Latest challenge in the workshop was one of these mixers.
The reported fault was faint motor boating on one channel and dead on the other.
First suspicions went to the switch mode PSU as the voltages were all well under.
Once apart (no easy job as everything has to be stripped) and the PSU removed, scorching was noted as can be seen. Not a very good design as all the power resistors are fitted directly in contact with the board.
Anyway, two of the black caps (top centre) had lost their capacitance, the lower one being completely open cct! On inspection these were 85c rated so no surprise there. After replacement with 105c caps things still were amiss and it was discovered that the 100v zener had gone short (D6 at the bottom centre).
On replacement all was well.
Since doing the PSU I came across this site which shows the same problem.
http://www.neufeld.newton.ks.us/electronics/?p=498
Thinking that was an easy repair I then discovered no audio output and to cut a long story short eventually had to replace 5 x TL074's and 6x5532's. I wonder what happened when the PSU started to fail as I would have thought the voltages would have got less and not caused any damage?
Still, another one brought back to life.
Midibob
The reported fault was faint motor boating on one channel and dead on the other.
First suspicions went to the switch mode PSU as the voltages were all well under.
Once apart (no easy job as everything has to be stripped) and the PSU removed, scorching was noted as can be seen. Not a very good design as all the power resistors are fitted directly in contact with the board.
Anyway, two of the black caps (top centre) had lost their capacitance, the lower one being completely open cct! On inspection these were 85c rated so no surprise there. After replacement with 105c caps things still were amiss and it was discovered that the 100v zener had gone short (D6 at the bottom centre).
On replacement all was well.
Since doing the PSU I came across this site which shows the same problem.
http://www.neufeld.newton.ks.us/electronics/?p=498
Thinking that was an easy repair I then discovered no audio output and to cut a long story short eventually had to replace 5 x TL074's and 6x5532's. I wonder what happened when the PSU started to fail as I would have thought the voltages would have got less and not caused any damage?
Still, another one brought back to life.
Midibob
Attached Files
