8
Troubleshooting tips - output failures
After a catastrophic failure, it is likely that the main supply fuse will be blown. Replace the
fuse and
very slowly
bring up the Variac while monitoring line consumption. It is likely that
substantial line current will be pulled due to shorted output parts.
Remove the main board from the unit and check for shorted output transistors. If one bad
output transistor is found, replace all six in the channel. When an output device shorts it can
place high current stresses on the other output parts. These output parts can fail over time. Since
long term reliability is paramount, please replace all the outputs.
All six of the 5W 0.51
Ω
emitter resistors must be verified for proper value. Any off-tolerance, or
open parts, need to be replaced. An off tolerance (higher resistance) emitter resistor will prevent
its related output transistor from “doing its share” and will place more stress on its mates in the
output section. Also verify the 6 base drive resistors (2.2
Ω
, 1/4W, fusible) are all OK. Verify that the
drivers and pre-drivers are not shorted. If one driver is shorted, replace it’s mate. Do the same
with the pre-drivers. Also check all the resistors surrounding the drivers and pre-drivers. Note: if
you change any fusible resistors, you must use identical replacement parts.
Check the VI limiters and detectors. It is not uncommon to damage these parts when the
amplifier fails in a spectacular way! Look for shorts on Q32, Q35, Q36 and Q39. Verify proper
value on the resistors that go to the bases of these devices. It is critical that these sections are
working correctly. Shorted transistors can cause some odd asymmetrical clipping problems.
Open parts will not allow the current limiting to operate effectively. If problems exist in these
sections the amplifier might fail into a short, or might clip prematurely when loaded to 2
Ω
.
All of the above trouble shooting (not including part replacement) takes perhaps 15 to 20
minutes to do. If you take the time, and do all that is indicated above, it will allow 99% of the
amplifiers to come up the first time! Trying to hurry, and skipping what is suggested, can lead to
a frustrating and time consuming repair.
Slowly bring up the supply and verify that line consumption isn’t excessive and that the
output is centered (no DC offset). It might be desirable to defeat the amplifier muting (connect
U4-14 & 12 together) temporarily so the amplifiers are active even when the supplies are still very
low. If the amp stays centered, verify that it will pass a nice clean sine wave. Remove any test
jumpers, and proceed to the safety test and the “bias and test procedure” section on the next
pages.