Yamaha G50-112 Schematic Needed

[rocknroller]

Yeah finding that schematic might be hard to do... So try referring to the G100 iii as it might help a bit for now. It's on the link below>>>
http://music-electronics-forum.com/t32980/

Main thing to try would be injecting signal into the power amp in jack to see if you get any sound through the output. A simple mp3 player or what not will do, just don't turn it up full blast. If you hadn't tried already plug a guitar cable from the pre-out to power amp-in. Lots of times an amp will sit and the jacks get all dirty and it cut the signal out hence "Dreaded switching jack."

Did try that - no sound with direct signal injection to PWR amp, or patching between preamp and PWR amp (and confirmed there is a valid signal coming out the preamp out)

[rocknroller]

Here is a partial of the G50 III from another thread. It looks very similar to the G100 so that is good. Would probably be a good idea to clean the send/return jacks etc etc...
[ATTACH=CONFIG]29592[/ATTACH]

Edit: Ahhhh!! I found it! It is under as G-20 III / G50 III. So as Enzo always says the schematic might be shared by two different amps. Well here it is...

Did a good dexox on them but no change and the direct signal inject to PWR amp leads me to think there must be a component failure in the PWR amp section someplace. I'm a re-educating elder now with electronics experience in my teens. Don't even remember my damn resistor code LOL! But getting back into to and and trying to learn how to diagnose and fix these types of things. So far, I've really only dealt with cap replacements, which have been easy to identify. This one will be a (re) learning experience for sure. Any thoughts where to start looking on the board at this point or easy tests to perform with a volt/ohm meter?

Also - any recommendations of some simple (lower cost) diag tools (beyond a volt/ohm/cap meter of course) for this and future diagnostics? I have a lot of broken tube radios, vintage stereo equipment etc and really am planning on making this my retirement keep-busy-but-still-productive hobby.

[Enzo]

Component failure or a connection failure or a broken copper trace, or, or, or.... You MAY have a bad part, but do not assume that from the start.


Is your speaker relay clicking on at power up?

[rocknroller]

Component failure or a connection failure or a broken copper trace, or, or, or.... You MAY have a bad part, but do not assume that from the start.


Is your speaker relay clicking on at power up?

Don't hear any relay click at startup. Interesting turn of events though. I turned the amp on/of about 3 times to make sure I didn't hear it - then the main fuse popped suggesting something finally went. Slight hint of smell of bad component around the caps but hard to isolate or be sure that's what I'm smelling. It's also interesting that the caps have dried glue on the board and I did have another stereo piece have a problem with dried glue becoming conductive. Wonder if the same has happened here. I could find another 2amp fuse but suspect at this point it will keep blowing until if find the root cause.

[The Dude]

What is the board designation of the fuse that blew? That might help us narrow down where the excessive current draw is coming from.

[Enzo]

Whenever you power up a circuit, there is a momentary current surge as everything charges up. If you watch a fuse element, many times you can actually see the little wire sag a bit then recover. Cycling an amp off and on repeatedly does not let the fuse have a chance to recover, and so it is not surprising the fuse went. There may be a problem, but it also might be from the cycling. It wasn't blowing fuses before, your issue was no sound.

I'd try one more fuse, if that blows, then we find out why, if it holds, then stop flipping it on and off.

If fuses continue to blow, look up "light bulb limiter", make one, and use it.

If the speaker relay does not come on, then the speakers remain disconnected. No sound.

Glue on larger parts is quite common and not likely to be involved.


So why does the speaker relay not come on? The relay drive circuit is just left of the power amp, IC111. If IC111 detects DC on the output, and it will not turn on the speaker relay. So look on the amp side of the relay to see if the amp is at DC or stable near zero. A convenient test point for that is the emitter legs of the output transistors. IC111 may want to fire the relay, but can't. Check power to it. Pins 9 and 5 are the power supply pins, are both powered? Pin 1 must be biased. AC from the main power supply is fed to D112, and that makes a small negative supply across C183 and R198, I'd guess maybe -20v or so. Got that? I see four small electrolytic caps around that IC. Has the plastic sleeve on them shrunk back? The thin plastic around them that has the printing on it.

[rocknroller]

What is the board designation of the fuse that blew? That might help us narrow down where the excessive current draw is coming from.

It was the main fuse - not one one of the 4 on the pc baord.

[rocknroller]

Whenever you power up a circuit, there is a momentary current surge as everything charges up. If you watch a fuse element, many times you can actually see the little wire sag a bit then recover. Cycling an amp off and on repeatedly does not let the fuse have a chance to recover, and so it is not surprising the fuse went. There may be a problem, but it also might be from the cycling. It wasn't blowing fuses before, your issue was no sound.

I'd try one more fuse, if that blows, then we find out why, if it holds, then stop flipping it on and off.

If fuses continue to blow, look up "light bulb limiter", make one, and use it.

If the speaker relay does not come on, then the speakers remain disconnected. No sound.

Glue on larger parts is quite common and not likely to be involved.


So why does the speaker relay not come on? The relay drive circuit is just left of the power amp, IC111. If IC111 detects DC on the output, and it will not turn on the speaker relay. So look on the amp side of the relay to see if the amp is at DC or stable near zero. A convenient test point for that is the emitter legs of the output transistors. IC111 may want to fire the relay, but can't. Check power to it. Pins 9 and 5 are the power supply pins, are both powered? Pin 1 must be biased. AC from the main power supply is fed to D112, and that makes a small negative supply across C183 and R198, I'd guess maybe -20v or so. Got that? I see four small electrolytic caps around that IC. Has the plastic sleeve on them shrunk back? The thin plastic around them that has the printing on it.

Replacement fuse is ok.
I'm not sure you are looking at the same schematic as me? The one a couple messages up in this thread for the G20G50 Series iii? I ask because you are talking about a IC111 then go on about pins 9 and 5, but on my schematic IC111 has only 3 pins. You also mention D112 but that part number is not present on my schematic (there is a D113 and D115 though)

[Enzo]

Sorry, you are right, I pulled up the 100 watt version. I'll use yours from now on.


On yours the relay control is IC110. From the AC is D111, which makes that -20v or whatever across C169 and R196. It is the same IC and circuit, just the part numbers are different. But the IC is the same so all the tests I described would be the same.

[rocknroller]

Sorry, you are right, I pulled up the 100 watt version. I'll use yours from now on.


On yours the relay control is IC110. From the AC is D111, which makes that -20v or whatever across C169 and R196. It is the same IC and circuit, just the part numbers are different. But the IC is the same so all the tests I described would be the same.

You said D111, but there is none on my schematic. Did you mean D113 or D115?
I did measure about -17.5 vcd across R196. Can't easily find C169 , as it's not clearly labeled on the board and there are several together and hard to read. Also weird that is is not listed on the schematic parts at the bottom with all the other caps, though is in the diagram itself. I may have to pull the board out and trace it out to find in on the board - but I suspect i should see a similar voltage there?

I did check the output transistor emitters voltage - 0 VCD

What else should I look at?

[g1]

D111 is shown straight up on the line from C169 (on schematic from post #45).
If you have -17.5V at C169 then that part of the circuit seems right. What do you have at pin 6 of IC110 ?
What about the 4 caps Enzo asked about in post #51 ?

[rocknroller]

D111 is shown straight up on the line from C169 (on schematic from post #45).
If you have -17.5V at C169 then that part of the circuit seems right. What do you have at pin 6 of IC110 ?
What about the 4 caps Enzo asked about in post #51 ?

Ah see that diode now.

Voltages for all pins of IC110
pin1 -0.63
pin2 0
pin3 0
pin4 0
pin5 -0.76
pin6 1.22
pin7 0
pin8 1.28
pin9 3.02

The cap sleeves don't look pulled back to me but I've posted a pic if you see something I don't. .

These are some of the other larger caps as well

[g1]

Check DC across D112, should be over 15V.
If so, check that resistance is zero ohms from tip of speaker jack to junction of R235/R236 (while on).

[rocknroller]

Check DC across D112, should be over 15V.
If so, check that resistance is zero ohms from tip of speaker jack to junction of R235/R236 (while on).

On either side it's 22 volts, but across it is 0 volts. (or practically 0.12)

[g1]

If there are 0V across it, then the relay is off.
As you mentioned there is no DC at the output transistors, and pin 2 of IC110 is 0V, so there is nothing wrong at the output, no signal telling IC110 to turn off relay.
So that leaves the IC110 and it's circuitry. Pin 5 I would have expected more negative, but I could be wrong. Check R208 and associated solder connections.