Have you checked Q905 and Q909 carefully to confirm that neither is leaking and that both transistors have two intact junctions?
I don't quite know what you mean?
You want me to test each transistor for continuity: like you suggest here?
http://www.bcae1.com/repairbasicsforbcae1/images/npnbipolartransistor00a.gif
As for leakage, how do I go about doing that?
You want me to test each transistor for continuity: like you suggest here?
http://www.bcae1.com/repairbasicsforbcae1/images/npnbipolartransistor00a.gif
As for leakage, how do I go about doing that?
Use the applet, for the type of transistor you have, on the following page.
Basic Amplifier Repair - Transistor Test Applet Link
Basic Amplifier Repair - Transistor Test Applet Link
Ok, I get conductivity every leg to every other leg on both transistors, 905 and 909.
The value changes quickly though, do you want me to attempt to report back with each combination of leg arrangements as the link?
Is the voltage on the emmiter of 909 a problem? I currently only get 1.5v here, where as on the base and collector I get 10v and 63 respectively?
The value changes quickly though, do you want me to attempt to report back with each combination of leg arrangements as the link?
Is the voltage on the emmiter of 909 a problem? I currently only get 1.5v here, where as on the base and collector I get 10v and 63 respectively?
The transistors need to be out of the circuit.
Post screen caps of the readings as instructed on the page/link I posted.
Post screen caps of the readings as instructed on the page/link I posted.
909 replaced. Sinked back up and powered up, sound came from speaker, then the breaker tripped. Cable warm. Felt the FET's, all cold (the 140's and 9140's) However the device on the end (FX50) was hot. The FS70 was not hot however.
I'm considering removing all of the driver part and replacing it. From Q905-912
I'm considering removing all of the driver part and replacing it. From Q905-912
You should have nothing larger than a 15 amp fuse in the B+ line. If an 80 amp breaker tripped, there is a serious problem which could result in the failure of the outputs (some of which are no longer available). If the outputs fail, the amp may not be repairable.
Are you sure that there are no solder bridges?
Are you sure that no transistors are shorted?
Are you sure that there are no solder bridges?
Are you sure that no transistors are shorted?
Ok i'll get a fuse setup. The transistor I installed had no bridges, no. I'll work my way around the board again, looking for a short, i'm pretty sure there are no solder bridges.
Is it best to remove the output parts of the board, so I can work more easily on the input side, without risk of damage to the FET's, Fx/Fs?
Is it best to remove the output parts of the board, so I can work more easily on the input side, without risk of damage to the FET's, Fx/Fs?
I pulled the FX50 (the one getting hot) here's the results:
With dmm on resistance I get; red lead on drain, black pin on source, 5.56Mohm.
Every other combination I get infinty.
I assume this means it's working ok? As it's a P type FET.
Which then leads me to think that it's further back, either the driver part still playing up, or the IRFP's?
With dmm on resistance I get; red lead on drain, black pin on source, 5.56Mohm.
Every other combination I get infinty.
I assume this means it's working ok? As it's a P type FET.
Which then leads me to think that it's further back, either the driver part still playing up, or the IRFP's?
It appears to be OK.
Did you use exact replacement parts?
Are you 100% sure that they're not installed facing the wrong way?
Did you use exact replacement parts?
Are you 100% sure that they're not installed facing the wrong way?
Yup, Exact parts and i'll double/triple check direction as it's a typical thing for me to do... 🙄 
I followed the gate signal backwards (from the fx) and there are no shorts anywhere.
I followed the gate signal backwards (from the fx) and there are no shorts anywhere.
Make a note of the position of the bias pot. If you set it fully counter-clockwise, does it still blow the fuse?
When transistors short, they rarely get hot (they generally make other transistors get hot), are you sure that none of the other output transistors are shorted or leaking?
When transistors short, they rarely get hot (they generally make other transistors get hot), are you sure that none of the other output transistors are shorted or leaking?
Yes, now both the Fx and Fs are warm. (the Fs moreso than the fx now??)
The output fets have no shorts.
The output fets have no shorts.
Yes.
Also I think something's a little funny with the other driver (fs70) so i'm pulling that as we speak...
Also I think something's a little funny with the other driver (fs70) so i'm pulling that as we speak...
Thought so: It's shorted... I think... 😕
I get:
Gate to drain, both ways... 35.8ohm
Drain to source... both way 3.7ohm
Gate to source, again both ways... 32.1ohm
I think thats out of limits?
I get:
Gate to drain, both ways... 35.8ohm
Drain to source... both way 3.7ohm
Gate to source, again both ways... 32.1ohm
I think thats out of limits?
Out of the circuit, gate to either of of the other legs should read infinity. If it reads anything else, it's defective.
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