JVC RX-450 silent

[Rich Brown]

Hello. I have a JVC RX-450 receiver that I got used but in generally good condition. Recently during a thunderstorm it emitted a loud snap (from the circuitry, not the speakers) and went silent. Nothing else in the house was affected, including my computer, which is plugged into the same circuit.

The front panel still lights up, and all indicator lamps including volume and balance work, but there's no sound from any input. I've tested everything with another receiver and all components including speakers are OK.

I'm now digging into the circuitry and have yet to find any obvious problem, There's heat discoloration on the case above the power transformer, but it didn't smell fresh when I first opened it up. I'm also unable to find any fuses in or around the output stage. I don't have a good schematic. Any help would be welcome.

 

[dave9]

Probably a power surge fried the rectifier diode(s) right after the transformer, or the output transistor(s) right before the output terminals, on the large heatsink. If there's only one bridge rectifier circuit for both the audio and the logic and display, then it's probably not the diodes.

 

[Rich Brown]

OK--there's a place to start. Thanks.

 

[73's de Edd]

Sir Rich Brown . . . . .

You have one of your low voltage supplies working with what functions you have.
Check the markup below and locate the designated transistors and see if the
2SD716's have ~ +45 VDC on their collectors when powered up
and the 2SB686's should have ~ - 45VDC on their collectors when powered up.
Meter negative is being connected to metal chassis ground.



73's de Edd
.....

 

[Rich Brown]

Sir Rich Brown . . . . .

You have one of your low voltage supplies working with what functions you have.
Check the markup below and locate the designated transistors and see if the
2SD716's have ~ +45 VDC on their collectors when powered up
and the 2SB686's should have ~ - 45VDC on their collectors when powered up.
Meter negative is being connected to metal chassis ground.

View attachment 41409

73's de Edd
.....

Thanks very much. Right now I'm recovering from ankle surgery and in a cast, and can't get to my workshop. I'm getting a temporary workspace set up and will get back to you as soon as I'm able to check this out.

 

[Rich Brown]

The mystery of my silent JVC RX-450 continues. The sound I heard from it was almost certainly created by an arc, but I can find no apparent damage--not even a scorch mark. If it was a power surge it didn't enter via the power cable. Nothing else plugged into that circuit was affected (well, my computer monitor blinked), and the fuse in the incoming power line is intact.

I'm beginning to wonder about eddy currents. Thoughts?

 

[dave9]

You have not stated anything that indicates to me that if there was a power surge that it didn't enter via the power cable. Everything has its own tolerance to voltage spikes, not frying other components is good, not evidence. Further a fuse may not blow before a transistor or diode fails open circuit, as it normally would from surge damage.

However, this is the wrong way to tackle this, theorizing and wondering about eddy currents. Consider that it is not a project which could have initial design flaws, that instead it was a working product for quite a while until the failure. Identify and replace the faulty component(s).

You need to whip out the multimeter and start tracing the relevant subcircuits. As 73's mentioned, you can measure whether there is moderate (45VDC was mentioned) voltage going to the output transistors. If not, trace backwards towards the diodes and transformer. You can check the lower voltage wires going to the other signal circuitry. You can pick an audio signal source (having live audio playing) and measure for low AC voltage all along the signal path, and measure for drive voltage at the base of the two output transistors 73's mentioned, measure for signal at any opamps if present.

There are several logical ways to tackle this. Start at the mains AC input and trace (probe) forward, start at the audio input and trace, start at the audio output and trace backwards, or pick a spot in the middle and based on whether expected power or signal has made it that far, trace forward or backward to where it stops.

 

[Rich Brown]

You have not stated anything that indicates to me that if there was a power surge that it didn't enter via the power cable. Everything has its own tolerance to voltage spikes, not frying other components is good, not evidence. Further a fuse may not blow before a transistor or diode fails open circuit, as it normally would from surge damage.

However, this is the wrong way to tackle this, theorizing and wondering about eddy currents. Consider that it is not a project which could have initial design flaws, that instead it was a working product for quite a while until the failure. Identify and replace the faulty component(s).

You need to whip out the multimeter and start tracing the relevant subcircuits. As 73's mentioned, you can measure whether there is moderate (45VDC was mentioned) voltage going to the output transistors. If not, trace backwards towards the diodes and transformer. You can check the lower voltage wires going to the other signal circuitry. You can pick an audio signal source (having live audio playing) and measure for low AC voltage all along the signal path, and measure for drive voltage at the base of the two output transistors 73's mentioned, measure for signal at any opamps if present.

There are several logical ways to tackle this. Start at the mains AC input and trace (probe) forward, start at the audio input and trace, start at the audio output and trace backwards, or pick a spot in the middle and based on whether expected power or signal has made it that far, trace forward or backward to where it stops.

Thanks for the detailed and practical advice. I've had to accept the fact that I can't do much until I can get out of this leg cast and back to my workshop. But I do now have a schematic, and I can start tracing out the paths you mention. I have checked the output transistors (powered down) and got some odd readings, but I'm taking them with a grain of salt. Thanks again.