So I am attempting to troubleshoot this beast the Yamaha EMX512SC. Lots of surface mount parts and very tough to take it all apart. Had to go buy a huge screwdriver just to get the front panel off, very long hole shafts.

The behaviour is that all the channels except ch3 are not passing a strong signal. There are LED's (Label Name: FCL) that are associated with each channel. All the channels except ch3 gradually light up the FCL LED as you turn up the compressor for that channel. I can observe on a scope the signal is passing through the channels, but it is very low. This is whether or not I scope from the speaker out or the main out(preamp). Channel 3 is loud and working great, except that FCL light is fully illuminated. The strange behaviour leads me to look towards the compression part of the mixer as a potential culprit for the problem.

I am going to take some voltages as well but just going slow first to understand which ground points I should choose on the preamp part of the amp. It is a switch mode power supply and I am not very experienced dealing with these types of units. Any help would be greatly appreciated and I have attached a link to the HUGE service manual, since it is way too big to upload here. Thanks.

http://elektrotanya.com/yamaha_emx512sc-emx312sc.pdf/download.html

Okay I have a Fender Showman solid state amp in for repair. The symptoms are Reverb and Equalization lights are stuck on, but cannot turn them off. The reverb was not passing signal but I am betting that was due to the faulty wiring in ribbon connectors between preamp and logic board. The EQ sliders are working fine, but cannot turn off the effect. This occurs whether you try the push buttons on the amp or the foot switch. Pushing the momentary switch on the amp and holding it down turns off the indicator light on the foot switch, not the light on the amp. I can only guess that the problem might be a TL607 or TL604 chip, which I know are rare birds. I am a bit confused on how to test the TL604 or TL607 chips to know if they are the culprits.

Also, to note is that the channel switching works fine. The effects loop switching was working, but has quit working now(indicator light stuck off). I had noticed continuity issues with both ribbon connectors that go between preamp board and logic board. I have straightened out that mess to make testing less of a guessing game and have good connections now.

Looking at the preamp schem page, look for P1 (Power IN) for the related voltages. (Strange that P1-1 & P1-3 should be +/-18vDC.)
P1-1 = 15.87vDC P1-3 = -16.65vDC
P1-4 = 14.75vDC P1-5 = -15.4vDC

Also look at P4 voltages. P4(on the preamp board) is what connects to the logic board. It connects to the P4 connection on the logic board. Notice on the preamp schematic on the switches it will have P4-6 for the reverb push switch for example. Here are some voltages with channel 1 selected on.
P4-1 = 15.77vDC (Which should be 18v)
P4-2 = 15.78vDC
P4-3 = 1.4vDC
P4-4 = 1.4vDC
P4-5 = 1.79vDC
P4-6 = 1.79vDC

So I need help understanding this logic board better. The switching logic on this amp is the most elaborate that I have worked on yet. Any help would be greatly appreciated, thanks. 

Edit: Just to note that all the TL607 and TL604 chips are getting -15.75vDC and 14.75vDC power on pins 5 and 8.

Now I know that Jazz Chorus amps are not known for their distortion sounds, but this one is not sounding like distortion at all. First off I have traced the signal through all the preamp circuit on channel 2. The master volume on ch2 was a bit dirty but cleaning that up got the pot working great now. Bass pot is also a tiny bit dirty too so it will need a cleaning no doubt. I intended to clean the pot first to make sure it was just a dirty pot and not a symptom. When you activate the distortion sound it is very scratchy when turning the pot, so I guess it needs a cleaning. Checked the distortion foot switch jack and it was not making great switching contact. I fixed that temporarily by cleaning and then clipping alligator clip to make sure it makes great contact. B+ = 42.3vDC B- = -42.3vDC approx. Clean sound is great on both channels and it is definitely something in the preamp that is not working well.

Anyhow I scoped a sine wave signal through the source and drains of Q6 & Q7 just fine. I can see even some clipping occur on the top parts of the signal when I dial up the distortion pot. C23 has signal on both sides with the dist. pot off and when turned on it will be reduced to a flat line as you turn it all the way up. I scoped the output and it is a nice healthy loud sine wave. Scope on the output does not show any significant clipping when I turn up the dist. pot all the way. So far all the transistors test good (Q1 Q2 Q4 Q5 Q7 Q8) and D4/D5 are good. So what sort of troubleshooting can I do at this point? I am really not sure what this could be at the moment so I am a little stuck. Any help greatly appreciated  .

My first action feels like I need to clean the pots more as it really helped the main volume (ch2) work correctly. So I will start with that next just to be sure. Also, the Serial # is 950166 and the schematic attached is accurate to this amp. I also remember Illaya had this exact circuit and felt he needed to repair the distortion. >>>http://www.ssguitar.com/index.php?topic=3388.msg25110#msg25110

He has an MP3 on his thread there so you can hear what the distortion sounds like. Now imagine my amp with the distortion turned up all the way sounds like 2% of the actual distortion is coming through mixed with the dominating clean sound. So it sounds like just clean with a few tiny crackles here and there, horrible crackles turning pot. So time to clean the pots but if not that, then what?

Okay I started some of this thread on MEF but that site is currently down or something...? Sorry to post in two places but I need a little advice at this point of the repair. I have a schematic that is close to the B300 but not exact. I am posting the output section of the amp here and deleting out components that are not in this circuit. Just a few components were not in this output section of the B300 and this edited version is very accurate. Not too concerned there but just thought showing what I am working on better with edited schematic. One components still on this page that is not on my board is C7. That cap is not on my board in the power output, but I did not delete it from this edited version.

I had a shorted 2SA1266 transistor, shorted collector to base, in the spot of Q4 on the schematic. Was getting 4.5vDC on the output. I matched a similar transistor(exact except for tiny bit higher in HFE) in it's place A1015 GR98 and it is doing it's job. Not really concerned about the HFE situation as I am only using it in place for some temporary testing. More concerned about my voltage readings on Q8 K D2059-y, one of the output chips. Here are my voltage readings of the output section.

(all measurements in DC voltage) Note: I am measuring about 350mv DC on the output.
Q2 E= -.63mv C= 35.75vDC B= -31.5mv
Q3 E= -.63mv C= 36.4vDC   B=17.8mv
Q4 E= 36.2vDC C= 1.48vDC B= 35.66vDC
Q5 E= .83mv C= 36.2vDC B= 1.48vDC
Q6 E= 350mv C= -36VDC B= -303mv
Q7 B= .83mv C= 36.4vDC E= 350mv
Q8 B= -35.82vDC C= 347mv E= -36.2vDC
Q9 E= -301mv C= 1.47vDC B= 319mv

So it looks to me that either Q6 is bad or it is Q8 that is bad. I tested Q6 (another 2SA1266) and it seems to be fine. I had another A1015 so I swapped it out just to make sure and Q8 still has HV on emitter and base. Here are measurements after the swap...

Q6 E= 350mv C= -36vDC B= -276.9mv
Q8 B= -35.7vDC C= 350mv E= -36vDC
Q9 E= -280mv C= 1.49vDC B= 338mv

Just wondering if someone can confirm my thoughts that Q8 is bad? I am pretty sure it is pointing towards that direction. Unless perhaps Q9 is the culprit? I had originally incorrectly thought that Q4 was part of the bias circuit but an MEF member informed me it is a predriver for Q5 and Q6. So is Q9 the main part of the bias circuit? Any help is greatly appreciated, thanks!

So I acquired this Marshall Lead 12 Japanese imported model amp and I am in the process of cleaning it up. My question has to do with line voltage rated for 100v(Japan) and what complications could occur running the amp on 120v(U.S.)? I have left the amp open on the bench as I might want to just change out the transformer, but first want some feedback. I have a Marshall 10CD power transformer and I am contemplating switching it. I realize that the Marshall 10cd amp would be 120vac stepped down 1:10 will be at  -/+12vac center tapped. I am thinking this is the way to go as the 100vac version might be over powered by the 120vac line. What is the best solution or do I even need to change out the transformer?

While searching for some info about the Marshall 10cd I came across a website and just have to link a page here for others to see. I know this amp is very simple but to mere beginners this web page here is very well put together. It really takes a simple amp and explains the whole thing in great detail. Of course the example illustrated it European 220vac.

http://www.electrosmash.com/marshall-mg10

Well I finally did it... I always said I am not gonna buy one of those ever! But... I did, oops. So, it was like ten bucks and I only bought it for parts etc. The main preamp logic board has a blown resistor and there is probably a good reason why. So I don't plan on fixing the logic board. Now I see a guy selling replacement boards on Ebay for this logic board and I am guessing that 1 in 10 of the lot he has are good, maybe all of them are bad. It's a Line 6 210 amp.

Well my reason for posting this is to ask if it is feasible to build some sort of preamp for this amp. Looks like plenty of room to put something in the chassis. I hate not trying to fix something, but I don't think those replacements boards are reliable if they are even working. The power amp is stereo out and I guess I would need to figure out how to send it signal. I mean without getting into too many more details. Does this sound like a feasible idea for fun? The power amp looks like it just receives analog signals and no digital. Anyone try this before?

Added: Also if anyone has a 35-00-0308 rev.D  Line 6 Amplifier Logic Board that is the part I would need to fix this thing.

So this is my first post here about an amp that I intend to fix . This EMX640 has been around the block the past decade plus! I mean it was the same PA that I used in many bands all through out 1998-2009. So it has been abused!! lol Lately my old lead singer in one of my older bands had been using it while out at small gigs. They came back from a break turned it on and it just blew a fuse. So, I have put a 6.3amp fuse in it since I have trouble finding a 7A fuse and I have those other fuses around from some tube amps. I mean I have only brought it up on bulb limiter at 40 and 60 watts. The light shined bright indicating a short somewhere... So now time for more info...

First thing I did was opened it up to have a look around and found four inductors on one side of the amp just totally warped. Anyhow I have some pics of these inductors included with this post and they need to be replaced. I am really looking first to test this power transformer to see if it is good still. I mean really how can I test this switching PSU safely without owning an isolation transformer? I am sure there are ways to do this but first would like to ask this community for good advice before I go forward.

Another factor to fixing this PA is the many many transistors attached to the heat sink. I mean these output transistors are totally stuck on this heat sink like glue!! So I busted out the hair dryer to see if they would lift off easily, and no way!! They are on there good and tight!! So it will be a mess just getting these things off to get to the main output board to replace parts(inductors). '

Also, running light bulb limiter, would it be wise to disconnect power from the from preamp to see if the short goes away? Also, I will check the status of the BR and other output transistors(especially those that are in line w/ the ugly inductors). Any thoughts on how to proceed on repair of this old PA would be appreciated! Thanks. Ask questions since I probably have left out missing details. You can see the transistors in the first pic attached and detaching these is looking to be problematic. Also, the main filter caps probably need replacement or at least new soldering. The glue that held them to the board is obviously cracked up bad!

Edit: another thing to note is that one pic shows what those inductors should look like... Also, note it the two 2Amp fuses that lead towards the preamp power supplies did not blow... Also, I was told that at least 3 input channels were not working very well or at all and that probably was related to the side of the amp w/ the deformed inductors.

http://www.fileden.com/files/2012/8/8/3334901/DSCF1026.jpg

http://www.fileden.com/files/2012/8/8/3334901/DSCF1028.jpg

http://www.fileden.com/files/2012/8/8/3334901/DSCF1029.jpg

http://www.fileden.com/files/2012/8/8/3334901/DSCF1030.jpg

http://www.fileden.com/files/2012/8/8/3334901/DSCF1031.jpg

http://elektrotanya.com/yamaha_emx-640.pdf/download.html