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Sharp GF-777 Low volume on left channel line in only


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#1 Mark1892

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Posted 09 September 2018 - 02:44 AM

Hi all, just in the process of working on a friends 777 that had a few issues but is now running perfect except one thing. I've repaired the power switch, changed the belts and cleaned the idler wheels, secured one of the tape heads as both screw were loose - now secure and adjusted and playing great. A thorough cleaning of all pots/record bar etc but the left channel is still very low on line in. If I turn the balance control all the way to the left you can hear it at a very low level, it was like this on both sides before I started so I've got the right channel to work. Tape and radio both play loud and clear through both channels. I've cleaned the plugs with deoxit and left to dry overnight but still no better.

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#2 stevenrob

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Posted 09 September 2018 - 12:11 PM

Perhaps you can verify the balance pot 



#3 Mark1892

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Posted 09 September 2018 - 03:55 PM

It works fine on radio and tape, could it be different for aux/line in?

#4 JVC Floyd

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Posted 09 September 2018 - 04:23 PM

Have you tried the mic inputs to see if it plays on both channels.

#5 Mark1892

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Posted 09 September 2018 - 04:33 PM

No not yet, the ones on the rear?

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#6 JVC Floyd

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Posted 09 September 2018 - 04:37 PM

I was thinking the phono mic input on the front.
I'm not sure if you sprayed the cleaner into the input selector switch on the back of the box but If you did not that is switch if its dirty can cause all kinds of signal problems.

#7 Mark1892

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Posted 09 September 2018 - 04:44 PM

I was thinking the phono mic input on the front.
I'm not sure if you sprayed the cleaner into the input selector switch on the back of the box but If you did not that is switch if its dirty can cause all kinds of signal problems.


I did but its quite stiff to switch between line in and phono so maybe that's the offending component, maybe I should revisit that switch first and give it a good clean. Thanks

#8 Superduper

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Posted 09 September 2018 - 08:48 PM

I bet the problem is with either the rear line-phono switch or the record bar needs further cleaning. On the former, it’s a pretty open switch but can get fairly corroded needing removal and disassembly to get a thorough cleaning. In the other hand, it’s probably one of the easier switches to do if it needs to be removed & disassembled.

#9 BoomboxLover48

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Posted 09 September 2018 - 08:59 PM

It is most likely the line in switch at the back like Superduper mentioned..... I have 3 GF 777s and all of them had the same problem to begin with.  :yes:



#10 Mark1892

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Posted 09 September 2018 - 11:43 PM

Thanks everyone I'll get rear line /phono switch stripped today for a good clean and it will hopefully sort it

#11 goodman

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Posted 10 September 2018 - 01:16 AM

This Sharp looks very good. I hope, you can fix line in problem.

You must check from time to time on ebay and can find original tuning knob.



#12 Mark1892

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Posted 10 September 2018 - 04:43 AM

This Sharp looks very good. I hope, you can fix line in problem.
You must check from time to time on ebay and can find original tuning knob.


Thanks Goodman, it's a friends which is still work in progress but if I spot one I'll let him know

#13 Mark1892

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Posted 10 September 2018 - 02:22 PM

This Sharp looks very good. I hope, you can fix line in problem.
You must check from time to time on ebay and can find original tuning knob.


Thanks Goodman, it's a friends which is still work in progress but if I spot one I'll let him know

Unless anyone in the UK has one they want to part with?

#14 Mark1892

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Posted 12 September 2018 - 12:28 AM

Hi again, so I spent a good portion of last night freeing up the selector switch on the rear so it's moving nice and freely now but still the same, I cleaned the line in sockets too. Just to recap over things, when in radio or tape mode both channels work great, when the 2 rca cables are fitted to line in and the selector is on line in - the left channel is very quiet but audible, the right channel is perfect. If I move the selector switch to phono - volume returns to both channels but at an amplified and distorted level. I've given the phono connections and switch another dose of deoxit but still the same. I'll look again tonight after work. 😕

#15 Northerner

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Posted 12 September 2018 - 12:35 AM

Sounds like the switch might need opening up

#16 Mark1892

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Posted 12 September 2018 - 12:48 AM

Will that have to be removed from the board for that Northener? Looks like a complete assembly.

#17 Mark1892

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Posted 12 September 2018 - 08:42 AM

Also I've tried plugging headphones in and only the right channel works, not sure if that's any good info or not.

#18 Superduper

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Posted 13 September 2018 - 09:20 PM

The headphones jack test has no bearing on the issue you are having.  This is because the headphone jack is after the power amp.  The fact that you are getting normal volume during other modes already shows this.  Symptoms suggest focus should probably be on area at or upstream of the mode switch.

 

IF you are certain that the line/phono switch is clean and not the problem, then you should check resistors R415 and R421.  22k Ω and 1.5k Ω respectfully.  Best to check out of circuit but a quick check in circuit could be of value.  If either resistor shows high reading in circuit, then there is definitely a problem.  If either resistor shows low, then best to recheck out of circuit.  If R421 shows very low, it's probably shorting which will shunt signal to ground.  You will find both resistors on the Line/Phono switch PCB near and between the switch and the L line-out jack.  How can you be sure that the line/phono switch is has good connectivity?  Just use a ohmmeter and test.



#19 Mark1892

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Posted 14 September 2018 - 12:26 AM

Thanks Superduper, it's not mine but would really like to get it back to 100% for my friend. I'll have a look over the weekend and hopefully I'll find something along your suggestions. The function switch is free moving now and seems to switch between line in/phone okay but will test it as well.