Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => The Newcomer's Forum => Topic started by: dropanchor812 on July 24, 2015, 11:15:22 PM

Title: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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.

(http://i.imgur.com/aomd8zhm.jpg)

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.

(http://i.imgur.com/NcSHoy3m.jpg)

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:
(http://i.imgur.com/O2T0gZHm.jpg)

Power Board up top:
(http://i.imgur.com/o8bCiWxm.jpg)

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.   

(http://i.imgur.com/Gn6D0w7m.jpg)

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! 

Title: Re: Fixing a Yamaha G100 112 ii
Post by: galaxiex on July 25, 2015, 01:39:55 AM
Here is a Yamaha G100 service manual.

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

Title: Re: Fixing a Yamaha G100 112 ii
Post by: Enzo 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?
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 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/-manuals-1-20a.html?alpha_filter_id=71)

http://www.owner-manuals.com/G100115II-service-manual-YAMAHA.html (http://www.owner-manuals.com/G100115II-service-manual-YAMAHA.htmll)
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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!
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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)
(http://i.imgur.com/EJNoSjd.jpg)

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! 
Title: Re: Fixing a Yamaha G100 112 ii
Post by: Enzo 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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! 
Title: Re: Fixing a Yamaha G100 112 ii
Post by: Enzo 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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!
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 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 (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... 
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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:
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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.
(http://i.imgur.com/K2gSy5H.jpg)

New output transistors, new insulators - Check.
(http://i.imgur.com/Drl8CJr.jpg)

New resistor and replaced nearby charred caps (far right of pic) - Check.
(http://i.imgur.com/PxriwKS.jpg)

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!

Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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!
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 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?
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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?
Title: Re: Fixing a Yamaha G100 112 ii
Post by: Enzo 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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.  ;)
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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.  :)
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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.

(http://i.imgur.com/1XsFcx4.jpg)

(http://i.imgur.com/UkwLezT.jpg)

Thanks y'all!
Title: Re: Fixing a Yamaha G100 112 ii
Post by: Enzo 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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!
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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!   
Title: Re: Fixing a Yamaha G100 112 ii
Post by: g1 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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:
(http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/o0cAAOSwuMFUZpBC/$_35.JPG)
Title: Re: Fixing a Yamaha G100 112 ii
Post by: Enzo 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 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?
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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. 
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: Enzo on September 17, 2015, 08:46:05 AM
Teemu's book is ALWAYS worth reading.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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!
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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! 
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt 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.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: dropanchor812 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!!!
Title: Re: Fixing a Yamaha G100 112 ii
Post by: phatt on September 21, 2015, 09:49:47 AM
 :dbtu: :dbtu: :dbtu:

Someone once stated that 90% of electrical problems are in connections,, whether they be plugs or soldered joins like you found it's all connections. The trick is finding them. :grr
Phil.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 on September 21, 2015, 02:56:31 PM
I love those old Yamaha amps! Nice work Dropanchor  :dbtu:
Title: Re: Fixing a Yamaha G100 112 ii
Post by: g1 on September 22, 2015, 08:14:04 PM
-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?
You can't follow the directions for this part without a scope, so you were not following the directions.  ;)
-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!
This reading is affected by the previous waveform adjustment.  Try moving the B4.7K pot either way and see if it helps you get the center voltage adjustment closer.
Title: Re: Fixing a Yamaha G100 112 ii
Post by: DrGonz78 on July 16, 2020, 02:35:22 PM
UPDATE: I had posted not accurate information about this amp, refer to post 12. This amp has a cap coupled output and without a load on the output it will float at 50v as the cap will not be able to charge up. I must have missed this bit on the schematics and did not realize. Hopefully this post will prevent any further confusion if referencing this thread while working on their amp.