JBL PRX635 Schematics?

[Yeoman]

Hello,
I have an atypical problem with a JBL PRX635 amplifier.
The HF amplifier seems to be oscillating causing the unit to heat
up. The usual scenario with these units is replace shorted output mosfet devices. So I decided to change out the mosfets on the HF amp on the outside chance that it might cure the current draw, no change. If anyone out there has the schematics to share or has seen a similar problem with these class D amps please let me know.
Thanks

[Andrew F. Ali]

Re: JBL PRX635 Schematics?

Might be one of the smaller pre-amp transistors/ICs causing the current draw. Look for an open copper trace, too.

[Khron]

Re: JBL PRX635 Schematics?

Would the last result from among these be what you're looking for?

http://elektrotanya.com/?q=showresul...oria=&kat2=all

[budm]

Re: JBL PRX635 Schematics?

Would the last result from among these be what you're looking for?

http://elektrotanya.com/?q=showresul...oria=&kat2=all

It does not show any Schematic.
It is the same technical manual at JBL.

[Longbow]

Re: JBL PRX635 Schematics?

Hello,
I have an atypical problem with a JBL PRX635 amplifier.
The HF amplifier seems to be oscillating causing the unit to heat
up.

Well, is the audio output oscillating? At what frequency? Class D amplifiers "oscillate" at 100KHz or higher.

The usual scenario with these units is replace shorted output mosfet devices.

This is an expensive way to troubleshoot. Besides, shorted FET's can be instantly confirmed with an ohm meter.

Something that would cause heavy current draw would be a faulty PWM drive IC. This chip also makes sure that the output devices are not turned on simultaneously. One or more chokes are required at the output. They are heavier than most other parts and might have become loose on the board.

[Khron]

Re: JBL PRX635 Schematics?

I'm starting to wonder just how different the electronics from the PRX635 are, from the ones in the PRX500 series - i DO have the service manual & schematics for those. The box in which the electronics are housed looks very similar...

[Longbow]

Re: JBL PRX635 Schematics?

Great, let's have a look at the output stage.

[Khron]

Re: JBL PRX635 Schematics?

Here are the schematics i have - the main board (power supply & power outputs) and the modulator daughterboard.

[Longbow]

Re: JBL PRX635 Schematics?

Now that's a serious Class D design! Not something you can slap together on your kitchen table.

[Longbow]

Re: JBL PRX635 Schematics?

First, assuming the failed amplifier uses the same full bridge output, which of the 4 output transistors are hot? All of them? After confirming correct rail voltages, compare signals at U165 pin 2 and pin 4 (scope channel 1 and channel 2) with no input. Adequate dead time? Anything look fishy? Next, have a close look at the 2 pairs of level shifter/driver transistors, especially if only one side of the bridge is hot.

[Yeoman]

Re: JBL PRX635 Schematics?

Hello,
Walked away from this one for a bit. Thanks for the replies! Same amp as the prx500 series. The HF amp actually delivers audio to the compression driver however with no input signal and the speaker load removed r183 and r184 quickly get very hot. With a load connected the amp gradually heats up to the point of thermal shut-down.

Thanks Longbow I will take a close look at U165 and the related driver transistors. I suppose that if the mosfets are not switching correctly it could explain what I am seeing.

[Yeoman]

Re: JBL PRX635 Schematics?

Ok I seem to have solved it. C180 and C181 on this PRX635 unit are 0.22uF instead of 0.47uF as indicated on PRX500 schematic and also as are in place on the MF amp and LF amp in unit in question. I thought that this might explain the 52.9 kilohertz at 2.25 Volts PP that was present at TP150 (modulator schematic) with no input signal and no load. When I tacked .3uF across existing C180 and C181 the oscillation disappeared. Does not appear to roll off high end significantly with the extra capacitance.