M90 tape apparently dea

Hi all,
I am currently trying to revive an dirty M90 someone has left in front of a TV shop (no joke!). I have replaced a blown fusible resistor and both belts, but I haven't had any luck in putting the tape to work so far. I guess that the tape is getting voltage, because the music scan light are working, but both the reel and the capstan motor do not spin. I hear the solenoid clicking weakly, whenever I press play or I turn the radio off.
Could someone of you suggest what need to be checked? I would really enjoy bringing this magnificent piece back to life :'-(
Regards,
Yatsuya

The solenoid clicking is a normal sign. The solenoid activates then waits for the mechanism to reset itself. If it does not, which obviously won't occur if the capstan motor does not spin, then the solenoid remains activated ad nauseum until the fusible resistor blows again, so don't run it like that.

Are you sure the motors are still good? If they are, then the next step is to check the driver transistors for the capstan motor : Q709 and Q710. The reel motor driver is IC702, BA6208A. It might be obsolete -- dunno.

Also, there are more fusible resistors. I forget how many but I think there are like 4 total. Also, are you sure you replaced the fusible resistors with the correct value?

It's a fairly complicated deck. Without personally looking at it, I'd say start with these.

I have replaced FR703 with the proper value. However, I am not sure that the motors are good. Is there any simple way to test this?

These are standard 2-wire motors, if I'm not mistaken. Just connect a 9v battery to the motor and see if it spins. Make sure connections are correct -- never reverse the leads or you might damage the internal speed controller.

BTW, if FR703 is blown, I highly recommend you check transistor Q712, which is the regulator in the circuit that FR703 is protecting.

i thought we could always reverse leads on DC motors ??
it would just run in reverse

Should the capstan motor start spinning as soon as I turn the radio on?

redbenjoe said:

i thought we could always reverse leads on DC motors ??
it would just run in reverse

Click to expand...

That would generally be true, IF there were no speed controller built in.

Yatsuya said:

Should the capstan motor start spinning as soon as I turn the radio on?

Click to expand...

On the M90, yes it will run immediately for one cycle until the mechansim resets, then it will shut off and return to the ready position. The exception is when the belts or loose or the solenoid not actuating all the way.

I'll have a closer look at the motors.

Superduper said:

BTW, if FR703 is blown, I highly recommend you check transistor Q712, which is the regulator in the circuit that FR703 is protecting.

Click to expand...

What is a simple way to check a transistor? It is enough to check the voltages indicated in the schematics?

Yatsuya said:

I'll have a closer look at the motors.

Superduper said:

BTW, if FR703 is blown, I highly recommend you check transistor Q712, which is the regulator in the circuit that FR703 is protecting.

Click to expand...

What is a simple way to check a transistor? It is enough to check the voltages indicated in the schematics?

Click to expand...

Maybe. Transistor is an active/dynamic device. The voltages and states will change depending upon what mode the boombox is in. Is it in play mode? Is it in Radio mode? Etc. Voltage specs are typically given for one state only and the schematic typically tells you what mode to put the boombox in before checking specs. I did not check the schematic so I'm not sure what it says.

There are several ways to check a transistor. (1) With a transistor checker (but then if you had one, you wouldn't be asking). (2) Using a VOM (the transistor is basically 2 diodes sharing one junction. -- google how to check transistors using VOM. (3) In circuit operation but this will require you to interpret the data and without an understanding of what is going on, this is harder.

I have now tested both motors with a 9V battery and they seem to be fine. I have then desoldered Q712 and measured with the diode meter. There is a short between emitter and collector. In addition to this, I get a zero reading when testing the hFE. The next step would be replace the transistor, I guess.

I love this guy! Norm, I am eagerly following this thread! Impressed is such an understatement! Yatsuya, you are in goooood hands!

" not just a mere man "

Go Man Go!

I can't wait for the out come of the Abandoned M90

Well, the transistor that shows a short between collector and emitter as well as zero hFE is definitely bad.

Transistors showing 0 hFE is essentially dead since hFE is the beta or gain. As a current amplifier, a zero hFE means that it is not amplifying. On the other hand, a short between emitter and collector should suggest that THAT path is always active since most loads are controlled by the collector-emitter gateway.

Well, let's see if Yasuga posts the results after replacing that transistor. Hope that was the fix, but there might be more to it.

Finally, I managed to get a replacement for Q712. According to a list I have found on the Net, a BD235 should be equivalent to the 2SD439. I went to the electronics shop, but they were unable to give me exactly what I was looking for. Instead, they suggested that I should take a BD241, which should be essentially the same transistor, only it is built as TO-220 (original is TO-126) and therefore manages a higher current. Now I have soldered it in, but unfortunately the capstan motor is still not spinning. I even noticed that, with the new transistor, the led of the photo coupler does not light anymore (before it was weakly lit).
Could it be, that it was not appropriate to replace the original with a type build for higher currents? What type should I take for a replacement?

NTE 373

Superduper said:

NTE 373

Click to expand...

Superduper, many thanks for your suggestion and all the help you gave me so far. As I do not live in the USA, to get a NTE 373 would be not so practical for me. Apparently, a BD139 should do the trick as well, I'll try to find this one.

Yatsuya. Sometimes, a blown transistor, especially if it's shorted, can end up taking out the driver circuits as well. Can you check the voltages of all 3 pins of the transistor in operation? (very very carefully -- if necessary, solder pigtails onto the bottom solder pad and test from the pigtails) Check one at a time and power down each time if necessary otherwise one slip and you could take out that transistor (or other components) again. I am wondering if the zerner diode (D720) has failed and the voltage is now too low.

Don't forget that a properly working diode has a natural voltage drop when forward biased. A transistor is a pair of diodes sharing one junction. With the C-E shorted on the previous device, the voltage drop might have been low, or non-existent which may be why your LED was barely working before. Now, with the diode working properly, it's possible that the voltage drop was enough to no longer light that diode.

Again, you should check the zener diode. When they fail, they almost always fail short meaning the regulated voltage will drop precipitously.

Superduper said:

Yatsuya. Sometimes, a blown transistor, especially if it's shorted, can end up taking out the driver circuits as well. Can you check the voltages of all 3 pins of the transistor in operation?

Click to expand...

The check should be done between one of the pins and the ground, right?

Yatsuya said:

Superduper said:

Yatsuya. Sometimes, a blown transistor, especially if it's shorted, can end up taking out the driver circuits as well. Can you check the voltages of all 3 pins of the transistor in operation?

Click to expand...

The check should be done between one of the pins and the ground, right?

Click to expand...

Yes. Do not use an analog voltmeter. Use a digital VOM with high impedance input.