Philips N4506 Tape Tension issues

De compact cassette was een schitterende uitvinding van Philips en werd een wereldwijd succes. Ook bandrecorders blijken een tijdloos populair medium.Kun jij ook geen genoeg krijgen van die sfeervol draaiende spoelen? Deel hier je magnetische passie!

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manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Hello, this is my first post on the Forum! I'm Colin and I live in Bishop Auckland, a small town in the North East of England.
I've been a Philips fan for many years - my father used to work for the valve maker Mullard, which was part of the Philips empire.
I am a great fan for the 'Plastic Fantastic' reel to reels, with a collection consisting of an N4511, numerous N4504s and three N4506s.

I am currently working on a near mint condition N4506, which I bought from a guy in Italy years ago. Unfortunately, the tape tension system does not work properly - the tape is slow to stop and hangs loose forming a loop. I have the service information, but have noticed that the diagram of the Tape Tension Unit U2, is completely different to the actual PCB unit from the tape recorder. Does anyone know if the design of the Tape Tension unit was changed at some point? - my PCB has five transistors, six resistors and five diodes.
Many thanks for any help you can offer! Colin.
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Will
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Berichten: 11991
Lid geworden op: 24 mei 2009, 22:56
Locatie: Het uiterste Zuiden
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That pcb/circuit has been modified a couple of times.

However, if the left tape reel doesn’t stop immediately and forms a loop, or brakes too slow, it is a mechanical problem.
Een verzamelaar ziet niet wat hij al heeft, hij ziet alleen wat hij nog mist.

Es ‘t good is moste ‘t good laote.
manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Hi Will,
Thanks for the quick reply. When Play is selected without a tape present, the left hand reel table does rotate in reverse, so I guess that proves that this reel motor is supplying back tension when a tape is running.
I have to say, the mechanical brake that acts on the edges of the reel tables does not work very well. It doesn't seem to apply enough pressure to brake the reels fast enough. I'll try increasing the spring tension a little bit. It is probably not relevant but, when pulled inwards and released, the right-hand tape tension arm returns to it's rest position instantly, without the slow travel that it should have.
Would you, or anyone else have a copy of the altered tape tension board U2 perhaps, just for my records?

Thanks, Colin.
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Will
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Berichten: 11991
Lid geworden op: 24 mei 2009, 22:56
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Hold on, my answer might have been
erroneous… :oops:
I’ll come back to it, have to verify some things first.
Een verzamelaar ziet niet wat hij al heeft, hij ziet alleen wat hij nog mist.

Es ‘t good is moste ‘t good laote.
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Thomas
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Lid geworden op: 16 jun 2005, 15:55
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The reel belts should have the correct thickness. The brake actually works on the belts that drive the reels. Too thin belts will not have good contact with the plastic brake strip.
Thomas Baur
20 jaar MFBfreaks: 2003 - 2023

Home: http://www.mfbfreaks.com
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Will
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Berichten: 11991
Lid geworden op: 24 mei 2009, 22:56
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Well, I was wrong. The mechanical brake is just an additional brake, similar to the handbrake/parking brake of a car.

I haven't found a 5 transistor U2 board in the N4506 docs yet. However, there is one in the N4512 service manual.
Since you own several recorders, I suggest to swap the U2 board with that of a working recorder and see what happens in both recorders.

It might just be a bad switch contact in one of the FF, Rew or Stop switches. So you could start cleaning these with IPA.

On this forum there is an old discussion about the tape tension circuit in these recorders and its different versions. I'll try to find that.
Een verzamelaar ziet niet wat hij al heeft, hij ziet alleen wat hij nog mist.

Es ‘t good is moste ‘t good laote.
manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Hello again Will,
Thanks for this, it would be great if you could find that tape tension thread! I've already tried swapping the U2 board with one of my other recorders, but no joy!

Hi Thomas,
I think I've got the wrong belts! The ones fitted have a round profile and are around 1mm diameter, so not much area for the mechanical brake to act on. I've just tried swapping them for some thicker belts (around 2mm) which are a lot tighter. The reels now stop a lot quicker, but there is now so much resistance that the machine will not rewind. These belts are too tight I think.

Best wishes, Colin.
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Will
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Berichten: 11991
Lid geworden op: 24 mei 2009, 22:56
Locatie: Het uiterste Zuiden
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The belt circumference/length should be 396mm minus approx 3%. So a belt diameter of 120 to 122mm is the right size.
Thickness is 2x2mm. A round belt of 2 to 2,2mm will also work.

Please note that the belts for this recorder series (N4506/N4504, what we call PF, Plastic Fantastic) are not the same as these belts in the N4418/N4510 series (Magno series). The latter reel-belts are approx 10% shorter, as the axes distance differs .

Please mount the right belts first, this might solve the brake problem.
Een verzamelaar ziet niet wat hij al heeft, hij ziet alleen wat hij nog mist.

Es ‘t good is moste ‘t good laote.
Ickxsz
Donateur
Berichten: 539
Lid geworden op: 15 nov 2021, 15:44
Locatie: Utrecht

Hello manpackman / Colin,

If you really want to be sure that you get the right belts:

https://shop.mfbfreaks.com/product/snar ... drecorder/

(no commercial affiliation)

Dirk
manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Hi Dirk,
Thanks, I've just ordered the correct belts from the webshop.
Best wishes, Colin.
manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Regarding the reel table drive belts, I have now found some more that I think are the right ones, they are a square section and about 2mm x 2mm - I will be able to confirm this when the ones I ordered earlier today from the web shop arrive. They work much better! In addition, I've changed the friction discs that sit behind the right hand tape tension arm - this moves slowly now, taking over a second to come back to its rest position from being fully compressed. When I stop the tape, this now takes up most of the slack.
I've found some other issues I need to address though as follows:

(1) - the Line In/Out selector switch has very poor contacts - on Playback, it causes the right hand channel to drop out. I'll remove the board with all the selector switches on it and clean them thoroughly.
(2) - the Line In and Line Out potentiometers on the top of the same PCB also suffer from intermittent contact.

(3) - All tape speeds are running fast by around 5%! Of course, this is not noticeable on tapes recorded on the machine, but anything pre-recorded, or recorded on another of my reel to reels sound very fast! So far, all I have done about this is to check Voltage B (which feeds the speed adjustment trimmers R65, R68 and R69), but this is normal at about 16.2 volts.

Best wishes, Colin.
manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Hi and thanks to all those who have replied so far.
Work on the N4506 continues - I've now got the correct reel table belts fitted and, in addition, I've increased the tension of the spring which applies the mechanical brake - it was only around 55g, so I've upped it to 70g by bending the tag as described in the manual.

You may remember, I was querying the tape tension board unit circuit - my board has five transistors, but the service manual for the N4506 only shows a three transistor circuit. I've now found a schematic and layout diagram for the five transistor version - it appears in an N4504 manual that I also have.
I had got the mechanical brake, fast forward and rewind working OK, but then I decided to perform the reel table adjustments as the heights were wrong. Unfortunately, the recorder will not wind now and rewind has got very slow too. To make matters worse, I'm getting tape loops forming again when I press the stop button! I do not think I've done anything wrong - it looks like setting up the reel tables correctly has just shown up a previously hidden problem in the tape tension or motor control circuits. So back to the drawing board, when I have time!
manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Oh, I forgot to mention a few little successes! I fixed the intermittent contacts in the Line In/Out switch by removing the switch slider and giving it a good clean. I also did the other switches (tuner, aux etc) at the same time.
I then opened up each of the miniature potentiometers that make up the bank of 12 and gave them a good spray with Kontact PR, before working then vigorously backwards and forwards. Problem solved.
The tape was running fast on all speeds, so I adjusted each in turn by playing back a 1Khz test tape. I then found a 1Khz tone on my phone and played this at the same time. It was then easy to adjust each preset pot for zero beat. Job done!
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gromsound
Berichten: 10373
Lid geworden op: 26 jun 2005, 22:58
Locatie: Haaksbergen
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it looks like setting up the reel tables correctly has just shown up a previously hidden problem in the tape tension or motor control circuits.
well it might be as simple as incorrect up-down play of the reel tables. they should move just a bit to run freely.
lijdt aan AMHD - alles moet het doen
manpackman
Berichten: 14
Lid geworden op: 27 dec 2015, 19:52

Hi. Again, thanks for the replies - sorry I've not had much time lately to look at the N4506.
I'm pretty sure the reel tables are now correctly adjusted - previously, there was over 1mm of up and down play in the spindle bearings. I oiled the bearings and motors at the same time and everything now seems to be free-running. However, I'll give everything a check over again before moving on.
The reason I suspect a fault in the motor control circuit is that the Wind/ Rewind was very weak when I bought this tape recorder around seven years ago.
Colin.
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