A friend reported intermittent issues with loud pops from one of speakers connected to his Cary 300SE monobloc setup. After some diagnosing swapping leads round to rule out the preamp, the issue stayed with one of the monoblocks. Suspected perhaps intermittent shorting of high voltage capacitor plates.
Brought them over and found quite a number of issues found, in both of them, so well worth a service.
First off, a couple of rattles, one of the springs responsible for ensuring a good contact with the 300B valve pins was floating about inside, this may have been a factor- poor connection?
However then on further inspection, we can clearly see a film capacitor that has got rather toasty – perhaps this is the ‘smoking gun’ we’ve been looking for? It did arc and short a bit when tested on the HV capacitor leakage tester, so maybe!
This was replaced with a JFX 2.2uF 400V premium film cap.
New valve bases were ordered and fitted:
The other monoblock had some other issues, another valve base to swap, 100uF 50V bipolar cap that had swollen, and a burnt out Dale resistor
Replacement bipolar cap (axial lead not available so had to make do with radial) and Dale RN65D 2K resistor fitted (not cheap but very good quality)
The bias was a little hot on these amps at over 110mA (running on our generous 250VAC), a little too hot for liking – a 220R resistor was added to the 1K cathode resistor previously to try to help with this (as some schematic showed 1.2K), however in the end it was decided to swap out the entire 1K Dale 25W mounted power resistor for a 1.5K:
This now brought the bias current down to a more respectable 95mA so the 300Bs no longer redplating.
After all these repairs done, there was still intermittent ‘popping’ going on – and it turns out one of the 300B valves had an issue, as the fault moved with that.
More in the next post!
