Hello,
I recently repaired and recapped a Musical Fidelity A300. One of the output transistors was blown. After putting everything back together I now have a loud humming coming out of one of the toroidal transformers that I don't remember it having prior. I have very limited experience with power supply issues. Any ideas on where to start? Thank you in advance!
I recently repaired and recapped a Musical Fidelity A300. One of the output transistors was blown. After putting everything back together I now have a loud humming coming out of one of the toroidal transformers that I don't remember it having prior. I have very limited experience with power supply issues. Any ideas on where to start? Thank you in advance!
More likely there is still an amplifier circuit problem, which is drawing excessive current.
Did you test all the transistors in the bad channel? Post the schematic here.
Did you test all the transistors in the bad channel? Post the schematic here.
I believe you are correct. All the transistors were tested and matched as close as possible. Other than the humming you can hear from across the room , the unit sounds great after the repair.
I unhooked the secondary leads from the humming transformer to the circuit board. The humming went away and both transistors were equally quiet. It is the left side only, so I know it is something I did on that side. My next step is to hook up the preamp section to see if it comes back, to confirm it is in the amplifier section. Thanks again for the assist!
I unhooked the secondary leads from the humming transformer to the circuit board. The humming went away and both transistors were equally quiet. It is the left side only, so I know it is something I did on that side. My next step is to hook up the preamp section to see if it comes back, to confirm it is in the amplifier section. Thanks again for the assist!
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There is a muting relay at the power amplifier input, so there is a brief delay upon power on/off.
Does the hum take the about muting time to happen, or is the hum instant after flipping the power switch?
This sort of problem is very unlikely to be in the preamp section.
See what the DC voltage is on the main power supply capacitors.
Does the hum take the about muting time to happen, or is the hum instant after flipping the power switch?
This sort of problem is very unlikely to be in the preamp section.
See what the DC voltage is on the main power supply capacitors.
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The hum increases during the muting time. When the front panel socket(SK4) is removed, the hum dies away.
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No...I am stumped and moved on to another project. I bought this MF A300 really cheap and was hoping I could fix it, which I sort of did. It is currently back on the "to-do" shelf.
I am still learning a lot about diagnosing and troubleshooting, and this issue seems to be beyond my current capabilities. I will probably have to get a more experienced pro involved.
I am still learning a lot about diagnosing and troubleshooting, and this issue seems to be beyond my current capabilities. I will probably have to get a more experienced pro involved.