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Started by PoorOtis, October 29, 2013, 01:51:21 PM
QuoteAfter the amp started to get into customer's hands, rumors circulated about a "hot rod" output amp chip that could be installed in the Cambridge 30 to double the output power to 60 watts. I know that some people changed the standard TDA2040 output amp chip to a TDA2050, but I would not recommend this modification. It is my opinion that the Cambridge 30 power supply does not have enough current to support the additional power draw from this chip. It is mentioned it here only as a part of the history of this amp.
QuoteA common Cambridge 30 service issue involves a severe loss of power and increased distortion caused by the failure of the tremolo opto-isolator. This gets a little technical, but I will try to explain it in simple terms.Most amps that have tremolo utilize an opto-isolator. An opto-isolator has two components. It has a small light bulb or LED and a photo resistor. The light bulb faces the lens of the photo resistor and is in a sealed case.A photo resistor varies its resistance to an electrical current depending on the amount of light striking it.The tremolo circuitry in the amp pulses the light bulb in the opto-isolator. These pulses of light cause the resistance in the accompanying photo resistor to rise and fall from 0 to as much as 2 million ohms. This varying resistance is applied against the guitar signal in the preamp, causing the signal to "pulse" in level.If the opto-isolator fails in the Cambridge 30, the resistance in the photo resistor rises to the high limit for the part, causing the amp to severely lose power. The amp is basically "stuck" in the quiet part of the tremolo cycle.This problem can be remedied by replacing the stock opto-isolator with either a Fender Opto (part number 16282) or a Vactral Opto (part number VTL53/2). I don't consider this a "home hobbiest" sort of repair. I suggest your local amp tech handle the "opto" exchange.
Quotethe output bumped up to 68 watts.
QuoteIt would be nice to show schematics of the changes but out of respect to all parties I'm not going to.
QuoteAn easy modification to achieve a higher output is to replace the TDA2050 with a LM3875 made by National Semiconductor and increase the output to 52 or 56 watts.
QuoteIf for some reason you are using different speakers or running a separate cabinet you can bump up the power to 68 watts. This involves much more work though. The TDA2050 can be replaced with a LM3886
QuoteThe Cambridge, of course, was also (in)famous for "Cambridgitis", which was caused by the choice of an inadequate stock optocoupler, which would fail when the amp was driven hard for 15-20 minutes.
QuoteFor Thomas Organ amps but may be useful