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09-03-2024, 05:37 AM
Thanks for sharing your troubleshooting experience with the 1972 Marshall 2040 Artiste. It's a great example of the kind of detective work often needed to resolve what might initially seem like minor issues.
To summarize, the primary hum issue was traced back to the phase inverter (PI) circuit. After elevating the heaters, which eliminated most of the hum from both preamps, a persistent 50Hz hum remained. Suspecting a ground loop, you reorganized all ground connections, but the hum persisted. Further investigation revealed the problem: AC noise was being picked up by the first grid wire due to untwisted heater wires running near the PI tube. Twisting and repositioning these wires reduced but did not entirely eliminate the hum.
The final culprit was the AC wires on the voltage selector switch, located too close to the PI circuit. Fixing this required disconnecting all wires, setting a fixed connection on the 230V tap, and moving everything away from the PI tube. This resolved the hum, but reconnecting the voltage selector switch proved challenging due to continued interference. You decided to leave the switch unconnected since the amp's owner would likely only use 230V, and secured the cables away from the PI, resulting in a dead-silent amp.
Additionally, a minor hum in the reverb circuit was found to be caused by RCA jack wires touching heater wires — another straightforward but critical fix.
Your approach highlights the importance of proper lead dress, grounding, and cable management to eliminate hum and interference, especially in vintage amplifiers. This write-up will undoubtedly help others facing similar issues with their amps.
Thanks again for sharing!
__________________
To summarize, the primary hum issue was traced back to the phase inverter (PI) circuit. After elevating the heaters, which eliminated most of the hum from both preamps, a persistent 50Hz hum remained. Suspecting a ground loop, you reorganized all ground connections, but the hum persisted. Further investigation revealed the problem: AC noise was being picked up by the first grid wire due to untwisted heater wires running near the PI tube. Twisting and repositioning these wires reduced but did not entirely eliminate the hum.
The final culprit was the AC wires on the voltage selector switch, located too close to the PI circuit. Fixing this required disconnecting all wires, setting a fixed connection on the 230V tap, and moving everything away from the PI tube. This resolved the hum, but reconnecting the voltage selector switch proved challenging due to continued interference. You decided to leave the switch unconnected since the amp's owner would likely only use 230V, and secured the cables away from the PI, resulting in a dead-silent amp.
Additionally, a minor hum in the reverb circuit was found to be caused by RCA jack wires touching heater wires — another straightforward but critical fix.
Your approach highlights the importance of proper lead dress, grounding, and cable management to eliminate hum and interference, especially in vintage amplifiers. This write-up will undoubtedly help others facing similar issues with their amps.
Thanks again for sharing!
__________________
