Roland JC-120 (Jazz Chorus) Pinpointing Short

[TimeBandit]

This JC-120 came to me with both outputs blown in one channel. Everything on the amp board "seemed" to test ok in circuit when compared to the good board, so I replaced the drivers 2SC1624 and 2SA814 with MJE15032 and 15033 and the outputs with MJ15024s in both channels. Powered up with a dim bulb tester while checking the bias voltage across one 0.3 ohm emitter resistor in each channel, and noted that the bias was rising very fast in the "bad" channel.

So I shut it down, rechecked the board and found that two of the 100 ohm 1/4W fusible resistors were measuring around 45 ohms each out of circuit. I pulled them and put in two new 100 ohm resistors and tried powering up again. The bias in the bad channel measures around 50mV, even with the current being limited by a 40W bulb. I also observed that the replacement 100 ohm resistor get very hot fast, when viewed through a thermal camera.

I took a break from it for the night, but will take another look in the a.m. If anyone has any suggestions, I'm all ears. Thanks.

[J M Fahey]

This JC-120 came to me with both outputs blown in one channel. Everything on the amp board "seemed" to test ok in circuit when compared to the good board, so I replaced the drivers 2SC1624 and 2SA814 with MJE15032 and 15033

Recheck pinouts match.
Very often Japanese and American do not.

and the outputs with MJ15024s in both channels. Powered up with a dim bulb tester while checking the bias voltage across one 0.3 ohm emitter resistor in each channel, and noted that the bias was rising very fast in the "bad" channel.

So I shut it down, rechecked the board and found that two of the 100 ohm 1/4W fusible resistors were measuring around 45 ohms each out of circuit. I pulled them and put in two new 100 ohm resistors and tried powering up again. The bias in the bad channel measures around 50mV, even with the current being limited by a 40W bulb.

Not too worried about that, if the only symptom.

I also observed that the replacement 100 ohm resistor get very hot fast, when viewed through a thermal camera.

That is something else.
They are across opwer transistor BE junctions, so should never get more than 1V across them, if that much, so no significant heating.

With NO speaker load, please measure voltage across:

*R52 and R53

* R48 and R45

* DCV to ground at speaker out.

* DCV from Q10 base to Q11 base or across Q6 CE (same thing) which is bias voltage.

[TimeBandit]

Ok, I powered it up on the DBT (40W bulb) to take the measurements, but R45 and R48 (hard to tell which for certain) hit 180F after less than a minute. So the following were taken with outputs pulled and quickly:

*R52 and R53
40mV and 70mV

* R48 and R45
13V and .003V

* DCV to ground at speaker out.
8.3V

* DCV from Q10 base to Q11 base or across Q6 CE (same thing) which is bias voltage.
1.5V

I also double checked the pinouts for the new drivers and outputs against the originals. All ok.

[g1]

How can R52 and R53 have any voltage across them if outputs are pulled?

[TimeBandit]

Duh, yep. Those two measurements were from the first power up with the outputs still installed.