Adding a little life back into this thread,...
I too have an old SP-8 that has the same problem as @tonescout . The voltages in the line stage, after the anode resistors, wander around, by at least 10-15V over a period of say 4-5 secs. This is with the inputs shorted. I just can't figure out how to make the thing stable. Audio wise, sound comes out, but it is full of spits and splats.
The original fault was that one of the cathode resistors in the MM stage had smoked out. I bit of detective work found that the caps in the RIAA feedback had shorted and overloaded the poor cathode resistor. I replaced these with like for like. Also, I found that all bar one of the zener diodes in the zener stack was O/C and one S/C. I replaced these too.
I also replaced the main PSU caps as these were the originals as well as the 12BH7 valve as it tested a little tried.
Turned it on and no smoke, and some audio 🙂 with numerous spits and spats.
Checking voltages around the cct indicated that the B+2 what slightly over voltage (+394V), the B+1 was under voltage (+398V) and the voltage from the top of the zener stack was about 50-60V undervoltage. I checked all the silicon in the PSU, no shorts, the 24V reg appeared to be running (23.8V), just the after anode resistors in the line stage wandering all over and the same oscillation on the pre-amp's output, lower voltage, but still.
I've attach a marked up cct diagram.
I was wondering what the conclusion was to @tonescout issue in this thread and whether it could be the same one that is ailing my SP-8.
Replaced:
Zeners diodes in the stake and elsewhere in the PSU
Signal diodes in the PSU
2N6316
2n5088 driver for the 2N6316
U1 and U2 TL071CPs
C34
R61 (out of 5% spec)
R68 (out of 1% spec)
I've also tried adding a simple RC (680R/220uF) network prior to the B+1 HT for the the line stage (V4/5/6). Voltages after the anode resistors for V4/5 wandered about just the same.
Also, I noticed that I'm measuring a +/- swing of about 15mV at the output of C9, into V5's grid. That does seem strange.
I'm wondering whether to add grid stopper resistors to V5/6 as is installed in the Mk 2 version of the SP-8.
I too have an old SP-8 that has the same problem as @tonescout . The voltages in the line stage, after the anode resistors, wander around, by at least 10-15V over a period of say 4-5 secs. This is with the inputs shorted. I just can't figure out how to make the thing stable. Audio wise, sound comes out, but it is full of spits and splats.
The original fault was that one of the cathode resistors in the MM stage had smoked out. I bit of detective work found that the caps in the RIAA feedback had shorted and overloaded the poor cathode resistor. I replaced these with like for like. Also, I found that all bar one of the zener diodes in the zener stack was O/C and one S/C. I replaced these too.
I also replaced the main PSU caps as these were the originals as well as the 12BH7 valve as it tested a little tried.
Turned it on and no smoke, and some audio 🙂 with numerous spits and spats.
Checking voltages around the cct indicated that the B+2 what slightly over voltage (+394V), the B+1 was under voltage (+398V) and the voltage from the top of the zener stack was about 50-60V undervoltage. I checked all the silicon in the PSU, no shorts, the 24V reg appeared to be running (23.8V), just the after anode resistors in the line stage wandering all over and the same oscillation on the pre-amp's output, lower voltage, but still.
I've attach a marked up cct diagram.
I was wondering what the conclusion was to @tonescout issue in this thread and whether it could be the same one that is ailing my SP-8.
Replaced:
Zeners diodes in the stake and elsewhere in the PSU
Signal diodes in the PSU
2N6316
2n5088 driver for the 2N6316
U1 and U2 TL071CPs
C34
R61 (out of 5% spec)
R68 (out of 1% spec)
I've also tried adding a simple RC (680R/220uF) network prior to the B+1 HT for the the line stage (V4/5/6). Voltages after the anode resistors for V4/5 wandered about just the same.
Also, I noticed that I'm measuring a +/- swing of about 15mV at the output of C9, into V5's grid. That does seem strange.
I'm wondering whether to add grid stopper resistors to V5/6 as is installed in the Mk 2 version of the SP-8.
I'm a complete newbie with valves, but I would guess that the change would be due to a change in the grid voltage of that valve. Should I try to ground the grid of V4 and see if that settles the after anodes resistor voltages?
I found out my instability in voltages was due to a leaky coupling capacitor(well actually a secondary bypass capacitor). I found some NOS Jensen 0.082uF I was using rated at 600V DC were not holding DC properly. I took them out and things returned to normal, but annoyingly they sounded better in circuit than out, so I have left them in on V1>V2 where they had less voltage annoyance and I took them out of the phono stage. Also previously the FETs used as diodes were also wrong (a repair man used the wrong ones and they failed causing strange voltage issues)
I also found with the phono stage I had to swap out a lot of the silicone to make it behave properly, and measure correctly. I just changed as much as I could and did not 'find' the root cause as it's quite cheap.
Happy soldering 🙂
I also found with the phono stage I had to swap out a lot of the silicone to make it behave properly, and measure correctly. I just changed as much as I could and did not 'find' the root cause as it's quite cheap.
Happy soldering 🙂