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Yamaha MX-1000 protection light, outputs all good, what's next?

Sir.Byrd said:
Maybe, I am not sure if that is from manufacturing or from the transistors mounted to it.
Why don't you pull the transistors and check them?
Click to expand...

Yeah, seems like a false alarm.

I checked them, they seem to be showing like any other good NPN / PNP type pattern in VOM
 
I have soldered outputs back in and assembled the whole thing. I have measured some voltages and need help to interpret them:


...............TP.......OUT......IN.......PR.....E.....-LB....+LB.......-HB.......+HB
---------------------------------------------------------------------
R...........95.7.......96.3.....97.6....103...0.....-48......48.2.....-103.2...102.6

L............96.........96.2.....97.6....103...0.....-48......48.1.....-103.2...102.6


By the time I was done measuring, -LB fell to about 44V and +LB to about 46V on both channels. Resistors R349 / R350 near VR302/VR301 adj pots got very very hot.
 
Everything looks good..except the 96-97volts on the IN , OUT. Should be about 0.0volts .
 
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Resistors R349 / R350 near VR302/VR301 adj pots got very very hot.
Click to expand...
90 plus volts on the input ,yeah those resistor are going to get smok'n hot.Shorted devices in the front end. Bad ground somewhere maybe. Solder bridges. Or you made a mistake reading the voltages. 97 volts is hi rail voltage on both the input and output of both amplifiers. Unbelieveable:scratch2:
 
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avionic said:
90 plus volts on the input ,yeah those resistor are going to get smok'n hot.Shorted devices in the front end. Bad ground somewhere maybe. Solder bridges. Or you made a mistake reading the voltages. 97 volts is hi rail voltage on both the input and output of both amplifiers. Unbelieveable:scratch2:
Click to expand...

I don't think I made a mistake measuring those (both IN and OUT), but I will verify again tonight.

Yeah, those resistors are almost smoking, they give off this typical hi-temp, faint heater smell. There is nothing else appears to be smoking or charred, though.
 
avionic said:
90 plus volts on the input ,yeah those resistor are going to get smok'n hot.Shorted devices in the front end. Bad ground somewhere maybe. Solder bridges. Or you made a mistake reading the voltages. 97 volts is hi rail voltage on both the input and output of both amplifiers. Unbelieveable:scratch2:
Click to expand...

Yep that's the voltage on those jumpers.

So, if it's identical on both channels, does it mean that there is some common cause to it?
 
Possibly..Look the main circuit board over with a magnifying glass for cracked solder connections or possible crack circuit board traces.
 
After a closer look, I found something that looks like a trouble spot. Based on schematics, IN terminal connects to Q515, Q513 transistors. I found them on the board and I noticed some burnt traces. See the pictures. The cap has a round spot around its neg leg on the board. Ceramic (?) cap has a burnt spot. nearby Q513 transistor area on PCB has strange scratch marks like someone wiped out smoke stain or something and wiped out transistor number.

Now, I will need to take this thing apart to get under those narrow PCBs again.

397807394.jpg


Things around Q515 seem to look clean; however, looks can be deceiving, I suspect.

So, will this make high voltage appear on OUT terminal or I should expect more fail components because of that?
 
R532 is burnt badly, it's junk. How didn't I notice this before?!:gigglemad

So, hopefully, it's going to be the only affected area.
 
Actually, it looks like someone was repairing this. Some resistors look different compared to the rest. I am going to check those just in case.
 
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I fixed it! Just traced voltages by schematic and had to solve a problem with a couple of unknowns. It's playing now with a pair of test (car) speakers @ ~ 3W output on LCD, sounds very, very nice. Everything seems to be 100% functional! It was a great learning experience and a nice reward!

Thanks a lot for all your suggestions, Audiokarma rocks :banana:
 
mrkoder said:
I fixed it! Just traced voltages by schematic and had to solve a problem with a couple of unknowns. It's playing now with a pair of test (car) speakers @ ~ 3W output on LCD, sounds very, very nice. Everything seems to be 100% functional! It was a great learning experience and a nice reward!

Thanks a lot for all your suggestions, Audiokarma rocks :banana:
Click to expand...

What actually was the problem?
 
I saw that -93V was missing completely and found out that two 1ohm resistors were open. I temporarily put just one (the only one I have so far) and the amp came alive. So, I need to get those and replace a temp.

However, here's the question. The amp was repaired before. I see that whoever repaired the amp, also replaced the same resistors, but on +93V side with 1/2W instead of original 1/4W.

Do you think it was a good idea? It's important, because now I am wondering if I should put 1/4W resistors or go with 1/2W as was done by the last repairman?

I suspect those resistors act somewhat like fuses when the amp is facing an imminent meltdown or something like this. So, putting original 1/4W kinda makes sense, but I am wondering if whoever put those 1/2W there actually knew what he was doing.
 
Most likely fusible or flameproof.Go with 1/4 watts. But pick up some 1/2 watts as well.
 
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There is a DC Offset discrepancy...

Left channel 0.053V
Right 0.005V

quite a difference. I wonder what might be the cause.

I don't think this amp has dc offset pots, very few adjustments, at least SM only mentions bias. I am setting bias almost exactly to 0.01V (+/- 0.001V) per service manual.

Checked all the voltages on V-Amp board and in some places (Emitter or Collector) voltages drift up to 1v, either left or right in the same channel. I haven't checked output voltages, I see that schematics shows 0V for all outputs' emitters.

Would checking voltage on those be a next step?
 
avionic said:
Are you hearing a "thump" in the speakers at power off or power on? Or when the protect relay clicks on or off? Replacing Q503 & 505 with a Hfe matched pair may reduce your offset.
2SA970BL
http://www.bdent.com/2SA970BL-p/2sa970bl.htm
Click to expand...

I don't think I heard any 'thump', maybe it was something faint. I will try again though.

OK, so, Q503 and 505.

I am getting almost 1.1V less on Q505 collector than spec'd (actual -91.5V vs -92.6V spec'd).

Q503 is actual -91.7V vs -91.3V spec'd, but I guess if HFE is off than anything is possible. I will get the replacements and give it a try. Thanks again and thanks for the link!!!
 
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