sherwood amp. problem

dzoni

Member
2019-11-25 9:05 am
Hello. I have some problem after repairing my sherwood ax 5090r amplifier. both channels were burnt and i replaced both of them, one channel with original output transistors (sanken 2sc3857 and 2sa 1493) and another channel with replacement-pmc 2sc3858 and 2sa1494 (becouse that 2 wast last original i could find availible in my country). Except output transistors, i replaced all other transistors with same types as original ones (from schematic). My problem is that the channel what i repair with original output transistors is now in great condition, sound nice and clear but the other channel (with replacement transistors) sound very distorted. Mu question is did i need to change drive transistors too or adjust idle current on thah channel (there is no bias adj. pot btw) or there is something else the main problem?
Thank you :)
 
That's the kind of thing that can happen when you blindly shoot the parts cannon at a problem instead of doing proper methodical troubleshooting. You don't really know what was defective in the first place, you have changed a whole lot of things and could have made one or more bad solder joints (or with some bad luck, lifted a pad) in the process, and you have not checked a number of other parts that could also be defective, like resistors or diodes. It is very common for resistors near the output to be damaged as well. The electrolytic capacitor in the feedback network's ground leg may not take too kindly to a major meltdown either.

Then there is the whole issue with your potentially-dubious output transistors - check whether the diode test says roughly the same as for the known-working ones in all directions. Counterfeiters have been known to relabel any kind of part with the same form factor at times.
 

dzoni

Member
2019-11-25 9:05 am
all resistors and diodes are checked (in good condition and exact value as in schematic), capacitors are changed, detailed checked pcb traces and solder joints (with magnifying glass), everything is ok. As i said, i repaired both channels in same thime with same components except output transistors (cant find two pair of original so i bougth one pair original, other replacement). So it must this be the problem. Measured e-e voltage across both chanel output transistors, on good one it is 10mv, on this (defective) it is 1mv.