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Old 24th Jan 2016, 11:04 pm   #1
Phil G4SPZ
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Default Philips N4418 stereo reel-to-reel tape recorder (1971-1975)

A friend passed this nice three-head, three-speed, four-track recorder on to me a while ago, and whilst it appeared to work after a fashion, the need to transcribe a valuable 43-years-old family recording prompted me to overhaul the machine and try to improve its performance.

On first inspection, the machine powered up, ran smoothly and played back pre-recorded tapes reasonably well, but with noticeable lack of treble response. The counter didn’t work, and most of the lamps behind the selector switches and the VU meters had failed, although those in the illuminated tape transport keys were working. I cleaned the heads and tried a recording. The machine would record for five to ten seconds, after which the recording would fade out and the erase function would disappear. I suspected a fault in the oscillator, which in this machine runs at 100kHz. My copy of the service manual didn’t show the internal circuitry of the oscillator, so a request was made for information via the Forum, and in less than 24 hours I had the full circuit! There was a 47uF electrolytic capacitor associated with a voltage regulator/switching transistor on the oscillator PCB, so I replaced the capacitor and hey presto, the oscillator function returned to normal.

I spent some time looking at the heads through a x10 magnifier, and it became obvious that this machine had seen a good deal of use. Wear on the heads and tape guides was significant, as can be seen from the photos. However, I saw that the paint securing the heads in position had been previously disturbed, so the heads would need realigning. I didn’t have access to a head alignment jig, nor a professional test tape for alignment purposes, so I had to make do with what was available. I have a Revox A77 in good condition, but the Revox is half-track stereo. I also have a calibrated signal generator and sensitive AC millivoltmeter, of which more later.

I recorded a test tape on the Revox, consisting of a 10kHz tone at 7.5 ips on a reasonably new BASF long-play tape, and used this to align the playback head on the Philips. After a certain amount of twiddling, I managed to align the azimuth of the playback head to reproduce the 10kHz ‘test tape’ at about -1dB, but every so often the tape would ride downwards off the guides. Annoyingly, this fault only showed itself when operating the machine vertically. I discovered that the upper capstan bearing is adjustable to ensure it is exactly at right-angles to the tape; careful adjustment cured the problem. I then tried making a recording on the Philips, using a 10kHz tone and monitoring the output from the playback heads, to try to align the record head to the playback head, and this produced fairly good results. There was however still some loss of treble.

Pulling back the felt pressure pads and looking very closely at the tape passing over the heads showed that the tape wasn’t sitting properly in the existing ‘wear channel’ in the record and playback heads. Whilst I could see that the head gaps hadn’t started to open up, there was a clear ‘step’ where the tape had worn the face of each head away. The wear pattern also showed that both heads had been misaligned, the record head tilted slightly forwards and the playback head tilted backwards. Without a proper alignment jig it was impossible to correct this perfectly accurately, but using a plastic ruler and a spirit level across the top of each head, a bit of judgement and a lot of trial-and-error, I eventually managed to get the heads aligned passably well with the tape sitting exactly between the upper and lower steps in the wear channel.

I ran through the test procedures in the manual, and immediately came up against a problem that suggested excessive loss of HF response somewhere in the pre-emphasis circuits. After scratching my head for ages, it eventually dawned on me that my AC millivoltmeter was at fault; far from having a linear response, it was falling off rapidly above 1kHz. As I was measuring AC signals around the 1 volt level, I switched to using a Taylor multimeter which I know to be linear on AC measurements up to 1MHz. After a final tweak of the azimuths and degaussing the heads, the results were very rewarding; at 7.5 ips the response at 20kHz was 5dB down, exactly as the Philips specification states. Considering the amount of head wear, that’s remarkable. The results at the two lower speeds weren’t quite up to spec, but significantly better than when the machine arrived. I have a few ‘World Record Club’ 3.75 ips mono tapes which I used to confirm correct azimuth alignment, and the machine reproduces them very well.

As mentioned earlier, the tape counter didn’t work, due to the belt having turned to ‘goo’ and deposited its sticky remains in various places. I bought a full set of new belts and replaced them all. I adjusted the left-hand reel height and lubricated the reel and capstan bearings. A generous Forum member supplied several replacement bulbs for the switch and meter illumination. The originals, rated at 19 volts 50mA, are wire-ended miniature lamps and I couldn't find any of the same type, and the replacements (23V 25mA) are slightly dimmer but acceptable. Fitting them is a fiddly and rather tedious job, but the results are worthwhile. The final task was to give the deck and cabinet a thorough clean, and as this machine has clearly been well looked-after by its previous owners it has come up very well indeed. Overall I’m very pleased with the end result electrically and cosmetically.

The N4418 was apparently in production from 1971 to 1975. My example bears the serial number 018677 and factory code WR07/302, which I think places its date of manufacture towards the latter part of the production run. By this time, hi-fi cassette decks were already serious rivals to reel-to-reel for home stereo recording; in 1974 I bought a brand-new Goodmans SCD100, itself a re-badged Nakamichi 500 with a very good specification. With their far more compact dimensions, cheaper and more convenient media, low noise performance thanks to Dolby B and the growing availability of albums in pre-recorded ‘musicassette’ format, it’s easy to see how the cassette started to displace reel-to-reel in popularity other than among professional users.

Phil
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Old 25th Jan 2016, 6:32 pm   #2
stevehertz
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Default Re: Philips N4418 stereo reel-to-reel tape recorder (1971-1975)

Lovely job Phil, well done. Funnily enough, I have just switched on my Revox A77 after many years lying dormant, but that's another story in the tape section of this forum. Yes, tape heads are very complex to align. They have insertion depth, height, azimuth, and angle settings. It's great when you get it right though. Interestingly, I did some similar test tone experiments some years ago. At 7.5" on the Revox a high frequency test tone (I forget what it was, probably 10K) would record and play back as pure can be. On a decent quality, well set up cassette deck you could noticeably hear frequency warble and 'fluty' volume anomalies when playing back. Yet, and this is the fun bit, on playback of recorded music on both machines you couldn't really hear the difference! It's an old theme of course, but we get more bound up on specs than simply 'listening to the music'. Anyway, I'm not saying there's a right or wrong to this conundrum, merely that it exists.

Well done Phil on a lovely restoration.
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Old 25th Jan 2016, 8:57 pm   #3
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Default Re: Philips N4418 stereo reel-to-reel tape recorder (1971-1975)

They look like the early heads. I seem to recall that later ones were black and were supposed to be harder. They had a little sticker on them with 'ferrite' printed on it.

Anyway glad to know you sorted it to a good standard and the head cover is still in place! That was always a casualty.

I remember them well. Philips claimed they could be used as a pre- amplifier as well for their Motional Feedback speakers.
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Old 26th Jan 2016, 12:07 pm   #4
Phil G4SPZ
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Default Re: Philips N4418 stereo reel-to-reel tape recorder (1971-1975)

Thank you both for your kind comments. It now seems that I was a bit premature in claiming complete success; I have been using the machine on long-term soak test, and a pronounced hum has suddenly developed on recordings...

So it looks like I will have to get it back on the bench again. There are numerous supply rails, feeding different parts of the circuit, all decoupled by electrolytic capacitors so perhaps one of those has succumbed to all the unexpected activity. But this is in the nature of vintage gear which hasn't seen use for decades.

Interesting info on the head construction, SB. Maybe someone who knows more about Philips serial numbers will be able to date this machine more accurately.
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