Yamaha emx 860 st with very weak signal

[Mr Johnny Birchwood]

Hey all, I got a Yamaha EMX 860 ST powered mixer in last week that wouldn't power up. L channel was blown, which wasn't hard to diagnose as one of the B1647s(Q114P) was burnt to a crisp . Main diode bridge D1 was bad too, which took down the mains 12A fuse. I replaced the bridge and all 6 transistors in the blown L channel. Q108P, Q110P, Q114P were replaced with new B1647s and Q107N, Q109N, Q113N were replaced with new D2560s. The amp will power up just fine now, but I barely get any signal on any channel, L, R, or MON. With the master and input levels all the way up and the channel pad toggle disengaged I get a very faint signal in the speaker on L R and MON. The master volume LED level indicators do not light up at all. The Green LED power light comes on very faint and flickers when a signal is introduced through a microphone into one of the input channels. The red LED indicator on the digital signal processing controls lights up very bright when toggle is engaged.

I'm still working on my solid state diagnostic chops, and my tired eyes can no longer trace all the paths on the schematic(attached below, see pp50, 51). The customer is a very pushy and impatient fellow, and he refuses to understand why the amp wasn't done last week(just got the transistors and bridge in today, btw.) I'm hoping that this is enough information for one of you solid state gurus to at least point me in the right direction for parts to check. I just finally got an oscilloscope and probe but haven't fired it up and learned how to use it yet. I just use the dead-reckoning method with a DMM, ohm's law, and a calculator. I just want to get this thing out the door ASAP. Any guidance here is much appreciated.

Thanx

File is too big to upload, here is the link to the service manual at elektrotanya: YAMAHA EMX860ST Service Manual free download, schematics, eeprom, repair info for electronics

[DrGonz78]

I have one first question that was not stated on your post... Is there any DC on the output at all? That needs to be determined before we can move towards work on the preamp. As you just fixed the output board with new output transistors it would be nice to have all the info that it is not putting any substantial amount of DC on the output. Simple to start and I am sure greater minds will move forward... To me it sounds like more damage was done(obviously), but were any preamp fuses blown?? Probably not... So that makes me want to look at the state of those regulators 7915A types. Are you getting +15 or -15 to the preamp?? Look at connection from the Power board CN101 (pg14 pin9 & pin11 if my eyes are working LOL) and test the voltage there... You will see that those regulators must be part of the equation. Also, check any of those diodes that are in that area too as they might have been damaged too. It looks like that is what is feeding into the preamp... So that is where I was looking on the power board to start even before getting to the preamp. I mean you could look for -15 and +15 on all those IC chips as another way of answering that question. Those regulators need to be looked though IMHO.

Also, don't let this guy rush the process... I say SEND the guy the schematic and tell him to look it over... Tell him if he wants it done right then he needs to respect the process. Some people are just really pushy, but he should recall that he was the one that led this amp to failure. People who are impatient are usually the ones blowing up their amps IMHO. Oh well that is me just venting as you should be too!! Don't do anything I just said as that is unprofessional but if the guy looked at schematic I think he should do you a solid and back off a bit.

To me looking at the power board first to make sure it is passing +15 and -15vDC to the preamp is a good place to start... Well that is my thought to start and way more experienced posts will follow I am sure...

Edit: Very well could be the shorted bridge took out chips on the preamp side but would just like to know if power board is passing the preamp voltage correctly before going over to check the preamp.

[Enzo]

I don't want to hear about not knowing solid state. What would you do if this were a tube mixer? You do the same thing, you isolate the problem.


SO you fixed a blown amp channel. That leaves other channels that were working before and were not damaged this time? And now they don't produce. You have an indicator light that is dim, yes? Or signal lights that are weak? You say not much of anything comes out the left, right, and monitor line outs? If the mixer preamp can't make output, then there is nothing to feed to the power amp to be heard.

DIm lights? Check power supply. RUn signal into an input channel and find it in the master section. Inject signal at various points in the signal path, starting at the power amp inputs and working back.

[km6xz]

The channels and the master section, neither of which indicate they are working from the lights, have one thing in common, the low voltage power supply, Measure the plus and minus 15 volt rails. I guess you have already checked the fuses F102 and F103 which are for the low voltage AC source from the transformer.

First things first however. Unplug the unit and get the manual out for the scope and read it. Do not connect loads or speakers to a solid state amplifier that is believed to be faulty because that violates the first rule of professional repair; "Do no harm". You are risking the value of this unit for the customer by not following reasonable and standard practices such as making sure there is no problems before attaching loads.
You are not going to like this part: How would you feel going to a mechanic or doctor and paying for a service that the person was unqualified for? The customer trusts you to know what you are doing and you do not. That is fundamentally and ethically wrong. Why does he pay $65 or whatever an hour for other than competency. You should have offered to have it done by sending it to a service center with the proper tools, training and experience. This forum is made up of both hobbyists and professional techs and the techs offer a lot of useful tips and guidance to the hobbyists to help them repair their own units, ones that they bear sole responsibility for if they ruin it. It is not really a place for people wanting to charge for services that can't perform at a professional level.
Repair has a shady reputation already considering it is a field that relies on customers trusting that their valuable equipment is in the hands of someone who really knows it, is qualified to diagnose and rectify a problem they claim to be able to handle. Did you tell the customer that this was not within your range of knowledge or experience? How should be feel if he read this forum and found out?
By not being familiar with the fundamental tools for diagnosis, a working knowledge of a scope, it would be safe to assume you do not have the other basic tool needed for successful repair and testing of a piece of audio equipment such as low distortion calibrated oscillator, a metered Variac, suitable dummy loads and proper metering.
Sorry but I cringe when I read of people charging unsuspecting customers for things that can't be delivered; expertise in the subject of concern.

[DrGonz78]

The truth in that last post is hard to follow, but I will try. Stan really is putting it out there that when "US" more hobbyist types take on projects beyond our "Scope" that it may lead to trouble. As far as making it a business I would guess you will learn very quickly how it can burn. On one hand it is great when we take a leap in one direction to learn the trade but don't forget a leap can lead to a fall. I have worked on a few projects for "friends" and some "clients." The friends had broken amps and I tell them "hey if I can't fix it then the labor is free." Now that is not a business it is a hobby. However, if I fix the amp it is lesson learned and paid for by a "friend." If not they pay for parts and I figure a way to get some money back to them... That has yet to happen... Luckily I have fixed all the projects sent to me by my "Friends" and a couple real clients. I have gotten lucky and they had faith in me with no false promises. I buy all my amps for the most part and fix them... In the future after years as a repair hobbyist I will feel more confident in my repair abilities. To anyone who takes a chance on me I will do my best and guarantee my work.

Stan brings up a good point as his experience and others here must be recognized fully. Just realize that there is a risk in taking on repair work as a trade. Be up front with the client or "Friend" and realize that it might bite you in the butt. I have a feeling this post will end up w/ a happy ending and a good lesson to be remembered. It's not rocket science but it is still a professional business. When someone expects pro results, there is a huge burden put upon any hobbyist trying to pay their dues.

[Mr Johnny Birchwood]

@KM6XZ Look dude, there is only one 'qualified' repair shop in my town and the guy takes minimum 6 weeks to get anything done. I didn't set out to do full time amp repair it just sorta happened that people found out I fix stuff, and now I am swamped with work. I told the dude that I may or may not be able to fix this thing, so unless you are actually gonna help out here, I would appreciate it if you would BUTT OUT, and let the rest of us get to work. There are separate forums for rants and raves.

I have so far only encountered ONE amp that I could not fix. I may not have all the fancy tools, but I get 90% of repairs done within a day or two of getting parts. Never had a complaint. I bid by the job, not charge by the hour. My customers overall are very happy that there is SOMEONE that can work on their shit and get it done in LESS THAN 6 WEEKS, and that is the SERVICE THAT I PROVIDE. The guy that brought this Yamaha in was gonna throw it in the dumpster anyway, I told him I could MAYBE fix it, and I will eat the $30 for the transistors if I CAN'T fix it. SO THERE. I haven't fired up the scope yet, as I have been too busy fixing peoples amps for them in a reasonable amount of time and for a reasonable amount of money. Maybe if I start screwing up people's amps really bad, and they stop bringing me work, I will have some free time to study my oscilloscope manual.

As for the other suggestions, the helpful ones, I will check for DC on the outputs and check the 15v rails. If either the + or - 15VDC is not present that will narrow down the problem. Neither of the preamp fuses was blown. I probably shouldn't have even posted this thread, I was tired and frustrated last night after a very long day of work and parenting. I'm pretty confident I can track the problem down. I will give y'all an update once I get back to it.

[Mr Johnny Birchwood]

And since I don't represent myself as a professional repair shop, I have no problem telling the guy to fix it his damn self if he keeps bugging me. If he calls again before it's done I'm just gonna tell him to come pick up his broken amp. It would be worth the $30 I would lose in parts. That may have been the best suggestion yet, @DrGonz78!!!

[Mr Johnny Birchwood]

I guess maybe it IS my fault. The first amp he brought in, I fixed within 24 hours and charged him $45. He was used to dealing with the 'professional' repair shop and now he thinks anything can be fixed instantly. Maybe I shouldn't have done such a good job the first time.

[Mr Johnny Birchwood]

Now I feel REALLY stupid. In my haste to re-assemble this thing last night, one of the ribbon connectors to the preamp board worked itself lose. Everything works fine now. L channel, R channel, Monitors. I called the guy and told him his amp was done. $100 parts and labor, which is precisely what I quoted him last week, IF I could fix it.

I also told him, just now, that he could have the work for free, parts AND labor, if I could watch him delete my number from his phone and he promised NEVER to call me again, EVER, FOR ANY REASON. He elected to pay the $100. I also told him that from now on I would charge him $60/hr for unnecessary phone calls, billed in increments of 15 minutes. That's why I love the music business so much: I have made it a continuous 'professional' habit to tell EVERYBODY to F*** OFF, and to this point it has only HELPED my career.

Sorry If the tone of my posts is a bit on the grumpy side today, I've been SWAMPED with work and bustin' my hump to get everybody's gear ready for their gigs this weekend. I'm sick of this repair stuff, I think I may just focus on mods and custom work, where my friends and 'clients' fully appreciate what I can do for them. Oh yeah, forgot to mention, I do have a metered variac which I built myself out of parts I salvaged from the junk bins at the science dept of the local university. It works pretty good.

[Steve Conner]

Glad you got it fixed. I'm sure that at some point even km6xz has assembled some complicated mixing desk with a cable unplugged, and had to take it all apart again.

Sounds like you may be trapped between a hobby and a career. If you feel swamped, you may want to ask yourself what it is that drives you to fix other peoples' gear. Then jack your prices up until the demand comes down to a manageable level.

[Mr Johnny Birchwood]

Glad you got it fixed. I'm sure that at some point even km6xz has assembled some complicated mixing desk with a cable unplugged, and had to take it all apart again.

Sounds like you may be trapped between a hobby and a career. If you feel swamped, you may want to ask yourself what it is that drives you to fix other peoples' gear. Then jack your prices up until the demand comes down to a manageable level.

That is the problem in a nutshell. I KNOW that I'm not really qualified to do professional repair on solid state stuff, so I don't really feel justified in charging more. The demand remains high, however. Maybe I just need a vacation. For any fellow hobbyists considering a career in repair, I hope you are reading this thread very carefully.

[Steve Conner]

At least you can read and have a conscience. I've been a member of the MEF since it started (and the two previous forums it grew from) and I've seen things that would make your hair stand on end.

I once was sick of a customer that I was doing a programming project for. I couldn't handle his demands on top of my day job, and I decided I had to get rid of him before I went crazy. He called me up and asked me to go out on site to fix the data logging computer which had crashed yet again, due to him buying a cheap machine from Best Buy instead of an industrial-grade PC. I said that I now had a $1000 emergency call-out fee. He paid it! However he took the hint and thankfully our relationship ended soon after.