Hi all, new member here after some advice please!
I have a Sound craftsmen A2502 stereo power amp, 250W. It's an American made solid state amp made around 1981.
The main capacitors x2 were replaced by a pro a year ago but since then it just stopped outputting any audio from both channels. If I turn the volume up I get buzzing from each channel.
So far I have:
Checked fuses OK, the dc voltage is correct both negative and minus. Checked for good continuity from both inputs past the volume controls to first device.
Unsure where to next check? What is confusing me is that both channels exhibit same issue.
Can someone please give me some pointers on where I should be looking to test next based on the attached schematic?
I have a good quality multimeter and a Fluke 123 scope meter for test equipment.
Note I'm new to electronics repair however am an electrician and industrial automation engineer so safety wise I'm happy with working on it, just it's not my usual cup of tea.
Thanks in advance!
I have a Sound craftsmen A2502 stereo power amp, 250W. It's an American made solid state amp made around 1981.
The main capacitors x2 were replaced by a pro a year ago but since then it just stopped outputting any audio from both channels. If I turn the volume up I get buzzing from each channel.
So far I have:
Checked fuses OK, the dc voltage is correct both negative and minus. Checked for good continuity from both inputs past the volume controls to first device.
Unsure where to next check? What is confusing me is that both channels exhibit same issue.
Can someone please give me some pointers on where I should be looking to test next based on the attached schematic?
I have a good quality multimeter and a Fluke 123 scope meter for test equipment.
Note I'm new to electronics repair however am an electrician and industrial automation engineer so safety wise I'm happy with working on it, just it's not my usual cup of tea.
Thanks in advance!
Attachments
Input a sine wave signal from your computer sound card and trace the signal through the schematic to see if you can determine where it disappears. Also check the +/- 70 volt rails for good clean power. If that is all good, most likely failures are power transistors and electrolytic capacitors.
just that already...
All the capacitors or just the main (psu)
start by looking at your biases to see if you are within the values given by the diagram, take the opportunity to see if nothing heats up abnormally on the pcbs, then you have what seem to be test points on the diagram (in the small diamonds in A and mA).
I will not be surprised if one or more k135 and j50 have failed, you will see once you have already checked the rest of the previous points.
I find it interesting that the buzz volume increases with the volume control. Does amplitude rise smoothly with control rotation? No buzz with control at minimum? What does your scope tell you? There should be very little DC present at speaker outputs, probably less than 100mV.
I'm thinking the amplifiers might be ok and am suspicious of a ground problem between input jacks and board input. Try to localize problem before tearing into amp circuits.
Good luck!