A wile back I posted how impressed I was with Modwright- they replaced the innards of my LS100 pre, though I was the 3rd owner.
However... headphone huma dn balance not working so eventually took it to my tech (a forumite but never on here).
Here's where we are...
Hi Dan,
Thanks for the reply. Herewith my findings.
Balance control: As per your partial schematic, the balance control should be grounded..... but it isn't. The diagonal track goes nowhere. I have added a link, see last photo and the control now works as expected. By the way, there was no nut securing the washer on the shaft, although it wouldn't have made any difference to the balance function, even had it been there.
Headphone hum: As I suspected, this is *power supply related. There is insufficient headroom for the L7812CT regulator at 220v ac (=14v reg in), which is more or less what we get here in SA. Boosted with my Variac to 230v ac (=14.75v reg in), the ripple (on the 12v rail) disappears as the regulator comes back into 'regulation', but this is the absolute minimum for ripple-free operation. My choice would be 15v under worst case conditions.
As the set is marked for 240v operation, one could argue that minimum supply requirements are not being met, however, a 10% tolerance is not unreasonable. This gives a lower limit of 216v, (228v @ 5%) definitely not enough for correct operation in either instance.
* Dan, having said all that, I'm assuming that excessive loading by the tube heaters isn't pulling the supply down. I'm not really a valve guy, but I have read that some tubes (heaters) draw considerably more current than others. Of course that's 'forum wisdom', I don't know how true that really is. Adrian loves the sound of this pre and as it's virtually new (after the main and rear boards were replaced), I think we can rule out malfunction elsewhere in the unit.
Short of replacing the entire main toroid, the only workaround I see is fitting a separate transformer for the heaters / headphone amp. If the ripple is not affecting the heaters (or anything else) in a negative way, one could just power the headphone amp (on it's own) off a dedicated supply. One could make do with a much smaller transformer in that case. Certainly, there is plenty of space in the enclosure. Another possibility (headphones only) is to use the 'ripply' 12v and run it through a second regulator, maybe a LDO device. That should give a usable 10..ish volts. You would need to advise on possible component changes to suit, if this option is at all possible.
Grounding: I only found out a few years back that anodising, done correctly, is not electrically conductive, so grounding onto anything anodised is not really a good idea. If you've been having grounding issues, this might be why. For what it's worth, Bose mount amplifier power transistors directly onto anodised heatsinks without any insulators at all, and they get away with it! I did check the grounding on the headphone socket and there is continuity to the chassis, and the socket is really tight.
.... So the balance is sorted but, in the absence of a response from Dan (so far- apart from 'thanks') I would like advice on the headphone output hum.
However... headphone huma dn balance not working so eventually took it to my tech (a forumite but never on here).
Here's where we are...
Hi Dan,
Thanks for the reply. Herewith my findings.
Balance control: As per your partial schematic, the balance control should be grounded..... but it isn't. The diagonal track goes nowhere. I have added a link, see last photo and the control now works as expected. By the way, there was no nut securing the washer on the shaft, although it wouldn't have made any difference to the balance function, even had it been there.
Headphone hum: As I suspected, this is *power supply related. There is insufficient headroom for the L7812CT regulator at 220v ac (=14v reg in), which is more or less what we get here in SA. Boosted with my Variac to 230v ac (=14.75v reg in), the ripple (on the 12v rail) disappears as the regulator comes back into 'regulation', but this is the absolute minimum for ripple-free operation. My choice would be 15v under worst case conditions.
As the set is marked for 240v operation, one could argue that minimum supply requirements are not being met, however, a 10% tolerance is not unreasonable. This gives a lower limit of 216v, (228v @ 5%) definitely not enough for correct operation in either instance.
* Dan, having said all that, I'm assuming that excessive loading by the tube heaters isn't pulling the supply down. I'm not really a valve guy, but I have read that some tubes (heaters) draw considerably more current than others. Of course that's 'forum wisdom', I don't know how true that really is. Adrian loves the sound of this pre and as it's virtually new (after the main and rear boards were replaced), I think we can rule out malfunction elsewhere in the unit.
Short of replacing the entire main toroid, the only workaround I see is fitting a separate transformer for the heaters / headphone amp. If the ripple is not affecting the heaters (or anything else) in a negative way, one could just power the headphone amp (on it's own) off a dedicated supply. One could make do with a much smaller transformer in that case. Certainly, there is plenty of space in the enclosure. Another possibility (headphones only) is to use the 'ripply' 12v and run it through a second regulator, maybe a LDO device. That should give a usable 10..ish volts. You would need to advise on possible component changes to suit, if this option is at all possible.
Grounding: I only found out a few years back that anodising, done correctly, is not electrically conductive, so grounding onto anything anodised is not really a good idea. If you've been having grounding issues, this might be why. For what it's worth, Bose mount amplifier power transistors directly onto anodised heatsinks without any insulators at all, and they get away with it! I did check the grounding on the headphone socket and there is continuity to the chassis, and the socket is really tight.
.... So the balance is sorted but, in the absence of a response from Dan (so far- apart from 'thanks') I would like advice on the headphone output hum.
