[Closed] 1977 Philips N1700: Player No #1 repair

N1700 belt check , bringing a new dimension to my over the shoulder threads/posts.

Edit: forgot to state on the video which belt was what

p.s.
Re vids; I don't find it easy or comfortable but do hope to get better at it. If you find me or the youtube vids boring or unnecessary please say as I will not be offended and will stop.

Posted by: crustytv

p.s.
Re vids; I don't find it easy or comfortable but do hope to get better at it. If you find me or the youtube vids boring or unnecessary please say as I will not be offended and will stop.

I find them very interesting and informative. An extra dimension is added to the threads.

Hi Chris,

I also find them interesting and useful.

Keep it going.

Another positive vote for the videos from me too! It’s good to actually see the things you are working on in action, especially with mechanical bits like this. Maybe one day I might have a go at some videos!

I see the difference of the 2 switches, these ones in the N1700 look similar to those found in 70’s and some 80’s music centres for the radio waveband selector switches. This machine looks promising!

 Regards 

Lloyd 

I've just got myself a video editing software package. I've processed and had a play with the previous belt check video, learning as I go but here it is. Just added some titling, text annotations and incidental intro/outro music.

I'll be filming the actual belt change later today or over the weekend

Very posh  ? .

Latest update:
22 minute video showing the changing of drive belts, on the Philips N1700. I'm sure you all might find paint drying more entertaining and be far more adept at the change. However after searching I cannot find another such video out there so maybe it will be of use to someone.

Enjoyed watching, my hands don’t have enough dexterity for that work.

Well some mixed results after the belt change and par for the course for me, steps forward followed by steps backwards.

A test signal in the form of test card F injected into the player, the player power on button was pressed, it powered up and E-E was established. During the power on the head drum assembly rotated into position whilst threading the tape, all good thus far. Powered down, head assembly rotated back and tape de-threaded.

Next tape testing; power button pressed, threading started and the head assembly rotated once again into position. Fast forward tested, it worked. Rewind tested, it worked. Now the moment of truth, I pressed play. The tape started playing and the content was displayed on the TV. It was in and out of sync at first but soon established itself albeit occasionally losing sync. Very pleased! I powered down, the head assembly rolled back and the tape de-laced.

I dug another tape out powered up but this time the head assembly was very sluggish to rotate into position, it did finally get there. Tried to power off but the head assembly did not rotate back. Tried to press the power button again but it does not have any effect. Pulled the plug, still no de-thread or head assembly roll-back. Re-plugged in and the head just resumes spinning as does the spool drive.

I'm now left with being unable to power the machine off which in turn would initiate the de-thread procedure hence the head roll-back. When the cassette tray is ejected the drive stops and the head slowly spins down to a halt. When the cassette tray is pushed back down even without a tape installed, you can see the light sensor actuate and the spool drive start along with the head spin up. It would appear to be locked in thread-up run mode which is only interrupted when the cassette tray is opened and due this state no threading action takes as its thinks its already in that state, so no play.

I now need to figure out what on earth has caused this and why. I'm suspecting perhaps stuck relays, missing or incorrect voltages in the threading circuit but to be honest I'm not sure, it seems a good place to start though. Also because it thinks its running the 30sec stop mode switch off is not taking effect.

A little further investigation reveals some interesting results and perhaps clues to what is causing the odd problem as described above. Here's a video of the initial testing of the VCR after the belt change and subsequent video tests. Followed by the faults as detailed above.

Basic description of the N1700 operation is described here

Good progress so far Chris. When you consider the age of these beasts, there must be some sticky dried grease somewhere. There was a name for the stuff Philips used which in most cases was a translucent beige or light brown colour. I just can't remember what it was but I doubt it's still available. I think if it was me I'd just clean all the old stuff off and use a decent car lubrication grease. Either that or a spring is missing somewhere.

Today Repair Update #1

Problem found and fixed.  ? 

Today Repair Update #2

Play test & record test, thoughts about possible servo speed and concerns about heads.

Also worn out Flywheel brushes replaced

Hi Chris,

The black and white lines running the length of the screen, that looks a bit like a barcode, is due to the way the dropout compensator (DOC) on Philips machines work, it constantly recycles the same line of video that is stored in a delay line in the absence of off tape FM, now whether this lack of FM is down to a worn head issue, or due to severe mistracking for whatever reason is unknown at this point in time, it could even be an electronic type fault in the head amp department due to the age of this machine.

Again, a known good recording would be highly desirable at this point  😉

Ps. it could even just be due to knackered tapes, however I doubt all of the tapes you have would be knackered to the same extent.

On a more practical note wrt to belt changes and servicing VCRs in general, magnetising your screwdriver would help enormously, just try to keep the magnetic bits away from the ACE head, also a couple of these: https://uk.rs-online.com/web/p/spring-hooks/3514495/   or similar, obviously the ones from RS are very expensive! you should be able  to pick them up for a few quid each from elsewhere on the net, these help immeasurably with belt changes, and also removing/adding back springs too, which is what they were originally designed for.

Posted by: Red_to_Black

Hi Chris,

The black and white lines running the length of the screen, that looks a bit like a barcode, is due to the way the dropout compensator (DOC) on Philips machines work, it constantly recycles the same line of video that is stored in a delay line in the absence of off tape FM, now whether this lack of FM is down to a worn head issue, or due to severe mistracking for whatever reason is unknown at this point in time, it could even be an electronic type fault in the head amp department due to the age of this machine.

Hi Baz, 

as ever thanks for your sage advice, its at least given me a clue as to where to make a start with investigations and that's always a tricky point when you're blazing a lonely trail with little or no experience of the device in front of you.

I've just been reading up on the function of U732 (FM Processor) and U734 (FM Demodulator). It certainly sounds like the area to make my start and the black and white lines I see and you describe, certainly point in this direction and are mentioned in the details below (highlighted in red).  ? 

Luminance Playback

The specific luminance playback section is made up of U732 & U734. Its function is to separate the chrominance signal from the luminance signal modulated in FM and next, to demodulate it.Moreover, it must fill up drop-outs free from interference.

FM Playback Processor U732

U732 has the following main functions: Suppressing the chrominance signal, FM gain controlled amplifier and drop-out detector.

Suppressing the chrominance signal

Before the head signal is supplied to U732, it is affected by the suction filters C710, S705 and C711, S706, which are adjusted at 562kHz resp. Consequently, the chrominance signals in the neighbourhood of the 562.5kHz chrominance carrier are crushed, so that they cannot cause interference in the luminance channel. Next, in U732, the signal is fed to a high-pass filter via an emitter follower.  

With R711 the high-pass filter is adjusted.  Adjusting R711 involves compromising.  On one hand, it is important that the FM sidebands in the range between 1 and 2 MHz are not suppressed too much, because the FM sidebands in this range are important for the resolution of the detected luminance signal.  On the other hand, the amplitude of these FM side bands may not become greater than that of the FM carrier proper.

This is evident, when the playback frequency characteristic of the video heads is considered.  The conclusion is that, proportionally, the FM carrier is reproduced with a much smaller amplitude than the FM side-bands in the range between 1 and 2 MHz.  If the amplitude of the side-bands exceeds that of the carrier, the limiter in the U734 gets stuck, so that the information is lost.  This symptom is recognisable in the picture in the shape of short, black horizontal stripes after a black-and-white changeover.  Moreover, the high-pass filter suppresses the chrominance signal.  

FM gain controlled ampifier

In the subsequent gain controlled amplifer, the FM signal is amplified in a manner that there is always an FM signal of constant amplitude available on the output of 15U732.  The control voltage for the gain controlled amplifier is produced by rectification of the FM signal.  After the gain controlled amplifier, all signals with frequencies exceeding the playback frequency range of the video heads (approx .5MHz) are suppressed by the low-pass filter.

Drop-out detector

The function of the drop-out detector is to recognise a drop-out in the FM signal and derive a switching pulse. In its turn, this switching pulse actuates the drop-out compensator, so that the drop-out in the output signal is filled up.  The drop-out detector is an envelope detector.  The output signal of the detector triggers a Schmitt-trigger controlling an electronic switch in U734.  The sensitivity of the sensor detector is factory adjusted.

FM-demodulator, U734

The FM signal at 15U732, controlled in amplitude, is next supplied to U734.  In U734, the FM signal is amplified, limited and demodulated.  Via a low-pass filter (3MHz) by which the FM - carrier remainders, still present, are suppressed and an amplifier, in which also video de-emphasis is effected, the video signal is fed to 1U734.

1U734 is connected to 17U507 via plug connection F42, L52.  From this point on, the luminance signal follows the same signal path to the aerial output as in recording position.

Drop-out compensation

If a drop-out occurs in the FM-signal reproduced, it will be detected by the drop-out detector in the U732,  The Schmitt-trigger delivers a signal, so that the electronic switch in U734 switches over.  The output of the limiter is now interconnected with the input via the 64us delay line. 

At the moment of switch over, a 64us delayed FM - signal will be present at 11U734.   A line duration is also equal to 64us, from which it follows that the drop-out is filled up by the information of the preceding line.  Because in practice the information of two subsequent lines is practically the same, the drop-out is filled up invisibly.

In case of a long drop-out (longer than 1 line duration) the last reproduced correctly will be repeated again and again.  As a result of reflections in the 64us delay line, the signal quality slightly decreases on every repetition.

I did read the bit in red, however we have nearly full fields of Vertical B&W lines, this is a sure sign that the DOC is working overtime, this was the case even with Philips VHS machines with the same symptoms.

Usually this is a sign of knackered or dirty heads, it usually signifies poor/non existent off-tape FM.

Now there could be other causes of poor FM, a scope check at the relevant test point would confirm this is the case.

The capstan servo appears to be running ok judging the sound playback.

If it were me with access to another machine I would try a substitution of the entire drum assembly, and if possible the head amp assembly in as big a chunk/block as I could as a means of speedy diagnosis, I would not be too bothered about any fine setting up as they should be near enough for diagnostic purposes, of course if an immediate improvement was apparent then start by eliminating smaller chunks.

Of course that is not to say any of the other machines you have are any better in this department, a case of suck it and see I am afraid. 

Try a good head clean first. Be very careful when doing this. Best when the head is spinning and hold the cleaner steady against the drum with light pressure. Also clean the audio, sync and erase heads.

On the subject of small tools for belt changes and springs etc... I recently acquired from a certain auction site a most excellent set of aluminium crochet hooks for change from a fiver. I'll happily share the link if Chris is OK with that.