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 Post subject: 8 track player/recorder modification
PostPosted: Mar Fri 02, 2018 5:13 am 
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I have a JVC 1250R 8 track player and recorder.

I noticed how some 8 track tapes have a lot of the conductive part of the foil sensing tape gone in spots.

That got me to thinking. A lot of 8 tracks use the track change sensor to directly operate the track change solenoid.

Mine does just that as seen in the schematic.

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What I want to do is use the track change switch and sensor to operate a transistor so that the sensor doesn't have to pass the current of the solenoid which may prolong the life of the foil tape.

B+ is 25.8Vdc, but drops down to 11.5 Vdc when the track change solenoid is actuated

How would I do that without disconnecting the switch and sensor from ground?

I'd like to use only one active device if at all possible, but if I need two that will work as well.

EDIT:

The schematic is actually wrong as one side of the sensor does go straight to ground.

The CONT PLAY-REPEAT-AUTO EJECT switch must then be between the solenoid and one contact of the sensor.


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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Fri 02, 2018 5:37 am 
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I would rather use a relay, there are some fairly small 12 volt types available, as well as the automotive ones but space could be an issue.

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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Fri 02, 2018 6:06 am 
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I thought about that, but I want to pass as little current through the sensor as possible.


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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Fri 02, 2018 6:30 am 
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Put in a PNP emitter follower.

emitter to solenoid.
Base to sensor
collector to ground.

Add a 10k between base-emitter to help shut off.

something like a 2N2907A or 2N2906A will do.

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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Fri 02, 2018 2:22 pm 
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That's exactly what I did this morning.

Found a 2N3741 and tried that.

Didn't use any resistor though.

Just took the base to ground and the solenoid actuated.

I may try a resistor if it is absolutely necessary.

The sensor is just two contacts and the track change pushbutton switch is basically the same.

When I would ground the end of the solenoid the switch and sensor connects to I'd see a little spark when grounding and ungrounding it so I can only imagine what that would have done to foil sensing tape over time.

With the transistor in circuit grounding the base produces no spark whatsoever.


EDIT:

Tried a 6.8K resistor and the transistor wouldn't turn on enough to operate the solenoid.

Went back to just shorting the base to ground.

I suppose most 8 track decks didn't do this because it meant an extra component plus another part to potentially fail.


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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Sat 03, 2018 2:38 am 
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LOL, I've never seen foil on a 8-Track fail from useage... I've seen the contacts worn so badly it didn't operate or maybe double change, sometimes the foil falls off due to dried out adhesive...

Problems such as deterioration of foam pressure pad, or rubber rollers that turn to jelly are far more of a concern...

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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Sat 03, 2018 3:37 am 
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Next step would be to remove the mechanical sensor and replace it with a reflective LED/Phototransistor sensor connected to your transistor. It would be touchless. I guess they didn't have those back when Bill Lear invented this doohickey.


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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Sat 03, 2018 3:42 am 
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I would use a mosfet. High input impedance so virtually no current save for a brief peak having to pass through. The sensor would just be one end to positive voltage and the other to the gate of the mosfet. The mosfet gate/drain just goes where the sensor used to be connected. Make sure to add a gate drain resistor.

In reality though I doubt this is what causes that stuff to wear down.

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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Sat 03, 2018 5:53 am 
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I've seen a few tapes where the foil had some spots missing.

Figured it was from the current passing through the foil.

I did clean the head and sensor then removed the capstan and cleaned then relubed it once it was installed. Never did that during the initial restoration so I thought now was a good time.



EDIT:

Tested with a tape last night and the results are good.

Before when it would change tracks using the sensor I would hear a bit of staticky noise (with the tape I was using to test, but don't know if all tapes did that) which might have been the foil contacting the head and sensor. Now the staticky noise is completely gone.

Was thinking of replacing the track indicator lights with LED bulbs since they are the screw in type, but after measuring around 5 1/2 Vac on the bulbs which are rated at 8 Vac I decided to leave them alone as I doubt they will be failing anytime soon.


Hadn't thought of a MOSFET, but when using the transistor I got no visible spark whatsoever when I shorted the sensor contacts or actuated the track change switch so I'm pretty well satisfied that the problem is solved.


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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Sat 03, 2018 5:24 pm 
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Foil tapes do fail fairly often in my experience and many times on the very first track change when played again for the first time. When I get a new tape I play it until the first track change and immediately pull out the tape to replace the foil. About 50% of the time the old foil breaks on the first track change.
I've observed the failure is due to one of two reasons.. either adhesive failure of the foil sensor tape backing which is rare or more likely due to flexing of the foil at the very small gap between the two pieces of tape which eventually leads to a break from flexing in that area. Think about it.. take a piece of metal and keep bending it.. what eventually happens? That is why I've seen some folks also place some clear reinforcing tape on the backside of the new foil for additional strength. Me.. I don't bother with that step. Also I've observed that many of the foils themselves appear very thin either because they were made thin originally or just due to wear. Either way the friction and stress of the movement of the head during the track change I believe is what breaks the tape at the gap.
On the worry of the electrical current through foil causing failure... I've never seen any evidence that this creates an issue.


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 Post subject: Re: 8 track player/recorder modification
PostPosted: Mar Sat 03, 2018 5:33 pm 
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I've seen some sensing foils (maybe just a conductive metallic looking coating on a non-metal backing) whee some of the coating is missing.


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