Power and control of Mitsubishi LT-70 Linear Turntable

Pretty sure that pin 26 is just digital; IC152 (CPU?) outputs a signal, IC151 inverts it, and Q221 inverts it again.

My guess: Let 25 and 26 float.

Ground pin 24 to power supply ground in an easily-disconnectible way, and press the "Start" button on the turntable. If nothing happens right away, disconnect pin 24 from ground.

If grounding pin 24 didn't do anything, connect it to +12 V in an easily-disconnectible way, and press the "Start" button on the turntable. If nothing happens right away, disconnect pin 24 from

+12 V.

I bet "STP" means "Stop". On the receiver schematic, it seems to be a diode-OR of three things, one of which is labeled "TUNER" on IC702. The receiver may have used this to stop the turntable when some other source (tuner, tape) was selected. My guess is that this would have shown up on pin 24 of J106 at the turntable.

"SYNC" may be a way for the turntable to tell the receiver that it was ready to go, so the receiver could unmute, start a tape deck, or something like that. On the receiver schematic, the "SYNC" line goes (among other places) into the "muting" block, through diode D7B3. My guess is that this is either pin 25 or 26 on the turntable.

"AF" I'm not sure of. On the receiver, it goes to "A in PLAYER" on IC702, and also seems to be an input to a diode-OR. It may tell the receiver that the turntable is present and on, whether or not a record is currently playing. My guess is that this is either pin 26 or 25 on the turntable - whichever one SYNC isn't.

Matt Roberds

Hi Matt,

Thanks for the analysis - very helpful.

Pin 26 measures +12V, so looks like it is an output signal, probably the "SYNC" as you say.

The other two, pins 24 and 25, look to be inputs.

I tried grounding pin 24, putting it at -12V and at +12V, with no luck - nothing changes.

Pin 25 - same story. (In the DIN plug this is pin 3, and is shielded by the braid ground. The wire gauge is also thinner.)

It does seem that pin 24, the "STP" pin, should be the one that signals the turntable to play or not. But connecting it to any rail of the PSU doesn't do the trick.

Perhaps this is more subtle.

Julian

Good catch. I didn't think of trying to measure the voltages when nothing is hooked up.

I would advise against connecting it to -12 V unless you have pretty good documentation that negative voltages are OK on that input. The digital logic levels in most consumer devices are 0 volts and some positive voltage; +3.3, +5, and +12 are popular but it can be almost anything.

From the turntable schematic, I would expect pin 25 to be at about +9 volts, if the turntable is in the same configuration as it was when the schematic voltage readings were taken. There should be a note somewhere that describes the conditions: for the turntable, record loaded/not loaded, playing/not playing, etc; for the receiver, tuner/turntable/ tape/aux selected, volume muted/low/high, etc. This note might be in the part of the manual that you don't have, though.

It might be a sequencing issue; maybe STP has to be in a particular state when the power comes up in order for the turntable to recognize a change in state of STP later. Or, worse, maybe it expects the receiver to flick STP off and on once at powerup... something like that.

It's possible that STP is some kind of serial data output from the receiver, but the fact that it participates in that diode-OR seems to argue against that, for me.

If you can get at pin 13 of IC151 in the turntable, or R152, you might measure that point with a voltmeter to see if it tracks changes in the state of pin 24 on J106. With pin 24 at 0 V, the IC pin should be about 7.3 V; with pin 24 at some higher voltage (probably approaching

+12 V), the IC pin should probably go to around 0 to 0.3 V. If this doesn't happen, IC151 may be screwing up.

Matt Roberds

I made a discovery by accident: if I push the plinth/platter on its left ha nd side, everything starts working!

It suggests either that there is a dirty switch somewhere, which isn't maki ng properly unless the plinth is pushed, or that the plinth isn't engaging into its fully closed position.

On the plinth, at the left hand side, are two buttons: "Power" and "Open". With the unit closed, when the Power button is pushed, the plinth slides ou t. Oddly, with the plinth out, pressing the Power button doesn't make the p linth slide back in - should it?

So, I've been gently pushing the plinth back in. It seems to latch when it' s fully in, but clearly it's not making the switch (whose location I can't see) close. Also, in this closed position, the plinth can be moved from si de to side somewhat: this doesn't seem right, as surely the plinth should b e rigidly fixed while a record in being played?

I guess I really need the full manual to understand what is going on. But I am now quite optimistic.

Julian

Sounds reasonable. It might be a mechanical switch; sometimes the metal leaf of the switch is bent, and sometimes the thing that pushes on the switch is bent or missing. Or, it might be an optical switch; these are usually a U-shaped piece of plastic with an LED in one side and a phototransistor in the other; a tab or blade slides into the slot of the U to interrupt the light. (The LED is often infrared, so it may be working fine, even if it doesn't look like it's lit.) Another variation is an LED and a phototransistor in the same flat package; the light reflects off a shiny spot on something that moves over the package.

Also possible. Are there any rubber belts in the drivetrain for the plinth? If so, they may be stretched/floppy and not quite driving it as far as they should.

If it's all gear teeth, it's possible that one side or the other has slipped a tooth, which means the plinth isn't being driven as far as it should be. The service manual may have directions on how the gear teeth should be aligned at assembly; if you decide to change it on your own, mark or take a photo of how it was before you start, so you can put it back that way.

I don't know. I have never owned or used a turntable where the playing surface slid out like that, so I don't know what the usual sequence is.

Depending on exactly what's moving, this might be OK. If the plinth and tone arm / cartridge support move together, then this is just the suspension of the turntable. If the plinth can move relative to the tone arm, then that may require attention... but I'd suggest getting the open/close/latch operation working first.

Does the schematic show a solenoid or motor that would be involved with sliding the plinth and maybe latching it?

Matt Roberds

Julian. The turntable slides open and shut manually on a track loaded with ball bearings. It is not motorized if it doesn't want to shoot open when you realease the latch you will want to reassemble and grease the tracks so it slides readily. There is a ribbon spring to propell the drawer out.

As for the elusive switch you are looking for....it tells the turntable the drawer is fully closed and safe to operate. It is on the left underside o f the turntable. To access it remove the matt on the record platter. You w ill see access holes for a screwdriver. Rotate the platter to see and remov e all screws. The turntable unit is now free from the sliding track. Prot ect all wiring! Carefully lift up. Notice a metal stop on the left track, this engages that pesky switch. Bend it a bit forward toward the front of the unit. It should now make contact with the switch and activate the pla yer when reassembled and closed. Common problem, the stop has been pushed back too far after 30 years of opening and closing.