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A customer dropped off a Randall RM50 yesterday and said immediately before it failed completely the preamp tubes were red plating. Intriguing, to say the least. Anyhow, he's usually pretty good with his observations and rather than challenge him on the spot I first wanted to find out what was going on. The preamp heaters are fed off a 7812 regulator and unusually it was passing the 17v input right to the output - I've never seen that happen with a regulator. The 'red plating' he witnessed was due to the heaters running at 17v. The 12V fan that cools the modules with their horizontal tubes had also packed in. Mystery solved.
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Very good detective work !!It's not just an amp, it's an adventure! -
Probable dry joint on the second pin of the 7812 will do that, otherwise the regulator will shut down by design.
Interesting fault!Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can.Comment
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I thought it would be a cracked joint and reflowed them. No change. When I removed the regulator I checked the resistance and the input-output measured 3 Ohms and the common pin open. It's a first for me.Comment
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