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Old 7th Feb 2009, 1:28 pm   #1
Hermit6345
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Default JVC QL-7 Turntable

I have an elderly JVC QL-7 Quartzlock DD turntable which I bought new in about 1978. It performs faultlessly. However, I want to be able to play my 78 collection through my HI FI system. I don't have a manual for it, so has anyone ever managed to carry out a mod to the FG Quartzlock system to make it run at 78 rpm? I can see that theoretically it must be possible, but can it be done practically? With no circuit diagram, it's difficult to see how it works except in the broadest terms, presumably with a signal from the motor being compared with the osc. signal and the motor being speeded up or slowed down in response to any error signal received from the comparator. I don't want to wreck what is a nice turntable either!

Ian.
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Old 7th Feb 2009, 1:53 pm   #2
mickjjo
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Default Re: JVC QL-7 Turntable

This thread may be helpful:-

https://www.vintage-radio.net/forum/...ight=turntable

I don't know if this idea would work with your JVC though .

Regards, Mick.
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Old 7th Feb 2009, 3:01 pm   #3
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Default Re: JVC QL-7 Turntable

Yes, my Trio is not quartz-locked as far as I know.

Conversion was incredibly easy though, and it still works well.

It's sat here doing very little so I might be persuaded to part with it... PM me if interested.

Good luck,

Nick.
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Old 7th Feb 2009, 3:05 pm   #4
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Default Re: JVC QL-7 Turntable

Hi Mick,

Thanks for drawing my attention to that. One useful thing that came out of that was finding the 27 page manual! Printing it out as I type this. Next thing is to see if I can understand what it does. If I can't there are probably loads of people on here who can! Just seen your mail Nick, will see how I get on and may come back to you yet!!

Ian.
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Old 7th Feb 2009, 11:17 pm   #5
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Default Re: JVC QL-7 Turntable

It should be possible, if it's a brushless DC motor using PWM control. The motors used in turntables are natively very fast (several hundreds of rpm at 100% duty) and so are running at a very low duty cycle in normal operation; the shorter the duration of each burst of energy applied to the turntable, the smoother the motion as the armature isn't trying to jump into a stable position. There will be a feedback loop, with the PWM voltage set according to the difference between the frequency seen at the Hall effect sensor and a reference frequency. If you can find a way to feed a different reference signal into the circuit (even a piece of wire hanging out the back and plugged into a signal generator), then it should lock to that. The only worry is if this would be taking the frequency comparator outside of some hard limit, but you'd have to try it.

Another way would be to use a computer: play the 78rpm record at 45rpm, use Audacity or similar software to speed the track back up (there's a menu option specifically to do this) and burn it onto CD.
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Old 8th Feb 2009, 11:04 am   #6
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Default Re: JVC QL-7 Turntable

AJS, thanks for your input. I now have the circuit diagram. I think it works very much as you surmised. There is not much of a description in the manual and I think that some of the dividers are contained in what seem to be custom ICs.

I hope that I won't fall foul of the mods if I put a cct diagram on the bottom, but it's not available from Paul anyway.

It's all controlled by a 9.504 MHz xtal. This is divided down by a divider, which has two division ratios, to produce a 100 Hz signal, which is applied to the Phase Comparator.

The division ratio is controlled by a signal from the two speed control touch switches. (45 and 33.3) This speed select signal is also sent to the Phase Comparator.

The output of the Phase Comparator is sent to the motor drive circuit. The motor has an internal frequency generator, the output of which, after being shaped, is sent as the second signal into the Phase Comparator and will be 100Hz when the motor is running at 33.3 rpm All seems straight forward so far. However it's a matter of finding out what does what on the circuit! Obviously I want to retain the two lower speeds as well as 45/33.3 I had thought of the computer method but that is not very convenient.

Ian.
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Old 8th Feb 2009, 11:30 am   #7
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Default Re: JVC QL-7 Turntable

Wow, Ian, that's much more complicated than my old Trio (a mere 10 transistors and no ICs).

Nick.
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Old 16th May 2009, 5:35 am   #8
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Default Re: JVC QL-7 Turntable

Hi Ian

I'm the beginner and jus got this QL-7 from my mom. However, it runs faster after playing for an hour. My friend tried to fix it but couldn't. He said that ICs are the crucial problem and the number on them were gone. I am wondering if you may help suggest these?

Sorry for your inconvenience if information above could be used. (I don't clearly understand it)

Prem.
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Old 16th May 2009, 5:36 pm   #9
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Default Re: JVC QL-7 Turntable

Hi Prem, Welcome to the Forum.

Sorry to say I no longer have that turntable. I would agree that some of the ICs are difficult to source. Best I can do is if you send me a Private Message with your email address and I will send you the complete manual for the QL-7.

Ian
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