Thanks very much for your help so far.
One .22 ohm resistor was not conductive (oops), and the other one had a high resistance. I replaced both of these.
Powering the amp on again caused the same kind of damage that originally happened.
It damaged the two 100ohm resistors (r152 and r154) and made c120 very hot. I have more 100ohm resistors so it is not a big problem.
The protect light was on during this.
Any clues?
One .22 ohm resistor was not conductive (oops), and the other one had a high resistance. I replaced both of these.
Powering the amp on again caused the same kind of damage that originally happened.
It damaged the two 100ohm resistors (r152 and r154) and made c120 very hot. I have more 100ohm resistors so it is not a big problem.
The protect light was on during this.
Any clues?
Hi miekedmr,
Okay. treat this as an entirely new repair. Replace Q122, C120, the resistors and the drivers and ouputs. Measure all the other remaining resistors even if they look great. Ensure continuity between the driver bases and either the E or C of the bias transistor (for broken traces).
Check Q118 and Q120 while you are at it. If they are defective then remove and test all the other transistors. Test VR102 as well.
When you power this up again, use a variac or lightbulb in series trick. Allow no more than 1/2 amp to flow to start. Have your meter set to measure bias current as you power up.
-Chris
Okay. treat this as an entirely new repair. Replace Q122, C120, the resistors and the drivers and ouputs. Measure all the other remaining resistors even if they look great. Ensure continuity between the driver bases and either the E or C of the bias transistor (for broken traces).
Check Q118 and Q120 while you are at it. If they are defective then remove and test all the other transistors. Test VR102 as well.
When you power this up again, use a variac or lightbulb in series trick. Allow no more than 1/2 amp to flow to start. Have your meter set to measure bias current as you power up.
-Chris
does it sound right that everything was working again except for the bias transistor, and that caused everything to fry again?
so far, the outputs are dead, the bias was dead, which i never replaced the first time, the drivers are fine, q118 and q120 are fine, c120 is dead, both 100ohm resistors are dead, and so are both .22ohms
I was foolish to try to avoid doing things the right way the first time, and now I am back to the start. I will do it right this time and take all the parts out of the board to test them and put something in series like you suggested when I power it up.
so far, the outputs are dead, the bias was dead, which i never replaced the first time, the drivers are fine, q118 and q120 are fine, c120 is dead, both 100ohm resistors are dead, and so are both .22ohms
I was foolish to try to avoid doing things the right way the first time, and now I am back to the start. I will do it right this time and take all the parts out of the board to test them and put something in series like you suggested when I power it up.
Its certainly possible for a bad bias transistor to kill the outputs/drivers etc.
Cheers!
edit: to clarify - the bias transistor acts to partially turn OFF the driver/output transistors. If the bias transistor is bad, the drivers/outputs turn fully ON, creating a shorted path from power supply + to power supply -. The result (usuallly) is the output trannies (and all components in-between, such as the low-valued emitter balancing resistors) go 'poof'.
Cheers!
edit: to clarify - the bias transistor acts to partially turn OFF the driver/output transistors. If the bias transistor is bad, the drivers/outputs turn fully ON, creating a shorted path from power supply + to power supply -. The result (usuallly) is the output trannies (and all components in-between, such as the low-valued emitter balancing resistors) go 'poof'.
just ask Variac. He's a "Respectable Social Guy" even though he's called "Variac the Destroyer".
Thank you.
I replaced the parts mentioned. The protect light is off when it turns on. The right channel is not drawing a lot of current --nothing is burning up. There is no output, however. Nothing happens with input and a speaker hooked up. I noticed a voltage drop of .02V across the outputs which is not on the working (left) channel. I'm testing other things now.
I know the protect circuit works because I can set it off on either channel with a short on the output.
I replaced the parts mentioned. The protect light is off when it turns on. The right channel is not drawing a lot of current --nothing is burning up. There is no output, however. Nothing happens with input and a speaker hooked up. I noticed a voltage drop of .02V across the outputs which is not on the working (left) channel. I'm testing other things now.
I know the protect circuit works because I can set it off on either channel with a short on the output.
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