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Old 11th Mar 2020, 6:07 pm   #1
roverleffe
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Default Philips N4418 wrong speed

Anyone having problems with speed adjustments? one can adjust the speed But the are all too fast!
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Old 11th Mar 2020, 10:06 pm   #2
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Default Re: Philips N4418 wrong speed

I am not familiar with this model. A quick look at the Service manual indicates that all 3 speeds can be adjusted by the relevant potentiometer/trimming resistor.

If speeds cannot be trimmed down sufficiently, then can only assume that problem may lie with the Capstan motor/Impulse head and/or the associated Speed Control Unit.
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Old 12th Mar 2020, 12:35 pm   #3
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Default Re: Philips N4418 wrong speed

Ok thanks I have to check the speed control unit again difficult to find any faults when its working but too fast. A potentiometer in series to the motor will slow down the speed but that way the speed is not stable
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Old 12th Mar 2020, 9:02 pm   #4
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Default Re: Philips N4418 wrong speed

Check the impulse head. Basically it uses a magnetic ring driven by the motor which sends pulses to the head (hence impulse head). These are fed back to the control circuit so they form part of a control loop. After all these years it's entirely possible that the magnet has become weak.
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Old 13th Mar 2020, 5:37 pm   #5
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Default Re: Philips N4418 wrong speed

if so how to solve that how to know if it´s too weak?
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Old 13th Mar 2020, 7:32 pm   #6
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Default Re: Philips N4418 wrong speed

You need to look at the circuit...that gives a waveform which you would then check with an oscilloscope....that's how we did it at Philips 30 years ago. Don't know how you would check it without a 'scope.

Check head for continuity first.
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Old 13th Mar 2020, 8:15 pm   #7
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Default Re: Philips N4418 wrong speed

Thanks no problems I do have a Scope but can´t see a pulse info anyware in the schematics? I know I have a pulse
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Old 13th Mar 2020, 10:10 pm   #8
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Default Re: Philips N4418 wrong speed

Hmmmm it's 30 or more years ago. We may have had more information available than just what was in the manual. I know there was a separate Circuit Description...maybe that showed the waveform. If you have pulses then it shows that the head and probably the rotating magnet is good. You need to check the feedback circuit.
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Old 14th Mar 2020, 4:31 pm   #9
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Default Re: Philips N4418 wrong speed

Solved!

Solved thank you all. The pulse from the head was only 20mV and the reason the wheel on the motorshaft with the magnetic ring was not in line with pickup head mounted a bit off on the motor shaft.

Happy Days!
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Old 14th Mar 2020, 11:53 pm   #10
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Default Re: Philips N4418 wrong speed

Yes I am also struggling to see the head pulse info in the schematics.
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Old 15th Mar 2020, 11:55 pm   #11
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Default Re: Philips N4418 wrong speed

....You just triggered my memory ref the low pulse fault....it was always one of the first things we checked.....a note from Eindhoven was sent to the service workshops advising us to check that the rotating magnet was in line with the head poles. It's quite possible that we measured the pulse ourselves and wrote it in the manual....it's a very long time ago. There was a separate publication called the Circuit Description which, as the title implies, explained how it all worked. Possibly THAT had the pulse waveform shown.
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Old 16th Mar 2020, 10:38 am   #12
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Default Re: Philips N4418 wrong speed

Quote:
Originally Posted by roverleffe View Post
Solved!

Solved thank you all. The pulse from the head was only 20mV and the reason the wheel on the motorshaft with the magnetic ring was not in line with pickup head mounted a bit off on the motor shaft.

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That is great news, what level of pulse voltage improvement did you obtain with better alignment ?
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Old 19th Mar 2020, 6:43 pm   #13
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Default Re: Philips N4418 wrong speed

Around 400 mVolt
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