Hi,
Quick question for anyone that is familiar with these units.
When I turn the treble control into the boost portion the output shows a large 33kHz signal on both channels. The reason for my inquiry is it starts to appear exactly where it says "AES" on the treble control position. What is this? Is it normal? I can't see it being good sending a 33kHz oscillation off to a power amp.
Thanks,
-bird
Quick question for anyone that is familiar with these units.
When I turn the treble control into the boost portion the output shows a large 33kHz signal on both channels. The reason for my inquiry is it starts to appear exactly where it says "AES" on the treble control position. What is this? Is it normal? I can't see it being good sending a 33kHz oscillation off to a power amp.
Thanks,
-bird
Seems to be oscillation due to the high impedance and high gain in the circuit. If using a 2V digital source,
you could bypass the entire line stage and connect the selector switch directly to the output coupling
capacitor/volume control/CF. If using phono, the treble control appears to incorporate HF boost
for the various eq curves. The necessary boost could possibly be added instead in the phono stage.
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The unit is anachronistic. 
IMO, source selection modernization is very much in order. Switches in mV. level signal lines are a source of trouble. So, hard wire a pair of RCA jacks to the low level section, which is permanently configured for RIAA. All other I/Ps should be line level and source selection should be done only at line level. A suitable 2 to 6 position (user selectable) Lorlin brand rotary switch can be bought from Mouser for approx. $5.
You have a chassis and plenty of sockets. Lot's of ways to twist and turn are available. A 1 bottle/channel variation on Max Robinson's "unity gain" Baxandall tone control setup would make totally defeatable tone controls easy.
BTW, what did you do for a PSU? Pilot intended that unit to be energized by the power amp. Also, DC heaters in the low level section is a very good idea.
IMO, source selection modernization is very much in order. Switches in mV. level signal lines are a source of trouble. So, hard wire a pair of RCA jacks to the low level section, which is permanently configured for RIAA. All other I/Ps should be line level and source selection should be done only at line level. A suitable 2 to 6 position (user selectable) Lorlin brand rotary switch can be bought from Mouser for approx. $5.You have a chassis and plenty of sockets. Lot's of ways to twist and turn are available. A 1 bottle/channel variation on Max Robinson's "unity gain" Baxandall tone control setup would make totally defeatable tone controls easy.
BTW, what did you do for a PSU? Pilot intended that unit to be energized by the power amp. Also, DC heaters in the low level section is a very good idea.
I think the sp-215 doesn't have a power supply and this unit the sp-216 has one, so I didn't need a separate power supply.
This unit is staying original and is used only used with vintage FM receivers. There was a hum in one of the channels which I fixed but while on the bench I was playing around with it and noticed the oscillation. I am thinking of putting a grid stop resistor snug against the pin. Maybe 10k CC.
This unit is staying original and is used only used with vintage FM receivers. There was a hum in one of the channels which I fixed but while on the bench I was playing around with it and noticed the oscillation. I am thinking of putting a grid stop resistor snug against the pin. Maybe 10k CC.
It would be odd if that helped on only a 30kHz oscillation, but you never know.
Maybe try a larger size, 20k or more.
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100k is starting to roll the oscillation off but I don't think this is the way to fix the problem.
I would like to get this thing stable. What about lowering the value or R30 and R79 to increase the feedback, possibly bypass them with a small value cap? There's got to be positive feedback happening somewhere.
I would like to get this thing stable. What about lowering the value or R30 and R79 to increase the feedback, possibly bypass them with a small value cap? There's got to be positive feedback happening somewhere.
Actually they're passive tone controls.
Depending on the construction technique used, the 3.3M feedback resistor on the first tube
might pick up enough HF to cause oscillation. If removing it fixes the problem, the circuit could be
left that way, or the 3.3M could be relocated to directly at the first grid.
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Lifting the 3.3Meg resistors doesn't get rid of the oscillation.......BUT bypassing them with a small value cap does.
That's a fairly standard technique used with op-amps. High-value feedback resistors can cause HF boost and instability due to parasitic capacitive losses. You may find that keeping those resistors well separated from grounded objects will solve the problem, or that only a small gimmick (twisted wires) capacitance in parallel is sufficient.
Someone else was in the amp before me, maybe things were moved around. All the large multi section can caps were replaced. In that specific area the coupling caps were replaced with large Russian PIO caps with metal bodies. The 3.3Meg resistors runs right along one of them.
Seems like it's a circuit where layout and lead dress is important. I'll fudge around a bit in there and see what happens. Probably just easiest to leave the caps in there though.
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Have you replaced all of the electrolytics? The ones filtering the 150V are particularly suspect as they are part of the Pilot scheme to bias many of the amplifier stages. (They use a combination of fixed + cathode bias all through the cathode resistors 100K/680 ohm.)
I agree with you, this is not a unit to "improve" at a certain point you recognize that some pieces have collector value and this is one of them.
Components that are appreciably different than the originals may result in stray capacitances that were not a problem with the original parts particularly in circuits with lots of feedback.
I agree with you, this is not a unit to "improve" at a certain point you recognize that some pieces have collector value and this is one of them.
Components that are appreciably different than the originals may result in stray capacitances that were not a problem with the original parts particularly in circuits with lots of feedback.
Yes someone else replaced all the electrolytics and the majority of coupling caps.
Yes this is a piece where I just want to get it properly working, no huge circuit changes.
I surmise this is what's happening.
Done tinkering with it today. I will button it up and do some more testing and then give it a listen. Hopefully she's cured.
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