Leak Stereo 20 hum and buzz problem

[ifsixwasnine]

Hi all,

After hearing a mates stereo valve amp i thought gotta get me one of those,
enter 1 leak stereo 20. Over the last few months all caps have been replaced along with all out of spec resistors(as a newbie this was an easy intro into servicing and understanding the basics of how valve amps work) anyway all was working well untill the other evening. The amp was playing for a few hours, i then decided to swap the interconnects for a pair that a friend had given me, when i turned the amp on a hum had developed on the left channel
swapped the interconnects back but still the hum??
Now the hum(and a slight buzz) is there with nothing connected to the inputs also with them connected.

When power is turned off the hum stops imediatley(it does not decay)

It also is effected by moving my hand near to inputs, also if i move near to V1.
I have inspected for bad solder joints by probing with a plastic probe all seems ok but when i probe the the positive wire and asociated resistors( 1meg/22k) at the left channel rca a thud can be heard through the speakers almost like microphonics??
I have tried different valves to no avail.
I have replaced the offending resistors,
I cannot understand why a change of connects would cause this and i suppose i could take it to a repair man but i would learn nothing from this, so have come here for a few pointers into tracking this problem down.
many thanks
DM

[DF96]

Swapping interconnects may have exposed a faulty RCA socket. The interconnects themselves are not to blame. There is a bad connection somewhere, probably in a ground. Or it could be too good a connection (which was absent earlier) causing a hum loop. Has the chassis acquired or lost a ground connection?

[ifsixwasnine]

have now replaced rca sockets
all the previous symptoms are still there, except now when probing to find dry joints etc the wire that goes to pin 3 on the preamp socket (B+) now becomes microphonic. ie. heard through speakers when tapped?? no noise at the resistors now, hum and buzz increase when i move hand near said socket.
The circuit is original so no extra earths have been added/removed,

[M Gregg]

How close is this schematic to your amp?

Schematic - Leak Stereo 20 Tube Amplifier @ AmpsLab.com



Regards
M. Gregg

[bulgin]

Hi

I have a Stereo 20 which had similar refurbishments and upgrades over a period of time and there's no hum at all. Have you replaced the rca sockets at the left rear as these are devils to replace due to limited workspace. Are they correctly isolated and are the fasteners tightened firmly?

Also, are you running earthed ac to the amp?

bulgin

[SHiFTY]

You could try wiggling the valves around with the unit on- your input valve might have a bad connection. You could also try swapping a couple of the 12AX7s around in case it's a bad valve.

Otherwise sounds like you have a dodgy solder joint- I'd try quickly reflowing the connections at the input socket and first valve socket.

[ifsixwasnine]

Hi M Gregg

I think mine is the later model (grey chassis) the same schematic with R12/13 470k
C5 0.1 also R5 has a 300pf cap in paralell.
I am researching the causes of this problem and can confidently take measurements, solder etc so any suggestions /advise would be welcomed.

bulgin hi,

both phono jacks have been replaced, the speaker terminals also they have teflon spacers to isolate them and tight.
Yes the mains plug is earthed.
Mine was sounding good before i changed the interconnects, i do not think this caused the hum, i think something is breaking down (have reflowed all solder joints)
I think its time to get the dmm out, but having a plan of attack is taking some time.

regards DM

[pointy]

as i always say you could try this group........ hjleak : "H. J. Leak & Co. Ltd."

[ifsixwasnine]

Hi SHiFTY

Have wiggled and replaced all valves except the rectifier,
hum increases when V1 touched?
all solder joints have been reflowed.

regards DM

[DF96]

Is the hum still there with the input shorted to ground? Is the feedback loop intact - does the amp seem much more sensitive now than it used to?

Do you have an oscilloscope? You need to find where the hum is entering the circuit.