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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 8th Jul 2020, 3:36 pm   #1
jim221
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Default Philips N4520 inapropriate operation of auto stop.

hello, I have had this machine about 5 years now with no problems. Now it seems to be auto stopping every few minutes. it is the same on both 31/'2 ips and7ips. I have run out of things to try has anyone got any Ideas? according to the counter the speed is locked ok and it works as it should ie the pressure roller drops and the all the buttons reset to the correct position.What can I try next.according to the manual a blip in the power supply could do it but I cannot see anything untoward when it does happen.Ideas please

Jim
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Old 8th Jul 2020, 3:44 pm   #2
Andrew2
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Default Re: philips n 4520 inapropriate operation of auto stop

I'm not sure how the auto-stop works on this machine (I'm no Philips expert), but I recall many R2R's from that period used a reed switch and magnet to supply pulses to the stop cct. If that's the case, it could be a worn-out reed sw.
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Old 8th Jul 2020, 5:58 pm   #3
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Default Re: philips n 4520 inapropriate operation of auto stop

Quote:
Originally Posted by Andrew2 View Post
I'm not sure how the auto-stop works on this machine (I'm no Philips expert),
Can you post the auto-stop circuit? You could be right about a reed switch and rotating magnet....possibly located under the right-hand reel disc. Other systems might use an LED with a rotating disc that has alternate light and dark areas which cause pulses to be received by a photosensor.
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Old 8th Jul 2020, 8:48 pm   #4
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Default Re: Philips N4520 inapropriate operation of auto stop.

The service manual is on HiFi Engine.
The schematic shows an optical emitter detector pair, looks like a slotted unit which would work with a castellated vane on the spool spindle.

In fact there two pairs providing two digital pulse trains. These are on panel 2A
Emitter D282 Detector TS282 and buffer transistor TS284
Emitter D281 Detector TS281 and buffer transistor TS283

The two signals are used to detect direction according to the direction of the edge on the second signal while the first signal is in the high state. They also give spool speed information which I guess feeds in to the logic for auto-off

If you find this board (panel 2A) you should be able to check the vanes are moving with the spool and that TS284 and TS283 have the pulse train signals on their collectors when the spool is moving.

Best Regards
Chris

Last edited by unitelex; 8th Jul 2020 at 8:54 pm.
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Old 8th Jul 2020, 8:53 pm   #5
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Default Re: Philips N4520 inapropriate operation of auto stop.

The slotted opto units seem to be pcb mounted so most likely location of this PCB is under the spool hub.
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Old 8th Jul 2020, 8:54 pm   #6
Lucien Nunes
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Default Re: Philips N4520 inapropriate operation of auto stop.

IIRC, there are a number of signals that can trigger the stop solenoid to unlatch the function buttons.
a) Tension arm position too low (insufficient tension)
b) Too long between counter pulses from optical sensor
c) Power switch-off
d) Counter memory stop

These signals are combined by simple diode-or logic on the transport control board to operate the solenoid driver. Again IIRC they act only to release the transport control buttons; anything else that happens is a consequence of the button being released.

My first port of call would be to monitor the signals arriving at the or'ing diodes to discover which of them is going high at the time of an unexpected stop. A picture of the transport control board would be helpful, as my manual is 100 miles away.

E2A, The counter opto interrupter is operated by its own tape roller between the heads and the RH tension arm. From the OP I think we infer that the counter is counting OK, i.e. pulses must be present at the opto outputs. One of the two pulse lines is used to reset a monostable that trips the stop when it times out, so if the stop events are related to the counter, either a) the pulses are weak, capable of operating the counter logic but not resetting the monostable or b) the monostable itself is faulty. But I think the tension safety is more likely to be the errant signal

Last edited by Lucien Nunes; 8th Jul 2020 at 9:05 pm.
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Old 9th Jul 2020, 4:12 pm   #7
jim221
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Default Re: Philips N4520 inapropriate operation of auto stop.

Hello again, sorry for the delay in getting back to you but suffering from arthritis in my hands takes a little longer these days. I have placed a couple of attachments which I hope you will find helpful. I no longer have an oscilloscope as it was too heavy to lift so I hope we can find some other way of communicating. There 24v into the board and when play is pressed this reduces to approx 11.5 v. I will try to do the attachment as last time I had to have several goes at it. I have included the one with 25 0 -25 scale as I assume this tells how to set up the tension rollers. I don't understand German perhaps some kind person could enlighten me.Please let me know if you need more information
Regards
Jim
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Old 9th Jul 2020, 9:10 pm   #8
Lucien Nunes
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Default Re: Philips N4520 inapropriate operation of auto stop.

As it happens, I speak German. It says:

9. Tension levers

9.1 Take-up lever
The tension necessary to hold the lever in the 0° position must be 1.35 +/- 0.04 Newtons (138 +/- 4 grams force)
The tension is measured at point A, and adjusted by altering the attachment point of spring 98.

Connect a voltmeter between points B & C.
With the lever at :
+25° the voltage must be >1.8V.
-25° the voltage must be >1.2V.
0° the voltage must be 0 +/- 0.1V
Adjustment is achieved by slight rotation of bracket 73 using D, after loosening the M2.5x16 screw.

9.2 Back-tension lever
The tension necessary to hold the lever in the +5° position must be 0.8 +/- 0.02 Newtons (82 +/- 2 grams force)
The tension is measured at point A, and adjusted by altering the attachment point of spring 74.

The tension necessary to hold the lever in the -10° position must be 1.4 +/- 0.05 Newtons (143 +/- 5 grams force)
The tension is measured at point A, and adjusted by altering the attachment point of spring 105.

Connect a voltmeter between points B & C.
With the lever at :
+25° the voltage must be >1.2V.
-25° the voltage must be >1.8V.
0° the voltage must be -0.5 +/- 0.1V
Adjustment is achieved by slight rotation of bracket 73 using D, after loosening the M2.5x16 screw.

Place a full 7" tape reel on the right-hand spindle. Engage rewind and set the WINDING SPEED control at minimum. The lever must be in the -10° position at the end of the tape. Adjustment is achieved using R308 on the 4D board.

By all means check your tension adjustments, and if you find the voltages far adrift then hopefully that will be the cause of the stopping. But I would still suggest checking on the control PCB for the incoming voltages at the diodes, to ascertain which of the possible causes is relevant. Post the schematic of that board, probably titled 'Steuerung' or similar, if you need guidance.

Last edited by Lucien Nunes; 9th Jul 2020 at 9:21 pm.
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