42
Some tips
Different sounds can indicate different problems.
Deep hum without any buzz - this is 120Hz hum, and is usually encountered in DHT amps. Usually a
touch up of the hum balance pot will eliminate it
Buzzy hum, like a bee - This usually means a lifted ground connection. Usually fixed with a careful
examination of solder joints and an arbitrary re-solder job
Scratchy sound, hissing, crackling, popping - usually a bad tube, but if just one of these four types of
noises is heard, it can also be caused by a bad capacitor or resistor.
Popping on start-up - usually a grid short in the output tube causes this. It will often clear itself and
the amp will play OK.
Microphony, a tendency to ring when tapped or a loud musical transient plays - again this is most
often due to a bad tube, but on rare occasion a capacitor can create this symptom too.
No sound - something isn't getting voltage or it isn't getting signal. Before you dive in to tear things
apart, check the obvious - are your speaker cables hooked up properly? Interconnects? Is every-
thing switched on? We see some slightly embarrassing moments at Bottlehead meetings, where the
equipment is often swapped in and out of the system at a frantic pace. About 95% of the time a
"dead" piece of gear just isn't hooked up right.
Muffled sound - this usually means something is not getting all the voltage it is supposed to get, and
that you need to get inside the equipment and start taking voltage measurements.
Pay attention to the LEDs on C4S boards
If a board doesn't light up, be sure to check out your component numbers, orientation on the PC
board, and solder joints. To test the transistors, put your meter leads across each of the three possi-
ble combinations of transistor leads and test for resistance. If you get a very low resistance reading
at any pair, the transistor is blown. Reversed LEDs are another fairly common problem.
Last tip -
If you see smoke, SHUT IT OFF!!!
Doc B.
To Next Page
Loading...