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

M

mrkoder

Member
Hi,

I got this amp recently, nice cosmetics, but protection light is on.

R349 / R350 resistors ( 5watt or something like this) get very hot when the amp is on. I did measure some voltages off wires coming from the top board (RE, LE, RO, LO, E, MU) and the voltages slowly decrease to practically almost 0 from the time of turn ON, but I didn't keep the amp ON for very long.

I went straight for checking output transistors with VOM and they all check out good, 470 - 590 ohm range B -> C / E (NPN / PNP accordingly).

I am kinda puzzled right on how to proceed next. I only soldered out the otputs, and if they are good, is there any sense to continue checking other transistors on those two boards? Or what to look at next. All fuses are good. I did tap on relays, but no change. Front meter lights up but shows 8888.8 or something like this.

If anyone has been fixing "stuck-in-protection" problems (but good outputs), please advise on what to do next. Thanks.
 
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mrkoder said:
Hi,

I got this amp recently for $10, nice cosmetics, but protection light is on.

R349 / R350 resistors ( 5watt or something like this) get very hot when the amp is on. I did measure some voltages off wires coming from the top board (RE, LE, RO, LO, E, MU) and the voltages slowly decrease to practically almost 0 from the time of turn ON, but I didn't keep the amp ON for very long.

I went straight for checking output transistors with VOM and they all check out good, 470 - 590 ohm range B -> C / E (NPN / PNP accordingly).

I am kinda puzzled right on how to proceed next. I only soldered out the otputs, and if they are good, is there any sense to continue checking other transistors on those two boards? Or what to look at next. All fuses are good. I did tap on relays, but no change. Front meter lights up but shows 8888.8 or something like this.

If anyone has been fixing "stuck-in-protection" problems (but good outputs), please advise on what to do next. Thanks.
Click to expand...

Have a real close look at the big caps.Especially around the base of the caps-look for signs of liquid leakage
I did measure some voltages off wires coming from the top board (RE, LE, RO, LO, E, MU) and the voltages slowly decrease to practically almost 0 from the time of turn ON
Click to expand...
power supply problem
Front meter lights up but shows 8888.8
Click to expand...
Check all the fuses with a meter.
 
Protection comes on for other faults besides output transistor failure/shorting, I would check the bias and DC offset (you will have to measure offset from a point before the relay, because it is blocking output to the speaker terminals) to see if they are wonky.
Do you have a service manual yet?
 
Yeah, I was pretty excited, but then plugged it in and saw the light on, oh, well. Otherwise, it would be something close to a miracle.

Thank you very much for the suggestions, I will go through fuses and check those cans. It's kind of good news - outputs look good, but, now what... I hope I didn't mess anything else there in the meantime, it's kind of awkward layout with those PCBs in two layers.
 
Sir.Byrd said:
Protection comes on for other faults besides output transistor failure/shorting, I would check the bias and DC offset (you will have to measure offset from a point before the relay, because it is blocking output to the speaker terminals) to see if they are wonky.
Do you have a service manual yet?
Click to expand...

I see, makes sense. I do have a service manual. I will put this step on the list, as well. Thank you!
 
I need to solder back those outputs then, before checking bias and DC offset.

Is it OK to keep those heatsinks OFF for this kind of troubleshooting?
 
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RE---right speaker ground
LE---left speaker ground
E ---chassis ground
RO--right speaker output
LO--left speaker output
MU--protect
All of these should measure pretty close to 0.0volts
in reference to ground.
-LB --- low B- = -40v
+LB --- low B+ = +40v
-HB --- high B- = -94v
+HB ----high B+ = +94v
All referenced to ground.
voltages are key..
 
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avionic said:
RE---right ground
LE---left ground
E ---chassis ground
RO--right output
LO--left output
MU--protect
All of these should measure pretty close to 0.0volts

-LB --- low B-
+LB --- low B+
-HB --- high B-
+HB ----high B+
voltages are key..
Click to expand...

Ah, OK. Those could be accessed from underneath, I believe. I didn't measure those yet. Thank you!
 
avionic said:
Have a real close look at the big caps.Especially around the base of the caps-look for signs of liquid leakage power supply problem
Check all the fuses with a meter.
Click to expand...

Big caps look clean and nice shape. 4 fuses on the PCB all show up at 0.0xx ohm in VOM. I mean, I took them out, of course, to measure.
 
mrkoder said:
i need to solder back those outputs then, before checking bias and dc offset.

Is it ok to keep those heatsinks off for this kind of troubleshooting?
Click to expand...
is it ok to keep those heatsinks off for this kind of troubleshooting
Click to expand...
--no-- Not unless you want to replace $50 worth of outputs per channel.
 
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This amp is not very tech friendly.The way they mount those two output boards to the main board really sucks for troubleshooting those boards.
Chances are the problem is on the main board.
You said you measured zero volts on RO and LO..So if your fuses are OK and there is zero voltage on the RO & LO.Then its a power supply/protect circuit problem most likely.
 
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Yeah, I think I saw those voltages dropping fast to 0.

OK. I was soldering outputs back in and was about to put a heatsink and I saw one suspicious mark on one metal plate that holds those middle small transistors against the heat sink. Here's the image. What do you think? What are those transistors? Only one side has this mark. The other one is very clean.

397779988.jpg
 
Those transistors are Q316, Q312, Q310, Q314. I looked at transistors and they don't look like burnt, actually. I mean, I didn't take them out, yet.
 
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?
 
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