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Hi,
A friend of mine brang me that amp because the fuse kept blowing a few seconds after switching the amp on.
The schematic is here https://drive.google.com/open?id=0B6...m9DXzAzdWhXWGM
I have disconnected the supply of the amp board to that made me conclude the issue is on that board (and not on the PSU). Visually, nothing looked burnt or damaged so far.
I the disconneted all 8 power transistors to check for continuity or shorts, they all looked fine.
Then I had this "brilliant idea" : why not power the amp board with the power transistors out to see if the fuse still blows ? By doing that I also thought it wasn't necessary to connect the signal connector, not seing that it provides a different ground to the board (both grounds are actually linked through a 10R resistor). Power on... BAM ! Q310, the lower driver transistor, also connected to the heatsink, explodes ! I imediately switched the power OFF, and notice that R304 also burnt.
Looks like it's time to recognize that this one is (at least) a little bit too complicated for my own skills and it's time to get some help to understand the details of that schematic, both for my own education and to avoid doing other mistakes....
What I can say so far :
- Q301A/B, along with the current source/mirror Q302/303, form a differential amp. It takes the input signal and applies some NFB through R318 from the power output
- Q306/307 forms another current mirror/source, I don't get what it's used for
- Q308 and the surrounding components form the push/pull biasing network, seems temperature compensated by TH302
- IC301 protects the speaker and controls the output relay
- Q311 and Q312 look like they stabilise the biasing, though I don't understand how very well. Q311 can only draw DC through Q305, and Q312 through Q307. Their interaction is mysterious though
- The part I understand the least is Q304/Q305 : the arrangement reminds me of a diff amp but I don't get what are the signals involved : why is the collector of Q305 connected to the driver's base (Q309) ? What's the role of this arrangement ?
Thanx in advance !
Best regards.
Eric
A friend of mine brang me that amp because the fuse kept blowing a few seconds after switching the amp on.
The schematic is here https://drive.google.com/open?id=0B6...m9DXzAzdWhXWGM
I have disconnected the supply of the amp board to that made me conclude the issue is on that board (and not on the PSU). Visually, nothing looked burnt or damaged so far.
I the disconneted all 8 power transistors to check for continuity or shorts, they all looked fine.
Then I had this "brilliant idea" : why not power the amp board with the power transistors out to see if the fuse still blows ? By doing that I also thought it wasn't necessary to connect the signal connector, not seing that it provides a different ground to the board (both grounds are actually linked through a 10R resistor). Power on... BAM ! Q310, the lower driver transistor, also connected to the heatsink, explodes ! I imediately switched the power OFF, and notice that R304 also burnt.
Looks like it's time to recognize that this one is (at least) a little bit too complicated for my own skills and it's time to get some help to understand the details of that schematic, both for my own education and to avoid doing other mistakes....
What I can say so far :
- Q301A/B, along with the current source/mirror Q302/303, form a differential amp. It takes the input signal and applies some NFB through R318 from the power output
- Q306/307 forms another current mirror/source, I don't get what it's used for
- Q308 and the surrounding components form the push/pull biasing network, seems temperature compensated by TH302
- IC301 protects the speaker and controls the output relay
- Q311 and Q312 look like they stabilise the biasing, though I don't understand how very well. Q311 can only draw DC through Q305, and Q312 through Q307. Their interaction is mysterious though
- The part I understand the least is Q304/Q305 : the arrangement reminds me of a diff amp but I don't get what are the signals involved : why is the collector of Q305 connected to the driver's base (Q309) ? What's the role of this arrangement ?
Thanx in advance !
Best regards.
Eric