Repair help - AC50 loses volume when turning up treble or bass

[klarsen01]

Hi all,
I have a '66 AC50. It was working fine Saturday at a gig but when I fired it up today it had very low volume & was kind of crackly. After messing with it a bit I discovered that the volume came back if I turned the bass and treble controls on the normal channel all the way down. If I turn up either one I lose volume.
If I plug into the Brilliant channel it works ok as long as the tone controls on the normal channel are all the way down. If I turn up either of the tone controls on the normal channel it cut most of the volume on the Brilliant channel. I usually jump the channels together so I need the normal to work correctly.

Anyone have any idea where to start troubleshooting this? I usually work on my own gear but I've never run accross this issue before.

Schematic: http://www.backfromthesixties.co.uk/SiteAssets/vox_ac50/AC50%20Circuit%20Diagram.jpg

Thanks
Kirk

[John]

Sounds like they are dumping signal straight to ground. Try using jumper wires to bypass the pots and/or cap and resistors. Hopefully you'll find a dodgy one in there somewhere.  Other guys will have better suggestions I'm sure!

[PRR]

I'd replace V1 V2 first. It *may* be squealing super-sonic when not full-down. That's not very likely, but tubes are socketed......

Then I'd eyeball real good for anything out of place. Including cracked pot lugs and joints as john suggests.

[klarsen01]

Swapped tubes & checked for bad connections yesterday. Didn't find anything. I'll go through it again today with a fresh head and see if I have any better luck.

Any other suggestions as to where to look are welcome.

Thanks for the help.

[stratele52]

For this work you need an Oscilloscope and a signal generator . Otherwise you are like a blind.

Check B+ with voltmeter .

 Try replace V3, Phase Inverter tube .

[klarsen01]

Started checking the caps in the tone circuit. The 50pf from the treble control was open. Replaced it and all is back to normal.

Thanks for the help!
Kirk

[jjasilli]

Or, use a flow chart.  E.g.:  http://geofex.com/ > -Tube Amp Debug Page > Low power or volume, or volume drops off

Note that turning up the tone controls is like turning up the volume. 

[stratele52]

Started checking the caps in the tone circuit. The 50pf from the treble control was open. Replaced it and all is back to normal.

Thanks for the help!
Kirk

Normal channel ?

It is the 47pf on schematic wich is a coupling caps from the output of V2 on normal channel to input V3. Sur if it is open you have no sound.

But why Brillant channel do not work properley ?

[HotBluePlates]

With C8 open, the signal sneaks back through R10, C7 and the resistance of RV3. So turning up RV3 puts more resistance in the path, potentially reducing output to V3.

The Bass pot (RV2) is 1MΩ to ground from C7. If you turn the bass up, it becomes more like 10kΩ to ground due to R11. That could also explain loss of signal with the Bass pot turned up.

[stratele52]

+1 HotBluePlate