So I have found that Q217N, Q218P, & Q223N are shorted and bad. I have done some searches around the net to find replacements. I have found one source that sells them in pairs...

http://www.kpcomponents.ca/product_info.php?manufacturers_id=17&products_id=885&osCsid=ytkymhlvumj

So this might be the way to go, but just looking up I noticed it's in Canada and shipping might prove to be expensive. Anyone have a good idea where to buy B1647 and D2560 pairs? thanks. Also, should I be matching up the pairs on that side of the board to all have new output transistors? Or should I just replace the three blown transistors only...? What do you guys recommend?

Also, I am thinking about replacing those inductors for good measure if my friend wants them changed out. I found perfectly matching parts on Digikey. However, how do I test these parts to see if any of them went bad? As Enzo said it would be a surprise if they were not working, as only the insulation was now melted.

@Enzo Yes I think my brain was farting when I was writing this yesterday... :duh I really meant that I should disconnect the power supply altogether to test it out of circuit. The end result was positive but I always like to start there to not have a surprise later on.

I was able to jostle those transistors free by way of a screwdriver. I barely tore the insulator but any ideas where to get this material insulator as to replace? I always find that my search for insulators in general is hit or miss at times. A lot of times the different sites are just not stocking the exact ones I want or need.

It says (JU) 7a 125v fuse on the schematic and that is what I found in the amp. So, not too worried about that right at the moment.

@Roly & @Enzo Those inductors really heated up causing the solder on the legs to have loose connections. There are some solder joints on the board that will need to be touched up and I am looking at those transistors. The BR seems fine but a couple transistors are suspect. However, tomorrow I will start removing them from circuit for better testing. I will also look into checking out the speakers but they are not here right at the moment.

Will get back here with some more findings on those transistors tomorrow. I really appreciate all the knowledge that has been passed along on these forums this past year!! Thank you all :dbtu:

Happy New Year Everyone :tu:

So this is my first post here about an amp that I intend to fix xP. 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

@Roly I think this is what you are referring to...? I have no idea here... I don't think I have seen something like that before! :lmao: >>>reference WTF.jpg

Edit: I think this is a strange version of this schematic and it even says "Trade Secret--Patent Pending"

Double Edit: I wonder if the output transistors are really bad yet. I think there could be something pushing a smaller amount of DC on the output here. Or JoeCool is right and one or both of the power amp IC's are shot! Not quite dead but if they are headed to dead then they will heat up good n hot. So turn on the amp (speaker disconnected as Roly said) for a minute and then see if one or both output power amp IC's are really hot.

Maybe this StudioPro112 TT schematic will help??

I say in any amp you need to always be looking for cold/cracked solder joints and bad traces. Sometimes bad filter caps can cause some serious problems creating hum/buzz sounds or flabby sounding tone. First replace the solder completely on the filter caps. Inspect the caps to see if they are bulging on top or leaking on the bottom. Maybe the amp sat un-played for long time? Yeah ditch the caps but first just try to see if it is the solder joints.

BTW: When you did the "Whack It" test were you feeding audio into the amp? As you are trying to get the amp trigger a loose connection and see if that audio signal is interrupted by this beating.

What year was the amp made around? Did either of the tech guys replace the main filter caps?

Quote from: panfilero on December 15, 2012, 04:26:36 PM
1. I just picked whatever kit they had at my local electronics store and it happened to be that one
2. I have two 4 ohm speakers, one hooked up on each channel
3. the guitar is tied in to both input channels

I think there is some kind of ground loop here between the two channels. Try one speaker and one channel at a time to make sure that it works. Which is where I think Roly is headed too. At least this needs to be tested channel 1, and one speaker. Good luck! :tu:

Late Edit: After re-reading this all over again I have to admit that I am probably wrong about ground loop between two channels so ignore that comment if you haven't already...  :lmao:

Two things strange about this noise thing... which we need more info about...
1.) "It's the same noise I hear if I leave nothing plugged up to the input of this amp"
2.) "I hook up an mp3 player to this and no problem, sounds fine"

Just had to add more to this post and to exclude my lost thought about ground loop between channels.

Just to note about amps input impedance and the guitars output impedance. Your guitar's output impedance might be somewhere between... lets say... 9-17k ohms. In the manual of the amp that you have built it states the input impedance is 150k ohms. So, even a Fender G15 has a 1M ohms as its input impedance. So your amp is low comparably to many amps and not higher impedance. (Note: I hope that can help us get away from that being the cause of the hum.) I hope my information is accurate too.  :duh

One thing that I would want to know is the amp able to be hooked up to only one channel? I would want to know if the problems is still occurring if only channel of the amp is connected at a time.

Edit: One thing to note about that manual is that it is called "Input Sensitivity" and I took that as being input impedance. Please forgive my ignorance if that is not accurate, thanks.

I had a problem with a solid state amp where the clean channel was almost mute sounding. When I switched the amp to overdrive it let a little more signal through but was nearly mute too. The culprit was one solder joint on a leg of a tone cap through the signal path. So, I think it can effect both channels if it shares the preamp down the signal path. Meaning that amp shares bass, treble, or mid EQ or possibly each channel has it's own separate EQ paths.

Also, have you tested anything voltage wise on the amp yet? If you see good numbers everywhere on the amp it might point to a connection on the signal path. Check if you have +/-15 vdc on any opamps on the preamp side.

I think the schematic might be online... Although it would be good to know what exact model amp we are trying to fix. If you have the schematic then please post it or show us a link. Of course cold/cracked solder joints come to mind instantly as a possible cause of this symptom. Also, may want to just clean the headphones jack to make sure to cover that base.

Clean the headphone jack to make sure dirty contacts are not shunting the speaker signal. So it works with headphones but the pots are dirty. Get some Deoxit or other type of cleaner and spray out the dirt build up in the pots. Also, would be a good idea to make sure the speaker output wires are really connected.   

If you are at all unsure or worried about taking live voltage readings then keep researching all you can about safety precautions. Like the one's that say take off all your rings on your fingers before testing voltages.  :tu:

How you described the testing is correct: Ground is typically your chassis and it is a good idea to just clip it in place. At that point, powered off, I like to set DMM for a continuity test to make sure I have grounded through to the circuit board. Clip black probe to chassis and test for continuity. For example, test continuity from chassis to something you know is ground on the board. Then yes start to record voltage readings in each different place on the board. If you are unsure about something ask before testing but it is really straightforward for the most part. As you do it more you get more comfortable but just never get too comfortable and always be on your guard.

<Mensur...> I think he meant to solder or clip a wire to the master (red channel) middle leg that then led straight to the hot (tip) part of the input jack as to force the full signal into input on that channel. Well that would be interesting to know if that works.

<J M Fahey> As Juan is saying to look at the Green channel and the voltages that are sought out on that channel and then compare and contrast any discrepancies since they are almost identical. If you look at the Green channel there are voltages posted in the schematic. However, on the Red channel of the schematic there is not much to show for voltage so you have to cross reference the circuit. These circuits are almost the same or are identical as Juan said... So, look closely and cross reference any voltages in a way to find a bad component or loss of voltage.

What I meant about about a break in a leg on the Green cap is in reference to the green part on the leg being cracked off. I am not saying the whole leg is cracked off the circuit... just to note. I have seen these types of tone caps w/ cracked insulation that lead to total loss of effect signal in (for example) a Boss DD-3. That one ceramic cap killed the entire delay effect from working.

I think Juan made a great point that looking at the components of the amp is not gonna fix it in most cases.

If I was to question one two things by your pictures alone it would be by the double potted tone potentiometer and those two IC chips(not sure what those are right of the Tone Clipping pot...) I think they are IC's I have an RG80 combo but it does not have that component there so I have no idea what it is used for... Anyway that thing looks cracked or something...

Another thing thing is the green 1KV 202M signal cap looks like the leg has torn and might be leaking... But that is probably not the problem but still can be replaced later on. However, I know that those ceramic caps when the legs are torn can cause a huge loss to signal overall. Not always the case but something to chew on just by looking at pics.