No Voltage at Tape Motor

[Old skewl]

I have a Aiwa CA-70. When I got it, the belt had broke and the playback head was stuck in the engaged position. Anyway I replaced the belts, but I do not have voltage at the motor. I checked the motor leaf switch and it works. It seems to have low voltage(2-3 volts) across the leaf switch terminals. Skippy said that when the belts break, sometimes the motor or the motor power supply burns up. Any ideas? Where should I look next?

 

[baddboybill]

Could be a fusible resistor

 

[Old skewl]

I checked 4 of the 5 fusible resistors on the main board and the resistance is ok. 5th one I can't get to with out removing the board from the box or maybe getting a new multi meter.(I am still using my old analog meter with a broke-off probe.) Any other ideas? I will definitely try to check out the other resistor. Does it sound right that I have low voltage across the motor leaf switch?

 

[Superduper]

.............Does it sound right that I have low voltage across the motor leaf switch?

Don't know what you mean. That expression is used so loosely by non-techs that it's difficult to understand exactly how you did the test. You also shared no data.

 

[Old skewl]

Sorry! With leads on the terminals of the motor microswitch, I am getting about 2-3 volts. Maybe I am doing something wrong.

 

[Superduper]

Sorry! With leads on the terminals of the motor microswitch, I am getting about 2-3 volts. Maybe I am doing something wrong.

I still don't know what you mean. Are you saying using both leads of the meter, one at each terminal of the switch? Or are you saying one probe on a common test point (such as ground) and the other at the switch terminals, alternating?

 

[monchito]

basicly thats what it sounds like ,, should he be getting atleast 12 volts?? if the ground probe is on a ground point and the red probe on the leaf switch??

 

[Old skewl]

I was testing the voltage with the negative lead on one terminal of the switch and the positive lead on the other terminal of the switch. Sounds like I was doing it wrong. I bought a new digital auto ranging multimeter last night. I will take a voltage reading the correct way and let you know what I get. Maybe I can access that 5th fusible resistor with my new meter probes.

 

[Superduper]

I was testing the voltage with the negative lead on one terminal of the switch and the positive lead on the other terminal of the switch. Sounds like I was doing it wrong. I bought a new digital auto ranging multimeter last night. I will take a voltage reading the correct way and let you know what I get. Maybe I can access that 5th fusible resistor with my new meter probes.

That is not a proper way to conduct a voltage test.

(1) You did not differentiate whether the switch was in the open or closed position. The test results will differ.
(2) If the switch was closed, you are measuring "voltage-drop" across the switch. Although this test has it's purpose, it does not sound like that's what you are testing for.
(3) If the switch was open, then presuming that circuit was "hot", then you are testing with the meter in series in the motor circuit. The internal resistance of the meter itself should prevent the motor from spinning even though the circuit has been completed by introducing the meter into the circuit.

First, to properly test for voltage at various points in a circuit, you should have the negative probe at a good power ground. The positive probe can be probed at various locations to determine the voltage at that point in the circuit. For you, you should probably start at the motor itself and trace backwards until you find good voltage. Then determine the cause of the voltage loss. If you find good voltage at one terminal of a switch and poor or no voltage at the other terminal of a switch, then the switch is either in the open position or not closing properly, or the contacts are worn.

 

[Old skewl]

Superduper, thanks for the lesson on proper testing for the lack of voltage. I did test the 5th fusible resistor on the Main CB yesterday. All 5 have proper resistance. I will work my way backwards from the motor and pray I can find the fault.

Thanks again! I will report my findings!

 

[Old skewl]

I have 18.6 volts going into the motor switch and after cleaning the switch I have 18.6 volts coming out. Unfortunately I still do not have voltage at the motor. It does not seem like that complicated of a circuit. The board is still bolted to the tape frame. My only other guess is that there may be a break in the circuit board. Am I missing anything? I was not sure how to measure the semi fixed resistor. All other resistors on the board read ok.

 

[baddboybill]

I have 18.6 volts going into the motor switch and after cleaning the switch I have 18.6 volts coming out. Unfortunately I still do not have voltage at the motor. It does not seem like that complicated of a circuit. The board is still bolted to the tape frame. My only other guess is that there may be a break in the circuit board. Am I missing anything? I was not sure how to measure the semi fixed resistor. All other resistors on the board read ok.

In order to check power at motor the micro switch has to be closed position. 18.6 volts sound awfully high since most motors are 12v. with exception of some being 6-15v What is the dc input power of the actual box? Or how many batteries does it use. Each battery is approximately 1.5v

 

[Old skewl]

13.5V, 9 D-Cell batteries. Here is the diagram of the tape board. Thought is seemed high too.

 

[Superduper]

The 18.6v reading is probably due to being powered on AC. A 18.6v system voltage reading for a 13.5v boombox powered from the mains is not at all uncommon. I have seen Sanyo M9994's (12v boombox) read 19 volts when powered by AC with no load.

Oldskewl: I would suggest at this point that you simply pass this boombox off to a technician qualified to do the repair. The fact that you weren't even able to take a simple voltage test suggests to me that you don't really know what you are doing. This is a 4-wire motor. You said no power at the motor. I have little confidence in your test method to determine voltage at the motor. How did you test for it? Also, what you are looking at is a wiring diagram. The one you should be using is the schematic diagram, which is a far more useful tool to see the circuit path at a glance. If you can't determine whether or not the motor even has voltage, there is no reason to mess with the variable resistors. That variable resistor does not control voltage to the motor.

Don't take this the wrong way. This is not intended to be a knock on your diagnostic abilities. Not everyone is a rocket scientist -- I sure know I'm not. That's why I leave the rocket designing to the scientists. In your case, you don't have the basics down yet and without the fundamentals, it's really hard for others to help you since the fundamentals really helps you to follow along. You are right, it's not that hard of a circuit, and what I told you before about starting at the motor and working your way back -- the diagnostic process really IS that simple. There is some circuitry between the motor and the switch, right? YOU have the boombox. YOU have the schematic or other assistive diagrams. YOU are the man on the spot, so this is something you really need to do on your own. Unless we are also looking at and have that boombox in front of us, and/or the diagrams, all we can do is provide you with general guidance. When we say trace the circuitry between motor and switch for continuity, you must be able to do that on your own since without the schematic and boombox in front of us, how can we give you step by step? EVERY boombox is different. Even if I worked on that exact model, on that exact issue 6 months ago, how could we possibly remember?

BTW, if this comes across as harsh, don't take it personally. Those that know me, also understands that I probably have as much tact as a tree.

 

[Old skewl]

No offense taken Superduper. Obviously this is over my head, which is why I was looking for some help. I would like to find a tech to work on the box, but have not found a tech that is willing to take it on. This was my grandfathers box and I had the same box as a teen. I thought I was just going to replace some belts and get this baby up and working again. I thought I had enough knowledge to to perform some basic tests, and needed a little guidance.

I guess I will put it all back together, unless someone else has some insight or knows someone that will work on it. I have the schematics if someone needs to see them.

BTW I placed the negative lead on the white wire and the positive lead on the green wire to test the motor. I even tried the negative lead on a common ground and the positive lead on the green and then the white. Obviously all were done with the switch closed.