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Many amps use the Vactrol VTL5C3 optoisolators for tremolo. They are getting harder to find and more expensive. Two years ago they were like 5 bucks, now they are 8.50. I've gotta make a digi-key order and they have some Optek optoisolators, I'm wondering if those would work. They are $2.20.
Some of the specs overlap, but I'm not sure what is relevant, in say a Vox Cambridge with Cambridgeitis. I have one of these I'm working on. I'll just put in an OPtek OPI 1264 and see what happens, if necessary, but I thought somebody here might be able to shed a little light on the differences between these two optoisolators: Here's the spec sheets:
http://www.optekinc.com/datasheets/OPI110-113-1264.PDF
http://www.qsl.net/w/wa1ion/vactrol/vactrol.pdf
Second topic: Vox Cambridge 30 Mods
I wish this were the Cambridge 30 twin (2, 10-inch speakers), but it's the one with a single 10 inch. It uses a TDA2050 amp chip for about 30 watts. Channel 1 is similar to the Vox Pathfinder which is a decent sounding SS amp. Channel 2 puts the Channel 1 output through a 12AX7 to "warm it up". I dig this amp for what it is, and I like to tinker, so with $50 invested so far, I figured I'd swap in a LMI3875 chip (52 watts), fix the tremolo (common problem in Cambridge), and see if I could get the footpedal to work.
I have the footswitch working intermittently. The problem is not with the footswitch or jack as far as I can tell (tried 2 different switches and both jacks on the amp [there are 2 footswitch jacks on the Cambridge one on top and one on the back]. I was hoping the tremolo was on the footswitch, but it isn't. The footswitch will toggle CHannel 1 or 2, and the Reverb on /off (which is useless, IMHO).
First thing I noticed when I cracked it open, was that someone has already been in there, and soldered some caps on the back of the board. And isolated the input jack (there are divots filed into the copper pad to solate a few legs of the input jack.
They also replaced the dodgy optocoupler in the tremolo circuit. However, they used the wrong one, and they were lazy and soldered to the old legs, rather than remove the old, which is odd, since they clearly had the PCB out at some point, makes me suspect that the optocoupler fix was after the original mods. I replaced this with a proper VTL5C3, and the trem sounds great.
I took out the TDA2050 and soldered in a LM3875. I need to tweak a few variable resistors (i think) in there. THere's a lot of ambient hum now that wasn't there before. I think I've got the gain dialed way up. THe thing is LOUD now, though and I think way more than the stock speaker can handle, so I'm going to swap in the Emi Ramrod that's in my Pro jr. right now after I get an alnico speaker reconed. The solder job I did was kind of sloppy and the jack complex on the back of the amp tends to mash the pins and wires on the 3875 chip and cause nastiness.
You can see a ton of thermal compound on the heatsink. This was because there was a block of aluminum mounted to the stock finned heatsink. I lapped that, but when mounted, it caused the LM3875 chip connections to get mashed by the rear jack complex. So I took that out for now. I ordered a new chip and some proper terminals that i can use to wire it up so that the amp would be gig worthy. Right now, I'd worry about the unused pins, etc on the 3875 chip bumping each other. I did a sloppy job soldering it in.
I put an Emi Ramrod in there and have been trying to dial this amp in. There's a couple trimpots that affect the tremolo and overall gain. Haven't quite got that sweet spot yet. IN fact I"m not digging this amp all that much. I don't know what the preexisting mods do to the circuit, maybe negative feeback loop related. I'll have to sit down an look at the schematic and see if I can decipher anything (unlikely). In the meantime, thinking of putting the original TDA2050 back in, as I like the amp a lot better before I swapped in the LM3875.
Realize this was a lot to digest, but this forum kept coming up when I google searched topics related to this amp, so I figured I might as well join up and ask.
Some of the specs overlap, but I'm not sure what is relevant, in say a Vox Cambridge with Cambridgeitis. I have one of these I'm working on. I'll just put in an OPtek OPI 1264 and see what happens, if necessary, but I thought somebody here might be able to shed a little light on the differences between these two optoisolators: Here's the spec sheets:
http://www.optekinc.com/datasheets/OPI110-113-1264.PDF
http://www.qsl.net/w/wa1ion/vactrol/vactrol.pdf
Second topic: Vox Cambridge 30 Mods
I wish this were the Cambridge 30 twin (2, 10-inch speakers), but it's the one with a single 10 inch. It uses a TDA2050 amp chip for about 30 watts. Channel 1 is similar to the Vox Pathfinder which is a decent sounding SS amp. Channel 2 puts the Channel 1 output through a 12AX7 to "warm it up". I dig this amp for what it is, and I like to tinker, so with $50 invested so far, I figured I'd swap in a LMI3875 chip (52 watts), fix the tremolo (common problem in Cambridge), and see if I could get the footpedal to work.
I have the footswitch working intermittently. The problem is not with the footswitch or jack as far as I can tell (tried 2 different switches and both jacks on the amp [there are 2 footswitch jacks on the Cambridge one on top and one on the back]. I was hoping the tremolo was on the footswitch, but it isn't. The footswitch will toggle CHannel 1 or 2, and the Reverb on /off (which is useless, IMHO).
First thing I noticed when I cracked it open, was that someone has already been in there, and soldered some caps on the back of the board. And isolated the input jack (there are divots filed into the copper pad to solate a few legs of the input jack.
They also replaced the dodgy optocoupler in the tremolo circuit. However, they used the wrong one, and they were lazy and soldered to the old legs, rather than remove the old, which is odd, since they clearly had the PCB out at some point, makes me suspect that the optocoupler fix was after the original mods. I replaced this with a proper VTL5C3, and the trem sounds great.
I took out the TDA2050 and soldered in a LM3875. I need to tweak a few variable resistors (i think) in there. THere's a lot of ambient hum now that wasn't there before. I think I've got the gain dialed way up. THe thing is LOUD now, though and I think way more than the stock speaker can handle, so I'm going to swap in the Emi Ramrod that's in my Pro jr. right now after I get an alnico speaker reconed. The solder job I did was kind of sloppy and the jack complex on the back of the amp tends to mash the pins and wires on the 3875 chip and cause nastiness.
You can see a ton of thermal compound on the heatsink. This was because there was a block of aluminum mounted to the stock finned heatsink. I lapped that, but when mounted, it caused the LM3875 chip connections to get mashed by the rear jack complex. So I took that out for now. I ordered a new chip and some proper terminals that i can use to wire it up so that the amp would be gig worthy. Right now, I'd worry about the unused pins, etc on the 3875 chip bumping each other. I did a sloppy job soldering it in.
I put an Emi Ramrod in there and have been trying to dial this amp in. There's a couple trimpots that affect the tremolo and overall gain. Haven't quite got that sweet spot yet. IN fact I"m not digging this amp all that much. I don't know what the preexisting mods do to the circuit, maybe negative feeback loop related. I'll have to sit down an look at the schematic and see if I can decipher anything (unlikely). In the meantime, thinking of putting the original TDA2050 back in, as I like the amp a lot better before I swapped in the LM3875.
Realize this was a lot to digest, but this forum kept coming up when I google searched topics related to this amp, so I figured I might as well join up and ask.