MTX JH1200

Jump to Latest
#62
No longer true, it is still in protect.

I find a 1K Ohm resistor between Pin #4 and ground that is measuring 1.7K Ohm in circuit.
The collector of Q10, an MMBTA56, is also directly connected to Pin 4 of the 494. I don't see anything else that is directly connected to Pin 4.
 
#63
The LM339 drives the transistor. There are 3 inputs to the LM339 (thermal, high-voltage shutdown and low voltage shutdown). Any of the three inputs can shut the amp down. If the LM339 is defective, that could also prevent the amp from turning on. If you can't determine where the fault is, post the DC voltage on all pins of the LM339.
 
#64
LM339
1: 93.2mV
2: 4.65V
3: 12.78V
4: .623V
5: 3.095V
6: 4.97V
7: 3.094V
8: 4.07V
9: 4.97V
10: 4.97V
11: 8.17V
12: 1.3mV
13: .672V
14: .672V

I take it that Pin #2 is shutting the power supply off and that is because of Pin #4?
 
#68
I have removed the power supply driver board and find that pin #9 attaches to the collector of Q3 (MMBTA06) that is between the LM339 and the themistors. The thermistors appear to be working fine.

I followed pin #6 to a 10 Ohm resistor and then to the collector of Q15 (MMBTA56). I lifted the A56 and it checks out fine. I have not traced out where the base of Q15 goes yet.

If I am having a failure of the thermal circuit, wouldn't the thermal LED be lit instead of the voltage LED?
 
#69
I traced that out further and found a solder joint that needed touched up. I re-installed the Power Supply Driver board and we are powered up, no faults.

I have the rectifiers removed, I will reinstall those and go back to driving a strong 100Hz signal and see if there is anything at the gates of the outputs without the outputs installed now.
 
#70
First of all it was Pin # 6, as you suggested, that I traced back to the defective solder joint that was sending the amp into protection.

I have nothing at the output gates. While I was checking that, a resistor fell off of the driver board.

I think the equivalent of R118, a 1K Ohm resistor on the Sony print, is also a 1K Ohm resistor, R132, on my board and it is getting hot enough to melt the solder which caused it to fall off.
On the Sony print, it is connected between -Rail and the bases of the drivers Q27, Q28, Q29, & Q30 and the collector of Q31.

I measure from main ground, the negative rail, ~-44VDC, on one side of that resistor which is the same as the collector of a MMBTA56 which should be correct. I then measure ~+.850V on the other side which is also the same as the bases of the above MMBTA56 and the PZTA42, my MTX drivers. According to the Sony print I should be expecting ~5V dropped accross that resistor. Neither of those drivers are shorted between any leg and I measure ~0 Ohms between the PZTA42 collector to main ground, which should be correct also.

This now makes a total of three resistors that are getting hot enough to burn and melt the solder and this is without any of the outputs installed.

What should I be looking for next as I can't seem to find defective transistors, but I find higher than expected voltages accross those three resistors?
 
#71
I am not exactly sure what I did, but today none of the resistors are heating up because they are dropping the expected voltages and I have the correct voltages at the various points I was checking from the above post. I also have my 100Hz signal at the high side gates.

I am going to install a pair of the original outputs on each side now.
 
You must log in or register to reply here.
Top Bottom