Philips N4450 - service info for mods?

[real2reel]

Hi all,

Last week, I obtained a Philips N4450 - a well-specified stereo tape recorder (autoreverse, 3 speeds, quarter-track, 6-heads, 10.5in. spool capacity, mechanical timer, in-built 20watt per-channel amp) from the early 1970s. It was in good condition, and even the cover was intact! The belts had turned to 'goo', which had to be painstakingly removed using Swarfega (as discussed here) and nail polish remover! I then fitted replacement belts that I acquired from a very quick eBay seller in Germany (dk3he1). The transport was found to work erratically, until I unplugged and replugged some of the deck connections and modules to 'clear' decades of oxidation and so on. It then worked, although the reverse-direction pinchroller failed to make contact with the capstan. This was traced to dried grease on a cam. After replacing the 'blown' switch-illumination bulbs with green 5mm LEDs and 680-ohm series resistors, cleaning the tape path and demagnetising the various record and replay heads, I was rewarded with a startlingly-good sound quality from my old tapes. Not bad for a 35-year old machine! Just to be on the safe side, I'll replace the electrolytic capacitors in the power supply at some point in the near future.

And so to the nub of my gist. Despite its otherwise-advanced specification, this machine relies on those horrible pressure-pads to ensure intimate head-to-tape contact. As the pressure pads wear out and/or cake up with oxide, they develop 'hard spots' that can accelerate head wear (and what's the chance of finding replacement heads for this model? The belts were a lucky enough find!). High-quality R2R recorders (including subsequent Philips decks) don't use pressure pads - instead, they gently energise the motor that drives the supply-spool turntable, and the minimal reverse-direction torque thus generated facilitates 'back-tension'. I experimented with the machine, manually 'dragging' the supply reel and disengaging the pressure pads. The results were audibly just as good as they were before.

This is where some kind forum-reader comes in! I'm ideally looking for a e-mailable schematic or service manual for the purposes of modifying the N4450's mechanism-control circuit so that back-tension rather than pressure pads keeps the tape in contact with the heads. It shouldn't be too difficult; in the rewind and fast-forward modes, the opposite reel is energised at low current to avoid spillage. Once this has been done to my satisfaction, I will remove both sets of pressure pads. The plates to which they're fitted would, however, be left intact because they act as some kind of hum-shield. Needless to say, I will share details of my modifications if they're successful. I note that several users have posted queries relevant to the N4450 here! (if anyone has a scan of the instruction manual, that would be greatly appreciated too!).

Thanks for taking the time to read this!

R

[Bob955i]

Sorry, can't help with schematics, but another reason for ditching the pressure pads is so that you can use back-coated tapes like Quantegy, which although relatively difficult to source, are certainly easier to get than shiny-back stock.

Back-coated tape will squeal like hell on a pressure pad equipped deck

[reel-to-reel-man]

Replacement heads, I managed to get all six from one of the members of this forum, so here is a good start to look for them I have had about 6 of these great machines, somewhere I have the manual and diagrams, I will see where they are. To get you started, in the back of the machine, there ia a board on the left hand side with loads of plug in boards, the two lower plug in boards with trimmers on them are for the reel tention, thats a good place to start, remove the pads and them up a bit.

Just out of interest here is my N4450

http://www.vintagerecorders.co.uk/VR...age.asp?IDS=30

[timewave]

R

I have Service Manuals Parts 1 & 2 and will try to scan them at the weekend.I could also scan the English section of the Operating Instructions if you require it. Advice on how to send them by e-mail will be needed though since I've never done it before.

I've yet to make a start on my machine. It's still in the airing cupboard. Good to know the German belt supplier is ok though.

Cheers

John

[timewave]

Think I've sussed this PDF malarky now but how many dpi for e-mail purposes?

John

[Sideband]

Oooooh!! Happy memories!

I repaired a fair few of these when I was in the Workshop at Philips. That would have been around 1974-1979. Unfortunately I have no service info now and the few odd spares I had went to a member of this forum.....!

One of the most common problems with these was failure of the bridge rec (2xBY164) in the power supply. There are four of these because of the + and - power supplies. The official mod was to add another two BY164 in parallel to the two already fitted. I used to drill extra holes in the PCB to take the extra recs and if both supplies had failed, it ended up with eight BY164's! Never failed again !

Heads were also prone to failure...there was a jig available to aid setting them up.

I actually repaired one of these last Saturday....the bridge rec had failed! I carried out the mod above...it had never been done.

Great sounding amps using 2N3055's which seem almost indestructable.


Happy days indeed!


Rich.

[timewave]

R

If you still require cct/service info on N4450 please send me PM with e-mail address.

John

[real2reel]

Many thanks to Timewave for taking the time and scan and send me the service info! I'll now indulge in a bit of experimentation!

[Sideband]

Originally Posted by real2reel

Despite its otherwise-advanced specification, this machine relies on those horrible pressure-pads to ensure intimate head-to-tape contact. As the pressure pads wear out and/or cake up with oxide, they develop 'hard spots' that can accelerate head wear

I'm ideally looking for a e-mailable schematic or service manual for the purposes of modifying the N4450's mechanism-control circuit so that back-tension rather than pressure pads keeps the tape in contact with the heads. It shouldn't be too difficult; in the rewind and fast-forward modes, the opposite reel is energised at low current to avoid spillage. Once this has been done to my satisfaction, I will remove both sets of pressure pads.

Hmmmm. Yes I understand the priciple of this but you may find that the wind/rewind could be greatly affected. I seem to recall that there was a fairly precise adjustment required when setting up the back tension so that the wind/rewind was very even on the spools. When the tape is moving one way, the opposite motor provides back tension. This was also controlled via the tape tension levers each side via a small leaf switch connected to each lever.

Good luck anyway. It will be interesting to find out if you are successful.


Rich.

[TopTip]

Originally Posted by real2reel

Hi all,


And so to the nub of my gist. Despite its otherwise-advanced specification, this machine relies on those horrible pressure-pads to ensure intimate head-to-tape contact. As the pressure pads wear out and/or cake up with oxide, they develop 'hard spots' that can accelerate head wear (and what's the chance of finding replacement heads for this model? The belts were a lucky enough find!). High-quality R2R recorders (including subsequent Philips decks) don't use pressure pads - instead, they gently energise the motor that drives the supply-spool turntable, and the minimal reverse-direction torque thus generated facilitates 'back-tension'. I experimented with the machine, manually 'dragging' the supply reel and disengaging the pressure pads. The results were audibly just as good as they were before.


Thanks for taking the time to read this!

R

I am curious to see if you ever managed to make the modification for removing the pressure pads?...I would think it is more difficult on this machine than others with hysterisis motors where you simply apply DC to the (normally) AC motors and they become increasingly more difficult to turn. Here you need to apply DC in the "wrong" polarity, which unfortunately has the nasty effect of making the reels spin out of control at the end of tape. Any other modifications you thought of? I was not sure how good the heads are on this machine -- sound was OK but a bit muffled, even after hours of setting azimuth, zenith, etc.

[ben]

instead of playing with the electronics, wouldn't it be easier to rig up something akin to the backtension band setup in video recorders? just a thought....

[TIMTAPE]

Apparently this is a 6 head machine. When using back tension alone there is a certain minimum angle of "tape wrap" around each head for reliable tape to head contact. The more heads you have in a machine, the longer the tape path has to be to allow enough wrap for each individual head, with the tape path describing an arc.

This is not so much a problem on a large machine where space isnt an issue but on a compact machine it might be an issue. On the Akai GX77, also a 6 head from memory, they split the heads into two groups of 3, each group facing the other, and with an idler to change the direction of the tape 180deg. I'm not sure if this was purely for compactness or if the tape wrap issue was a factor too.

I'm just thinking Philips may have had a good reason for using pressure pads on this model if there wasnt enough tape wrap angle available for all 6 heads. With pressure pads you can even have the head faces in a straight line but of course the pads will be doing all the work.

Another design issue is that the more heads you have, the more friction in play and record.

I doubt there would be many professional grade machines with 6 heads. The design seemed limited to consumer or prosumer decks.

I would just be wanting to confirm that back tension alone would be sufficient for optimum tape contact on all 6 heads, and without compromising reliable tape speed right to the end of the reel. There is a limit to how much back tension can be applied before tape speed becomes unreliable. How much "wiggle room" you have with that 6 head design will be interesting to see.


Anyway, my 2 cents worth.

Cheers Tim.

[TopTip]

Originally Posted by TIMTAPE

The more heads you have in a machine, the longer the tape path has to be to allow enough wrap for each individual head, with the tape path describing an arc. On the Akai GX77, also a 6 head from memory, they split the heads into two groups of 3, each group facing the other, and with an idler to change the direction of the tape 180deg. I'm not sure if this was purely for compactness or if the tape wrap issue was a factor too.

Cheers Tim.

Good point and Philips has also done the same, see picture: http://www.makarateyp.com/MG/N4450/N4450Kafa1.JPG from a fan's site. So back tension should work. Also these are not particularly large wrap heads. In the end though, given none of these machines will clock thousands of hours of future use, leaving the pressure pads in place probably is neither here nor there. It just looks unprofessional.

BTW -- is this not the slickest, most beautiful tape recorder ever produced? The only possible competition is the never-beyond-prototype Beocord 5000: http://www.bang-olufsen.com/UserFile...iginal/468.jpg
I have 20 of the most beautiful R to Rs ever produced (including the Braun TG 1020, the ASC 6000, Beocord 1200, the Crown 800...) and I still have to stop and stare each time I pass by the N4450 in my living room.