JVC RC-M80 Tape Capstan Motor running with power off

[Cpl-Chronic]

Hey boombox fixers & lovers. I have a decent M80 & replaced the aftermarket chinese motor with a genuine JVC motor from an extra 838 deck I had salvaged years back. so far all is good & I got the speed right & azimuth lined up. All functions of the deck work well but the capstan motor is running all the time now, when plugged in, even with the power switch off. I know the deck does an initial power up when turned on then stops after a second or so. Why is my deck motor running all the time now. Waiting to put it back together but wondering if there is a stuck leaf switch or solenoid by bumping it around while working on it &/or how do I cycle it out of it's stuck position of ON?

Any advice would be greatly appreciated. =)

Thanks,
Cpl

 

[BoomboxLover48]

I was wondering if you tried to reset it by turning the flywheel till you hear a click. I have worked on M80 deck and cannot remember this problem. I think it has to do with deck resetting! I could be wrong.

Member superlew has also worked on the M80 deck. He was so helpful when I was fixing mine.

JVC RC M80 belts! | Boomboxery

 

[Cpl-Chronic]

Hey! Thanks for the reply but yes, tried that & fiddled with it but still running constantly when plugged in & power off. It has to be a stuck leaf spring or something. Still looking for help on this.

 

[Tinman]

When that one resistor blows on the M90, does the deck constantly run with the power switch off or on?
I believe that resistor used to blow when I plugged it in with the power switch on.
It took me a few times to figure it out.
I ended up soldering the resistor on the back side of the board to make it easier to access just in case it blew.

 

[Superduper]

This is a solid state controlled deck. There are no leaf switches. You may have a shorted transistor. The motor is connected directly to the positive rail through a regulator transistor and the ground for the motor is sourced through a control (switched) transistor. Get yourself the service manual and test/replace those transistors to fix. Again, there are no switches for this deck.

Unless you are referring to the reel motor (this is a 2 motor deck). In that case, the reel motor is powered via an motor control IC. However, since it is running when plugged in, then the capstan motor (previous scenario) is the likely issue.

 

[caution]

When that one resistor blows on the M90, does the deck constantly run with the power switch off or on?
I believe that resistor used to blow when I plugged it in with the power switch on.
It took me a few times to figure it out.
I ended up soldering the resistor on the back side of the board to make it easier to access just in case it blew.

FR704 is the fusistor for the solenoid. Strange that yours wasn't mounted on the back side, that's where it should be. It burns up because the flywheel belt failed and isn't turning the mode gear to its reset position, which shuts off the solenoid. It takes about one second. Anything longer than that and FR704 starts to fry.
Cpl is talking about the M80 though, not the M90

 

[floyd]

When that one resistor blows on the M90, does the deck constantly run with the power switch off or on?
I believe that resistor used to blow when I plugged it in with the power switch on.
It took me a few times to figure it out.
I ended up soldering the resistor on the back side of the board to make it easier to access just in case it blew.

The motor only runs constantly when the power switch is on . you can't stop the motor running without turning off the power. I
guess that's what the problem is. I know The m90 will not do anything unless you press the power button .

 

[Cpl-Chronic]

I don't have a service manual for the M80. Can anyone tell me where this FR704 fuse-istor is & how to test if its blown? Also, what can I use as a replacement if mine is blown? Thanks again for the help guys. =)

 

[Tinman]

I don't have a service manual for the M80. Can anyone tell me where this FR704 fuse-istor is & how to test if its blown? Also, what can I use as a replacement if mine is blown? Thanks again for the help guys. =)

No, that resistor is in the M90.
I just couldn't remember if the motor ran with the power switch off like your M80 is doing.
I thought maybe it could be a similar problem.

 

[Tinman]

FR704 is the fusistor for the solenoid. Strange that yours wasn't mounted on the back side, that's where it should be. It burns up because the flywheel belt failed and isn't turning the mode gear to its reset position, which shuts off the solenoid. It takes about one second. Anything longer than that and FR704 starts to fry.
Cpl is talking about the M80 though, not the M90

I've had several M90s and from what I can remember, all of them were mounted on the same side as all of the other components.
I soldered mine on the back or under side so it can be replaced without having to take the board out to access it.

 

[caution]

Strange! I've never seen it mounted on the top on any of the M90s I've worked on, it's always been on the back with a piece of tape blocking it from the PCB and aramid sleeves on the leads. In fact if you look at the pic of this area of the deck in the service manual, it's an early engineering sample and doesn't even show FR704 yet, so it was a late addition.
The M80 has no fusible resistors.
Here's a pic from an M90 I swapped out Mr. Crispy with a new one.

 

[Tinman]

Hmm.
It's been a while since I've been in an M90.
I do remember several of the resistors having the sleeves on them but I swear my 704s were on the other side because I thought it was an ingenious move on my part by mounting them on the reverse side.
It's possible they mounted them both ways.
Ugh, that pic reminds me of when I recapped one of mine.
What a congested mess that corner is.

 

[Cpl-Chronic]

Well, nothing on the cassette boards looks fried like the picture above...lol...

That thing looks like a burnt hot dog on a stick...I bet Super is right. I've got a fried transistor or something, I guess.. It's a head scratcher because all the controls work like normal. Play, FF, Rewind, Pause, Rec, Stop all function like new, including music search & the sound is quite good too. I just can't figure out what I did to cause this. No idea & the deck was working right before I last took it apart.

 

[Tinman]

Strange! I've never seen it mounted on the top on any of the M90s I've worked on, it's always been on the back with a piece of tape blocking it from the PCB and aramid sleeves on the leads. In fact if you look at the pic of this area of the deck in the service manual, it's an early engineering sample and doesn't even show FR704 yet, so it was a late addition.
The M80 has no fusible resistors.
Here's a pic from an M90 I swapped out Mr. Crispy with a new one.

Ok, I thought maybe I was misremembering but here's an old pic before one of my M90 teardowns.
So apparently they did solder them both ways.
I've worked on four M90s and I believe all of them were on the top side like this one.

 

[Tinman]

Well, nothing on the cassette boards looks fried like the picture above...lol...

Of course not, that'd be too easy, lol.

 

[caution]

Ok, I thought maybe I was misremembering but here's an old pic before one of my M90 teardowns.
So apparently they did solder them both ways.
I've worked on four M90s and I believe all of them were on the top side like this one.

That's wild, thanks for sharing that. I've worked on three M90s and they all had them mounted on the back. I don't even remember holes in the board for them, they just soldered to pads on the back.

 

[Cpl-Chronic]

Ok, so no idea what could be the problem on mine? All functions work great but the motor runs as soon as it is plugged in & even with the power switch off. It's got to be something simple.

 

[Tinman]

Like super said, it sounds like it's shorted somewhere and you'll probably need the s.m. to trace it.

 

[Superduper]

This is a solid state controlled deck. There are no leaf switches. You may have a shorted transistor. The motor is connected directly to the positive rail through a regulator transistor and the ground for the motor is sourced through a control (switched) transistor. Get yourself the service manual and test/replace those transistors to fix. Again, there are no switches for this deck.

^ THIS ^. In particular, check X809, X813. That's all I'll say on the matter, you'll need to do the work on your own. If you don't know how to check a transistor, youtube has tons of videos on how to check transistors. If a transistor tests bad, shorted, change it. If you don't like this answer, and hope for a simpler solution, you can keep asking but the actual fix isn't going to change or get any easier no matter how many times you ask as this is a logic controlled solid state deck. The other possible cause could be if you (or someone else) provided a "true" ground directly to the motor (-), perhaps by way of a jumper. However, since that's a circuit modification, it's "presumed" that isn't the case in my assessment, since I can only assume factory original and have no way to account for undisclosed circuit modifications.

 

[Cpl-Chronic]

Ok, thanks Super. I'm definitely a noob when it comes to troubleshooting circuits unfortunately. I try to sort it out instead of pestering you to death. loL!