'm not really sure where this puts things at the moment.  In summary, the power amp seems to be working as it should, the +15 supply is running at half-ish power, and I still don't know why the signal can't be heard off of that first opamp, but it can be heard everywhere else. 

I skimmed some of Teemu's book, but I haven't been able to really dig into it.  I will definitely be looking into it a little more to see if there's an explanation for all this.  Thanks! 
« Last Edit: September 20, 2015, 01:05:10 AM by dropanchor812 »

phatt

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Re: Fixing a Yamaha G100 112 ii
« Reply #41 on: September 20, 2015, 06:46:59 AM »
Yes your -8.3 VDC rail does not look right but then your +15 supply is reading +16.2 VDC at the emitter of TR203 which is the SAME node as the +15VDC supply rail.  So Both can't be right.  xP

How about you simply check you have both rail voltages present at pins 1 and 8 on all the NJM5448 chips. If they are all way out then regulator is suspect, if only one chip reads bad then rails are fine and you have a local problem around that chip or a dead chip.

IC3 has a different pinout and supply is +/-20VDC. (pinout is on schematic)

If all the above is ok then you may have a dead passive or a broken trace on the pcb.
But you need to establish that you have DC voltage at all stages before you can move forward.

My best guess from your last observation is that all rails may well be fine but you have a local problem around that chip.

Hey in spite of a few stuff ups you are doing ok. :tu:

Phil.

DrGonz78

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Re: Fixing a Yamaha G100 112 ii
« Reply #42 on: September 20, 2015, 07:28:28 AM »
Definitely something funny with your measurements. Phatt laid out one major discrepancy but also note that the collector voltages for TR201 and TR203 are one and the same. They are connected together so both should read the same voltages exactly. TR201's collector reads 22.1vdc and TR203's collector reads 16.05vdc. As Phatt described we see there is 16.2vdc on the emitter of TR203 which contradicts your measurement of -8.3vdc on the +15 supply. Please check your measurements and describe exactly where you are placing your DMM probes.
“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 #43 on: September 20, 2015, 08:32:19 AM »
Yep thanks Doc, I missed that one :-[  the more eyes the better. :tu:

Regards using probes, be aware that just resting a probe on a point can give quite different results,, there is a knack to using the equipment. 8|

This is an old amplifier and in many places there will be corrosion on the surface even when it looks clean. Corrosion and scum is only semi conductive and will give you false readings.
You often need to Gently wiggle and twist the probe into the surface to get full electrical contact with the metal before you get accurate readings.

Of course if you just push hard there is a very good chance you will slip off and likely sparks will result,, so a steady hand is needed.   
Phil.

dropanchor812

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Re: Fixing a Yamaha G100 112 ii
« Reply #44 on: September 21, 2015, 01:10:48 AM »
Please check your measurements and describe exactly where you are placing your DMM probes.

On the DC board I've been taking all of my measurements on the legs of the components themselves- since the board is mounted that way to the chassis, with the output board connected via those long pins on the underside of the chassis.  I can try to maneuver it out in order to take my measurements on the solder points, it will just be a bit cumbersome making sure things don't touch things they shouldn't  :-\  But oh well, I decided to take on this project, right?

(a few minutes later)

Alright, got the dc board off the chassis, the emitter pin's solder joint on TR203 had broken, so I reflowed that joint.  Looks good.

(Connects everything back together)

WE GOT VOLTAGE PEOPLE.  Before reflowing that joint I measured the pin 1 and 8 on the 4558's and I was getting ~7-9vdc on pins 1 and 8.  Now here are my voltages for them:

Ehh, maybe I'll just plug it in just to see if it works now that I know voltage is getting to all the IC's....

(plugs in signal through input)

IT'S CHRISTMAS!!!

(proceeds to play guitar through input, checks both inputs, fiddles around with all controls, checks direct out, reverb, push/pull pots, etc.)

It works.  It actually works.  Unspeakable joy.

Well gentlemen, it looks like it works!  Now I just need to make these MA board adjustments to make sure all that's where it should be before I box it all back up.
-The idling current is measuring right.
-I don't notice a difference in sound coming from the speaker when I adjust the output waveform when following the directions in the manual.  Is this something I'm just going to need an scope to measure?
-I can't get the center voltage down to ~45v as directed in the manual.  When the trimmer is fully counter-clockwise it only gets down to ~53v.  I'm afraid to turn it fully clockwise because I'm afraid I might blow something, but as it is fully counter-clockwise the amp still sounds great!

I really still am in disbelief that it actually works.  Thank you guys so much for all your help, I wouldn't have figured it out without you.  If you're ever in the Louisville, KY area hit me up so I can buy you a beer!!!