sound city 120 - I may have 'highlighted' a fault !!!

[jbmdisco]

I have a friends sc120 amp in. It came in with a 'rumbling' sound which presented itself when increasing the 'middle' control. I swapped out the ax7 tube associated with the mid channel with the bass to eliminate a tube fault. With no difference in the rumbling and tubes back in their respective sockets I have then fitted a new pot - as the problem seemed to come when increasing this. Again the fault was still present. I have decided to scope out the pre-amp to try and locate the rumble having narrowed it to somewhere in the 'mid' circuitry. However I decided, as there is some high voltages present in this amp, to check voltages in the pre-amp stage before connecting the 'scope. I connected my DVM across a couple of resistors to the rear of the mid control pot. Almost immediately there came a loud ground type hum which despite switching off and on again, is now ever present and hence I appear to have created another fault on top of the original rumble.
Could the use of a DVM have caused this further fault or have I accelerated the progress the original fault ?

[g1]

Possibly you have disturbed an "iffy" solder connection. It may or may not be related to the original fault. And it may not be at the exact resistors you were probing. Sometimes the pressure of the probes will cause the board to flex and disturb poor solder joints.

[Chuck H]

A bad solder joint was my first thought also. Like g1 said. Re solder and test. Poke around with a wooden stick to look for any other loose connections. In this case I think it would be a good idea to clean all non fixed contacts as well. Tube sockets, jacks, pots, etc.

[jbmdisco]

I keep a drumstick handy for these purposes. I have located a number of potential solder faults - a good job for the weekend.
Thanks guys.