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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 25th Feb 2005, 12:23 pm   #1
humorist2751
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Question Philips N7300: spurious triggering of autostop

Hi all. This Philips model - late 1970s vintage? - uses metal stopfoils on the tapes between the oxide length and the leaders in order to automatically stop the machine's transport. It seems to be a switch-closing system rather than the reflective one of (say) a Revox, for no bulb or other illumination can be seen on the tape.

However, last night when I was listening to a recording made some years ago on Agfa PEM 368 tape, the autostop triggered when it shouldn't have (the oxide length, not the stopfoils, was in the relevant place at the time). Even more mysterious, upon rewind and trying again, the same thing happened in the same place!

Visual examination of the tape shows nothing odd - i.e. no obvious signs of contamination with metal dust etc. that might cause the autostop to trigger. The phenomenon happens in both directions, and seemingly regardless of the operational mode (play, rewind, fast forward etc.).

I believe that I have played this tape on this machine before without problems, but I couldn't swear to it (the tape was originally recorded on an Akai GX4000D, which doesn't have the same type of autostop system, so wouldn't have been affected by any putative contamination).

Any ideas, anyone? Is the autostop perhaps a little "hair-trigger", and if so, how does one adjust it (the manual doesn't seem particularly forthcoming on this - I suppose that the answer is there in the schematic somewhere, but this rather hairy logic stuff is a little beyond little ol' valve-based me).
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Old 25th Feb 2005, 2:36 pm   #2
reelguy
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Smile Re: Philips N7300: spurious triggering of autostop

I am not familiar with this model but as you mentioned it had logic circuits in it ,Is it possible that it has recordable index marks for stopping purposes similar to video recorders? Maybe index marks recorded accidently... Only a thought. Now I have to find out about this recorder you have mentioned N7300. reelguy
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Old 25th Feb 2005, 2:59 pm   #3
Sideband
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Default Re: Philips N7300: spurious triggering of autostop

As far as I remember on these when I used to service them, the end stop consists of a solenoid and some electronics. I think it's quite simple and uses the stop foil on the tape to trigger the solenoid. The tape guides are earthed and the 'sensor' is connected to another guide which is insulated from earth. A wire is connected from this to the trigger circuit. When the foil shorts the sensor to earth via the earthed guide, the solenoid releases the mechanism.

It's a very long time since I saw one of these and I don't recall any problems with the end stop. I believe this model uses separate wind and rewind motors. If so, I would suggest you check that the motors are free. The sensing circuit monitors the current through the motors in various modes. The drive for the motors is through a balanced bridge circuit. If the current becomes excessive and the bridge is unbalanced, then the solenoid will be triggered and release the play mechanism. Most problems in this area were caused by excessive 'drag' usually caused by dry/worn motor bearings. This might account for why the tape is stopping in the same place each time. It just happens to be at this point that the balanced bridge becomes sufficiently 'unbalanced' due to excessive current and triggers the stop mech....if you see what I mean!!


Rich.

Last edited by Sideband; 25th Feb 2005 at 3:04 pm.
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Old 25th Feb 2005, 3:03 pm   #4
Stewart
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Default Re: Philips N7300: spurious triggering of autostop

I think these machines represented Philips' last bash at a reel to reel back in 1981. They can be too clever for their own good!

The machine monitors a number of functions and will trigger a stop if not happy with what it finds. The tape tension arms are worth investigating as their position is certainly monitored by the machine - are they free to move OK? Beware of moving their position whilst running a good tape by the way - the spool tension can be cranked up by the servo to very high levels! Likewise if working inside the machine don't disturb the inductor assembly (used as a frictionless sensor) mounted on the tension arms - the setting is quite critical!

I'm not sure that the stop foil method was still in use by this time but will check in the service manual later and report back.

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Old 25th Feb 2005, 6:49 pm   #5
humorist2751
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Default Re: Philips N7300: spurious triggering of autostop

Hi again gents. I'm afraid it's "oeuf sur le visage time"; funny how often a "silly" isn't spotted until wiser heads have already been consulted...

This model has a "zero stop" function. When this function is enabled, the mechanism will be stopped when the tape counter hits zero, and this is exactly what happened! I must have inadvertantly pressed the "zero stop" button somehow; I certainly wouldn't have done so deliberately.

Personally, I've never found a use for this function anyway - which is probably why it didn't occur to me to check it straight away.

My humble apologies for being such a pl*nker, Rodney!
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Old 25th Feb 2005, 11:50 pm   #6
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Default Re: Philips N7300: spurious triggering of autostop

I'd forgotten about the zero-stop function. This and other models used it. It was only of any use when you started a new recording after an old one.....press zero-stop at the commencement of the new recording then when you rewind to playback, the machine will stop at the point you zeroed the counter. Well that was the theory anyway! Rich.
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Old 26th Feb 2005, 10:31 am   #7
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Default Re: Philips N7300: spurious triggering of autostop

Quote:
Originally Posted by Richard_Newman
I'd forgotten about the zero-stop function. This and other models used it. It was only of any use when you started a new recording after an old one.....press zero-stop at the commencement of the new recording then when you rewind to playback, the machine will stop at the point you zeroed the counter. Well that was the theory anyway! Rich.
This is, of course, a very common feature on video cassette recorders, including the earlier models with mechanical digital counters like your Philips.

It's not unknown for these to "stick" at 0000 if the reset button is pressed a bit halfheartedly, causing the mecahnism to stop within a second or two of any mode being selected. The cure is simply to press the reset button a bit harder, thereby freeing the counter. My 1970s Pioneer audio cassette deck did this to me the other day, and I was delighted that the cure was so straightforward

Many autostop circuits of this vintage are based on a circular magnet fixed to the take-up hub, which momentarily closes a reed switch once per revolution, thus keeping a capacitor charged. Failure of the reed switch' contacts or a leak in the cap will cause spurious triggering, as will slippage of any coupling belt (on models which have the rotating magnet mounted remote to the take-up hub). Newer units use a hall-effect sensor instead, which though more reliable, is never above suspicion.
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