If all resistors are in specs maybe coupling capacitors are keaky , C40 in parallel with C3 or the output caps C41 and C4
You can pull out one end
You can pull out one end
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unfortunately C40/C3 and C41/C4 have been replaced with new Paper in oil capacitors (the same as the line stage has) and I very much doubt they are faulty equally on both sides of the valve Right and Left channel
There is something you overlooked ... the circuit is fairly simple with few components
Remove the diode you put ( the JFET is disconnected ) , remove the 6DJ8 tube so only V2 is in circuit . Still the same ? 🙂
Remove the diode you put ( the JFET is disconnected ) , remove the 6DJ8 tube so only V2 is in circuit . Still the same ? 🙂
R5 is almost 0.5 MegaOhm. C3 and C4 are new caps, so probably not excessively leaky. Any dirt or flux residue buildup where C3/C4 were replaced, or in the vicinity that could change the grid voltage? If you replaced V2, then something external is driving the current through that tube higher than it should be.
Maybe it's a good idea to give the area around the V2 grid circuit a scrub and the whole PC board a rinse with 91+% Isopropanol.
Maybe it's a good idea to give the area around the V2 grid circuit a scrub and the whole PC board a rinse with 91+% Isopropanol.
Phono is almost always quieter than other sources, and was even back when SP8s were new. This is partly because getting enough open loop gain, even with the best optimized circuits (two 12AX7 stages with an output follower, like yours, McIntosh, Marantz, the big boys), to do RIAA compensation is not possible at arbitrarily high closed loop gain. And partly because it doesn't matter - that's what the line stage with volume control was for.
If you really need to satisfy your concerns, you will need to separate the anode of V2 plus its 301K anode resistor from the following FET plus grid of V3. (Pull V3 because it will lose bias.) One way is to snip open one side of the FET and pull V3. Another is to make a small cut across a solderable part of the trace (and still pull V3), maybe in the solder pad at the anode pin socket. This second method would also exclude any board contamination, etc. issues. Things downstream of V2's anode are super high impedance, so should have no effect on anode voltage, and this test will tell us if that is true.
If the test shows us no significant change in anode DC voltage then we will know that no fault is occuring, and we can relax.
All good fortune,
Chris
Thanks for all the amazing support and guidance, I would not have been able to save my old SP8 without it 🙂
So if I remove the soldered back together joint on the FET (after the last snip and test with a diode) on one channel and then remove V3 and test I can compare one side/channel with no V3 and the FET and the other with no V3 and no connection to the feedback circuit through the FET.
I will also do an alcohol wash on what is accessible as the PIO caps are large and obscure some of the circuit (and I don't fancy removing them just for a wash!)
Great!
Rich
So if I remove the soldered back together joint on the FET (after the last snip and test with a diode) on one channel and then remove V3 and test I can compare one side/channel with no V3 and the FET and the other with no V3 and no connection to the feedback circuit through the FET.
I will also do an alcohol wash on what is accessible as the PIO caps are large and obscure some of the circuit (and I don't fancy removing them just for a wash!)
Great!
Rich
You will test if the plate voltage of V2 is still low 110V or "normal" 160V .
Doing that only V2 is in circuit and then you have 3 resistors , grid , plate and cathode and input coupling C40 and C3 . Very simple circuit .
And don't forget to keep the RIAA input shorted so no AC signal is amplified that could change the DC static voltages
Doing that only V2 is in circuit and then you have 3 resistors , grid , plate and cathode and input coupling C40 and C3 . Very simple circuit .
And don't forget to keep the RIAA input shorted so no AC signal is amplified that could change the DC static voltages
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I'm not 100% certain that the FET would survive being forward biased, so snip both channels' FETs. The FETs are only kept reverse biased by V3's current flow. Also, the channel with a forward biased FET (diode) would give a scarily low V2 anode voltage. Unnecessary drama.
All good fortune,
Chris
You could alternatively just scrub the V2 grid circuit area with an old toothbrush, making sure the alcohol doesn't carry contamination elsewhere on the board. But if you can hold the board over a sink (or outside - it's just alcohol), rinsing after scrubbing is a lot easier. The alcohol shouldn't hurt any components, but I would check first how the PCB coating (if any) reacts to alcohol.
So I gave it as much of a clean as I could with neat IPA, and sadly not any difference. Still sitting at 110V at the anode V2 🙁, and 120V at cathode of V3
Cathode of V1 is 1.4V and cathode of V2 is also 1.4V
Very strange! 🤔
Cathode of V1 is 1.4V and cathode of V2 is also 1.4V
Very strange! 🤔
The more accurate numbers are after leaving it to warm up for half an hour
B+ 387
Anode V1 167 : 173
Cathode V1 1.401. : 1.398
Anode V2 118. : 117.6
Grid V2 0.8. : 0.78
Cathode V2 1.355. : 1.351
Cathode Grid V3 : 127.3 : 126.9
It's still a bit of a mystery ?!?
B+ 387
Anode V1 167 : 173
Cathode V1 1.401. : 1.398
Anode V2 118. : 117.6
Grid V2 0.8. : 0.78
Cathode V2 1.355. : 1.351
Cathode Grid V3 : 127.3 : 126.9
It's still a bit of a mystery ?!?
That is a heck of a lot of G1 voltage. If true, coupling caps are suspected. Are these original or some "boutique" replacement?
All good fortune,
Chris
All good fortune,
Chris
" Boutique " They are Arizona Blue Cactus PIO capacitors coupling V1 to V2
They are also in the line stage with no problems matching the voltages (different vuF values to match the design specification)
They were all also over the place when I started with the old metalized polypropylene caps.
They are also in the line stage with no problems matching the voltages (different vuF values to match the design specification)
They were all also over the place when I started with the old metalized polypropylene caps.
You write 'cathode grid V3', and then give two voltages, but you also give two voltages for 'cathode v2' and others above.
what is the cathode v3 voltage, and more important what is the grid v3 voltage?
I will try this later, basically removing the connection of R70 would be an easier way of achieving this if that is acceptable?