Last year, I snapped up a Vox Pathfinder 15 (non-reverb) combo amplifier from ebay. The seller was a charity shop, and the amplifier was listed as "untested", so I was taking a bit of a gamble, but figured I'd have a good chance of being able to fix it it it wasn't working, and cosmetically, it looked almost mint.
When it arrived, I plugged it in, and as you guessed, it was not working. Barely any sound, and what sound there was was horribly distorted. One thin I did notice was the the power on LED was not aligned correctly with the hole in the back panel, so I did quickly took it apart, and realigned the potentiometers and LED indicator, but didn't have a look in detail at the circuit. I put it back together, but the problem remained.
My guess was that the problem was the famous "cambridgitis" (just google that term and you'll find all about it). Basically, a problem where the optocoupler that controls the tremolo circuit fails. A common failure on early Pathfinder 15 amps, and a pretty easy one to fix. So I figured I'd fix it when I found time at some point.
Anyway, I finally got around to it, and took the amp apart. My first surprise was that as I disassembled it, the "Tremolo Depth" pot fell out of the chassis. It was broken, assumedly at the same time as someone in the past had clumsily taken it apart and reassembled it (hence the LED not in the hole as per when it arrived). Easy fix - replace the pot.
My second surprise was that the version of the amp I had did not have the optocoupler at all, it was the version of the circuit with a FET controlled tremolo and a CA3080 OTA in the circuit, so it wasn't a case of cambridgitis at all, probably more likely due to the broken depth pot.
The third surprise and bonus was that the circuit board was obviously the same as used for the Pathfinder 15R (with reverb) because there was an unpopulated reverb section (more on this in a later post I hope).
I replaced the depth pot and reassembled the amplifier and it worked like a dream. The tremolo isn't very deep, and I can't see myself using it other than on full depth, so I'm going to see if there is a modification I can do to improve that, but otherwise, it's a nice little amplifier.
When it arrived, I plugged it in, and as you guessed, it was not working. Barely any sound, and what sound there was was horribly distorted. One thin I did notice was the the power on LED was not aligned correctly with the hole in the back panel, so I did quickly took it apart, and realigned the potentiometers and LED indicator, but didn't have a look in detail at the circuit. I put it back together, but the problem remained.
My guess was that the problem was the famous "cambridgitis" (just google that term and you'll find all about it). Basically, a problem where the optocoupler that controls the tremolo circuit fails. A common failure on early Pathfinder 15 amps, and a pretty easy one to fix. So I figured I'd fix it when I found time at some point.
Anyway, I finally got around to it, and took the amp apart. My first surprise was that as I disassembled it, the "Tremolo Depth" pot fell out of the chassis. It was broken, assumedly at the same time as someone in the past had clumsily taken it apart and reassembled it (hence the LED not in the hole as per when it arrived). Easy fix - replace the pot.
My second surprise was that the version of the amp I had did not have the optocoupler at all, it was the version of the circuit with a FET controlled tremolo and a CA3080 OTA in the circuit, so it wasn't a case of cambridgitis at all, probably more likely due to the broken depth pot.
The third surprise and bonus was that the circuit board was obviously the same as used for the Pathfinder 15R (with reverb) because there was an unpopulated reverb section (more on this in a later post I hope).
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| broken DEPTH pot and unpopulated reverb |
HI! Do you have the tremolo schematic? I think it's awesome
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