To lighten the stress on the zener string, one could increase the value of R66.
I also trimmed R68 to get the U2 output voltage to within the value designated on schematic. This stopped the overheating of V8.
I did notice some instability after a while, with U2 output toggling at subsonic frequency, so I added a 25uF cap across the output to ground and that seems to have stabilized it.
I also trimmed R68 to get the U2 output voltage to within the value designated on schematic. This stopped the overheating of V8.
I did notice some instability after a while, with U2 output toggling at subsonic frequency, so I added a 25uF cap across the output to ground and that seems to have stabilized it.
I increased R66 to 220K to lessen the stress on the zener string.
I was chasing down a subsonic (0.2Hz) oscillation and found that replacing D13/15 helped cut that amplitude down by over 90%.
The stabilizing of subsonic oscillation was also achieved with the addition of a R/C network with a 6.2V zener at the grid of V8. Prior to adding this, there was a large amplitude swing at subsonic frequencies at grid. Completely stabilized by the zener/cap. Prior to addition of parts, this voltage would swing between 1.3V and 21.6V at a subsonic rate, causing phase modulation of the audio output, V8 runs MUCH cooler now, as well.
I'm capped at 4 hours labor by customer budget, so this will have to be the final solution. Posting in case it helps others with this oscillation problem.
I was chasing down a subsonic (0.2Hz) oscillation and found that replacing D13/15 helped cut that amplitude down by over 90%.
The stabilizing of subsonic oscillation was also achieved with the addition of a R/C network with a 6.2V zener at the grid of V8. Prior to adding this, there was a large amplitude swing at subsonic frequencies at grid. Completely stabilized by the zener/cap. Prior to addition of parts, this voltage would swing between 1.3V and 21.6V at a subsonic rate, causing phase modulation of the audio output, V8 runs MUCH cooler now, as well.
I'm capped at 4 hours labor by customer budget, so this will have to be the final solution. Posting in case it helps others with this oscillation problem.
There's some really useful information in this thread.
I've just bought a cheap SP-8 what was smoking one of the cathode resistors (1.82k) on the first valve on the MM input. I tracked down the problem to two capacitors in that channel's RIAA feedback network. The 750pF cap was shorted and the 330pF cap, well wasn't a cap, O/C. With both removed the resistors in and around the MM stage measure similar, one of the 39k cathode resistors on the MM's 6DJ8 has obviously over heated as the other 1.82k cathode resistor smoked. The 39k is measuring 35k now. Both caps and resistor will be changed when the parts arrive. I'd suggest that the RIAA caps were over voltaged and failed.
I've also found today, I've not turned it on yet still checking components, that all but one Zener diodes in the Zener ladder are open circuit, and one is short circuit. So, they are all set for replacement. I guess this is why the RIAA caps have too much voltage applied, but I'm not sure.
I'm struggling to find some decent pictures of a SP-8 Mark 1-3 insides to compare mine with, mine has had all the ARC mods up to the Mark II, which has a different PCB.
I've just bought a cheap SP-8 what was smoking one of the cathode resistors (1.82k) on the first valve on the MM input. I tracked down the problem to two capacitors in that channel's RIAA feedback network. The 750pF cap was shorted and the 330pF cap, well wasn't a cap, O/C. With both removed the resistors in and around the MM stage measure similar, one of the 39k cathode resistors on the MM's 6DJ8 has obviously over heated as the other 1.82k cathode resistor smoked. The 39k is measuring 35k now. Both caps and resistor will be changed when the parts arrive. I'd suggest that the RIAA caps were over voltaged and failed.
I've also found today, I've not turned it on yet still checking components, that all but one Zener diodes in the Zener ladder are open circuit, and one is short circuit. So, they are all set for replacement. I guess this is why the RIAA caps have too much voltage applied, but I'm not sure.
I'm struggling to find some decent pictures of a SP-8 Mark 1-3 insides to compare mine with, mine has had all the ARC mods up to the Mark II, which has a different PCB.
I've been doing a little bit more investigation with this SP-8 and whilst the MM HT is stable, the line stage HT is oscillating +/-6V and never reaching 402V, peaking at 398V, despite there being plenty of HT from the transformer 665V.
I'm going to try inserting an additional few uF and series resistor, about 0R47, into the HT supply, post regulator to see if this stabilises the line HT.
I do need to change the PSU caps as these are still the original ones, just waiting for Mouser to deliver. And I'll try a couple of things from this thread to try to stabilise the PSU.
I'm going to try inserting an additional few uF and series resistor, about 0R47, into the HT supply, post regulator to see if this stabilises the line HT.
I do need to change the PSU caps as these are still the original ones, just waiting for Mouser to deliver. And I'll try a couple of things from this thread to try to stabilise the PSU.
Not sure why I can't quote your post @LuvValve ...
The problems with my SP-8 were primarily fixed by four changes. Adding a 47uF cap across the B+1 (402V rail), changing the 200uF PSU caps, swapping out the valve bases the old ones are very loose, and adding grid stoppers to V2/3/5/6/7/8, especially for V7 and for V8 the valves in the PSU.
Interestingly the Mk 2 SP-8 has these grid stoppers but not the Mk 1.
So you were completely correct.
The problems with my SP-8 were primarily fixed by four changes. Adding a 47uF cap across the B+1 (402V rail), changing the 200uF PSU caps, swapping out the valve bases the old ones are very loose, and adding grid stoppers to V2/3/5/6/7/8, especially for V7 and for V8 the valves in the PSU.
Interestingly the Mk 2 SP-8 has these grid stoppers but not the Mk 1.
So you were completely correct.
Well, I think we are there (I've said that before, but I've put the base cover on now, so it has to be done)....remeasured ripple this morning, all the other crap in the office turned off and unplugged and it's down to 0.6mV on both HT rails.
Plenty of changes, mostly silicon in the PSU section due to the zener stack diodes being kaput, new HT PSU caps, rebuilt MM stage (fixing the shorted RIAA caps and cathode resistor on V1), re-valved with Sovtek 12AX7LPS and Sovtek 6922s (the latter it came with and test okay), fitted grid resistors as fitted to the Mk 2 version of this pre-amp, new valve bases, replaced the 555's tant cap, replaced carbon comp resistors were they had wandered high, replaced the two 1.82M resistors I found O/C (the O/C ones were decent Dale RN70 resistors too), cleaned all the pots and switches and rebalanced the voltage divider R52/R51/R68 (changed R51 from 332k to 348k) to bring the HT voltages up to their correct levels.
It now plays music, doesn't generate any erroneous noises, just has a wicked amount of gain...
Hopefully posting this will help someone else with a misbehaving SP-8.
Plenty of changes, mostly silicon in the PSU section due to the zener stack diodes being kaput, new HT PSU caps, rebuilt MM stage (fixing the shorted RIAA caps and cathode resistor on V1), re-valved with Sovtek 12AX7LPS and Sovtek 6922s (the latter it came with and test okay), fitted grid resistors as fitted to the Mk 2 version of this pre-amp, new valve bases, replaced the 555's tant cap, replaced carbon comp resistors were they had wandered high, replaced the two 1.82M resistors I found O/C (the O/C ones were decent Dale RN70 resistors too), cleaned all the pots and switches and rebalanced the voltage divider R52/R51/R68 (changed R51 from 332k to 348k) to bring the HT voltages up to their correct levels.
It now plays music, doesn't generate any erroneous noises, just has a wicked amount of gain...
Hopefully posting this will help someone else with a misbehaving SP-8.