Hi guys,
I have a friend who gave me a low powered PA amplifier to repair which is blowing the main rail fuses after it got slightly wet while turned on, and the music just stopped, no bang or anything so he turned it off and used a hairdryer straight away to dry it.
After finding a C-E short on one of the output PNP transistors I desoldered the lot - the rest of the unit powers up fine and nothing else seems affected (yet!)
I always replace the entire output stage when I do a repair and I don't have any 2sc5200/2sa1943s on me, only mjl21193-94 - Is it safe to replace the output stage with these? The transformer puts put 35-0-35 so im guessing it has about 50v rails - so I am pretty confident but still need that little peace of mine.
There are 2 complimentry pairs per channel.
Thanks very much for your help.
I have a friend who gave me a low powered PA amplifier to repair which is blowing the main rail fuses after it got slightly wet while turned on, and the music just stopped, no bang or anything so he turned it off and used a hairdryer straight away to dry it.
After finding a C-E short on one of the output PNP transistors I desoldered the lot - the rest of the unit powers up fine and nothing else seems affected (yet!)
I always replace the entire output stage when I do a repair and I don't have any 2sc5200/2sa1943s on me, only mjl21193-94 - Is it safe to replace the output stage with these? The transformer puts put 35-0-35 so im guessing it has about 50v rails - so I am pretty confident but still need that little peace of mine.
There are 2 complimentry pairs per channel.
Thanks very much for your help.
It should work OK. The main difference is that the Motorola parts are much slower -- I think Ft is around 4 MHz instead of 30 MHz. This may or may not cause problems with HF stability, especially with capacitive loads. I think a closer substitute would be the MJL1302/3281. Check the data sheets to be sure.
Hi,
Thank you to everyone for their help.
The substitution worked perfectly, I have had the amplifier on test for a few days on different loads/speakers and am happy to report is has been totally stable.
I checked the predriver for the output stage and they all tested ok, so I soldered a few resistors across the rail fuse holders and tested the DC and AC values on the speaker output with a multimeter and scope and all were very low under no signal conditions.
I also read the datasheets and previous threads which gave some insight about the max frequency difference and in this case it didn't seem to be of concern as the heatsinks stay cold, and only getting warm at higher volumes.
My friend has had no problems and is very happy with the result, I sprayed the board with a PCB cleaning solvent to ensure no further corrosion occurs.
So once again thank you all very much for giving me piece of mind and highly regarded advice in this issue.
Mike
Thank you to everyone for their help.
The substitution worked perfectly, I have had the amplifier on test for a few days on different loads/speakers and am happy to report is has been totally stable.
I checked the predriver for the output stage and they all tested ok, so I soldered a few resistors across the rail fuse holders and tested the DC and AC values on the speaker output with a multimeter and scope and all were very low under no signal conditions.
I also read the datasheets and previous threads which gave some insight about the max frequency difference and in this case it didn't seem to be of concern as the heatsinks stay cold, and only getting warm at higher volumes.
My friend has had no problems and is very happy with the result, I sprayed the board with a PCB cleaning solvent to ensure no further corrosion occurs.
So once again thank you all very much for giving me piece of mind and highly regarded advice in this issue.
Mike
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