Re: Roberts R707
Valid points, David.
Per safety concerns, a 996, or even a PP9 battery, would contain enough energy to do something awful in worst case circumstances. I should clarify though, I do not sell these items on, or even allow them to be used without my own supervision.
Interestingly, one of my R505s behaved similarly to your description. Once other (trivial, think ‘switch cleaner’) faults were rectified it worked nicely for a few seconds, then audio went all wobbly, and the AC187/188 overheated, emitting actual smoke. On checking, the (adjustable) dc bias conditions were miles off, so transistors going into runaway. Simply resetting the bias and quiescent current per service manual restored normal operation, sounded good again, and AC187/188 stayed cool. I assumed the bias presets had simply been ‘tinkered with’, but given your experience, regardless of the fact it is working nicely, I may check the electrolytic cap conditions in that set sometime soon.
I do not have oscilloscope or even a professional signal generator. But I tend to believe in a transistor audio circuit, as long as bias levels and quiescent currents are in spec, and everything works well and sounds good, there is not that much a ‘scope can add?
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