JVC A-X77 stuck in protection - UK Vintage Radio Repair and Restoration Discussion Forum

starone · 17th Oct 2020, 6:23 pm

I've got a JVC A-X77 that I use as my main amp that is not coming out of protection on start up. It was fine last week until I switched it off then the following day it just wouldn't work. I've got some basic skills but feel this may be a step too far without some guidance. I downloaded the service data and checked the voltages on the outputs, which appear fine. I've located the protection circuit and checked some voltages on the IC and found some discrepancies with the service data, particularly on a pin that should have 1.1 volts on it but has 50 volts. Unfortunately I don't know where to go from here, or how to track the fault down. I was considering taking it somewhere but repair shops for this kind of gear are thin on the ground locally. So was wondering if anyone can suggest the best way to proceed.

Thanks
Martin

paulsherwin · 17th Oct 2020, 7:20 pm

I think you've identified why the protection chip is shutting everything down. Presumably you have lost an output transistor, which has then produced the usual chain of cascading failures.

It's a pity, as these JVC 'Super A' amps are nice designs, being somewhat related to the Quad 405 of a decade earlier.

starone · 19th Oct 2020, 7:00 pm

Thanks for your reply Paul.

I've checked the outputs again and I'm pretty sure they're OK. My reasoning for this is that non of them test shorted in circuit and the voltages seem pretty close to those on the service data. I've got an A-X5 that I know has a shorted channel - both output transistors on that channel test short base to collector and both have rail voltage (50 volts) on all six pins. The A-X77 voltages are as follows:

Right

SA91215: B, 0.058v, C, 52v, E 0.035v
SC2922: B, 0.55v, C 52v, E, 0.05v

Left

SC2921: B, 0.60v, C, 52v, E, 0.02v
SA1215: B, 0.60v, C, 52v, E, 0.02v

I may be barking up the wrong tree but I've started to check the voltages on the transistors going back into the circuit. So far OK. The collector voltages though are about 3v lower than it states on the diagram. Not sure if this means anything?

Regards
Martin

kevinaston1 · 19th Oct 2020, 7:34 pm

This is not making sense

If the 50V n pin 1 of the protection circuit is not coming from the output, you need to make more measurements around this IC to find what is supplying 50V to pin 1

buggies · 19th Oct 2020, 9:42 pm

Quote:

Originally Posted by starone

Right

SA91215: B, 0.058v, C, 52v, E 0.035v

I don't know the circuit but should that base voltage not be 0.58?

17th Oct 2020, 6:23 pm	#1
starone Triode Join Date: Feb 2016 Location: Nottingham, Notts. UK. Posts: 43	JVC A-X77 stuck in protection I've got a JVC A-X77 that I use as my main amp that is not coming out of protection on start up. It was fine last week until I switched it off then the following day it just wouldn't work. I've got some basic skills but feel this may be a step too far without some guidance. I downloaded the service data and checked the voltages on the outputs, which appear fine. I've located the protection circuit and checked some voltages on the IC and found some discrepancies with the service data, particularly on a pin that should have 1.1 volts on it but has 50 volts. Unfortunately I don't know where to go from here, or how to track the fault down. I was considering taking it somewhere but repair shops for this kind of gear are thin on the ground locally. So was wondering if anyone can suggest the best way to proceed. Thanks Martin

17th Oct 2020, 7:20 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,967	Re: JVC A-X77 stuck in protection I think you've identified why the protection chip is shutting everything down. Presumably you have lost an output transistor, which has then produced the usual chain of cascading failures. It's a pity, as these JVC 'Super A' amps are nice designs, being somewhat related to the Quad 405 of a decade earlier.

19th Oct 2020, 7:00 pm	#3
starone Triode Join Date: Feb 2016 Location: Nottingham, Notts. UK. Posts: 43	Re: JVC A-X77 stuck in protection Thanks for your reply Paul. I've checked the outputs again and I'm pretty sure they're OK. My reasoning for this is that non of them test shorted in circuit and the voltages seem pretty close to those on the service data. I've got an A-X5 that I know has a shorted channel - both output transistors on that channel test short base to collector and both have rail voltage (50 volts) on all six pins. The A-X77 voltages are as follows: Right SA91215: B, 0.058v, C, 52v, E 0.035v SC2922: B, 0.55v, C 52v, E, 0.05v Left SC2921: B, 0.60v, C, 52v, E, 0.02v SA1215: B, 0.60v, C, 52v, E, 0.02v I may be barking up the wrong tree but I've started to check the voltages on the transistors going back into the circuit. So far OK. The collector voltages though are about 3v lower than it states on the diagram. Not sure if this means anything? Regards Martin

19th Oct 2020, 7:34 pm	#4
kevinaston1 Hexode Join Date: Aug 2014 Location: Featherstone, West Yorkshire, UK. Posts: 386	Re: JVC A-X77 stuck in protection This is not making sense If the 50V n pin 1 of the protection circuit is not coming from the output, you need to make more measurements around this IC to find what is supplying 50V to pin 1