Philips N4308/55 refurbishment (plenty of faults!)

[ricard]

Originally Posted by dazzlevision

What I wasn't aware of, was the need to avoid use of standard play tape on four track machines. Do you know why this is? I thought the main difference was the thickness of the plastic tape, not in the magnetic oxide coating upon it...?

The only reason (that I'm aware anyway) is simply that the stiffness of standard play tape tends to cause less-than-optimal head contact which is more problematic on 4- compared to 2-track machines.

There is a difference in thickness in the magnetic coating at least between certain tape thicknesses, not sure if that's the case between SP and LP tape though, I seem to recall it's not until DP or TP that the oxide thickness is decreased (simply to make the tape thin enough, perhaps also to get some sort of a physical balance between backing and coating thickness), but I could be wrong, or it could be different for different brands.

[SteveCG]

I agree with Ricard's comments - the mechanical stiffness of SP is the real issue with 1/4 track machines. SP will certainly work on a correctly set-up 1/4 machine, but you are prone to drop-outs. As a test you can try recording a 1 kc/s sine wave on your machine and then play it back and listen for the dropouts. They are quite noticable and are usually more prevalent on the outer 1/4 track (which is track no. 1 on most machines).

BTW: SP (for professional uses) has the advantage of being tougher, so can withstand the higher rewind speeds (and torques) of professional machines and has lower print-though than LP or DP.

If your recordings are lacking HF then it could be that your machine is designed for use with the later low-noise formulation tapes and that you are using a tape from the earlier formulation era.

However the HF sensitivity of tapes varied from make to make - and this is one of the reasons why manufacturers recommended a particular brand of tape to use on for specific recorders.

If you can try some different makes of tapes and they all show the same poor HF response, then indeed you could have a worn head. But as a further test, do you have any recordings made on different machine(s)? If so and they also play back sounding dull, then a worn head is looking increasingly likely. However there are more subtleties to consider - azimuth alignment; and if you are playing recordings made on different machines the replay characteristic.

[dazzlevision]

Hello Ricard & SteveCG,

Thank you for your further input and I shall, when time permits, try out your suggestions and report back here.

Regards,

Dazzlevision

[SteveCG]

Dazzlevision,

Another couple of thoughts:

1) I take it that you have given the rec/pb head a really good clean to remove any stubborn oxide?

2) When you are doing your frequency sweep then set the input level to -20dB. This is because the tape will saturate at the higher frequencies because of the pre-emphasis applied in the recording process. This pre-emphasis is greater at the lower tape speeds - hence recording studios using 15 ips so that they can capture high level, high frequency without too many problems. Doing the sweep at -20dB should give you a good indication of the machine's response.

[dazzlevision]

Hello SteveCG,

1) Yes, I have thoroughly cleaned and degaussed the tape path.

2) I have set the record level during my frequency response tests to avoid saturating the tape at the higher frequencies, when the record amp characteristic applies pre-emphasis at higher frequencies.

Regards,

Dazzlevision

[dazzlevision]

Hello,

Well, I have tried a reel of Philips Long Play High Output Low Noise (HO/LN for short) tape this morning, but the frequency response test was much the same.

I have noticed a cyclic variation on the playback amplitude and by observation, this is synchronous with the rotation of the pinch roller. I can now see that the rotation is eccentric, so I replaced the pinch roller assembly with one from a little used scrapped machine and this effect has now gone.

I then re-examined the locking paint on the Rec/Play head mount. Closer inspection suggested the azimuth screw had in fact been tampered with, so I played back a BASF "white noise" azimuth adjusting test tape (with many thanks to Nickthedentist!) and it was indeed significantly out. I now had an improved response at 10kHz but not up to the N4038's specification.

I then used my jeweller's loupe to examine closely the surface of the machine's R/P head and three others I have from scrap N4307/N4308s. It then became obvious that the machine's head surface wear was significantly worse that one of the others.

I find that, with these Philips heads, with a brown-beige coloured material surrounding the actual head laminations (which are the silver coloured "letterbox" areas; two on 4 track heads), exhibits a clearly defined area of wear. The width of the wear region was much less on one of my spare heads, so I bit the bullet and fitted the entire tape heads/tape guides/capstan bearing assembly into my N4308.This replacement assembly hadn't had any of the factory's locking paint disturbed, which is a bonus.

When I made a new frequency response recording on the Philips HO/LN LP tape, the output at 10kHz was greater than at 100Hz or 1000Hz, and the output at 14kHz (the upper +/- 3dB limit) was almost in spec.

So, I have now made significant progress. I had to slightly bend the R/P head (on it's mount) in order to get the tape to pass between the two projections fitted on its left hand side. This also keeps the tape perpendicular between the erase head's front face and the R/P head itself.

The tape now runs smoothly from the entrance guide post right through to the exit post, with no sign of "crinkling" or riding up and down on the pinchroller.

I think I will check the R/P head azimuth screw setting, as a precaution. I think I can then start on the electrical adjustments: record bias (separate preset pots for each track), the record level meter's calibration preset pot and the record amplifier's HF response peaking coil setting.

I might then have an N4308 that performs pretty close to the original Philips performance specifications!

Regards,

Dazzlevision

P.S. In case I haven't mentioned it before, there are two leaf spring type electrical contacts that are opened or closed according the which of the two tape speeds is selected. I did find these were very intermittent, so I have cleaned and re-tensioned them, in case they might be affecting the machine's overall frequency response.

[SteveCG]

Dazzlevision,

I'm pleased to read you have made progress!

As you may now be aware there are many factors that affect the overall Record and Playback performance 'loop' for a tape recorder - at least you don't have to align a 3 head machine!

[Jimmyhaflinger]

this kind of head is very prone to premature, uneven wear, the larger type of head fitted on some earlier N4308s is holding up much better

[ricard]

It's hard to see in the pictures, but if the head is severely worn the gap could be opening which would explain the bad frequency response with the old head. It's happened to me a couple of times that I've started to set up a machine but haven't been able to get the frequency response up at the high end, and closer examination has revealed that the playback head was in fact so worn that the gap was widening, even though I hadn't noticed it during a cursory inspection previously.

Being a combined record/playback head, the azimuth should not have any effect on the frequency response if you record and play back on the same machine.

Great you're making progress at any rate!

[dazzlevision]

Hello Ricard,

If you look carefully at the close up of the R/P head, you can see the region of wear to the beige coloured material - there are distinct vertical "lines" either side of the R/P pole pieces. The width of the worn area on the good head is much less.

I shall check the azimuth, as it will be playing tapes not recorded on it.

Yes, I am also glad to have made substantial progress. I am fortunate to have some little used spares from an N4307 machine that I dismantled in the 1990s.

I'm still after a speed change knob though.

Regards,

Dazzlevision

[ricard]

Originally Posted by dazzlevision

If you look carefully at the close up of the R/P head, you can see the region of wear to the beige coloured material - there are distinct vertical "lines" either side of the R/P pole pieces. The width of the worn area on the good head is much less.

Yes, it does look very worn, but the gap could still be ok dispite the apparent wear; if the gap is widening it should be visible as a (probably not uniform width) black line in the middle of one or both of the pole pieces.

I'm still after a speed change knob though.

I might have one lying around, I'm pretty sure I've dismantled one or possibly two of these machines at some time. I'll have to check exactly what they look like as if I've got it it's in a big bag of mixed knobs and buttons (unlike small mechanical parts which I tend to keep in bags marked with the model in question).

[dazzlevision]

Hello Ricard,

Thank you for your further comments.

I have put a "wanted" thread for the N4308/55 speed change knob here:https://www.vintage-radio.net/forum/...d.php?t=114834

There is a link to a picture of it.

Many thanks for looking,

Dazzlevision

[ricard]

I made a serious effort to look for my 'big back of buttons and knobs', but try as I might I can't find it at the moment. All my other tape recorder stuff is neatly tidied away in marked boxes, but the knobs just aren't there. I have sneaking suspicion at the back of my head that I moved them to a "better" place, but of course I can't remember where that "better" place is, having looked through all the "better" places where I tend to keep parts. I'll try to keep a lookout for it, although the way my memory is going, when I do find it, I'll not be able to remember why I was looking for it...

[dazzlevision]

Hello Ricard,

Many thanks for looking. I understand very well your comments about memory issues!

Regards,

Dazzlevision