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Had this monitor to repair..Couldn't find any documentation...so I went and took a crack at it.....Sometimes when it was first powered up it would make this awful loud buzz as if there was a major dc level applied to the speaker..the cone would almost jump out of the basket but it would only last for a second or two then it would be fine......leave it for awhile and it would do it again but only for a second or two....if I didn't leave it for too long and turned it back on... then it would not happen....
..I thought it might be a power supply filter problem....this is a bitch to work on..I had to remove the grill, speaker and the chassis that holds the power amp/power supply section. Then I had to remove the board from the chassis and insulate the chassis with a piece of heavy cardboard so that the board wouldn't short out against it while working on it...hooked up the speaker again with jumper wires.....soldered some solid wire to the filter caps and the ground connections so I could clip test leads to it..
..I checked the positive and negative supplies for both voltage and ac ripple.....it checked fine....I did some quick checks on the major semiconductors and everything checked out good....I don't know why, but for some reason I took out the relay to test it...I was thinking that the contacts were not making a good connection when power was applied and the relay was first energized.....The relay checked good as I used an external power supply and checked the contacts for a good connection with a resistance check..
.Then I thought that maybe the filter cap for the relay might be bad...removed it and checked for ESR and rated capacity...checked good...Checked for bad connections...I couldn't find any......Re-installed the relay and capacitor and checked it again.....for some reason it seems to be working....I checked the voltages on the power supply caps and I noticed that when the unit was turned off they would drop...but before that they would not....(can the relay cause this type of problem)....
..When power is first applied both the positive and negative supplies come up to 63 volts before the relay engages to connect the speaker and horn to the circuit.....when I turn it off the voltages drop to plus and minus 50 volts and then the relay de-energizes to disconnect the speaker and horn before the voltages drop any further.....and the power supply voltages drop to pretty much zero within say 10 or 15 seconds.....with no schematic it is difficult for me.....
there must have been a bad connection either with the relay pins that were soldered to the board or the capacitor pins...but they did look fine......I am going to leave this overnight and try it again in the morning......if anybody has anything to suggest feel free.....it is possible that it still has this problem but so far so good.....
Cheers......
..I thought it might be a power supply filter problem....this is a bitch to work on..I had to remove the grill, speaker and the chassis that holds the power amp/power supply section. Then I had to remove the board from the chassis and insulate the chassis with a piece of heavy cardboard so that the board wouldn't short out against it while working on it...hooked up the speaker again with jumper wires.....soldered some solid wire to the filter caps and the ground connections so I could clip test leads to it..
..I checked the positive and negative supplies for both voltage and ac ripple.....it checked fine....I did some quick checks on the major semiconductors and everything checked out good....I don't know why, but for some reason I took out the relay to test it...I was thinking that the contacts were not making a good connection when power was applied and the relay was first energized.....The relay checked good as I used an external power supply and checked the contacts for a good connection with a resistance check..
.Then I thought that maybe the filter cap for the relay might be bad...removed it and checked for ESR and rated capacity...checked good...Checked for bad connections...I couldn't find any......Re-installed the relay and capacitor and checked it again.....for some reason it seems to be working....I checked the voltages on the power supply caps and I noticed that when the unit was turned off they would drop...but before that they would not....(can the relay cause this type of problem)....
..When power is first applied both the positive and negative supplies come up to 63 volts before the relay engages to connect the speaker and horn to the circuit.....when I turn it off the voltages drop to plus and minus 50 volts and then the relay de-energizes to disconnect the speaker and horn before the voltages drop any further.....and the power supply voltages drop to pretty much zero within say 10 or 15 seconds.....with no schematic it is difficult for me.....
there must have been a bad connection either with the relay pins that were soldered to the board or the capacitor pins...but they did look fine......I am going to leave this overnight and try it again in the morning......if anybody has anything to suggest feel free.....it is possible that it still has this problem but so far so good.....
Cheers......
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