Troubleshooting Yamaha EMX5000 -Voltage Regulator Output??

[ewd]

I'm working on a Yamaha EMX5000 Mixer that is showing negative voltages all over the place. While doing voltage checks, I have noted some readings on a 7805 Voltage Regulator that doesn't look correct to me. IC103 (7805) indicates 53 VDC on the Input leg, 37 VDC on the Ground leg (Com) and 42 VDC on the Output leg. I have removed and checked Q136, R246, R247, and Zener Diode D150. I have also replaced IC103 with a new 7805 and checked all the other Zener Diodes and Caps in this voltage reg section and get the same measurements on IC103 even after replacing it. Are these voltages actually correct? IC104 & IC105 both appear to be outputting a little low. IC104 (7805) measures 16VDC on the input, -3VDC on the ground, and 2 VDC on the output. IC105 (7812) measures 16 VDC on the input, -3VDC on the ground, and 9VDC on the output. Q136 reads 53VDC on the base, 72VDC on the collector, and 53 VDC on the emitter
SM_EMX5000_E VR Circuits.pdf

[Jazz P Bass]

First off, when checking the voltages, especially the 5 volts, check them at the headers.

Make sure that your ground reference is true.
Yamaha uses extensive grounding schemes, so if the unit is apart on the bench, you may need to have everything connected.

That 5 volt circuit is a bit strange.
You cannot use the common terminal of the regulator to get a true voltage reading on the output pin, as it is not at ground.

[ewd]

Yes, I took the measurements in reference to the PCB ground (negative side of C162).

Since my original post, I was able to find a schematic reference elsewhere in the circuitry (that I had not seen earlier) tracing back to this 7805 regulator output leg, that has a header indicating 48 volts. That kinda threw me for a loop that Yamaha used a 7805 voltage regulator to provide a specified 48 volts.

I guess my voltage measurements (at IC103) are actually close to normal for this design.

[Enzo]

All a 7805 specifies is that the output pin will be 5v more positive than the ground pin. And it asks that the input pin not be more than (if I recall) 35v over the ground pin.

The thing to do, rather than just looking all over for voltages, is to track down the problem itself. In other words, if this 5v regulator was being used as the heart of a 48v supply, and you do have more or less 48v in the circuit, then there is no point in swapping out parts. The expected supply is working.


What is wrong with the amp that we are trying to repair?

[Jazz P Bass]

Schematic link for anyone interested: http://elektrotanya.com/yamaha_emx5000-12[20]_sch.pdf/download.html

[jbltwin1]

Linky no worky here. the whole link didn't get pasted in I guess. Copy and past the whole thing in and it works. Mike.

[ewd]

Thanks Jazz P & Enzo. What added to my confusion was that the datasheets I've seen for 78xx series regulators indicate that absolute max input voltage for 5-18V regulators is 35 volts, and for 24V regulators is 40V. I made an apparently incorrect assumption that this was an absolute input limit, and not in reference to the voltage differential between the input and ground pins of the regulator. Another learning experience. I did the part swap before I discovered that this regulator was part of a 48V supply network.

This mixer came in noted as "smoked". I started working on it awhile back, then had to put it off to the side due to higher priorities, and am again getting back into it. I initially found several bad C3856 and A1492 power transistors at that time, negative DC voltages in multiple locations on the positive side of the power amp, on the PCB ground, and in the IC103, IC104, & IC105 regulator network. I replaced the bad power transistors, and while doing voltage checks immediately after the transistor replacement, capacitor C162 (which couples the regulator section to the PCB/chassis ground connection), actually blew up in the middle of those voltage checks. I replaced C162 and then had to put this mixer aside.

I’m now just getting back into the mixer power amp and am going through it again to see where I’m at with the repair progress. I found what I thought were suspect voltages at the IC103 regulator, but now know they are correct, and the negative voltages at the regulator network are gone. I will be doing further checks when I get back into the shop this evening.

[J M Fahey]

I *think* the actual problem you have ("smoked") is blown outputs and what they carry to death with them.
Very much doubt you have a phantom power supply problem or similar ones.
And even *if* you did, anyway you must first concentrate on the main problem, using standard troubleshooting techniques, such as measuring shorted transistors, rail and speaker out voltages, biasing, blown ballast resistors, etc.
Until you get 0V dc at the speaker out, a few mA bias current, signal at the speaker when driving the power amp input, don't weaken your focus looking for "other" problems.
I understand your complicated schedule (same here ) but on these problems it's better to leave them aside until you can have, say, at least a full afternoon or equivalent for it, because these are complex systems and you must "get into it" , "understand what the h*ck the designer was trying to achieve" , etc.
And ... good luck