Yamaha emx512SC protection circuit

[rbilx]

Thanks for taking the time to respond to us about this problem. Here is what I've found - If you remove the back panel from these amps, (again, I have three of them) which I've done with all three to troubleshoot them, they won't stay on. As another poster pointed out, you need all the grounds in place for it to work correctly. I was always connecting all the ground wires together but I didn't assemble them with the panels on. I guess you need the rear panel screwed into all the brass standoffs for it to be properly grounded. I reassembled all three amps and all three stay on now.
The original problems on all three were a blown power supply. I had to replace R418, R424, IC401, IC402, IC403, Q401, R440 and on two amps - Q406 and Q407. Two amps had one of the outputs shorted and one amp didn't. I was sure everything was fixed and running like it should. That is why it didn't make sense they were not staying on.

[Jazz P Bass]

Thanks for the tip about the brass standoffs.
I pulled my hair out on one of these amps a few years back until I realized the grounding scheme Must Be Complete
Glad you got it worked out.
John G

[Enzo]

There are a lot of guitar amps that rely on board mount screws for grounds. Crates had for years the ground screws, and one or two of them screwed down over grounding lugs or bare wires.

When I mount a small DSP card or something, I always look at the corner screws to see if one of them also serves as a ground connection.

I have been bit enough in the past to always now check.

[1984ARC]

Grounding info for the EMX512SC - how it seems to work

For the record, in case it's of use to anyone, here is some info how I believe the grounding scheme works on the unit that I have been working on, and how to complete the grounding paths in order to get the unit working out of its case without any difficulties - e.g. to avoid the protection circuit operating. If attempting this, do be sure to take all necessary safety precautions to avoid possibly lethal shock.

There are two basic supply grounds from the SMPS:

• The one for the PA output-stage supplies (i.e. +-B and +-BL) - I'll call this GB.
• The one for all other supply voltages (i.e. +-15, +12 and +5) - I'll call this GA.

These two grounds are not interconnected on the SMPS board.

• GB connects from the SMPS board to the PA board via the black leads that run between the boards - one takes it from the SMPS to the OP board, then another takes it from the OP board to the PA board.
• GA connects from the SMPS board to the PA board via the ribbon cable that goes there (on pin 2). It also connects from the SMPS board to the Mixer board via the ribbon cable that goes there (on pin 2).

If you run just the SMPS and PA board outside the case, without the Mixer board connected, there are two problems:

1. The PA board is getting GB and GA - but it also needs two other grounds. These are the signal grounds, marked SG. They are separately fed to the PA board via pins 5 and 9 of the ribbon cable that comes from the SMPS board - but those grounds don't actually come from the SMPS. They are not interconnected with GA on the SMPS board. Rather, they come from the Mixer board (via its ribbon cable, on pins 5 and 8), where they are interconnected with GA. The SMPS board just provides a link-point for them, between the two ribbon cables. So, you can fix that problem either by connecting the Mixer board (by its ribbon cable), or by making a temporary link between the two SGs and GA.
2. Even having fixed the issue above (by either method mentioned), there is still no interconnection between GA and GB - and that interconnection is necessary. The reason is that, in the assembled unit, that interconnection is by the following route: a) The OP board's GB connects with the rear panel by contact with the two threaded lugs on the the OP board (held by their screws). b) The rear panel makes contact with the metal air duct (that sits around the PA board & heatsink), held by several screws. c) The metal air duct makes contact with the two larger "springy straps" that are screwed to its other side. d) Those straps make contact with the two aluminium foils that are attached to the Mixer board & front panel assembly. e) Those foils make contact with the GA at the Mixer board (that arrived there via its ribbon cable). So, if you have the Mixer board connected, you can fix this second problem by making a temporary link between one of the foils (e.g. using a fairly strong croc clip, to make good contact) and one of the two threaded ground lugs on the OP board. If you don't have the Mixer board connected, then as well as addressing the issue in 1. above you will also need to temporarily interconnect GA and GB, e.g. at the SMPS board.

[Note that, in my opinion, the 4 brass threaded stand-offs attached to the PA board (& its heatsink) are not part of the normal interconnect path between GA and GB - they just provide grounding of the heatsink (via the rear panel), which although possibly important (e.g. for stability) isn't essential for the unit to basically work. Neither are the two short black leads, with ring tags that attach to the metalwork near the fan, involved. They are capacitively connected on the SMPS board, and are perhaps for RFI suppression purposes only.]

For completeness I should also mention two other points:

• The Mixer board needs two other grounds besides GA, namely LGND and DGND. But it gets these, along with GA, as soon as you connect its ribbon cable to the SMPS (via pins 12 and 13 respectively), where they are interconnected with GA.
• The safety ground of the power inlet connector is wired only to the rear panel. Normally, that provides its contact with all the other grounds via the routes explained above. With no rear panel in place, you have no safety ground. (Though how important that is depends on how you are powering the unit. If, for safety, you are powering it through a true isolating transformer - which is highly advisable if you are running it disassembled - then a safety ground at the unit will not fulfill all its usual functions.) Always take proper account of all possible shock risks.

I hope these comments are useful, or at least interesting.

[Jazz P Bass]

Yamaha EMX Grounding Tips

Hope you do not mind, but I made a pdf file of this invaluable info.
Cheers.

[1984ARC]

Re-assembly

No probs, Jazz P Bass; thanks for that.

P.S.

When finally re-assembling the (complete) front panel unit to the main plastic shell, it is necessary to make sure that the two foils attached to the front panel unit are correctly located over the 4 central plastic fixing lugs of that unit, so that, when it's put into place onto the shell, the foils make proper contact with the springy grounding straps inside (ref GB-GA interconnection path explained above). The foils are meant to be held in place on those lugs by glue, but it's not very strong and they easily come off and wander about.

Likewise, check that all of the springy grounding straps are correctly located over the corresponding plastic fixing lugs inside the shell. The straps are meant to be held on those lugs by their formed shape and their springiness, but can become dislodged.

[geolang]

Exact same issue

No probs, Jazz P Bass; thanks for that.

P.S.

When finally re-assembling the (complete) front panel unit to the main plastic shell, it is necessary to make sure that the two foils attached to the front panel unit are correctly located over the 4 central plastic fixing lugs of that unit, so that, when it's put into place onto the shell, the foils make proper contact with the springy grounding straps inside (ref GB-GA interconnection path explained above). The foils are meant to be held in place on those lugs by glue, but it's not very strong and they easily come off and wander about.

Likewise, check that all of the springy grounding straps are correctly located over the corresponding plastic fixing lugs inside the shell. The straps are meant to be held on those lugs by their formed shape and their springiness, but can become dislodged.

Thank you so much 1984Arc. I've been working on an EMX512SC for two days now and it has held me up considerably because this is what I do for a living. Obviously I'm going to lose out on this particular repair, but I'm always up for a learning experience.

The unit I have been working on first suffered SMPS failure. I was able to get the SMPS voltages back after replacing R440, Q414, IC401, I402. But the unit would still shut off very quickly on pwr up. I narrowed this down to pin 11 (DC) on the connector from PS to PA. After lifting pin 11 and connecting pins 2,5, and 8 on the ribbon connector to the front panel, I was able to power up, and I've been all over the PA and found everything seemed to check out OK except that the two grounds which you labeled GB and GA had around 8.5V difference. This was causing the pin 11 to shut down the SMPS. I was puzzled by the fact that the two grounds GA and GB were not interconnected anywhere that I could see physically or in the schematic.

Now, thanks to your diligence I see that is because of the convoluted grounding scheme that Yamaha designed. Tomorrow I''ll try putting on the front and back panels and see what happens...

[geolang]

Another note - front panel must be screwed in tightly.

I replaced the back panel then attached the ribbon cable to the mixer board. The "GA" and the two other grounds get interconnected at the mixer board (pins 2, 5, 8), but as noted in a previous post, the "GA" and "GB" are not connected until EVERYTHING is put back together (or temporarily with a jumper). The amp was shutting down due to voltage difference between these two grounds. So, I mounted the mixer assembly with the four large screws that force contact between the springy straps and the foil. Plugged in, pushed the pwr button and it still shut off after 2 sec or less, every time. I removed the four large screws and opened up the unit again. I tried running an alligator jumper between "black" on the PS (this is "GB"), and the foil on the mixer (supposed to be connected to "GA"). Still no go. Then I ran the jumper to the sheet metal shield on the mixer board and this allowed it to power up. OK, so what's up?

Then, after disassembling the mixer board from the front panel I could see that the foil only makes contact to the ground plane on the mixer board by way of contact with the front panel which contacts the ground plane by way of the 1/4" jacks mounted in the front panel. I had the screws out of the front panel (since they only go into plastic I assumed they were not critical), so there was poor contact between foil and front panel, hence no connection from foil to ground plane, hence no connection between PS ground and all other signal grounds. Incredible. So, after complete assembly the unit works fine - finally. Thank you to everyone who contributed to this most bizzarre troubleshooting!

[Jazz P Bass]

What a convoluted grounding scheme.

I have always held Yamaha engineers in high regard, so they 'must' have had there reasons.

[farkasmusic]

great help

I replaced the back panel then attached the ribbon cable to the mixer board. The "GA" and the two other grounds get interconnected at the mixer board (pins 2, 5, 8), but as noted in a previous post, the "GA" and "GB" are not connected until EVERYTHING is put back together (or temporarily with a jumper). The amp was shutting down due to voltage difference between these two grounds. So, I mounted the mixer assembly with the four large screws that force contact between the springy straps and the foil. Plugged in, pushed the pwr button and it still shut off after 2 sec or less, every time. I removed the four large screws and opened up the unit again. I tried running an alligator jumper between "black" on the PS (this is "GB"), and the foil on the mixer (supposed to be connected to "GA"). Still no go. Then I ran the jumper to the sheet metal shield on the mixer board and this allowed it to power up. OK, so what's up?

Then, after disassembling the mixer board from the front panel I could see that the foil only makes contact to the ground plane on the mixer board by way of contact with the front panel which contacts the ground plane by way of the 1/4" jacks mounted in the front panel. I had the screws out of the front panel (since they only go into plastic I assumed they were not critical), so there was poor contact between foil and front panel, hence no connection from foil to ground plane, hence no connection between PS ground and all other signal grounds. Incredible. So, after complete assembly the unit works fine - finally. Thank you to everyone who contributed to this most bizzarre troubleshooting!

Thank you guys for the thread, I was going crazy after cleaning off an EMX212, and trying out without complete reassembly. Protection just shut down after 1-2 sec. I expected a heavy power supply or power amp repair. The unit must be fully assembled to get DC protection working properly, very sophisticated design.

[Jazz P Bass]

I was working on an EMX with the rear panel removed (as to access the bias pots) & it would trip after 2-3 seconds on.
In this case, there is a lug on the speaker out board that must be connected to the main circuit ground lug.
Fully assembled, the metal rear cover does the connecting.

[robbie1949]

Bugger of a design

Thank you guys for the thread, I was going crazy after cleaning off an EMX212, and trying out without complete reassembly. Protection just shut down after 1-2 sec. I expected a heavy power supply or power amp repair. The unit must be fully assembled to get DC protection working properly, very sophisticated design.

I would call this a bugger of a design, you cant stop the power supply from shutting down unless the unit is together, makes it hard to repair other faults in the amplifier without jumpering all the grounding points together. The service manual says nothing about this problem and I would have chased my tail around looking for a fault that was inherent in the design except for this forum, thanks for saving me time and helping me to make money out of this job. I used contact adhesive on the tin plate/foil to hold it to the plastic pillars to make sure when the amp was back together the gnd points would meet properly. I used to swear quite regularly repairing Yamaha organs with some of their designs, the amps are no better it seems........Rob

[Havy Vallarta]

hello, got the same problem. What I do is open the unit in middle and back . Spray with circuit cleaner spry.
Clean all the dust you can from in there. Close the unit. Put the unit in the sun if it's dry out. If not find a dry heat source an dry for 15 minutes, humidifiers are great. Hear dryer will work but about. And repeat 1 time every 2 years or sooner if u live and play
in humid place or close to the ocean.
Store unit inside plastic bag. Put a sock with a nut with handful of rice and change every 6 month. Good luck.

[bekim digital electronics]

Grounding yamaha amp

I think this issue of ground is very serious in this yamaha cos when i returned everything in place the machine worked properly

[GeeHowie]

Blown Power Amp--Now What

I admit to not having studied this closely, but on that 14-pin ribbon, pin 11, called DC, connects to the output bus of each channel, and if DC is ther, it will trigger either Q411 or Q413 to shut it down through the opto. "Optocouper" as they typo.

SO if ther is DC on an output, that will trip it out, and pulling the ribbon will defeat that.

Also there is the temp sensor, which will mute the amp, but doesn;t seem to involve the SMPS.

The soft start relay is across the +/-15v rails, so if they come on it should fire. But that doesn't prevent something from shutting down the SMPS, which of course would remove the 15v supplies and drop the relay.

Let us assume we are all experienced techs with some idea of safety. You need an isolation transformer to work on the primary side of the SMPS. Specifically if you want to connect any real world stuff to it, like a scope. But you can scope the secondary side all you like, Other than the freq it runs at, it is rectifiers and filters like any other. We know the 15v is going away and causing the relay to drop, but I doubt it is a direct failure of the 15v rails. On the secondary side, things are referenced to ground. On the schematic, anything left of that dotted line is dangerous.

The SMPS sits ther running if all the rest of the system is disconnected, right? You get +/-15 and the control voltages, and the two sets of HV rails? If so, it probably works. Wanna be hard core? Stick a load on each supply in turn and see if it handles it. The two control voltages don't even go to the power amp board, so I'd ignore them, at least initially. The loads are specified, so I wouldn;t drag more than half an amp from either 15v. And The HV rails are probably OK if they come up to voltage, but maybe 1 amp tops should be sufficient for test.

My money is still on a power amp. With the ribbon in place, watch pin 11. In the few seconds you have before it drops, what does pin 11 do? For that matter look at the output bus for each channel. In those few seconds, does it sit near zero, or does any of them go offset? That is on the output bus itself, not the speaker connectors outside. Little 2-pin connectors CN102,CN103 look to be test points for bias current monitoring, but would be handy points to connect to the output bus.

#Thanks to all for this thread: So, my SMPS shuts down after 1-2 secs. I've fiddled with the grounding, and the speaker outputs aren't shorted to ground. Disconnecting Pin 11 (DC) keeps the power on. so from the discussion it's most likely the PA, and I don't find any replacement boards for sale. So, any guidance on debugging the PA I've got, beyond looking for obviously damaged or burnt components? Other than CN102,103 are there any other accessible test points?