Vox Cambridge 30 low volume

[IanV]

Hi,

I have a Vox Cambridge 30 and have an issue with Ch2. Ch1 (Clean) works great as loud as expected, however Ch2 has very low volume - I am guessing low signal through the Ch2 pre stage. The treble pot, mid boost switch and bass pot work fine as does the gain/boost switch, and the volume control does increase the volume - but it is very low compared to CH1, and has background hum on it which also increases with volume.

I think the signal is OK at the gain circuit (basically a clipping overdrive) as the LEDs are flickering with signal input. I have built a tube screamer type pedal before now using LEDs for clipping and saw similar things, so believe it is after this.

I have some experience in electronics mainly on valve amplifiers rather than solid state though, and have checked things voltage supplies to the 12AX7, both heater and HV, and have swapped the 12AX7 just to be sure with the same result.

I have attached the schematic. If anyone has any ideas or pointers please holler.

I have access to pretty much any of the components in the circuit and a multimeter - no scope unfortunately otherwise I would be trying to trace the signal through.

Cheers

[IanV]

I actually fixed another one of these about 2 months ago with a similar issue, but that was an obviously leaking cap. THis is less obvious.

Cheers

[g1]

Can you post the DC voltages at pins 1,3,6, & 8 of the tube?

[IanV]

Can you post the DC voltages at pins 1,3,6, & 8 of the tube?

Hey @g1

The voltages are as follows:

Pin 1 - 129v
Pin 3 - 1v
Pin 6 - 132v
Pin 8 - 1v

I hope this helps.

[shortcircuit]

Hi,

I have a Vox Cambridge 30 and have an issue with Ch2. Ch1 (Clean) works great as loud as expected, however Ch2 has very low volume - I am guessing low signal through the Ch2 pre stage. The treble pot, mid boost switch and bass pot work fine as does the gain/boost switch, and the volume control does increase the volume - but it is very low compared to CH1, and has background hum on it which also increases with volume.

I think the signal is OK at the gain circuit (basically a clipping overdrive) as the LEDs are flickering with signal input. I have built a tube screamer type pedal before now using LEDs for clipping and saw similar things, so believe it is after this.

I have some experience in electronics mainly on valve amplifiers rather than solid state though, and have checked things voltage supplies to the 12AX7, both heater and HV, and have swapped the 12AX7 just to be sure with the same result.

I have attached the schematic. If anyone has any ideas or pointers please holler.

I have access to pretty much any of the components in the circuit and a multimeter - no scope unfortunately otherwise I would be trying to trace the signal through.

Cheers

[ATTACH=CONFIG]47702[/ATTACH]

replace the optocoupler

[IanV]

replace the optocoupler

Thanks you for the reply.

Optocoupler is fine and has already been replaced / upgraded by the looks of it. Tremolo works fine on both channels and has no impact on the volume for Ch1 which works as expected, and uses the same output stage, so don't think it will be that.

[g1]

IC3 is the selector between the 2 channels.
Run a constant tone to the input. Set up the clean channel so you are getting a reasonable volume at the speaker. Measure the AC voltage at the wiper of VR2.
Now switch to the gain channel. Measure the AC signal at the wiper of VR7. Is it much weaker than what you measured at VR2 wiper? If not, I'd suspect IC3.