Tone control PCB without capacitors in Cambridge Audio A1 mk3

[kesrmk]

Hi,
just curious if someone seen the tone control (Bass, Treble) circuit without capacitors? When opened for fixing Cambridge Audio A1 mk3 I could not believe what I saw...
Did CA invented new tone control circuit which does not require 2 pairs of capacitors and resistors for bass and the same for treble?

Kes

 

[PRR]

Do the knobs do anything?

Parts-numbering does not match the docs on HiFiEngine.

 

[kesrmk]

Knobs making some difference but not as expected.
Bass knob makes difference on lower freq. But treble - just a little, not sure if any difference.
So wonder how that even works for Bass...

Definitely need to see the other side of the board...

 

[tomchr]

Well, that's wonky. You can request the schematic for the amp from Cambridge. When I was repairing an A23 they happily shared the schematic.

Tom

 

[volume knob]

It looks like the board has been modified, capacitors seem to have been removed and replaced with resistors. I'm guessing somebody wanted to defeat the tone control

 

[Charles Wallis]

I am working on what looks like the same model. As you say there are no caps, despite them being marked on the PCB as shown in your pic. I found a schematic online but it does not match this version as it shows caps on the tone board along with +and - 15v voltage regulators. These have been replaced with simple 15 v zener diodes/resistor combinations for the IC power supply. My suspicion is that CA were cuttings costs at this point.
My one had a dud channel and plagued with motorboating and mains hum. I have replaced the two op amps, its a bit better but still distorted. The power amps are OK. There is not much else to replace. Any ideas?

 

[romeoleung]

Cambridge Audio A1 Schematic

 

[kesrmk]

Mine also has some mains hum, which not disappears even with 10000uF capacitors installed, so I have spend some time investigating, seems that mains hum is picked by tone control components which are too close to mains switch, and no shielding, therefore I guess CA removed caps from face side of PCB and installed them on solder side a bit far away from it's original place. Just to minimize hum. Unfortunately it is still there. Hum can be heard only on pause and in close distance. With distance 2m and more you hear something, but not sure if it is hum in speakers or rings in ears...

 

[Charles Wallis]

Eventually I got rid of the hum by improving the earthing. I have a seperate thread about that in the chip amp section. The tone controls sort of work but not very convincing. Not sure if that is what they were like when new.

 

[nigelwright7557]

Looks like someone has hacked it to get rid of tone controls to get straight through sound.
Some people hate anything in the signal path.

 

[peufeu]

Some people hate anything in the signal path.

I have an old Sansui with a busted tone control pot that's stuck in maximum bass boost mode, so that's on the list