M90 Successful Deck Repair HOW TO

[Lasonic TRC-920]

More M90 Deck Questions....

I do not want to perform this test, so I will just ask...

What happens if the deck is playing, fast forwarding or rewinding and you turn the power off?

Also, is there a sleep function on this box?

Is this deck shared with any of JVC's home or professional equipment?

 

[blu_fuz]

What happens if the deck is playing, fast forwarding or rewinding and you turn the power off?

I've been told this is the worst thing to do to a full logic deck, even in play mode.

 

[BoomboxLover48]

Great information you shared there Chris!

This will help a lot of M90 owners.

~Royce

 

[Superduper]

More M90 Deck Questions....

What happens if the deck is playing, fast forwarding or rewinding and you turn the power off?
Also, is there a sleep function on this box?

There is a timer function that allows the unit to play tape or record to tape when powered up following an external power timer switching to on. But that's it, there is no sleep function. Most boomboxes that have sleep functions relies on decks that constantly receives power. When activated, the deck also supplies power to the amplifier too, which is why a running deck can power any function including radio, even when the power switch is off. Once the deck auto-stops, not only does the deck shut off, but the amplifier shuts down too. On the M90, the deck does not function if the power button is off, so it can not perform that function.

I believe if you were to switch off the set while it is playing or doing anything else, everything will turn off but the deck will remain in whatever state it was when the plug was pulled. If it was playing, the heads would still be up and contacting tape. If FF or RW, no problem, since that function does not rely the capstan motor, instead it relies on the FF/RW motor. When the set is powered back up, the deck will automatically reset to the "ready" position. In fact, this is a normal feature of the deck, and every time the set is powered on, the deck automatically does a power-on reset. You can probably hear it when you power it up.

Another JVC deck that shares similarity with the M90 is the M80.

 

[Transistorized]

While we are in the process of learning on these decks, I have a question - possible myth.

I have been told that the M90s main drive motor spins all the time when powered up (even if just listening to the radio).

Is any of that true?

I can verify that the capstan does not. It powers up for a second or two during the reset cycle but then stops until powered down. When the radio is turned off it spins for another second during powering down...The main drive motor I am unsure of. I guess it's possible. Not sure if inertia of a currently spinning flywheel is required all the time for immediate action of the decks solenoid functions.

 

[Lasonic TRC-920]

What happens if the deck is playing, fast forwarding or rewinding and you turn the power off?

I've been told this is the worst thing to do to a full logic deck, even in play mode.

I remember reading somewhere the horror stories of doing this.

Great information you shared there Chris!

This will help a lot of M90 owners.

~Royce

Thanks, These community efforts always pay off. Hopefully others will find this thread and apply what is here.

More M90 Deck Questions....

What happens if the deck is playing, fast forwarding or rewinding and you turn the power off?
Also, is there a sleep function on this box?

There is a timer function that allows the unit to play tape or record to tape when powered up following an external power timer switching to on. But that's it, there is no sleep function. Most boomboxes that have sleep functions relies on decks that constantly receives power. When activated, the deck also supplies power to the amplifier too, which is why a running deck can power any function including radio, even when the power switch is off. Once the deck auto-stops, not only does the deck shut off, but the amplifier shuts down too. On the M90, the deck does not function if the power button is off, so it can not perform that function.

I believe if you were to switch off the set while it is playing or doing anything else, everything will turn off but the deck will remain in whatever state it was when the plug was pulled. If it was playing, the heads would still be up and contacting tape. If FF or RW, no problem, since that function does not rely the capstan motor, instead it relies on the FF/RW motor. When the set is powered back up, the deck will automatically reset to the "ready" position. In fact, this is a normal feature of the deck, and every time the set is powered on, the deck automatically does a power-on reset. You can probably hear it when you power it up.

Another JVC deck that shares similarity with the M90 is the M80.

It seems like such a strange thing to me. That JVC put all this effort into this deck and then left it with this huge vulnerability. Does this hurt the deck? Or does it just leave the heads on the tape? Does the tape resume playing after you push the power button? Someone (not me) needs to test this LOL

Also, strange about there not being a sleep function. I guess it was the trade off.

I knew about the M80 sharing this deck. Again, allot of work to not use it in many other systems, especially home systems.

While we are in the process of learning on these decks, I have a question - possible myth.

I have been told that the M90s main drive motor spins all the time when powered up (even if just listening to the radio).

Is any of that true?

I can verify that the capstan does not. It powers up for a second or two during the reset cycle but then stops until powered down. When the radio is turned off it spins for another second during powering down...The main drive motor I am unsure of. I guess it's possible. Not sure if inertia of a currently spinning flywheel is required all the time for immediate action of the decks solenoid functions.

I have not experienced that. I did a ton of testing with the box powered up and never saw that.

 

[Superduper]

As I said, EVERY time you power on the set, it will run the deck one cycle to reset the mechanism to the ready position. I don’t believe it hurts anything if the deck was left in the run position because someone yanked the plug while it was playing, especially since this shouldn’t be regular practice right? If someone is silly enough to tempt fate by making a habit of shutting down the set when the deck is playing, they probably deserve all the ills that befalls them. However, I presume anyone willing to fork out the big bucks to buy something like this, will probably be sophisticated enough to treat it properly.

As for whether a deck left in play mode will resume play when powered back up, the answer is NO, unless the timer feature is set to play. Otherwise as I said, the deck simply resets back to ready position.

Don’t forget, this set was designed to be used with external timers. Timers not only can turn a circuit on, it can turn power off too, so I’m sure JVC figured that into the equation, and possibly one of the reasons why the deck resets at every power on cycle.

While we are in the process of learning on these decks, I have a question - possible myth.

I have been told that the M90s main drive motor spins all the time when powered up (even if just listening to the radio).

Is any of that true?

I can verify that the capstan does not. It powers up for a second or two during the reset cycle but then stops until powered down. When the radio is turned off it spins for another second during powering down...The main drive motor I am unsure of. I guess it's possible. Not sure if inertia of a currently spinning flywheel is required all the time for immediate action of the decks solenoid functions.

The main drive motor IS the capstan motor so you answered your own question. The other motor only runs during FF or RW. On the other hand, there might be some truth to the myth, and it might happen when the deck is broken (slipping belt?). Not sure about M90 but most decks with a slipping belt might not reset leaving motor to run continuously until it blows.

 

[Radio raheem]

done this many times but not with such skill congrats on the m90 chris bro... and yea not on an m90 either

 

[Brutus442]

An invaluable post on the M90 yet again. Sharing your experiences and the assistance/ advice you received, is exactly why this hobby is so great.

Thanks for the pictures Chris...each one is worth a thousand words. Now get that gem outside this summer and BOOM it!

 

[Transistorized]

The main drive motor IS the capstan motor so you answered your own question. The other motor only runs during FF or RW. On the other hand, there might be some truth to the myth, and it might happen when the deck is broken (slipping belt?). Not sure about M90 but most decks with a slipping belt might not reset leaving motor to run continuously until it blows.

Much appreciated Norm. Wanted to wrap that up.

 

[Lasonic TRC-920]

Thanks for chiming in on all this Norm.

An invaluable post on the M90 yet again. Sharing your experiences and the assistance/ advice you received, is exactly why this hobby is so great.

Thanks for the pictures Chris...each one is worth a thousand words. Now get that gem outside this summer and BOOM it!

Summer is just starting and this box is gonna do some Rockin'

Thanks everyone

 

[docs]

And the issue Norm mentioned with the FR704 blowing rears it's ugly head on a unit I am working on.
Only this FR is blowing and I assume from what Norm says that this is related to the Solenoid circuit consisting of D719, C731 (1000uf/25v), C720 (470uf/25v), Q711 and D752.
I'll test these but when they check out ok, like i believe they will, then what?!
As predicted, with a leg lifted the caps and diodes all check out. It isn't as easy checking Q711 because there are no voltages marked on the schematic.

 

[caution]

FR704 burns up from pressing play without a belt. The mode gear can't advance to disengage the solenoid, which is only meant to stay on for a very short amount of time, maybe a second or so, because of the insane amount of current it draws and could overheat the batteries.

 

[docs]

Yeh Eric but this has good strong belts and fries that fr on power on. I think the solenoid transistor might be broke, doing diode tests on it are not ringing true so i’ve ordered one.
I can manually move the solenoid and rotate the flywheel to engage heads, and also to disengage, mechanically it looks legit.

 

[caution]

The IC701 table says pin 15 is "normally 'L' level. When mode is changed, momentarily 'H' level." If you have a scope, ensure you only see this pin pulse high and back down when you press play. It's only got to be on long enough for Q711 to let the solenoid kick the drive gear into action. If pin 15 is working, only creating pulses when told to, then Q711 may not be able to stay closed during low signal input from pin 15.

 

[docs]

Thanks Eric, i’ll pop the new q711 in and if not resolved will test this. Though if fr704 fries before touch the deck controls, will your statement still ring true?

 

[caution]

Before you reinstall Q711, disconnect the solenoid and see if pin 15 is low. Also check the level at the pad for Q711-B, it should also be low, it's tied to pin 15 through R733. Q711's base relies on this connection to pin 15 in order to stay low.

If it's low, then it's fairly safe to say that the control chip is fine and the non-solenoid-related stuff tied to pin 15 is ok, and not shorting to another line and unintentionally pulling things high. Now, reinstall Q711 and check pin B again. If it's still low, then you should be fine to reconnect the solenoid and try powering up, but only for less than a second - just enough time for it to do a normal reset. Feel the fusistor for warmth. It shouldn't even be warm if everything worked. If it is still getting warm, and pin 15 is staying low even at Q711-B, I suppose the coil could be shorted internally and has too low of a resistance but I doubt it. This all is assuming of course that the motor and belts are working at powerup.

 

[docs]

Ok I see, thanks.
I haven’t scoped it yet but will give it a try.
Am I expecting to see a flat line at both pin 15 and Base with solenoid connection wires desoldered?

 

[caution]

Yes, both sides of R733 should look low
The solenoid should not affect the signal coming out of pin 15. If you're seeing a difference on pin 15 when you disconnect/reconnect the solenoid you have bigger problems.

 

[docs]

I wasn't able to confirm this on the oscilloscope. I've replaced Q711, the fr no longer burns up but still no clunk. I guess this has several problems which are beyond my skills. 3. odd volts is on the base of q711 and pin 15 IC701 where I don't think there should be any volts. I noticed the capstan is spinning?