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Old 12th Mar 2019, 4:10 am   #8
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Cambridge Audio A500

As said before, it's unlikely that the visibly disrupted power device is the only fault. In dying, it likely connected its base to the power rail at least momentarily. Fault condition currents don't obediently follow expected signal flow paths and spread the destruction.

You have to find and replace ALL damaged parts before you apply power to it. Otherwise your new parts get destroyed by the remaining bad ones. You then think "Ah, there must be another bad transistor" and go checking elsewhere, believing that the part you just replaced must be good because it's new. In this way you get led round in circles needing to replace some parts again and again. This characteristic of transistor amp faults has convinced many repairers to never touch the things again! At £10 a pop, it soon gets very expensive.

Test all the transistors and diodes, check all the resistors. An ESR check of the electrolytics won't go amiss. Some commercial repairers had a policy of just fitting a full new set of semiconductors to a power amp channel. As the rest of the devices are a lot cheaper than the output devices and it saves a lot of time and uncertainty it made sense.

Are those two Rifa smoke bombs on the mains PCB?

David
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