I have been using the tube preamp for many years without problems and have some Mullard M8080 and CV4058 tubes to roll - the advantage is that this amp uses only 2 such tubes, so I thought the amp can live on for many years to come.
Unfortunately, the hope is short lived - the volume control is now malfunctioning. It may suddenly sound muted (as if the volume has been turned down) but the displayed volume remains unchanged and also render the volume changes ineffective, ie. pressing up/down volume button causes no change in volume although the display changes accordingly. And sometimes it suddenly becomes loud again. It mostly start of (on switching on) as normal, and then gets problematic after a few minutes of using. It is always the same for both channels L&R, ie both channels get loud or soft together. Its probably some transistor or capacitor failure but I couldn't trace the faulty part. I know it uses stepped attenuator principle with resistor ladder and transistor switches but I do not know how to trouble shoot.
If you have the schematic diagram or faced similar problem or have experience in repairing such faults, please kindly share with me.
Thank you and best regards,
LH Chang (firstname.lastname@example.org)
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