Help with an MV45(a?)
Posted: Sun Jan 08, 2017 7:48 pm
I recently acquired an MV45 from a friend. It had been in storage for quite some time. I removed the top cover and cleaned the dust from the chassis and tubes. I assumed that the amp just needed to be powered up for awhile before being used. I connected a pair of Polk bookshelf speakers and nothing to the input. It powered up ok, but I noticed a slight hum in the left channel. Then, there was some crackling coming from that channel and the bias adjust LED for one of the tubes was flickering. I surmised that the bias adjust pot may have needed some cleaning. I sprayed some cleaner on both pots for the left channel. I went through the procedure for adjusting the bias on each of the EL34's. After waiting, I powered the amp back up. After about five minutes, there was smoke coming out of the chassis. I knew what that was. The first thought that crossed my mind was, "just lost an electrolytic cap". After the smoke cleared, I opened the bottom and my suspicion was confirmed. I removed the cap and confirmed that it was gone, along with some other physical evidence on the underside corner of the chassis.
I jury rigged an appropriate value of replacement capacitance in its place, but it is evident that there is damage beyond that. Right now, I am guessing that I lost my negative grid bias on one half of the push pull in the left channel. That half shows an LED bias light that will not go out. It also seems that because of no grid bias, the EL34 starts to go into runaway. I do not leave the power on long enough to let the plate glow red, but that EL34 gets warm right away while the others are almost cold.
I may have left out some pertinent details. I have been getting the schematics from this website, so it is getting a bit easier to approach this. I am at the point where I may have to get at the top side of the printed circuit board that is on the underside of the chassis. I certainly would appreciate any ideas and/or getting pointed in the right direction.
I jury rigged an appropriate value of replacement capacitance in its place, but it is evident that there is damage beyond that. Right now, I am guessing that I lost my negative grid bias on one half of the push pull in the left channel. That half shows an LED bias light that will not go out. It also seems that because of no grid bias, the EL34 starts to go into runaway. I do not leave the power on long enough to let the plate glow red, but that EL34 gets warm right away while the others are almost cold.
I may have left out some pertinent details. I have been getting the schematics from this website, so it is getting a bit easier to approach this. I am at the point where I may have to get at the top side of the printed circuit board that is on the underside of the chassis. I certainly would appreciate any ideas and/or getting pointed in the right direction.