Any suggestions on treble issue on a Cambridge A5 amp reignited after 20 years?

[ClenchedLiver]

Hi, after some years in storage I unboxed my old hi-fi system, principally to play the old vinyl that I still have. All is sweet EXCEPT, the treble control on my Cambridge Audio A5 amplifier (prob about 20 years old) does not appear to do anything. The amp sounds OK to me, listening to vinyl and CD players. It has a direct selection button, for bypassing the bass and treble controls. By monkeying with that, I can work out that the bass tone control operates well. But I am sensing no difference no matter where the treble control knob is placed. I can get a new amp, but I am not a big fan of wastage if it can be avoided. I tried listening to drum-rich music with hi-hat sections that should be of high frequency. I cannot sense them zoning in and out when moving the treble knob though. Does anyone here with technical nous think it is worth me trying to get somewhere with this problem?

[Edward Huggins]

It could be that the "Tone Defeat" control, when switched out of circuit, is not restoring the variable Treble control's circuitry. If not that, then the passive components that are associated with the Treble Control, possible a capacitor may have failed? However, I suspect the former.

[paulsherwin]

As Edward suggests, if this amp was working correctly when it was taken out of use, then you almost certainly have a switch contact problem. Get hold of some Servisol 10 or similar contact cleaner, and apply some to all the switch contacts, exercising each switch vigorously afterwards.

[buggies]

I can't detect any effect with treble controls either - but my HF hearing response is probably 20dB down on 20 years ago.

[McMurdo]

possibly an open-circuit treble pot? If it worked when you put it away then it may have become tarnished internally

[ClenchedLiver]

Thanks for the suggestions everyone. George I did ask my son to listen too, coz I certainly have done quite a bit of Led Zep high volume listening over the years! He thought there was a slight treble enhancement when the treble control was turned right up, but I think he was talking himself into it. And when I blind-tested him he was really struggling to convince me. Anyway, I sprayed out the switches and all the potentiometers with Servisol 10 and gave them a decent work-out. Sadly, this has made no difference. So I'm still stuck.

[Craig Sawyers]

The tone controls only do +/-6dB on the A5. Specified at 100Hz and 10kHz.

I haven't found any measurements on line for the tone control characteristics.

Since this is probably a straightforward Baxandall, if the bass control works, so should the treble. Unless there is a really sneaky problem with the way the defeat works, and it is defeating only the treble bit as a result of a fault.

Can you check with a DVM that the switch is doing what it ought?

[ClenchedLiver]

Well, although I have a decent DVM and other tools, and I am fairly practical on mechanical engineering, I am a total rooky electronics wise. So although I would likely be able to implement a plan, I don't actually have one! I roughly know what components do individually, but don't intuitively grasp their relationships or how to unravel them.

[buggies]

How about using a PC?
Bung 10kHz into the amplifier and look at the variation of the output as treble control varied.
This program (for example) has an audio generator and 'scope:-
https://www.zeitnitz.eu/scope_en