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Hey folks,
long-time lurker, first-time poster. I'm working on a Mesa Mark IV A that suffered a seemingly common problem where R383 (100ohm feeds LD channel in switch matrix) had cooked and taken with it R381 and R382 (100 ohm artificial centre tap for filaments) as well as D8 and D9 (1N5392 15V supply). Anyone who's worked on these knows that to access this part of the board you basically have to unsolder every wire, which isn't super fun. Anyway, got it out, tested the three 8200uF 16V caps and they seemed ok, replaced the 100ohm resistors and the 1N5392 diodes, as well as VR12 which is a 12V regulator. This 12V circuit supplies the optos as well as the V1 heater. I put back the three seemingly good caps and replaced all the 2W and 1W resistors as well while I had the board out.
Got it all back together, which is unfortunately necessary to properly test it, and everything seems good except I'm only getting 5.6VDC out of VR12 instead of 12.6V so of course V1 will not turn on. The 15V supply seems to be good on the input, but it has the incorrect output. I looked at the schematic again and noticed that R383 is supposed to be a 1W resistor but the factory resistor was definitely 1/2W (carbon comp 9.5mm long) so now I'm thinking something could be fried in the switching matrix or it could be those 16V caps aren't as good as they appeared to be.
Does anyone have any insight into why this circuit isn't working right? Especially looking for a solution that I don't need to remove the board again, but I'm not too hopeful at this point.
Thank you for reading this
*Editing to add that the initial problem the amp was brought in with was that the lead channel was really quiet. This seems probably related to the burnt R383 but could a fault in the switching matrix cause a decrease in volume for a given channel? The R1 and R2 channels both seemed to be "working fine" but a problem with the low voltage supply should affect all channels equally because V1 is used by all channels.
long-time lurker, first-time poster. I'm working on a Mesa Mark IV A that suffered a seemingly common problem where R383 (100ohm feeds LD channel in switch matrix) had cooked and taken with it R381 and R382 (100 ohm artificial centre tap for filaments) as well as D8 and D9 (1N5392 15V supply). Anyone who's worked on these knows that to access this part of the board you basically have to unsolder every wire, which isn't super fun. Anyway, got it out, tested the three 8200uF 16V caps and they seemed ok, replaced the 100ohm resistors and the 1N5392 diodes, as well as VR12 which is a 12V regulator. This 12V circuit supplies the optos as well as the V1 heater. I put back the three seemingly good caps and replaced all the 2W and 1W resistors as well while I had the board out.
Got it all back together, which is unfortunately necessary to properly test it, and everything seems good except I'm only getting 5.6VDC out of VR12 instead of 12.6V so of course V1 will not turn on. The 15V supply seems to be good on the input, but it has the incorrect output. I looked at the schematic again and noticed that R383 is supposed to be a 1W resistor but the factory resistor was definitely 1/2W (carbon comp 9.5mm long) so now I'm thinking something could be fried in the switching matrix or it could be those 16V caps aren't as good as they appeared to be.
Does anyone have any insight into why this circuit isn't working right? Especially looking for a solution that I don't need to remove the board again, but I'm not too hopeful at this point.
Thank you for reading this
*Editing to add that the initial problem the amp was brought in with was that the lead channel was really quiet. This seems probably related to the burnt R383 but could a fault in the switching matrix cause a decrease in volume for a given channel? The R1 and R2 channels both seemed to be "working fine" but a problem with the low voltage supply should affect all channels equally because V1 is used by all channels.
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