Yamaha EMX88 no sound

[mikeskory]

Anybody want to think about this old mixer? I found it while cleaning our repair shop. No real reason to fix it other then to practice my trouble shooting plus the previous tech tried and failed to fix it. LOL! I have it up on the bench. we have scope, meter and parts. The initial problem was DEAD. I found bad fuses and blown bridge rectifier. That got the lights on but it pulls the light bulb testor and the longer I leave it on, the brighter the light gets. No signs of burning or explosions. The 4 3300mf power capacitors don't look perfect. One is bulgy a little on top and the white glue they used for vibrations is dried out. I had to jumper them as the printed circuit was loose. Again no signs of burning or over heating. The print was just a little loose.

[glebert]

Is that an EMX88s? I don't know if they made a different one with no "s" in it. I have an EMX88s and found the switch to from stereo to mono/monitor and bridged mode can be kind of flakey, so watch for that. Where are you injecting signal? Do you have mixer/preamp functionality?

When you say "no reason to fix it" is that because there isn't a customer paying to have it fixed? I actually think it is a pretty nice unit, and worth fixing, but then again I have been known to spend hours working on $50 amps because I don't want to throw them away.

[mikeskory]

Is that an EMX88s? I don't know if they made a different one with no "s" in it. I have an EMX88s and found the switch to from stereo to mono/monitor and bridged mode can be kind of flakey, so watch for that. Where are you injecting signal? Do you have mixer/preamp functionality?

When you say "no reason to fix it" is that because there isn't a customer paying to have it fixed? I actually think it is a pretty nice unit, and worth fixing, but then again I have been known to spend hours working on $50 amps because I don't want to throw them away.

Lol! Tes. It technically belongs to the shop. If I get it working the owner may spring for the broken controls and eq sliders. or I may use it in my bench. I mainly need practice isolating these types of problems. I mean other then the "shotgun method" or the "smoke test" method.

[mikeskory]

Is that an EMX88s? I don't know if they made a different one with no "s" in it. I have an EMX88s and found the switch to from stereo to mono/monitor and bridged mode can be kind of flakey, so watch for that. Where are you injecting signal? Do you have mixer/preamp functionality?

When you say "no reason to fix it" is that because there isn't a customer paying to have it fixed? I actually think it is a pretty nice unit, and worth fixing, but then again I have been known to spend hours working on $50 amps because I don't want to throw them away.

Lol! Tes. It technically belongs to the shop. If I get it working the owner may spring for the broken controls and eq sliders. or I may use it in my bench. I mainly need practice isolating these types of problems. I mean other then the "shotgun method" or the "smoke test" method.

[mikeskory]

After I put in the rectifier, the front panel lit up. The controls light up and the various features on the front light up or not as I press them. I put a signal into the channel 1 jack. I get no signal out of the mixer. It goes to a 15 pin jack on the mail board. I snipped the leads that are labeled S1 and S2 but still nothing from the mixer. Here is another clue. Their are 2 "limiter" lights on the front panel above the on/off switch. The R one is lit. Also, the light buld does NOT brighten or dim with the gain controls.

[glebert]

In this thread (http://music-electronics-forum.com/t40167/) they mention listening for the relays to kick in, and if they don't this means it is in "protect" mode.

[glebert]

Other threads mention that this is a pretty complicated unit to work on (I haven't opened mine up). Since it almost certainly has SMPS and Class D power amp sections if you can get it working that would certainly be good experience for working on newer technology amps.

[oc disorder]

Um Glebert , these models still had a torridal mains transformer so the psu is a little easier to digest!

Well spotted good advice re the relays.. they have a separate one for left and right so it seems both power amps have faults.

The fact that the bridge rectifier needed replacing indicates some very heavy current draw so I would suspect several output transistors mounted along the edge of the output board are now open or shorted.
Usually the ones in the middle 'cause they get hotter !

Pity both left and right appear to be kaput as it's much easier when you have a good amp side to compare readings with.

The smaller driver transistors that immediately precede the 2SA1492/2SC3856 should also be checked and the "fast" diodes also
which can be open or shorted.

Think they also have a long strip of thermal insulation between the o/p transistors and the heatsink. Be really careful when you remove the board as it's easy to damage it.. although Yamaha still supply a replacement .. well at least here in the land down under they do (Aust) !

You will need the manual which is in the post thread that glebert linked to.

[The Dude]

......The fact that the bridge rectifier needed replacing indicates some very heavy current draw so I would suspect several output transistors mounted along the edge of the output board are now open or shorted......

That^^^^^^ and most likely the channel that is staying in protect.

[mikeskory]

The relays do not kick in. They rattle faintly when I knock them so I guess they are not stuck together. Ok I'll look again at the transistors Thursday AM along the rails on each side. I'll see if I can tell if there is an obvious bad one with an ohm meter while they are in circuit.

[glebert]

Um Glebert , these models still had a torridal mains transformer so the psu is a little easier to digest!

Interesting. Hadn't looked at the schematic, since mine is working well, but it is fairly light for the amount of power it has. I considered using mine as a bass amp, bridged power into 8 ohms is supposedly higher than my Ampeg SVT 7 Pro, but it could be the a peak vs. RMS thing. Pretty strong amp though.

[mikeskory]

OK! We're not going to fix it but I feel I gave my boss a thoughtfull estimate. 2-6 a1492 trans, 2 c3856 trans, bridge rect, 4 .22 ohm 5watt resistors, 2 sliders, effects return control, 2 effects knobs. Cost approx $80-100 plus lots of time. For a real customers I would ask $300.

[52 Bill]

OK! We're not going to fix it but I feel I gave my boss a thoughtfull estimate. 2-6 a1492 trans, 2 c3856 trans, bridge rect, 4 .22 ohm 5watt resistors, 2 sliders, effects return control, 2 effects knobs. Cost approx $80-100 plus lots of time. For a real customers I would ask $300.

If you are serious about learning from this, why not buy it from your boss and fix it up yourself? If it is a boat anchor, maybe your boss will let you have it for a song and then your investment in it is your own time and the cost of the electronic parts. You don't need to replace knobs and sliders to have a working amp.

[Enzo]

Mike has a long history of electronic repair, I think he is mainly getting used to latter day stuff like this Yamaha. Yesterday he suggested I bring over my tech buddy, and we could all sit around some deep thinking repair while eating pizza. SOunded good to me.

[glebert]

If you are serious about learning from this, why not buy it from your boss and fix it up yourself? If it is a boat anchor, maybe your boss will let you have it for a song and then your investment in it is your own time and the cost of the electronic parts. You don't need to replace knobs and sliders to have a working amp.

And then in a couple of years you will have a house full of "projects" that you will get to "some day."