Otari MX-5050 plays fast. Solved !! Next, re-lapping, physical alignment, electrical adjustments and paint job!

[Nashou66]

Hi, I got this a few weeks ago and recently got around to testing it out. All functions work but it pays way way too fast. Speed control does nothing. I tried to get cleaner in the switches as best I could. So searching the web I decided to pull the capstan control board and test the caps. WOW all were very high ESR and some not detected as apart and some had pf values.

here is my unit back side exposed

Parts checker reading on some caps.

Also this model has two belts, one for the counter but there is one on the Take up reel going to some sort of encased switch/box?
Pic below

Could that be the reason it plays fast? the belt is loose.

So I replaced all the caps and still got the same activity, way too fast. I measured the voltages on the capstan board connector pins and the pitch control works, and the 5 volts is there but I get no 18 volt when switching the control. 24 volts is present on 3 or 4 pins.
So I guess I should move to the power supply. Also Not sure which schematics to use. there are so many variations. so I'm going back and forth between a couple. I think I got most I need.

Question, does the speed switch carry the voltage or does it actuate a relay that changes the voltage from 24 to 18?

Any help would be great.

be back in a few hours.

Athanasios

 

[Tinman]

Oh... you got the "Grandpa 5050". The second and third number of the serial is the year it was made. But IT has the Matsushita capacitors in it.... well known to die. AND leak. I generally don't advise a blanket re-cap of an otari (yet) but yours SHOULD have it before you even try to go further. You need a SOLID baseline to really troubleshoot it.

 

[Nashou66]

Oh... you got the "Grandpa 5050". The second and third number of the serial is the year it was made. But IT has the Matsushita capacitors in it.... well known to die. AND leak. I generally don't advise a blanket re-cap of an otari (yet) but yours SHOULD have it before you even try to go further. You need a SOLID baseline to really troubleshoot it.

Ok thats the kind of info I was looking for. Im just hoping it's not one of the unobtanium chips I was reading about.

I also dont like doing a recap until i fix the issue. But in this case I'll get to it. starting with the power supply. The capstan board is done.

Any other tips?

I'll add a pic of the info tag. Some of the lettering is wiped off.

Athanasios

 

[Tinman]

No..... but if you plan to KEEP that deck, re-cap it, at least the rear board first. You don't need to do the audio amp until you fix the rest.
THEN post back to finish troubleshooting things. I seriously doubt the capstan has an IC fault. Could be the pass transistor, OR the tacho head, and possibly the 3 phase coils inside the motor. Read up on otari capstan motor service. But these decks are in general not hard to fix. I HAVE seen the Tacho head go open. That causes full speed. It's basically a mono cassette deck head. You can measure it across the tach wires. Make sure it has resistance. If it does, you can scope the output and trace it through the capstan board. Both the motor and the board can be run outside to make it easier.

 

[Nashou66]

Well I recapped the flip down board except the 100uf 50 volt cap as I only have up to 35 volt, and still the same operation.

I also found a cooked resistor but it measured ok but I replaced it anyhow.

R712

So I'll check the resistance on the sensor a little later tonight.

Athanasios

 

[tapetech]

Burned resistor means something connected to it is defective. A resistor never burns all by itself.

 

[Nashou66]

Burned resistor means something connected to it is defective. A resistor never burns all by itself.

Oh, forgot to mention C706 leaked its guts , it's connected to R712. I bet that did it.

All the Voltage regulators near it seemed to put out the proper voltage.
I'll recheck them tho.

 

[tapetech]

Yes in all 5050 decks the main PS caps need to be replaced even if they appear to be working. They have a high failure rate.

 

[ToyDoctor]

There is an internal switch that you can opt for 3 3/4 + 7.5 ips and 7.5 + 15 ips. Find the switch and that may solve the speed issue. If it's on your model, it will be on the rear board. Some have made a hole in the back so they can switch without opening it up.

 

[Nashou66]

There is an internal switch that you can opt for 3 3/4 + 7.5 ips and 7.5 + 15 ips. Find the switch and that may solve the speed issue. If it's on your model, it will be on the rear board. Some have made a hole in the back so they can switch without opening it up.

Yep, thats not it , tried that first.

Nashou

 

[mcgjohn]

you may want to check the hardness of the pinch roller. If its not compliant you can get this speed issue. Also check the movement of the pinch roller mechanism to make sure its fully engaging with the capstan shaft.

 

[Nashou66]

you may want to check the hardness of the pinch roller. If its not compliant you can get this speed issue. Also check the movement of the pinch roller mechanism to make sure its fully engaging with the capstan shaft.

It is compliant enough I believe and I put Rubber renew on it a few times. and the arm moves freely.
to me it seems the reel motors spin to fast, I dont think its the capstan motor issue. watching my TEac and Pioneer in play mode with no tape they spin slower all set to 7 ips.

Athanasios

 

[tapetech]

The capstan motor controls/determines the tape speed. Reel motor speed/torque has almost no affect on tape speed in play. Is pinch roller tension good? If you put deck into Play with no tape, it should be difficult to stop the pinch roller by hand. Measure reel motor torque with an empty 10 inch reel and a linear spring scale. You should get about 70g on supply reel and about 85g on take up reel. If close, then nothing wrong with reel motor drive.

 

[Nashou66]

The capstan motor controls/determines the tape speed. Reel motor speed/torque has almost no affect on tape speed in play. Is pinch roller tension good? If you put deck into Play with no tape, it should be difficult to stop the pinch roller by hand. Measure reel motor torque with an empty 10 inch reel and a linear spring scale. You should get about 70g on supply reel and about 85g on take up reel. If close, then nothing wrong with reel motor drive.

Ok I'll check when I get home from work.

Nashou

 

[Tinman]

OK.... this is ridiculous... Does the capstan motor change speed and stay there when you change the speed selector switch OR does it keep running at the same speed REGARDLESS of where you set it?
That tells you if there is a mechanical issue or a problem with the motor feedback circuit. Do this in stop mode.

BTW, "I think" is NOT a valid diagnostic term.

 

[Nashou66]

OK.... this is ridiculous... Does the capstan motor change speed and stay there when you change the speed selector switch OR does it keep running at the same speed REGARDLESS of where you set it?
That tells you if there is a mechanical issue or a problem with the motor feedback circuit. Do this in stop mode.

I'll verify this when I get home .

PS: Should this be with Tape or not?

Nashou

 

[Tinman]

with tape, OR lift up the right tension arm so the motor runs. Let's find out if it's mechanical or electrical.

 

[Nashou66]

with tape, OR lift up the right tension arm so the motor runs. Let's find out if it's mechanical or electrical.

Ok hard to tell if was slowing down. So I added a white dot to the capstan wheel and recorded in slow motion then hit the low speed button hoping to see a change. I can not tell. to me it looks the same speed.

What say you gents?

 

[Nashou66]

Pretty sure its the tacho. I soldered wire to the orange and purple wire points on the control board that go to the
tacho.

This way it was easier to measure resistance and voltage and from what I read frequency. But I got only 99mv DC and even lower on AC ( not sure what the voltage would be DC or AC), also I got OL on the resistance.

Now where do I find this part?



Athanasios

 

[Tinman]

OK, the slow motion didn't tell me what speed it was running at. You WILL need to deoxit the HELL out of those switches, as they are NOTORIOUS for causing either a no-run, or no speed change like yours. Always start at basics. You clearly have NOT read up on the 5050, as all this has been documented to death.

The tach.... (sigh)..... you need an oscilloscope.... you are looking for a sine wave, basically. A meter tells you nothing. Did you even clean the switches? They can be INCREDIBLY stubborn. A squirt of contact cleaner and 3 presses won't do it. More like a HUNDRED! This is all documented.

BTW, R705 in your picture also looks smoked.