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Sharp GF-9090 tuner issue


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#1 jimmyjimmy19702010

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Posted 03 August 2017 - 02:20 PM

Hi Guys,

Unfortunately my beloved GF-9090 has developed an issue where the FM stations are drifting up the tuner dial where for example 104.50 is now at 107 and it's continuing to move up the dial. I've now lost FM 107.7 altogether.

Tuners are a black art to me!

Any suggestions? :-)

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#2 MyOhMy

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Posted 06 August 2017 - 09:53 AM

Is the tuning string slipping?  If the 'lost stations' direction is towards one end only of the scale, could the tuning string be snagging somewhere?  I'd open the box and rotate the tuning knob to find out what is happening with this as a first step.  It could be a simple problem such as the tuning indicator in the window not being secured properly to the tuning string?  



#3 BoomboxLover48

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Posted 06 August 2017 - 03:54 PM

Is the tuning string slipping?  If the 'lost stations' direction is towards one end only of the scale, could the tuning string be snagging somewhere?  I'd open the box and rotate the tuning knob to find out what is happening with this as a first step.  It could be a simple problem such as the tuning indicator in the window not being secured properly to the tuning string?  

Like Christine said it is be due to the slipping of the tuning cord. The tuning gang is not in alignment with the cord connected to the dial pointer. The tuning knob axle has several turns on it and when for some reason if that slips, same problems can result. 

 

I would manually turn the gang condenser wheel and try to tune in stations. 

 

This is nothing but slipping problem of the tuning cord. 

 

One way to correct is by increasing the tension of the cord. On the large wheel of the gang condenser we can move the spring and increase the tension. Basically we are stretching the spring and hooking it to another adjacent hooking point. 

 

I have changed tuning cords on so many radios over these years. The last one I did was on on a Sharp GF777. 



#4 Superduper

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Posted 06 August 2017 - 08:40 PM

In my opinion, it would not be a string issue. The variable cap has stops at the extremes. If string was slipping, the needle would stop at a point before the bottom (or top) of the scale.

That does not sound like the case as you described. A more likely situation is that some tuning components are either failing or their values drifted. These values affects the alignment and bandwidth range. The service manual will have instructions on how to adjust the high and low range. This will involve turning some of the transformer coils or varicaps but you'll need a rf signal generator to do this. Typically you'll use the rf generator set to a specified low range frequency (i.e. 88mhz) and make adjustments until the tuner receives the signal outputted by the generator. Then you'll tune to the high end (i.e. 108mhz) and set generator to that frequency, and tuner again adjusted to best reception. This is repeated several times. After adjustment the entire bandwidth tracking should be quite accurate.

However this adjustment is a waste of time if the components are not stable and their values continue to drift. If the local oscillator is not set to proper frequency, then the received frequency won't be correct either.

A lot of people think adjusting the needle position is a fix, but in reality, it only ensures that the needle is correct at that particular frequency because tuning to a different location of dial will result in the needle once again in wrong position. This is because tracking will always be off if tuner scale and actual receiver bandwidth is mismatched.

So in summary, you can fix this if you follow the precise steps prescribed in the service manual, you fix the causes of drifted component values or local oscillator frequency, and you have or have access to some pricey lab equipment.

#5 BoomboxLover48

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Posted 06 August 2017 - 08:51 PM

In my opinion, it would not be a string issue. The variable cap has stops at the extremes. If string was slipping, the needle would stop at a point before the bottom (or top) of the scale.

That does not sound like the case as you described. A more likely situation is that some tuning components are either failing or their values drifted. These values affects the alignment and bandwidth range. The service manual will have instructions on how to adjust the high and low range. This will involve turning some of the transformer coils or varicaps but you'll need a rf signal generator to do this. Typically you'll use the rf generator set to a specified low range frequency (i.e. 88mhz) and make adjustments until the tuner receives the signal outputted by the generator. Then you'll tune to the high end (i.e. 108mhz) and set generator to that frequency, and tuner again adjusted to best reception. This is repeated several times. After adjustment the entire bandwidth tracking should be quite accurate.

However this adjustment is a waste of time if the components are not stable and their values continue to drift. If the local oscillator is not set to proper frequency, then the received frequency won't be correct either.

A lot of people think adjusting the needle position is a fix, but in reality, it only ensures that the needle is correct at that particular frequency because tuning to a different location of dial will result in the needle once again in wrong position. This is because tracking will always be off if tuner scale and actual receiver bandwidth is mismatched.

So in summary, you can fix this if you follow the precise steps prescribed in the service manual, you fix the causes of drifted component values or local oscillator frequency, and you have or have access to some pricey lab equipment.

:bow:



#6 jimmyjimmy19702010

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Posted 07 August 2017 - 12:33 AM

Thanks for your help guys. :-)

I spoke with a local tech and he basically said a tuner that is 'drifting' is probably a worst case scenario as it would take sometime to find the faulty component. He also mentioned that new replacement parts would be the next challenge. The issue is slowly worsening over time. The annoying thing is that the FM tuner otherwise works great with a very strong stereo lock and excellent reception.

I must have been lucky in the past as up until this 9090 tuner issue, I've never had any issues with GF series tuners!?