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So I worked on my friends Vox AC15C1 these last two days. The original problem was the pilot light would light up on power up, but no sound. I checked all fuses and all checked out fine. I discovered that R80 between the B+1 and B+2 rails was blown. After replacing it i turned on the amp and was able to get a very faint and weak signal out of the amp. Then something started to burn. As I glanced over at the resistor i had just replaced (1K 2W), I saw that it had gotten so hot that it unsoldered itself from the PCB! Once it cooled I checked my B+ voltage and it measured 225 volts. Way too low. I measured 225 volts on one end of R80 and only 35 volts on the other end.
I had an idea i would encounter more problems after replacing the resistor, as from experience i've learned its very rare for a resistor to burn up, short, or blow open on its own. Resistance from B+2 to ground measured 696 ohms. I isolated the problem to be on the main PCB and on the B+2 rail. The B+2 rail feeds two different sections: The tremolo section and the grid resistors. The grid resistors are on the tube PCB's, which were not connected to the main PCB so that ruled out the grid resistors. R73 fed the B+2 to the tremolo section so i unsoldered one lead of R73 to eliminate that section. Still nothing. Didn't make any sense!
After glancing around for a couple of hours I noticed that one of the connectors soldered to the PCB had some corrosion on it. As I got a closer look I saw a small burn mark on one of the wires. I desoldered the connector and cleaned it up the PCB. Using a 40X microscope I looked at the PCB where the connector was and saw there was a small burn mark between two pads. The pad with the most damage happen to be the B+2 line! After cleaning up the burn mark my resistance from B+2 to ground went up to about 900 ohms. The pad where the B+2 pin on that connector had been had a short internally in the PCB. But not a big problem. My solution was to cut the trace that fed that pad and hard wire that wire to the PCB. That eliminated the damaged pad. My resistance from B+2 to ground now measured close to 1 Meg!
PROBLEM SOLVED! The moral of the story: Never assume that "it can't be that....it can't be this." A thorough inspection would have saved me some time. A microscope is a great tool to have. I would have never seen that with just my eyes!
I had an idea i would encounter more problems after replacing the resistor, as from experience i've learned its very rare for a resistor to burn up, short, or blow open on its own. Resistance from B+2 to ground measured 696 ohms. I isolated the problem to be on the main PCB and on the B+2 rail. The B+2 rail feeds two different sections: The tremolo section and the grid resistors. The grid resistors are on the tube PCB's, which were not connected to the main PCB so that ruled out the grid resistors. R73 fed the B+2 to the tremolo section so i unsoldered one lead of R73 to eliminate that section. Still nothing. Didn't make any sense!
After glancing around for a couple of hours I noticed that one of the connectors soldered to the PCB had some corrosion on it. As I got a closer look I saw a small burn mark on one of the wires. I desoldered the connector and cleaned it up the PCB. Using a 40X microscope I looked at the PCB where the connector was and saw there was a small burn mark between two pads. The pad with the most damage happen to be the B+2 line! After cleaning up the burn mark my resistance from B+2 to ground went up to about 900 ohms. The pad where the B+2 pin on that connector had been had a short internally in the PCB. But not a big problem. My solution was to cut the trace that fed that pad and hard wire that wire to the PCB. That eliminated the damaged pad. My resistance from B+2 to ground now measured close to 1 Meg!
PROBLEM SOLVED! The moral of the story: Never assume that "it can't be that....it can't be this." A thorough inspection would have saved me some time. A microscope is a great tool to have. I would have never seen that with just my eyes!
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