Help needed! JVC M70 tape and radio not working!

[BoomboxLover48]

My newly acquired JVC M70 works well on phono and line in connections. I can play Ipod on it but when I move the selector switch to radio I hear the amp working and responds well to the volume controller, treble and bass but no radio signals from any of the bands. No sound when you change from FM to SW or AM. When I depress play button the motor is not running, but audio response is there from both channels when you move the volume controller. The power indicator LED never lights, so also the dial light.

I cleaned the radio/tape/line in/phono switch with deoxit. Then I cleaned all the band switches and power on switch also the same way. When the selector switch is at "tape" position there is a loud hum from the speakers. This hum is gone now when I did some more cleaning on the tape/phono/radio/line in switch. The amplifier turns on when I press play, rew, or FF button, but motor will not turn. Normally when you power up M70 the flywheel make a slight turn, this is not seen. The LED power on indicator is off.

There is no radio signal coming to the amp. No power also on the tape section. I checked the small Cu colored spring leaf with contacts and it looks clean but cleaned it gently with a contact file.

Let me know what else I can try to revive the radio and tape section. I doubt the radio section is powering up or not. If I get the radio atleast working i am a happy camper. I can live with Ipod and radio..

 

[Superduper]

Royce,

The JVC M70 has several voltage regulators, all situated on the main board near the bottom close to the battery compartment.

When they blow, you will have no power at the rail powered by the respective regulator.

X505 and D505 powers the motor and dial lamps.
X503 and D503 powers the tuner.

If you've determined that those items are not receiving power, then you should look into those. If you know how to test the transistor-regulator and zener diode for proper operation, then you should test those components. If you don't know how to test them, then maybe it's easiest if you just swap them out. The orig regulators are probably obsolete but they respond well to replacements since transistors that perform regulator or switching functions are not picky, as opposed to active signal transistors that need specific biasing. If you are replacing them with substitutes, make sure they have sufficient capacity and the pins correspond properly since even though a substitute might have satisfactory electrical characteristics, the pin leads might not correspond.

 

[mmcodomino]

Royce,

The JVC M70 has several voltage regulators, all situated on the main board near the bottom close to the battery compartment.

When they blow, you will have no power at the rail powered by the respective regulator.

X505 and D505 powers the motor and dial lamps.
X503 and D503 powers the tuner.

If you've determined that those items are not receiving power, then you should look into those. If you know how to test the transistor-regulator and zener diode for proper operation, then you should test those components. If you don't know how to test them, then maybe it's easiest if you just swap them out. The orig regulators are probably obsolete but they respond well to replacements since transistors that perform regulator or switching functions are not picky, as opposed to active signal transistors that need specific biasing. If you are replacing them with substitutes, make sure they have sufficient capacity and the pins correspond properly since even though a substitute might have satisfactory electrical characteristics, the pin leads might not correspond.

PLEASE tell me the Conion C100 has something similar!
Because if it does, I just found my problem
My tuner gets no power at all and I believe even though the motors run, the tapes do not give any output and the VUs do not bounce
OH PLEASE

 

[hollyrockets]

I'm sorry your M70 is having issues.

:'-( :'-( :'-(

 

[BoomboxLover48]

I'm sorry your M70 is having issues.

:'-( :'-( :'-(

Hi Holly, This is my second M70. It looks so nice but got problems.

 

[BoomboxLover48]

Thanks Norm!

I will plan on testing and replacing them.

 

[BoomboxLover48]

X503 C=0.14V,E=2.13V,B=0.14V (faulty)
X504 C=16V, E=0.15V, B=0.14V (faulty) :annoyed:
X505 C=16V, E=0.00V, B=0.03V (faulty) :annoyed:

Where can I buy X503- 2SD636, X504- 2SD468 and X505- 2SD439 voltage regulators?

 

[Superduper]

Hold on a second here. Firstly, those regulators you identified may in fact be faulty. But they might be fine too. It looks to me from your voltage readings, like the zeners are all toast. The regulators depend on properly working zener diodes. How much resistance are you getting from B-E, in both directions?

Also, there must have been some form of electrically catastrophic event, to have wiped out all the voltage regulators in that box. Events such as lightning, power surge, introducing 240v to a 120 box, etc. etc. There may be other issues with this box. Hopefully, the regulators blew before signficant damage to the circuitry.

Anyhow, first step is to replace the zener diodes. Use 1 watters. Then power up carefully and immediately test emitter and base voltages. If you still are reading low voltages... power off. There are other issues.

 

[BoomboxLover48]

Here are the resistance readings:


X503 C=0.14V,E=2.13V,B=0.14V
Resistance- Open both ways

X504 C=16V, E=0.15V, B=0.14V
Resistance-4.98 Kilo ohms both ways

X505 C=16V, E=0.00V, B=0.03V
Resistance- 998 ohms both ways

X506 C=15.93V, E=15.25V, B=15.96V (good)
Resistance-3.9 Kilo ohms both ways

 

[Superduper]

Replace D504,505,506.
If afterwards, tuner still doesn't work, then replace D503 as well.

You are on your own now, buddy.

 

[BoomboxLover48]

Thanks! No problem!

I'll fix it!

 

[BoomboxLover48]

FIXED IT!!!!!




Special thanks for Superduper for his valuable input!

It was more complex..


wow! that was not easy at all..

Details later..


And the best of all was when ..I powered it ... Chicago's 87.7 was there with Christmas songs..... what can I ask for more?

 

[BoomboxLover48]

What happend to my dear brothers and sisters?

No reply yet!

 

[Superduper]

Glad you fixed it.

 

[baddboybill]

That is cool

 

[BoomboxLover48]

Replaced faulty voltage regulator transistors and the corresponding zener diodes. Mouser equivalents are the NTE and N shown to the right. X503 and X506 were perfect!

I'm glad Mouser Electronics had the replacements/equivalents.

Voltage Regulator/transistor ------------------- Zener Diode

X504 2SD468 = NTE293 ------------ HZ11B3 =1N4741A
X505 2SD439 = NTE373 ------------ HZ12B1 =1N4742A/ NTE5022

 

[pattrik522]