MC300 Power Guard Light on Right channel
- Thread starter william13
- Start date
So you are highlighting the missing green mask , two resistors from the right, just below the white molex connector.
Those are resistors r19,20, and 21 part of the load shown to the source of the balanced rh input.
Is this board out of the unit now or was this picture from years ago?
Those are resistors r19,20, and 21 part of the load shown to the source of the balanced rh input.
Is this board out of the unit now or was this picture from years ago?
I do not think so. A cracked or bad solder joint at this location is so early in the signal path the PG would reference it as coming from the preamp.
It would certainly sound bad but so would a vinyl pop, tape or cd drop out. As far as the PG circuit it is the source.
When you get the cover off take some good focased pics of the driver cards.
It would certainly sound bad but so would a vinyl pop, tape or cd drop out. As far as the PG circuit it is the source.
When you get the cover off take some good focased pics of the driver cards.
william13
That's not me
I understand. At first I thought it was the preamp also but was disappointed when I discovered it was the amp. Here is another symptom. When I turn the unit on, both PG Leds light up for a fraction of a second and then both go off. Then the right led lights up again (but not the left for a fraction of a second). This is with no input cables connected. I do not know what it is sensing when that happens.
Pictures will come tomorrow when I get help taking this off the shelf!
Pictures will come tomorrow when I get help taking this off the shelf!
MACKIE1975
Super Member
"When I turn the unit on, both PG Leds light up for a fraction of a second and then both go off"
This is normal. It is a self diagnostic.
I agree with C_dk that the unit should be diagnosed with power on.
This is normal. It is a self diagnostic.
I agree with C_dk that the unit should be diagnosed with power on.
william13
That's not me
Right ! But after that the LED on the right will go on one more time (ie twice) as opposed to the left which goes on once. It might not be a significant hint but I wondered if it lends any insight.
I plan to provide pictures tomorrow but let me ask a simple question. The unit is 15 year old and has twelve HOTs. They may or may not be the issue but does the age of the Amp itself make it good practice to put in new HOTs or is it best practice to replace these one by one as the years go on.
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pdm4606
Super Member
Way back when I built my first integrated amp, Fisher KX-100, it had a light come on if there was a problem. It was the arching in the output tube bases.
It finally welded one tube so bad that I gave it away and got a new integrated, Marantz 1060. Had it ever since without any lights going on.
It finally welded one tube so bad that I gave it away and got a new integrated, Marantz 1060. Had it ever since without any lights going on.
william13
That's not me
Will do!!!
I see no pretty blondes or brunettes on these circuit boards so I will admit to a HOTs moment.....
If the power has been off for a while you could swap the driver boards L for R and see if the issue swaps sides when you carefully bring the gain controls up from zero....when you swap them back make sure the PS has drained back down from 87 volts.....10 mins or so.
If the power has been off for a while you could swap the driver boards L for R and see if the issue swaps sides when you carefully bring the gain controls up from zero....when you swap them back make sure the PS has drained back down from 87 volts.....10 mins or so.
william13
That's not me
I thought they were Horizontal Output Transistors.
As far as swapping boards, I think that is a brilliant idea. Please step me through "make sure the PS has.....", I am not sure I understand how to do that.
I just uploaded the pictures of the central area over the capacitors; I know this is the area I should be most concerned about along with the horizontal transistors and the fuse in back and the relay. Where should I measure the 87V?
As far as swapping boards, I think that is a brilliant idea. Please step me through "make sure the PS has.....", I am not sure I understand how to do that.
I just uploaded the pictures of the central area over the capacitors; I know this is the area I should be most concerned about along with the horizontal transistors and the fuse in back and the relay. Where should I measure the 87V?
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william13
That's not me
Experiment done:!! The symptoms remain the same ie right PG still triggers and minimal meter movement. The left remains the same. Thank you for that idea!
Bill
Knowing the 87 +-voltage supplies have discharged you would swap the driver cards back. Knowing from the schematic that the 12+-volt and 87 volt supplies are common to both channels the next step would be the monitor the input into the driver boards to see if the waveform was being affected upstream. I would have routinely already checked the 12 and 87 volt supplies for their integrity in the normal coarse of checking the unit out.
We would now be over an hour of a techs time with $3-4 thousand dollars of test equipment and we still have not got to the most tedious task of checking out individual parts.
Monitoring the input is easy enough to do with a distortion analyzer and osciliscope and knowing the PG LDR is located on the driver board I would now be suspecting a failed output transistor and/ or a open emitter resistor.
Long ago I cut in half a old double ended alligator jumper lead and soldered in a old school Christmas tree 120 volt lamp. Clipped on the hot side of a filter cap and then to ground it makes for a great visual discharge path for a possibly charged cap. Crow baring with a screwdriver is way too startling and burns too many screwdriver tips.
We would now be over an hour of a techs time with $3-4 thousand dollars of test equipment and we still have not got to the most tedious task of checking out individual parts.
Monitoring the input is easy enough to do with a distortion analyzer and osciliscope and knowing the PG LDR is located on the driver board I would now be suspecting a failed output transistor and/ or a open emitter resistor.
Long ago I cut in half a old double ended alligator jumper lead and soldered in a old school Christmas tree 120 volt lamp. Clipped on the hot side of a filter cap and then to ground it makes for a great visual discharge path for a possibly charged cap. Crow baring with a screwdriver is way too startling and burns too many screwdriver tips.
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