JBL GTQ190 idle current

[Mote]

Another quick question.
Today I repaired JBL GTQ190 amplifier with distorted sound/constantly turning on and off.
At first it had distorted sound, then it started to switch itself on and off. With the help of Perrys tutorial I was able to determine that something was driving dead-time pin on TL494CN too high and therefore amp switched off, then dead-time pin got low and amp powered up until dead-time got again too high etc.
I was able to trace the voltage to one of predriver boards opamps output. After removing that board amp powers up fine, one channel where I have outputs in plays nice clean music.
But current draw is a bit higher than on other amps. I cant measure it but my current limiting lamp glows a bit when amp idles. PS FETs dont get hot, outputs do not get hot etc. Maybe its hungry?
It has 4 P50N06 FETs, rectifiers are FEP16BT and FEN16CT. Outputs 4x TIP35C and TIP36C, they are driven by TIP31 and TIP32.

Few pics of the filter boards and predriver boards, just in case someone needs them.

[Perry Babin]

Reinstall the modules and post the DC voltage on the op-amp on the faulty channel.

[Perry Babin]

You may have to remove R12 and Q6 from the defective channel. If you remove Q6, be careful, the amp may draw significant current.

[Mote]

This small idle current draw, which worries me, is not present instantly. At first amp does not draw current but after about 5 seconds of working current draw slowly starts to rise.

Removed R12 and Q6. I managed to break Q6. It says 1 GM on it, what is correct replacement? Google says MMBTA06?

Without them and all other components and modules reinstalled it does not draw more current than before.

Voltages on opamp:
01:11,7
02:7,6
03:6,3
04:-13,3
05:11,0
06:13,0
07:13,7
08:11,6

2 and 3 are inputs for this opamp, 6 is output.
Measurements are taken without input signal. Other 3 opamps have 1,5V on their output.

3 channels play clean audio. 1 ,to which I installed this last module, does not play audio. When speaker is connected amp draws maximum amount of current the lamp lets it to.

Please dont tell me that I am wrong and it is not the opamp .
After not being able to fix Hifonics Nemesis and Intersonic Maxound MX 220 I need to have some success.

[Perry Babin]

Yes. MMBTA06.

It appears that you have rail voltage on the output of that channel. Is that correct?

[Mote]

I had 13V on output of defective channel.
Output transistors had 13V on every leg. So I pulled them and amp had still 13V on the speaker terminals of defective channel. Pulled drivers, TIP31 and TIP32, those were OK. Traced that 13V back to sense pin of the predriver module. Replaced opamp which I thought was defective, reinstalled drivers and outputs and I have 0V on speaker terminal, 0,2V, 13V and 0V on legs of output transistors. Reinstalled Q6 and R12 to the protection circuit, it does not trigger anymore.

Idle current is the same which it used to. Put new fuse in my multimeter (I have a short circuit in my brain when it comes to measuring current. I know how to do it but I end up blowing the fuse in multimeter every time I do it).

Idle current is 0.76A. Other amps I have here draw about 0.25A.

One thing that I noticed is that after switching off, I hear a click from the amp. Click like something discharging fast. Could it be that one of the rail caps is defective?

[Perry Babin]

The sense line is the feedback from the output so it will have votlage relative to the output voltage. With the driver board out of the circuit, the voltage on the sense line should be the same as the votlage on the output.

0.76 isn't excessive.

To measure current, use a shunt resistor. The resistor would be connected in place of your lamp. A 0.1 ohm resistor works well because quick calculations are possible but any value can be used. The current through the resistor is calculated by the ohm's law formula I=V/R. If the voltage across the 0.1 ohm resistor is 0.076v, the current is 0.76 amps. The resistor needs to be rated to handle the power dissipation and is really only practical for low current testing unless you have a large resistor.

I don't know what the noise is.

[Mote]

Tested the amp, everything is working fine, ran it about half an hour at full volume. Did not even get hot.

Thanks for the shunt resistor tip.