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Old 11th Sep 2017, 7:33 am   #1
snanceki
Retired Dormant Member
 
Join Date: Sep 2017
Location: Stafford, Staffordshire, UK.
Posts: 1
Default Volume offset correction JVC RC-QW33

Hi I'm new here and I'm hoping that one of you can point me in the correct direction. From what I have read already it would appear that I have stumbled on an amazing amount of detailed knowledge. Although I'm a professional engineer my electronics diagnosis/repair skills are limited and I need a starter for 10 to get me on the right track.

Problem: JVC RC-QW33 Boombox. Fully operational with the exception that the volume control offset has changed resulting in the output being too high even when the volume is turned to minimum. This situation exists irrespective of whether the volume is changed using the Remote or the +/- buttons on the main assembly. Also irrespective of input source (Radio / CD/Tape etc). Likewise irrespective of whether main speakers or headphones are selected. It is important to note that the volume does change on request and that the digital indication of the level selected (1,2,3,4,5 etc) changes but the offset of the available range has changed and needs a reset.
I have the circuit diagram (Block and Schematic) but am struggling to determine exactly where in the circuit the offset volume (potentiometer/setting) is located.
Vol & Tone is controlled using a Sony CXA1792S Graphic Equaliser IC which when referring to its data sheet shows BIAS between pins 11 &12.
Could this be what I'm looking for?
I assume I can post images so if you require the circuit diagram and data sheet etc please let me know.
Your input would be most welcome. Stuart.
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Old 11th Sep 2017, 3:30 pm   #2
Mooly
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Join Date: May 2011
Location: Lancashire, UK.
Posts: 1,349
Default Re: Volume offset correction JVC RC-QW33

This could be difficult...

The CXA1792 is controlled (somewhat unexpectedly) via analogue control voltages rather than a digital bus system.

First thoughts with any problem like this are that the issue could be down to a corrupt system control memory. I can't see any obvious form of backup supply (such as a supercap) that keeps things alive but I think it still worthwhile to perhaps check around the system control IC (no power applied, batteries removed) and see if any voltage is present. If there is then I would first try discharging (gently via a resistor) the source of that.

If there is low level voltage generally around various points on the chip then (at your own risk... well the units anyway) I would try pressing tinfoil over ALL the pins to fully discharge everything. Leave a while and try again.

Beyond anything like that and you have to make a judgement call on whether the problem is the VOL/TONE IC or whether it is doing what it should in relation to its appled control voltage inputs. It looks like the DC control inputs are buffered by an opamp with the actual control signal coming from the system control chip.
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