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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 5th Nov 2019, 1:43 pm   #1
KeithsTV
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Default Philips N4520 alignment problems

Iím looking at a Philips N4520 (four track stereo tape deck) which has an annoying crosstalk problem that I canít seem to get rid of. Playback and recording are fine but if you record on side 1 of the tape, turn it over and playback side 2, with no recording on it, you can hear the recording from side 1. Itís faint and backwards but audible if the volume is turned up. It seems to be worse with recordings made on the recorder but playback of recordings made on other recorders seem fine.

Iíve adjusted the playback head height using a known good recording which has a blank section on side 2 and made sure there is no crosstalk. Iíve then made several recording on side 1 of a blank tape and played back side 2 adjusting the record head up or down each time depending on whether the crosstalk is in both channels or just the right channel. Adjustments have been ľ turn of the 3 head height adjustment screws each time. But I canít get rid of the crosstalk.

I have also played the recordings back on another tape deck and have heard the crosstalk suggesting it is the record head alignment that is out.

Iím sure the erase head is aligned correctly as if I erase side 1 the crosstalk disappears. I havenít touched the erase head alignment.

When adjusting the record head I can get to the point where the adjustment screws have no effect.

Can anyone suggest what I could do next or is there anyone who could check the alignment of the tape path.

Keith
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Old 5th Nov 2019, 1:55 pm   #2
Welsh Anorak
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Default Re: Philips N4520 alignment problems

I think you've proved that the problem is with the record head. It's possible that it's worn to an extent there's crosstalk, though a close inspection should reveal that. Aligning a three-head machine is of course more difficult than on a two-header, but the idea is to get the playback head absolutely correct then, using off-tape monitoring, align the record head.
You can rule out the erase head by erasing a portion of tape and check it's completely blank.
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Old 5th Nov 2019, 3:40 pm   #3
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Default Re: Philips N4520 alignment problems

If the heads are far out of alignment it should be visibly evident when looking at them with tape in the machine. It may help to make a piece of clear tape by removing the oxide using acetone. Normally the tops of the pole pieces for the playback and erase heads should be almost flush with the edge of the tape - the service manual may list exact measurements. I think the playback head should be a fraction of a mm inside the tape, and the record head flush (and the erase head hanging outside a smidgen).
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Old 7th Nov 2019, 5:31 pm   #4
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Default Re: Philips N4520 alignment problems

I've had another go at re-aligning the heads but the results are very similar, there's still crosstalk present.

I've attached pictures of the record and playback heads with and without the tape present as currently set up for minimum crosstalk. I'm confused as to exactly which part of the head is the active part as they don't look like any tape head I've seen before. Can anyone point me in the right direction as to where the active part is?

The service manual states the active part of the head should be just below the edge of the tape but if I set it up where I think the active part is the crosstalk is much worse.

The heads are supposed to be "long life" which I assume they don't wear as quickly as "normal life" heads but the deck is around 40 years old and I don't know how much use it's had. Is it possible that a worn head could cause the crosstalk problem?

Keith
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Old 7th Nov 2019, 8:34 pm   #5
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Default Re: Philips N4520 alignment problems

I've never worked on one of these machines, and I might be wrong but to me it looks like the top of the upper head segment is too high in relation to the top edge of the tape so far as I can make out from the photo's. When I used to service Sony decks at Sony the top edge of the upper segment was always set slightly below the top edge of the tape thereby leaving a guard band, the head heights for record and playback heads were the same as far as I remember. The top of the erase head segment was always above the top edge of the tape which ensured that any recorded info on a tape that had made it into the guard band was fully erased.

If I remember at Sony the top edge of the upper segment was adjusted to be exactly in line with the top edge of the tape then the head height adjusting screws were turned anti-clockwise by so many degrees, Sony used to give a figure for the number of degrees the head height adjusting screws had to be turned (anti-clockwise) to drop the head height down so that the correct distance between the top edge of the tape and the top edge of the upper head segment was achieved.

That might or might not be of any use....but good luck with it whicheverways.
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Old 8th Nov 2019, 9:00 am   #6
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Default Re: Philips N4520 alignment problems

I'll have another go today. I think if aligning the heads to just below edge of the tape cures the crosstalk problem on it's own recordings that should be OK.

Keith
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Old 8th Nov 2019, 10:41 am   #7
ms660
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Default Re: Philips N4520 alignment problems

I can't speak for the Philips tape path but when I worked for Sony the top edge of the upper head segment was set to approx. 0.025mm below the top edge of the tape, for record/playback head(s) and approx. 0.05mm above for the erase head.

Lawrence.
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Old 8th Nov 2019, 11:02 am   #8
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Default Re: Philips N4520 alignment problems

In a perfectly set up machine there will be some baseline crosstalk. The level of crosstalk was normally quoted in the specs as a db level at a certain frequency. Crosstalk should be much worse in the mid and lower frequencies.

Crosstalk is much more noticeable between adjacent tracks but the reversed nature of adjacent tracks was part of the 4 track design knowing that listeners wouldn't be as distracted by a voice or music they couldnt understand. After a while it became a quiet. meaningless babble.

These days we're used to digital recordings with unbelieveably good separation between tracks, and general absence of background noise. Analog tape cant compete with that and sometimes customers might need to be educated about these limitations.

In the field of "digital audio restoration" people can make digital recordings of older tape recordings or discs and try to "clean them up to modern standards", often with disastrous results.

Edit: in photos 1 and 2, tracks 2 and 4 look wider than 1 and 3. Very odd.
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Old 8th Nov 2019, 11:40 am   #9
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Default Re: Philips N4520 alignment problems

My apologies, should have said turn screws clockwise in Post#5 not anti-clockwise.

Also just found a reference for dropping by 0.025mm in one of the Sony manuals, 15 degree turn of the adjusting screws.

Lawrence.
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Old 8th Nov 2019, 1:47 pm   #10
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Default Re: Philips N4520 alignment problems

I've adjusted the heads with the active part of the head just below the edge of the tape and the crosstalk is much worse. So I've adjusted the replay head for minimum crosstalk on a known good recording and adjusted the record head for minimum crosstalk on its own recordings. I don't think I can do anything else. The crosstalk is only really noticeable in quiet passages and then only if the volume is turned up.

The other possibility is to record on one side only.

I did wonder about the apparently wider "tracks" on the heads but not having seen any other examples I don't know if this is "normal" for this recorder.

Thanks for all the comments and suggestions.

Keith
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Old 8th Nov 2019, 2:27 pm   #11
TIMTAPE
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Default Re: Philips N4520 alignment problems

I found another online photo of the Philips head's face and it looked the same.

What's puzzling to me though is that this is a 4 track stereo machine but the head looks like a 4 track/4 channel head but with the tracks 2 and 4 pole pieces wider.

The heads should only have track 1 and track 3 capability so the wider sections seem strange as they wouldn't normally be there or be visible.

Attached photo of an older style Phillips 4 track head. I guess the thinner section between the pole pieces represents some sort of magnetic shield between them.

Tim.
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Old Today, 9:27 am   #12
KeithsTV
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Default Re: Philips N4520 alignment problems

I made some measurements on the head and compared them with the spec for a 4 track tape and, although the measurements weren't deadly accurate, they seem to indicate that the head is in line the spec.

I've now adjusted the record head for minimum crosstalk. As the crosstalk is only audible in quiet passages on the reverse side and then with the volume turned up I'm going to call it a day.

Keith
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Old Today, 12:18 pm   #13
KeithsTV
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Default Re: Philips N4520 alignment problems

I'm wondering if the additional pole pieces, arrowed in the photo are anything to do with the problem. They are aligned with the "reverse" tracks so could there be some leakage from them into the "forward" tracks? (getting to the clutching at straws stage )

Keith
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Old Today, 1:34 pm   #14
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Default Re: Philips N4520 alignment problems

I wonder if they are not just an area of the same material as the 'live' tracks so as to get an even wear on the head assembly. Have you discovered whether they are connected to anything? I guess they would/should be connected to the frame/earth of the head assembly or the screen of the head cables.
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