I've got this amp on the bench and it wont turn on properly. When powered up, both the power and protect LEDs are lit. There is no voltage at the rail, and none of the larger transistors appear to be out of tollerance.
TL494 pin volts
Pin 1: 2.770
Pin 2: 4.34
Pin 3: 4.61
Pin 4: 0.019
Pin 5: 1.558
Pin 6: 3.481
Pin 7: 0.004
Pin 8: 13.73
Pin 9: 0.004
Pin 10: 0.004
Pin 11: 13.73
Pin 12: 12.83
Pin 13: 5.02
Pin 14: 5.02
Pin 15: 5.02
Pin 16: 8.96
TL494 pin volts
Pin 1: 2.770
Pin 2: 4.34
Pin 3: 4.61
Pin 4: 0.019
Pin 5: 1.558
Pin 6: 3.481
Pin 7: 0.004
Pin 8: 13.73
Pin 9: 0.004
Pin 10: 0.004
Pin 11: 13.73
Pin 12: 12.83
Pin 13: 5.02
Pin 14: 5.02
Pin 15: 5.02
Pin 16: 8.96
Q4's top right leg is leading to a series of diodes in the middle of the audio section. From here, these diodes are leading to 4 x 2690 transistors. Pictured, these Q109, Q209, Q309 & Q409 . All the diodes and transistors check out OK.
The top-left leg of Q4 leads to the Power and Protection LEDs, and also up through the external board (w/ the RCAs) but only over to the remote port. I'm guessing thats the LEDs for a remote control.
-More to come-
The top-left leg of Q4 leads to the Power and Protection LEDs, and also up through the external board (w/ the RCAs) but only over to the remote port. I'm guessing thats the LEDs for a remote control.
-More to come-
Lift the top 2 legs of Q4. Does it still go into protect?
If not, do all channels produce clean audio with no significant DC offset?
Be careful. If the circuit is connected as it appears, this will disable the overcurrent protection.
If it draws excessive current, power it up through a current limiting resistor (all transistors clamped down) and measure the DC voltage across each of the emitter resistors to see which channel is causing the excessive current draw. If you can't get to the terminals of the emitter resistors measure between the emitters of the transistors in each individual channel.
If not, do all channels produce clean audio with no significant DC offset?
Be careful. If the circuit is connected as it appears, this will disable the overcurrent protection.
If it draws excessive current, power it up through a current limiting resistor (all transistors clamped down) and measure the DC voltage across each of the emitter resistors to see which channel is causing the excessive current draw. If you can't get to the terminals of the emitter resistors measure between the emitters of the transistors in each individual channel.
Q4 is back in and conencted. I mis-read the previous transsitors; Q109, Q209, Q309 & Q409. These are actually not hooked into the center diodes. Instead, its Q116, Q216, Q316, & Q416 which are 3189 transisfors. Connection is from the diodes through a 10K to the center legs of 3198s.
I'll pull them one at a time and check everything shortly.
I'll pull them one at a time and check everything shortly.
The transistors that you're looking for likely have their emitter connected to one leg of the emitter resistor. The base likely connects to the other side of the emitter resistor via a resistor. The collectors of these transistors should be connected together via diodes and possibly resistors. The diodes would go to a transistor (like Q4) that would shut the amp down when an overcurrent fault is detected. Are these the transistors Qx16? If not, look for transistors connected as described.
You may want to read the 'Protection Circuit Troubleshooting page' if you haven't done so already.
You may want to read the 'Protection Circuit Troubleshooting page' if you haven't done so already.
Yes Q4 seemed to test OK when I had two of it's legs lifted.
Seems theres a bunch of components attached to Q4's top right leg including Q3 center, R12, and R13. Lifting Q3 doesnt help the amp.
R13 leads to Q2. Taking Q2 off the board allows the amp to power up. Q2 testing fine on the bench.
Do you have a schematic for this board?
I'll trace Q2 back to where it connects through.
Seems theres a bunch of components attached to Q4's top right leg including Q3 center, R12, and R13. Lifting Q3 doesnt help the amp.
R13 leads to Q2. Taking Q2 off the board allows the amp to power up. Q2 testing fine on the bench.
Do you have a schematic for this board?
I'll trace Q2 back to where it connects through.
Q2's left leg leads to 4 x 47k reisstors. These resistors (Left in photo Rx64) are attached to speaker terminal outputs.
With Q2 removed and the amp powered, it's left leg reads ~6vDC. Center leg reads ~12vDC, and right leg reads 0vDC.
I'm thinking this is the DC offset protection circuit. Powered, should all speaker terminals have +13.7vDC on them referencing ground? Same from center tap.
With Q2 removed and the amp powered, it's left leg reads ~6vDC. Center leg reads ~12vDC, and right leg reads 0vDC.
I'm thinking this is the DC offset protection circuit. Powered, should all speaker terminals have +13.7vDC on them referencing ground? Same from center tap.
Voltage is less than 0.020 across speaker terminals.
Secondary ground I think if the right side of R60 and R61 (Pictured above, top left of rail caps). If thats the case, then R60 shows 0.050vDC referencing amp ground, and R61 is showing ~27vDC.
I checked the center legs of the rectifiers and D43 has +45vDC while D42 has -17vDC. I verified these same voltages across the output transistor rails.
So, the problem looks like rail voltage is offset.
Checking the power supply, the gate resistors have a clean +10v sqaure wave on them. The drain has a +25v clean square wave. Seems like everything after the transformer has a positive DC offset.
Is this amp reversed from what I know? Basically the above rail voltages show +-31vDC if I reference the amp's positive terminal... That is just wrong.
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