This 2nd gen 225HCCA is stumping me for a moment, but I may figure it out soon enough.
Both channels muting unless I remove the 2N5639 JFet for that channel. Then the channel unmutes and will play just fine. Re-installed.
100k resistors nearby are good. I traced back to the PS side, where there is a 1m resistor and another 2n5639. Pulled that only but the amp still stays muted. From that 2n5639 goes back to an optocoupler.
Cant seem to figure this one out otherwise. Maybe the 2n5639 are bad at the channels? Measuring 26 ohms between drain/source seems a bit too low for this muting jfet. Otherwise diode checks OK.
Thank you
Both channels muting unless I remove the 2N5639 JFet for that channel. Then the channel unmutes and will play just fine. Re-installed.
100k resistors nearby are good. I traced back to the PS side, where there is a 1m resistor and another 2n5639. Pulled that only but the amp still stays muted. From that 2n5639 goes back to an optocoupler.
Cant seem to figure this one out otherwise. Maybe the 2n5639 are bad at the channels? Measuring 26 ohms between drain/source seems a bit too low for this muting jfet. Otherwise diode checks OK.
Thank you
Using the secondary ground as a reference, what's the DC voltage on the gate of all 3 of the muting jfets?
What's the DC voltage on the terminals of the muting optocoupler.
What's the DC voltage on the terminals of the muting optocoupler.
Im pretty sure the Muting MC20 optocoupler may have failed. Is K3N26 a good sub?
L&R JFets
1. 0.016
2. 0.016
3. 0.639
Middle Jet
1. ~6vDC Floats a bit
2. 0.016
3. -21.76
MC20
1. 1.232
2. 0.006
3. 0
4. ~5.5v Floats
5. ~5.5v Floats
6. ~6.0v Floats
L&R JFets
1. 0.016
2. 0.016
3. 0.639
Middle Jet
1. ~6vDC Floats a bit
2. 0.016
3. -21.76
MC20
1. 1.232
2. 0.006
3. 0
4. ~5.5v Floats
5. ~5.5v Floats
6. ~6.0v Floats
There is a high value resistor (221k?) connected to pin 1 if the 'middle jfet'. Confirm that it's within tolerance.
The optocoupler has the same voltage on pins 4 and 5 so it may be OK.
The optocoupler has the same voltage on pins 4 and 5 so it may be OK.
There is a capacitor that is connected to the 221k resistor, if I'm not mistaken. It may be leaky (physically and/or electrically).
Something is preventing that cap from being charged. You'll have to look at every point in that circuit to see where the problem is.
Desolder the cap-end of the 221k resistor to see if it's going down to -21v.
Desolder the cap-end of the 221k resistor to see if it's going down to -21v.
I believe I found the issue. Small cap just under the orange wire. Though it measures OK out of circuit, pulling it allowed the amp to power up fully. Its the cap next to the 1M resistor in the attached photo.
Each time you power the amp up, from now until you finish the repair, confirm that the voltage is charging to the same (-21?) each time.