Greetings,
Trying to get this amp up and running after stupidly inserting one of the output ribbon connectors off by one pin (no locating mechanism on the header). This exploded the amp with energizing - D'OH!! (This was over 10 years ago).
I did this after a cream-puff fix for thermal distortion happening after warmup. The pre-drive transistors Qx17 and Qx19 were visually drifting off their heatsink (chassis, I believe). So, I removed them and re-fixtured the hold-down tabs for positive contact pressure - piece of cake, until plugging in said connector.
I've already replaced about a dozen parts, the +/- 15V bridge diodes bought it the worst, along with their traces. Only one of the HS NPN transistors was damaged (replaced).
Amazingly, the front-end opamps are working, and I can get the output to translate voltage to the speaker terminals, but as soon as a load is connected, the positive rail is truncated; does not seem (sound) to be drawing current. I've been pulling my hair out comparing it to the 'good' channel, but cannot locate a smoking gun (poor choice of analogy).
The worst part is having to work with ballpark schematics. The MA-600/1200 docs are quite a bit off in the Last Voltage Amp stage, as I found another closer to what I have: "MR MAIN". One part I cannot find, and is present on either schematic, are the LM339 comparators, and there are a handful of them, supposedly.
I've been reading other very helpful posts on bias voltages, and such, but still up against a wall.
Any help would be greatly appreciated. I'm a Design Eng Tech with 40yrs exp; 7 were at a music store selling Crown amps.
Trying to get this amp up and running after stupidly inserting one of the output ribbon connectors off by one pin (no locating mechanism on the header). This exploded the amp with energizing - D'OH!! (This was over 10 years ago).
I did this after a cream-puff fix for thermal distortion happening after warmup. The pre-drive transistors Qx17 and Qx19 were visually drifting off their heatsink (chassis, I believe). So, I removed them and re-fixtured the hold-down tabs for positive contact pressure - piece of cake, until plugging in said connector.
I've already replaced about a dozen parts, the +/- 15V bridge diodes bought it the worst, along with their traces. Only one of the HS NPN transistors was damaged (replaced).
Amazingly, the front-end opamps are working, and I can get the output to translate voltage to the speaker terminals, but as soon as a load is connected, the positive rail is truncated; does not seem (sound) to be drawing current. I've been pulling my hair out comparing it to the 'good' channel, but cannot locate a smoking gun (poor choice of analogy).
The worst part is having to work with ballpark schematics. The MA-600/1200 docs are quite a bit off in the Last Voltage Amp stage, as I found another closer to what I have: "MR MAIN". One part I cannot find, and is present on either schematic, are the LM339 comparators, and there are a handful of them, supposedly.
I've been reading other very helpful posts on bias voltages, and such, but still up against a wall.
Any help would be greatly appreciated. I'm a Design Eng Tech with 40yrs exp; 7 were at a music store selling Crown amps.
Wrong/reverse voltages usually mess up electrolytics and transistors so I would check them first.
Dont stop at first fault as sometimes one component going can take others with it.
Dont stop at first fault as sometimes one component going can take others with it.
Newark has LM339AN (dip package with legs) stock in UK. Digikey has them in the USA.
@nigelwright7557; Thanks; yeah, completely understand. I've checked everything static and energized, and compared to the 'good' channel. I ordered some replacement transistors, but would rather discover the 'ah-hah' moment for what has failed in-circuit, instead of blind replacement. The biasing is 'off', so I'm still down that rabbit hole.
@indianajo; Hello, I meant I can't find them on the circuit board they're supposed to be operating. There's nothing on either schematic to indicate they are on a board other than the Main.
@indianajo; Hello, I meant I can't find them on the circuit board they're supposed to be operating. There's nothing on either schematic to indicate they are on a board other than the Main.