Hello,

This continues my previous post. Bearing problem solved (in fact I mixed oil and grease!), thank you everyone!

So when I move the carriage shaft by hand (by turning the carriage motor with two fingers) so that the carriage moves to the left, the deck wakes up.

I can change speeds over.

I can lower/lift the arm and it poses no problem at all. Movement is nice and gentle.

I can get the arm back to the right in small increments.

If I press stop the arm goes quickly to its rest and the deck stops normally.

But the arm doesn't move when I press "PLAY", no voltage seems to arrive at it. The shutter and arm lamps are fine. 

Do I make myself clear enough?

Thanks in advance!

Jacques

When the platter is rotating and the deck is "on", will the carriage move to the left if you press << ?
If not, it could be a problem with the servo motor control, only allowing the motor to run one way.

Martin

No, it doesn't move to the left at all, only to the right.

I'm going to test TR24(25) because I don't see or measure any dry joints in the area or at the connectors. By the way I can't find the PCB layout for my 4002 (the one with the connectors, serial (edited) 054865).

What else is there to this motor control?

Jacques

There are 8 transistors, grouped together on the board, forming the H-bridge circuit for the motor control.
If one is bad, replace all eight.

Your board may look slightly different, there are countless versions, but most have the transistors here:

Martin 

Thank you for your time Martin!

This is my board exactly. What equivalents do you advise?

For instance, BC 547/557 for command, 2N 2222/2907 for power? (you guessed, I have them at hand!)

Jacques

If they are equivalent types and correct NPN/PNP types, they will
be fine, I'm sure. But don't just replace blindly, check if they are the
problem before replacing.

Martin

The transistors are all good...

What next please? I have re-soldered everything and still nothing. I just don't get it. I've been studying the diagram (not that complicated on the paper!) but I can't find anything.

By the way, my 4002 has some funny kind of phototransistor (in a transparent TO92!). Must be the ancestor of all this!

Any ideas then?

Jacques

OK, next thing could be the start button not providing a signal to the electronic switch.
Again, there are many different versions, but check the diodes (and zeners) and capacitors
around the switch transistors (designated 1TR1 in many versions), check the base voltage/signal
when you press start.

Martin

Okay, the deck lights up when I press "play", the motor turns, but it stops as soon as I release the button.

I will check the diodes and come back to you.

Thanks

I have tested the electronic switch: it is alright in all its functions.

OK.
DC voltage across the servo motor when you press play ?

Martin

No, voltage arrives at the first and second transistor (+21.8 v at collectors and +14 v or so at base of the first transistor) when I press 'play' but somehow never manages to power it through. No voltage at the emitter of the second transistor (metal), however tested good. The motor works normally when I press 'back' or 'stop' though (after manually moving the carriage to the left). 

Jacques

The H-bridge directs voltage to the motor, either through the top left two and bottom right two transistors
- or -  the top right two and bottom left two. That gives the polarity change across the motor that controls
its direction.
Either way, the current is controlled by four transistors that all need to function.

I suggest you go ahead and replace all eight.

Martin

Thanks Martin, I had got that...

One hour later... Done!

but no cigar... Same symptoms. Exactly. 

Jacques

Right then. 2N1711 2N2907 don't work at all here. The BC142/143 used here are audio drivers. I've ordered 2 of each. 

Jacques

Hi,

The LED in the carriage position circuit is good but it doesn't light up. The mechanical switch works. Any ideas?

Is this related to the previous problem?

I can't find any schematics for my deck (SN 054865) which makes it harder to comprehend!

Jacques

Don't know if you are aware that it's an IR LED.
You will need a digital camera or similar to see if it lights up.

Martin

Dillen:

Don't know if you are aware that it's an IR LED.

Ah yes! It's because I saw all these posts on how to replace the bulb with a white LED, I was confused! And the service manual only says "LED"!

Thanks Martin!

Jacques

Hi,

Well there's something I don't get. The NPN BC142 (top-left on diagram) keeps burning, losing its base-emitter junction.

Why? The motor, tested, is able to rotate CW or CCW without any problem. Other transistors (new) seem to pose no problems.

Jacques

The low power transistor? Sounds like it's getting excessive drive current. Check the diodes around it, perhaps the base is now stuck high...

tournedos:

The low power transistor? 

The BC237b is fine (it is new, like the 7 other transistors, well, except that burnt BC142...).

The diodes are fine too... hence my puzzlement...

And where are these 10nF capacitors? Does anybody have the real diagram for later decks?

Jacques

Ah, and the bakelite underneath that specific BC142 has darkened. It is not burnt but it is discoloured indeed. It must have heated up before going belly up. My new transistor burnt instantly. It didn't even get hot! No obvious short to the ground. 

Sorry if these are too obvious, but...

How did you determine that the diodes are fine?

And just to make sure, did you test the motor by actually running it with a power supply instead of just turning it with your fingers?

Thanks Mika. Don't be sorry, you're actually helping!

Well I tested the diodes with my diode tester.

Yes the motor was tested running with a PSU, CW and CCW, and it works fine.

I'm really ç"$`ù%  at that deck right now.

Jacques

No problem Jacques... but I think I'll shut up for a while now and ponder this over, it is indeed odd!

The idea I was after with the diodes was that if they simply conducted in one direction but not in the other, their threshold voltage might still be off and prevent the circuit from working correctly...

Have a break yourself as well, one often becomes blind at obvious things once you have stared at a problem for too long