So after listening happily since above post I decided it was time to investigate why there was distortion on one channel of phono stage (shows how much I listen to vinyl nowadays...to be honest, lost interested when I fitted a Benz Micro Gold to my LP12/774 arm...really need to refit one of the Asaka's
).
Anyhow, pop open the pre - voltages appear OK, although B+ (260v) to one phono valve seems a little unstable - consider going to cellar to dig out my scope...but don't want to...tap my fingers on top of PSU and bingo, correlates to a spike on DVM. Repeatable. Looks like a dry joint...but see above, its all point to point.
Open up PSU and start tracing - nothing obvious at first sight. But work out what the replacement transformer is doing - each of the original B+ transformers also has a heater winding for the PSU. The replacement is only a heater transformer, the B+ line is now shared for the phono stage from 1 transformer.
As I mentioned above, the transformers are mounted on rubber strips which have gone a little gooey, allowing the transformers to lean over a little. One of the affected B+ AC wires to the strip board had broken (I assume in transit, transformers could flap about alarmingly), but was still making contact to the strip board lug - enough to provide power, but with distortion! Resoldered, tightened down all the transformers to stop them flapping about and swapped the phono stage valves (different date code [late 82 & mid 83 I think] JAN 5751s) with some matched Electro-harmonix 12AX7s.
Result - into my fourth album now, even with the Benz Micro it sounds stunning
So, I have a few questions;
- What is suitable heat resistant rubber to replace the green stuff in the photos above?
- Given a B+ of 260v - what size of transformer do I need to replace the faulty one? What should secondary be? I'd replace both transformers anyhow, and stupidly didn't measure any of the existing transformers (heater and B+ windings). DOH, guess I need to pop PSU open again...
- The line stage has matched Philips ECC82s - I have a set of matched Edicron ECC82s in my collection - worth trying them?
- Then a question on Glenn's design - for the PSU section he chose AC heaters, with a dedicated winding for each of the 4 rails. For the line and phono stage valve heaters he has a single transformer winding, converted to DC in the PSU and 2 wires running to the pre-amp, then with a further regulator per channel (you can see them strapped to base of case by input sockets above) - is it worth taking this further? There's space in PSU for another heater transformer, so I could use one per channel. And I could then have dedicated regulators per valve in the pre? Or I could just add another 2 regulators in the pre to give one regulated rail per valve? Worth it or overkill?
Oh, and I just realised that the two PSU sockets on pre are exactly the same - one provides B+ power to phono stage, one to line stage. I'm going to swap them around so I have dedicated transformers for line stage B+, and phono stage can use a shared one for now. Up until now I have listened to very little vinyl...although that is now going to change I think. I may even finally commission the TD-124 tomorrow
Thanks, Richard