JVC JA-S71 Protection / Power Supply Troubleshooting Help

[Beavs]

Need some help as the amp won't come out of protection mode and my troubleshooting knowledge has a long way to go.

Got this amp knowing that the left channel was out. Turned out to be the same issue others have had with the wire connection to the volume pot (see post #5 here: https://audiokarma.org/forums/index.php?threads/jvc-ja-s71-right-channel-out.807344/). Solder connection was fine but the left channel wire snapped at the solder joint and was loose (see white wire in pic, should connect to 2nd pin from bottom). Soldered the wire back to the pin and both channels were working now.



Then I went to set the idling current and DC offset per the SM. Idling current was no problem, but the left channel would not go lower than about 200mv. Did some inspection/measuring for components around the trimpot VR601 and turned out that diode D603 was shot. Replaced D603 and DC offset was set per the SM and everything was looking to work fine.

Hooked the amp up and let it play for the day. Then day 2 came and had been playing it for a couple hours when my wife paused the music because of a work meeting. Amp was left on but no input feeding to it. Few hours later I went to start the music again and no sound. Turned the amp off and then back on and the protection relay wouldn't engage. I've been testing components around the relay as well as the power supply and everything seems to be measuring fine as far as caps, resistors and diodes. Haven't pulled any transistors yet. Odd thing that I'm seeing is the voltages on the power supply board.

Voltages seem to be within the ballpark until you get to Pin 9 but the R813 and C805 seem to measure fine...just not getting the expected voltage drop from D803 listed voltage to Pin 9 listed voltag. Image pasted below or PDF attached for measurements I'm seeing. Any help on where to go from here would be appreciated.

 

[Beavs]

I figured there would be no DC offset without the relay engaged, but measured anyway and both channels were 10mv or less. Measured around the protection circuit and it's looking like maybe X627 is suspect? Also had thought D613 was suspect from earlier measurements so I had pulled that diode out and it tested fine so I put it back in. Take a look at these measurements and let me know what you think...the +44 vs the schematic listed at +26.4 is what had me suspecting the power supply, but that could have been a bad assumption on my part.

 

[Beavs]

check dc offset. IF both channels are below .1V then compare the measured voltages to schematic voltages of the protect circuit.

Also, no change in DC offset voltages when adjusting either of the trimpots which seems right without the relay engaged.

 

[Robisme]

Check the DC on the relay contacts. Not the coil.

If that's ok, then maybe the relay driver transistors failed.

Rob

 

[Beavs]

Check the DC on the relay contacts. Not the coil.

If that's ok, then maybe the relay driver transistors failed.

Rob

DC offset at relay contacts were both near 0V and adjustable with the trimmers. I'll definitely pull X627 to check that transistor. Any of the other transistors in the protection circuit area of the schematic (X625 through X630)?

Also, the voltage at pin 9 of the power supply seems way off and feed directly into the protection circuit, but oddly the resistor and capacitor both tested fine:

 

[llwhtt]

The voltage at Pin 9 is high because the relay is NOT energized. The base of X629 is being held low because of the voltage on the collector of X627. If you pull X627 the relay should energize. Now is X627 bad or something turning it on? The voltages on the base and emitter suggest that X627 is bad.

Craig

 

[Beavs]

Alright, well I think I found the issue but this just helps to demonstrate why I have such a hard time diagnosing these types of issues. I pulled and tested all transistors X625 through X630 and all tested functional (though I can't speak to whether these hFE and Vf values would suggest any of them should be replaced):

X625, 2SC1775, hFE 515, Vf 667
X626, 2SC1775, hFE 471, Vf 655
X627, 2SA872, hFE 375, Vf 677
X628, 2SC1775, hFE 545, Vf 666
X629, 2SC1775, hFE 564, Vf 664
X630, 2SC1509, hFE 123, Vf 638

After they all tested to be functional I soldered them back in and the possibility exists that there was a bad solder joint on one of these and that resolved the problem. However, I had already tested caps C622 and C623 in circuit and they tested fine, but I could tell these had been replaced at some point because they were cheap Xicon caps whereas all other electrolytic caps appeared to be original Elna caps, so I removed them while I had the board accessible to replace with Nichicons I had on hand. When I removed the old ones I could see that the negative pad on on C622 had been lifted at some point. I etched some of the coating off of the connecting trace to get some decent contact with the new solder (pic taken after etching the coating off):


After getting the new caps in and powering on, relay engaged and the voltages that had been off before came back in line and it's working fine now. So, could have been a bad solder joint on one of the transistors that wasn't apparent (doubtful as they all appeared fine when pulling the transistors), or it was more likely this lifted/broken pad on C622.

And back to where I started off this post, this is what baffles me when trying to diagnose issues...looking at the overall schematic it is not apparent to me at all as to why a broken trace between C622 and C623 would lead to the voltages I was seeing near pin 9 of the power supply or at X627, X629, and the relay. Any Circuit101 tips on how to understand that better would be great.