Akai CS-33D motor stopping after a couple seconds

[undsoweiter]

I recently acquired an old Akai CS-33D.
Initially the rewind was not working at all and the ffwd was rather dodgy.
I opened it up to give it a clean but it has now developed a problem where the motor will stop after a few seconds whilst trying to play, ffwd or rwnd.
I stripped the motor and lubricated the bearings to no avail.
I also tried some different belts and that made no difference.
Could it be an issue with the auto-off sensor under the counter, would that cause the motor itself to stop?
One thing worth noting is that someone has at some stage added a varispeed control to the motor although I must say they seem have done rather a nice job, could that be the issue?
Any advice welcome.
Thanks

[vinrads]

Hi the modification would be the first thing I would look at ,is it possible to see how the motor was originally wired if not you will need a circuit, some pictures may help. Mick.

[RogerEvans]

I have limited experience but on one deck I was repairing the problem was that the hubs needed lubricating. Even though the spools turn the tape tension is sufficiently high that the end of tape sensor thinks that the motor has stalled. I don't know this model so this suggestion may be a red herring.

Roger

[Ted Kendall]

Several Akais of this period have a sensor linked to the tape counter which switches the deck to Stop if it isn't rotating. There may be a winking circle of orange lights indicating counter action - if not, look at the counter itself to see if it is advancing. If not, it may be jammed or the belt driving it may be stretched or missing.

[undsoweiter]

Thanks for the suggestions everyone.
So am I right in saying the sensor would actually switch off the motor then if it believes the tape has ended? It does seem like this is what might be happening, the motor runs for a couple of seconds then goes off

The counter does seem to be progressing ok with the moving light thing next to it.
I did also try changing the belt for the counter in case that could be it.

Here's a pic of the inside of the unit

[undsoweiter]

Here's another pic showing the mod to the motor, I'm not 100% but I believe they've simply swapped out a trim pot for wires leading to a larger case mounted pot.

[DMcMahon]

Not familiar with this model but a quick look at the schematic shows 2 switches that have control of the motor power circuitry.

SW2 switches bridge rectifier (diodes D5-D8) output (shown as + 10.4V) through to control transistor TR2. SW2 is called motor switch, I assume it is activated (closed) when tape motion control keys are pressed.

SW7 (a reed switch) controls the switching of TR3 and together with TR4 feeds a tape run control signal through to TR2, the emitter of TR2 driving the motor. Being a reed switch presumably it is switched by a magnet, maybe incorporated in the Tape Run indicator/tape counter which is on the top panel. I assume this is the end of tape/tape not moving control.

So assuming that SW2 is not switching off quickly I would monitor SW7 to see what happens to it when the motor stops.

David

[undsoweiter]

Thanks David.
Yes there is a reed switch underneath the counter mechanism that seems to be activated by a rotating magnet.
I do not have a meter with me just now to test it but that seems potentially the culprit. I will see if I can get hold of one.
Do you know what could be causing it to fail to switch correctly assuming that is the issue?

[teetoon]

Hello. I had this problem with the same model deck a few months back.
There are 2 power supplies 19v and 5.9v. They each have a transistor to control the voltage.
One of these had a short between 2 of the legs.
It's the 2 transistors bolted to the metal surrounding the board.

I'm fairly sure I used something like a TIP 42 ( the same polarity whatever it was ) and it's been fine since.

David.

[undsoweiter]

Was it this one?

[teetoon]

Hi. That is one of the 2. It should measure wrong if it's faulty. There's one each side of that board.
And just to correct myself, it was a TIP 31C I replaced it with as it was available and the right polarity.

Edit: I had a look and it's the one towards the middle of the deck.
I believe it was the 19 volt rail that was missing. I tried to post the circuit but it won't let me for some reason.

Edit 2 : I couldn't see the upload button. It's changed since I last did one.
Hope this helps.

David.

[undsoweiter]

These 2?
Polarity is NPN so would TIP41C be appropriate?
I've also attached the full service manual.

[teetoon]

Yes. I just edited my post and stated that it was infact a TIP 31C I used.
I did download the service manual at the time.

David.

[DMcMahon]

Originally Posted by undsoweiter

Here's another pic showing the mod to the motor, I'm not 100% but I believe they've simply swapped out a trim pot for wires leading to a larger case mounted pot.

You may well be right.

Some cassette recorders with DC motors have a motor speed adjust trim pot at the end of the motor body, sometimes accessible through a hole when there is a cover over the end of the motor.

David

[undsoweiter]

Many thanks David

[teetoon]

Glad to have helped.
I did a lot of head scratching with it as I couldn't see why it was stopping when the 5.9 volt rail was there. It's not until you study the circuit that you see the 19 volt rail needs to be there as well to keep the motor running.

Edit: You are right about the speed trim pot. I adjusted mine after repair to make sure it was correct.
I have a very basic way of doing this, I play a tape in a deck with a mains motor and sing the note to myself and then put the tape in the machine I'm adjusting.
Always worked for me. I've also done it by tuning a guitar string to a note in the music and compared it that way.
I learnt all this before I had access to signal generators and frequency counters etc.

David.