SL-1200 DC Power Supply DIY

[Co&]

Just for interest - when I recapped my SL-1200Mk2 I checked all removed capacitors and everyone has passport capacity with tolerance +5..-10% and suitable ESR (I compared ESR with new ones). However, caps should be revised first of all anyway.

Hi Khom,

I think I did not pay enough attention here. Did you record the desired ESR for each cap? I'd be very interested in the types and values you used.

Boy does my deck sound great now...yet only for the first 60 minutes or if I'm lucky 90 minutes. Then something is warmed up enough to start oscillating and mess up the entire sound. This must be related to the board and the caps since I've had very different regulators in place which all exhibited this same issue.
I did take some measures and almost thought the issue was solved only to discover a bit later that it persisted.

My assumption is that the filter and signal couple caps (around FG, 6680 and 6675) do not play a role in the oscillation and it is all associated to the caps used in the power supply or armature coil decoupling. But I might be mistaken.

I would like to solve this issue any help is appreciated!

[Ianmac]

We have made good progress in the last few weeks

Can I try and help others coming to this thread with a brief summary of where we are with the techie PSU

The early part of this thread covers the removal of the existing stock transformer and the various options with regulators.
In my book the only way to completely eliminate transformer vibration and stray magnetic fields from the transformer is to remove it.
To avoid further repetition I'll start from a situation where unregulated or regulated DC is fed into the techie.

Level 1 Build a good 21v regulator ( Reg 1) to supply the electronics. Remove the Techie regulator components. Fit this inside the Techie (the old transformer well is ideal.)
Level 2 Build a second identical 21v regulator (Reg 2). Cut one track on the main PCB and power the Control electronics from Reg1 and the Motor Drive electronics from Reg 2 . Again fit this inside the Tfr well
Level 3 Build a Sub regulator (Sub 1) to feed Q203 the FG Amp. Sub 1 is 9v and fed from Reg 1. I managed to mount this on the main PCB
Level 4 Build a Sub regulator (Sub 2) to feed IC 302 Pitch control on /off. Sub 2 is 5.6v and fed from Reg 1. I have my sub regs mounted on the two screws used by the original Techie regulator/heatsink.

At this point we are close to getting the Platter rotation as good as we can.

Level 5 Build a Sub regulator (Sub 3) to feed IC 301 Pitch control IC301 .Sub 3 is 9v and fed from Reg 1

Level 5 seems to me to be a bit of overkill as it is only active when Pitch Control is in use and I have never used PC when using my deck for Hi Fi

The Bare Bones system
If you do not want to use your TT in Pitch control mode
IC302 and IC301 and associated fader /switch are redundant.
They can be disabled or removed.
One links Pin 2 and 3 on IC201 to ensure normal operation.

We are all using different Regulators I/C's but achieving similar end results.

In my case
Reg1 and Reg2 are identical both set to 21v I used LM3081 regulators
Sub reg 1,2, 3 and 4 used LM3082 regulators.
These regulators are from the same family and the output voltage is set in theory by one resistor but in reality an additional multi turn trimpot is need to accurately set the voltage

[dimkasta]

OK feel free to call me a moron...

A while back I changed C210 to 550nF following some random "mod" advice I found somewhere.

I changed it back to 220nF and the tt is rotating normally once again

I will leave it running for a couple of hours on my bench to see how it behaves and then try to have a listen

Co&
Do you still have D301 installed? I remember reading somewhere it can overheat and create issues.

I also remember reading a suggestion to put a small heatsink on 6680.

On the regs part, I would check the datasheets to see what the chips like to see on their output. There might be an oscillation that is slowly heating up a reg, pushing it to its overheating protection zone. Such temperature should be easy to feel with your fingers if you do not have a DMM with a thermocouple.

[Ianmac]

Good you've got it fixed.
Ill keep watch on the temperature on my 6680

[dimkasta]

OK my sl1200 plays sweet music for about 1 hour now.

I cannot comment on what effect removing the pitch circuitry has though. I have new speakers, new pre and new amps from the last time I played some vinyl.
Not to mention everything needs to break in a bit, including my ears.

I can say that I like what I hear so far. Most things I expected to hear on my favorite recordings are there, and I cannot perceive any issues on the sound or on the speed stability.

[Khom]

I think I did not pay enough attention here. Did you record the desired ESR for each cap? I'd be very interested in the types and values you used.

Boy does my deck sound great now...yet only for the first 60 minutes or if I'm lucky 90 minutes. Then something is warmed up enough to start oscillating and mess up the entire sound. .................
My assumption is that the filter and signal couple caps (around FG, 6680 and 6675) do not play a role in the oscillation and it is all associated to the caps used in the power supply or armature coil decoupling. But I might be mistaken.

Hi, Coen

I didn't saved measured ESR data for old capacitoors, just comparen them with new ones. New capacitors are (AFAIR) mostly CapXon, sseveral Rubicon's and Hitano - I'm don't worried about 'audiofile' caps because of I measure them before soldering and anywhay the final arbithre is sound, any wrong component instantly degrades it.

I agreed with you that FG and PLL filter details can't affect to your issue (at least while both stages are in non-selfoscillating mode). It rather looks like caps or even regulator warming. My 317 has about 50 C temperature after work one hour+. Another (and may be first!) possible weakness is soldering, try to revise all soldering points.

We are deepened into such technical discussion here... is it correct?

[Co&]

OK feel free to call me a moron...

A while back I changed C210 to 550nF following some random "mod" advice I found somewhere.

I changed it back to 220nF and the tt is rotating normally once again

I will leave it running for a couple of hours on my bench to see how it behaves and then try to have a listen

Co&
Do you still have D301 installed? I remember reading somewhere it can overheat and create issues.

I also remember reading a suggestion to put a small heatsink on 6680.

On the regs part, I would check the datasheets to see what the chips like to see on their output. There might be an oscillation that is slowly heating up a reg, pushing it to its overheating protection zone. Such temperature should be easy to feel with your fingers if you do not have a DMM with a thermocouple.

Great you got it fixed!

D301 is still in, it could be the source of excessive noise. If time permits it will be replaced by a LM431 low noise zener.

I think C217 also may be causing troubles since the C209 (100uF) already has very low ESR. If this doesn't work out I will move the cap to the pins on the 6680.

The CCS in place of R201 gets way too hot for its dissipation. Oscillation may be at order but there is already an extra 100R resistor between the CCS and the 6680 that should damp this. Replacing that one too for a new one I tested at the desired current and dissipation. This new CCS doesn't even nearly get as hot.

Temperature of all big chips seems ok (lukewarm).

[dimkasta]

What chips are you using for regulation again? 317 right? There is some silly inductance in those swirly pcb traces. Moving caps next to the chips might be a good idea. Including output ones

What cap are you using on C216?

I would also check R304 CCS. This one should get hot as well. And it seems a bit lower than specified on the schematic ( if my calculations are correct ~28mA )

D301 is still in, it could be the source of excessive noise. If time permits it will be replaced by a LM431 low noise zener.

Try a series reg too.

[Co&]

@khom, @dimkasta: Thanks for thinking along. The TT is turning for some hours now and sound is great even after warming up.

These are the (planned) mods I did:

- replaced D301 with LM431 (2k resistors for set)
- disconnected C217
- replaced the 317 CCS for R201 for a new one (but this doesn't matter)

and.... I thought of a new tweak and implemented a first step while the TT was open anyway. I've called it the "final frontier tweak" since I boldly went were no-one has gone before (I think).

The idea is that the reference signals seem to matter much to the TTs performance. Now the 6675 injects a signal (OSC) into the position coils and the current for this signal is provided by the 21V "bus". My hunch is that a compromised 21V will interfere with the precision of the detection signals that are generated by the coil secondaries. Iow the 21V supply has improvement potential.

The only thing I did today is cut the 21V trace at the first coil and put a separate wire to the capacitor at the output of the 317 that cares for the 21V. This means that the oscillators current still runs via the ground on the PCB, but it seemed to me a simple first step.
Now I cannot isolate the effect of this tweak since I did it together with the above ones. There is some clue though that it the theory works because I first connected it to the output pin of the regulator and moved the wire later to the capacitor (which is still near the pcb). To me the sound obviously changed by this action.

@Ianmac: I think you might add another regulator to your list.

[Ianmac]

Trying to reduce the reg count !
I already have a dedicated reg supplying only the 6675.
Havent used a LM431 Ill have a look as soon as I get time. What are the advantages of constant current to a Zener over a low noise voltage stabilizer for the Cmos Chip IC302?

I'm having difficulty " hearing" the effects of additional tweaks. Any way we can measure the platter rotational accuracy and stability?
Would a 1K tone from a test record into a spectrum analyser help as LF and HF speed variations should show up as sidebands ?
I have ARTA and a pico scope and access to an unmodified Techie