Rare Teac TN-400 turntable - speed control repair

sqlsavior

Lunatic Member
I've had a Teac TN-400 Magnefloat turntable since '79 or so.



It went into the closet for a speed-control problem after a few years. Dug it out a couple years ago and got it going, sort of, but it still has speed issues. The adjustment knobs are all the way in one direction, but that is not enough to get the strobe steady. There is a slow drift, and a pulsing to it as well, at maybe 1 Hz or so. So, I've finally dug into it, and hope to get it fixed...



The Teac has a DC motor, and old-fashioned, discrete components, so it should be fixable, I hope.



I cleaned the two pots at the top of the picture ^^^^, which seemed to help - no more wild variations in speed while adjusting, and it settles and stays anywhere in the range now, but still runs a fraction too slow, and still has the pulsing to it as well. There are 3 capacitors on this board - 820 uF, 220 uF, and 10 uF.



The two pots are actually wired to this board with two smaller caps on it, 1 uF and 4.7 uF.



The capacitance meter I got won't measure the bigger two, but the smaller three are all off:

1 uF -> 1.7 uF
4.7 uF -> 6.7 uF
10 uF -> 14.1 uF
 
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I would like to help, but I need a more legible schematic.

Thanks, Mike. That's what I was afraid of. I don't have the original, just a picture in a word doc. I tried rotating and sharpening again, and it made some things better, but it's still pretty fuzzy. If anyone out there can supply a better image, that would be great.

I'm hoping the recap does the trick. I noticed two trim-pots accessed through holes in the motor housing, marked 33 and 45. My plan is to replace the capacitors, set the bigger adjustment pots (the ones controlled by the knobs on top) to the middle of their range, then try adjusting the little trim pots so that the strobe stops drifting. Hopefully the knobs will then be more useful, and if my prayers are answered, the pulsing will be gone as well...

 
The size and lead spacing on those Panasonic film caps could be a problem... There's a 4.7uF Wima with 5mm lead spacing that might be a better fit, but only rated at 50V.
 
Should be good to go on the recap your readings indicate it even with meter error. check solder joints etc. gen lube etc. while in there.
 
Well, after having my tools order shipped 2nd day air, and then leaving it unopened for a day, I went for economy shipping on the capacitors. Mistake. ETA is Nov. 8th, election day. My brother is sending me photocopies of the original schematic, so I should have a legible image up by then.

Now I'm trying to figure out which neon bulbs to order. These are 1" long. They are wired in parallel, each with a 10K resistor in series with it, across the line voltage, for the strobe. Any idea exactly which ones and/or where to source them? A1A? D2A?



Thanks for all the replies. I really want to get this working right!

:music:
 
Neon lamps like those have gotten very difficult to source. I would replace 'em with LEDs, but additional support components will be required. Let us know if Wojo doesn't come through.
 
I think the schematic says this is for 100V power supply. Is your unit for 100V or 120V. Maybe that's what blew the bulbs. I would look at the datasheet and use the recommended resistor.
 
I think the schematic says this is for 100V power supply. Is your unit for 100V or 120V. Maybe that's what blew the bulbs. I would look at the datasheet and use the recommended resistor.

The sticker on the electronics box says 117V, 50/60 Hz. The bulbs are not blown; they are just dim because they are nearly blackened inside. They still work fine, however.
 
what's that black plastic thing with the wires going to it beside the motor, is it the transformer? If not it may have some electronics in it. Also, is it possible to take a cover off the motor? those speed controls probably are on a board which may have other parts on it.

I suspect you may need to dig into this a little and figure out how it works... or you could get lucky and change the electrolytic capacitors and clean the controls and have a working table.

In any case it's nice, and deserves to work. I would start by changing all the electrolytic caps (including those under covers), and use contact cleaner on all the potentiometers. If that doesn't work time to start figuring out what's going on.

This looks like an AC motor, not a DC motor, although it is direct drive (DD). The speed is determined by the controller for the direct drive motor, which is probably under the cover around the motor.
 
Probably been determined by op that suspect caps will be replaced. contact dwojo.. not the greatest web site designer but if he doesn't have or modify I'm sure his bulb list sources are long. Might consider if bulbs are clean maybe too bright a bulb is annoying. might like a low level light listening style.
 
My cap order came yesterday and I installed them last night. The electrolytic caps are smaller than the originals.



The new film caps are larger than the original electrolytics, of course.



The effect of these five cap replacements is that I can now get the 33 rpm strobe stopped near the middle of the adjustment range. The 45 rpm remains outside the adjustment range, however. Sill haven't tried its trim pot yet.

what's that black plastic thing with the wires going to it beside the motor, is it the transformer? If not it may have some electronics in it. Also, is it possible to take a cover off the motor? those speed controls probably are on a board which may have other parts on it.
...

You are correct. Another board mounted inside the motor cover.



Phase II begins...
 
The size and lead spacing on those Panasonic film caps could be a problem... There's a 4.7uF Wima with 5mm lead spacing that might be a better fit, but only rated at 50V.

Thanks. I went with the Wima 4.7 uF for the motor board replacements, and tantalum 5% for the 10 uF values. They should be here soon. I'm not sure whether this recap will eliminate the pulsing of the strobe - with two examples of this TT in dozens of AC situations, it's always been that you can get the strobe to stop drifting, but not to stop waving or pulsing.

Probably been determined by op that suspect caps will be replaced. contact dwojo.. not the greatest web site designer but if he doesn't have or modify I'm sure his bulb list sources are long. Might consider if bulbs are clean maybe too bright a bulb is annoying. might like a low level light listening style.

dwojo seems to specialize in low-voltage incandescent bulbs. I'm going to try a different neon lamp and corresponding resistor first.

I am curious about what I would need for LED replacements, however.
 
Thanks. I went with the Wima 4.7 uF for the motor board replacements, and tantalum 5% for the 10 uF values. They should be here soon. I'm not sure whether this recap will eliminate the pulsing of the strobe - with two examples of this TT in dozens of AC situations, it's always been that you can get the strobe to stop drifting, but not to stop waving or pulsing.



dwojo seems to specialize in low-voltage incandescent bulbs. I'm going to try a different neon lamp and corresponding resistor first.

I am curious about what I would need for LED replacements, however.

Power them through a half wave rectifier and dropping resistor with no filter capacitor, off the secondary of power transformer. I converted an LED flashlight into a strobe I use for servicing record players, works just as well as the old neons. BUT the LED colour spectrum would look just wrong on that old beauty, a neon isn't impossible to find, it deserves to stay original.

That board surrounding the motor is what should control speed. I read up a bit on direct drive servo theory and its super complex. If you have a scope and frequency counter could do some more more in depth troubleshooting - for example check tachometer output.

Also don't forget low hanging fruit like bearing lubrication.
 
If I remember correctly frequency voltage to power LEDs in Technics SL1210 is around 100Hz or just little more..
 
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