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 Post subject: JVC 1250R 8 track speed issue
PostPosted: Dec Mon 03, 2018 5:10 am 
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Location: Warner Robins, GA
I have a JVC 1250R 8 track deck I restored with new electrolytic caps, motor cap and belt. I did not replace the motor grommets.

I noticed it sounded off speed and I just confirmed my suspicions tonight when I ran it on my variable frequency AC supply I made for a Sansui record player.

Requires a frequency of 57.8Hz to make it the right speed. The motor cap I replaced with one of the same value as the original got quite warm in operation.

If that cap was off value would that affect speed like it would in some Sony reel to reel decks or is the cap simply to start the motor?

Or could the round belt not be the right thickness?

If it is the belt then I'll save a preset with the proper frequency on the function generator I use with the supply and just run it on the power supply.

What I need to do is find a test tape or another 8 track deck running that the right speed and make my own with a 1000Hz sinewave or perhaps pay a member to make the tape for me.

Recording a tape would require being sure the deck I use is at the proper speed, but I'm extremely pitch sensitive so it won't be hard for me to know if it is off speed.

I like the JVC because it is a great sounding deck.

Also did a mod where I put a transistor between the track change solenoid and track change sensor so that there would be minimal current going through the sensing foil on the tape. It's a bit quieter between track changes as a result given before it was noticeable with a staticky sound in the audio when the sensing foil contacted the sensor plus the transistor extends the life of the foil's conductive surface.

Downside is I cannot use one of those cassette adapters, although I never intend to use one anyways as if I have to play a cassette I'll get a proper deck for that.


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Tue 04, 2018 6:54 pm 
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Location: Fayette County, Pa
The cap should not get warm in normal operation. Since you put the proper value one in, I would try a different one to confirm the problem. I will say though I have seen that problem on those motors if a winding has a few turns shorted. If that is the case, you need a new motor. If it was simply a matter of wrong size belt or pulley alignment there would not be an issue of heating buildup. It would simply run off speed.


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Tue 04, 2018 9:13 pm 
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Location: Warner Robins, GA
The original cap itself was going bad like the motor cap in some Sony reel to reel decks does so it was replaced durimg the restore with the proper film cap. Now unless it is a syncronous motor the cap value might affect its speed so perhaps the cap is near its tolerance.

If the motor had shorted turns id expect it to get quite warm in operation if it ran at all.

Maybe it's the belt thickness as I am thinking that can affect the speed as the belt won't sit far enough down in the pulley and will ride more along the sides.

I do need to replace the motor grommets which were bad and I used washers to keep the motor secured in place. Doubt that could make it run too fast.


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Tue 04, 2018 11:27 pm 
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Location: Boston, MA USA
A failing motor run capacitor can make a motor drag and run slow, maybe very slow! It cannot make it run fast. Suspect your problem is the belt.

-David


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Wed 05, 2018 5:06 pm 
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Location: Fernandina Beach, FL
dberman51 wrote:
A failing motor run capacitor can make a motor drag and run slow, maybe very slow! It cannot make it run fast. Suspect your problem is the belt.

-David

Or... sometimes pinch roller does not apply proper pressure on the capstan. This typically happens on my (previous) Sony TC-630, and TEAC machines which need complete cleaning and re-lubrication of the mechanics. Of course, with an 8-track, it can be difficult to access this whilst it is running.

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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Wed 05, 2018 7:04 pm 
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Location: Boston, MA USA
On 8-tracks the pressure roller is within the cartridge. If the pressure is incorrect the cartridge has failed.

-David


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Wed 05, 2018 7:23 pm 
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dberman51 wrote:
On 8-tracks the pressure roller is within the cartridge. If the pressure is incorrect the cartridge has failed.

-David


Or on machines with an adjustment for pressure it may need a tweak if the problem is with most every cartridge.

The worst are the cartridges with the plastic pressure roller.


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Thu 06, 2018 3:24 pm 
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dberman51 wrote:
On 8-tracks the pressure roller is within the cartridge. If the pressure is incorrect the cartridge has failed.

-David

True. Thinking this through a little more (had coffee now!) in an 8-track, I suspect that running slow would be the failure with the capstan/pinch roller. Make sense?

Reel-to-reel machines usually move the pinch roller. 8-tracks move the capstan?

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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Thu 06, 2018 4:38 pm 
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The capstan stays stationary.

Some have an adjustment that determines how much pressure the cartridge exerts on the capstan.


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Thu 06, 2018 7:46 pm 
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Location: Boston, MA USA
8-track cartridges move the pressure roller by virtue of moving the entire cartridge. When you insert a cartridge into an 8-track player, the tape is pressed against the stationary capstan by the pressure roller mounted in the cartridge behind the tape. Meanwhile the tape is pressed against the head by the pressure pads in the cartridge behind it. All this was motivated by the intended application for 8-track -- music in automobiles with minimum driver distraction. Grab a cartridge, shove it into a slot on the dash, and music issues forth.

4-track cartridges were different. Derived from the existing Fidelipac broadcast cartridge, the pressure roller is mounted in the player, underneath the cartridge. When the cartridge is inserted (RCA) or PLAY is pressed (Gates/Harris) the player swings the pressure roller up against the tape and capstan. Simpler, cheaper 4-track auto players had a lever to pull to engage the pressure roller.

-David


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Sat 22, 2018 7:27 am 
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Maybe it's the belt thickness as I am thinking that can affect the speed as the belt won't sit far enough down in the pulley and will ride more along the sides. <==========

There's the problem. A belt that rides on the high end sides of the motor pulley will play slow. Get the right belt that fits. May need to be just wider to fit in the middle of the pulley.
Adams sells 8 track belts. You have to hurry with an order though. Will be closed at the end of this month.

http://www.adamsradio.com/


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 Post subject: Re: JVC 1250R 8 track speed issue
PostPosted: Dec Sat 22, 2018 11:10 pm 
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Location: Warner Robins, GA
That would make it play fast I do believe. A belt that is too thin will make it play slower.


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