Vox AC15 Fuse Failure Diagnosis and Repair
Customer Complaint
The amplifier arrived with a complaint of repeatedly blowing the main power fuse during power up.
Equipment Information
This particular AC15 is a Vox AC15CC1 model which was produced from 2005-2010 or so. these amps were the first designation of the AC15 line to get rid of the vacuum tube rectifier and replace it with a solid state full wave rectification circuit using two diodes. This change was largely due to the desire to produce an all tube Vox Amplifier at a lower cost to the customer.
Initial Inspection
Immediate fuse failure often points to problems in the power supply of the amplifier. The main fuse in the amplifiers IEC socket was was failing upon power up, but the secondary fuses for High voltage line (B+) and the valve heater circuits were still intact.
Upon initial inspection, there were no obvious visual signs of component failure. In order to rule out faulty tubes, i removed the tubes from the amplifier and tested the circuit to find the same fault present.
Diagnostic Process
After determining that the tubes were not at fault, I disconnected the secondary of the power transformer from the circuit and verified the correct resistance across the transformer secondary. I then carefully powered the amplifier on. With the transformer secondary disconnected, the fuse did not blow meaning that the power transformer was healthy. This essentially left only the rectification section as the prime suspect. I checked the continuity and resistance of all filter capacitors and well as dropping resistors and found no issues. lastly I tested the continuity of the two rectification diodes. A healthy diode should only conduct in one direction but using the diode test function on my multi meter, i found that one of the two rectification diodes was conducting in both directions indicating a fault. the kind of fault would make the fuse blow before any other part of the circuit received power so this was the cause of the issue.
Repair
Once I identified the fault, I removed the PCB from the amplifier chassis (not very easy on these vox amps...) so that i could have access to the back of the circuit board and desoldered and removed both diodes. Replacing both diodes is the best choice because it lowers the chance of a future identical repair if the other diode fails like the first one. It also allows for a better balance of the rectifier since I will be using slightly different diodes to those used in the original manufacture of the amplifier.
I replaced the original diodes with a new pair of 1N4007 rectifier diodes and carefully reassembled the chassis of the amplifier. after reattaching the power transformer secondary and reinstalling the tubes, I powered up the amplifier and verified that it was working as intended.
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