* Tuner Issue * - Sony CFS-85S

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jimmyjimmy19702010

Member (SA)
May 5, 2012
3,578
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Sunny Brisbane, Australia
Hi guys,

Looking for a little help on an issue I have with my Sony CFS-85S:
View attachment 6636

When I got this box, I pulled it down and re-belted the deck and found the deck now worked beautifully. The Line In on this model is also very sensitive and works well. The amps on this unit are very good with a nice and even sound coming from both channels. The only real issue was the tuner. I had no AM, SW1, SW2 and only mono on FM. No amount of switch cleaning etc could rectify the tuner issues.

Fast forward a few months and a scored another CFS-85S. Upon receiving this new Ebay find, I realised this box was in far worse condition than my existing unit but it did appear to have a good tuner.

So I did a tuner swap and everything appeared fine with all 4 bands working with an FM stereo. The only problem is that when I turn up the volume, the tuner seems to start having sensitivity issues. On one very close FM station, the tune light and stereo lights stay lit and everything is fine. On every other FM station in Brisbane, the FM stereo light flickers depending on the load on the amp. It gets worse if you are using the deck at the same time:

Low volume on FM stereo mode:
View attachment 6637

Low to Mid volume on FM stereo mode:
View attachment 6639

Over 50% volume on FM stereo mode:
View attachment 6638

The 'operate' light beside the LED meters also dims quite a lot during the bass hits.

On AM mode, the 'Tune' light is only lit during pauses in conversation. It extinguishes the moment someone talks.

I am planning to do a full re-cap of this unit soon but just wanted some input as to a possible cause of these issues.

Thanks,

James... :-)
 

Superduper

Member (SA)
Tuner needs proper voltage to work 100% and is very sensitive to voltage changes. In fact, most tuners are fed from a regulated rail even as the rest of the BOOMBOX works fine on unregulated power. Might want to check quality of the connections (especially power) to the tuner and/or switches. If issue continues, then you might wanna install a tap to monitor voltage to tuner as you crank to see if it drops. If so, then you need to correct the source of the power drain or connection issues or regulator performance.
 

jimmyjimmy19702010

Member (SA)
May 5, 2012
3,578
26
38
Sunny Brisbane, Australia
Superduper said:
Tuner needs proper voltage to work 100% and is very sensitive to voltage changes. In fact, most tuners are fed from a regulated rail even as the rest of the BOOMBOX works fine on unregulated power. Might want to check quality of the connections (especially power) to the tuner and/or switches. If issue continues, then you might wanna install a tap to monitor voltage to tuner as you crank to see if it drops. If so, then you need to correct the source of the power drain or connection issues or regulator performance.
Thanks for the quick reply. I had to do a very thorough spray/switch back and forth to get the band select switch to reliably switch to AM. So you're suggesting maybe this switch could be the possible cause of an inadequate voltage getting to the tuner.

Could bad caps be another possible cause?
 

Superduper

Member (SA)
Function switch is one possibility. Connectors that wires the tuner is another (I don't know this BOOMBOX in particular, just speaking generalities). Bad caps.... Maybe. One of the failure modes on caps is to short. If a filter or bypass cap were to begin shorting, then it would shunt power to ground turning it into a pig. But I would guess this isn't as likely as one of the other scenarios. Also most boomboxes provide a regulated voltage supply to the tuner (~ 5-9v or so). This regulator could be getting weak. What I would do is if this tuner is on a separate board, figure out which wire is the power and ground ones. Solder test points to allow test leads to clip to it, then test live voltage while running to confirm and see if issue manifests with fluctuations in voltage.
 
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