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Old 13th Jun 2020, 7:16 pm   #1
Kyle__B
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Default Video Detector has a second function - Philips F6

Nosying at this old telly again, been wondering something about it for a while so I think it's time I ask. I have my own idea but more interested in you lot's.

I'm looking at the video detector U487, 5.5v goes in at pin 5, goes through the diode and comes out pin 6 as video, makes sense to me.

But there's also pin 7 in there, connected to the video detector with a couple of caps, and has a coil you can twiddle.

A connection goes all the way to pin 7, through a ceramic cap, then to between the triode and pentode of the sound amplifier valve B401 (a PCL86).

It's not providing the audio signal, that comes from it's own separate section. Pin 7 goes to the sound section long after volume and tone control etc.

So question number 1:

wozzit for?

question number 2:

if, hypothetically speaking, someone removed U487 and fed the telly direct video, how would that someone go about recreating what it's pin 7 was doing?
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Old 14th Jun 2020, 6:42 am   #2
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Default Re: Video Detector has a second function - Philips F6

To remove sound on vision?

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Old 14th Jun 2020, 7:01 am   #3
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Default Re: Video Detector has a second function - Philips F6

The 5.5Mhz tuned circuit will reduce any ‘Dot crawl’ in the vision signal but why the feed to the audio stages I don’t know.
Any hints in the service manual or have you just the circuit.
I would have thought the colour sub carrier would have caused more ‘dot crawl’ than the inter carrier sound.

Something to think about while in this ‘lock down’ we are currently in.
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Old 14th Jun 2020, 8:54 am   #4
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Default Re: Video Detector has a second function - Philips F6

The level of 5.5 MHz intercarrier going from that post-vision demodulator trap/transformer to the audio output valve grid should be near-zero when the receiver is properly tuned, with the IF traps doing their job of taking out the 33.4 MHz sound carrier, of which only a negligible quantity would reach the vision demodulator. Off-tune though, the situation would be different, and the level of 33.4 MHz sound carrier reaching the vision demodulator and hence 5.5 MHz intercarrier reaching the audio output valve would increase.

That 5.5 MHz signal, carrying not only sound FM, but also Nyquist FM and vision AM (including harmonics from the demodulation process) would be rectified at the sound output valve grid. Two thoughts on this: the first, less likely I think, would be that the rectification process generates enough DC bias to mute the output valve. Thus it would remain muted until correct fine tuning of the receiver is achieved. The other is that the rectification produces audible buzz components that do not diminish into inaudibility until correct fine tuning is reached. Thus the sound channel would form an aural tuning indicator; that is, tune for minimum buzz.


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Old 14th Jun 2020, 9:52 am   #5
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Default Re: Video Detector has a second function - Philips F6

The answer to this mystery is revealed if you look a little further back to the sound IF amplifier. The signal fed to the BF194’s base comes from the tuned circuit U480. U480 is coupled to the choke S462, which is used to feed HT to the PCL805 part of the audio output stage. The choke has little effect at audio frequencies, but at 5.5MHz it will present a significant impedance in the anode circuit of the PCL805. Thus, the valve will offer a little gain at that frequency.

Going back to U487, the signal is passed through the 5.5MHz bandpass filter, which picks off the sound carrier then passed via pin 7 and C524 to the grid of the PCL86. Signal from the PCL86 anode is passed to the grid of the PCL805 and appears at the PCL805 anode and S462.

It’s a very unusual way of getting the intercarrier signal from the vision detector to the sound IF amplifier.

To feed video to the stage directly, forget about what is happening around pin 7, unless you are still using the intercarrier signal to pass the sound to the audio stages.
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Old 14th Jun 2020, 10:19 am   #6
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Default Re: Video Detector has a second function - Philips F6

Also of interest is the BA102 vari-cap diode connected to pin 2 of module U483.
Obviously there to modify the response of the 33.4Mhz trap.

DFWB.
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Old 14th Jun 2020, 10:42 am   #7
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Default Re: Video Detector has a second function - Philips F6

So the next question is why? Perhaps they needed another IF stage for the sound and this was a cheap way of delivering it.
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Old 14th Jun 2020, 4:54 pm   #8
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Default Re: Video Detector has a second function - Philips F6

Ah but it's going into the sound amplifier long after any volume or tone control has been done. So whatever it's giving the sound, it's giving it at a constant level no matter how loud or quiet the telly is.

Most of what Synchrodyne said flew over my head, but he mentioned buzz. That has my attention because I've had a longstanding battle with a constant level of speaker buzz which doesn't change with volume control - but does change with the distance of the chassis to the neck and yoke of the picture tube.

My hypothesis was that pin 7 to the sound amp was for cancelling out that interference.
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Old 15th Jun 2020, 3:22 am   #9
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Default Re: Video Detector has a second function - Philips F6

Quote:
Originally Posted by frsimen View Post
The answer to this mystery is revealed if you look a little further back to the sound IF amplifier. The signal fed to the BF194’s base comes from the tuned circuit U480. U480 is coupled to the choke S462, which is used to feed HT to the PCL805 part of the audio output stage. The choke has little effect at audio frequencies, but at 5.5MHz it will present a significant impedance in the anode circuit of the PCL805. Thus, the valve will offer a little gain at that frequency.

Going back to U487, the signal is passed through the 5.5MHz bandpass filter, which picks off the sound carrier then passed via pin 7 and C524 to the grid of the PCL86. Signal from the PCL86 anode is passed to the grid of the PCL805 and appears at the PCL805 anode and S462.

It’s a very unusual way of getting the intercarrier signal from the vision detector to the sound IF amplifier.
Agreed. I was looking for something more sophisticated, whereas Philips had in fact regressed to using what was effectively a reflex amplifier, handling both the 5.5 MHz intercarrier and audio output. So the 5.5 MHz IF transformer immediately following the vision demodulator was in fact a conventional intercarrier take-off.

Apparently the technique was used in other Philips TV receivers, as evidenced by this block schematic:

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I imagine that this system might have been prone to cause more sound buzz than usual for intercarrier systems. Any non-linearities in the audio output valves would result in local demodulation of the 5.5 MHz intercarrier, at this stage unlimited and so carrying AM (with AF buzz components) transferred from the vision carrier. Normally the limiting action in the intercarrier IF strip reduces somewhat AM-origin buzz, although it does nothing to reduce Nyquist FM buzz.


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Old 15th Jun 2020, 5:48 am   #10
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Default Re: Video Detector has a second function - Philips F6

That all sounds very technical but it doesn't give me any understanding at all of what's supposed to be going on here.

I have no idea what an intercarrier is or why you would want one. A complete audio signal is there at plug B3. After disconnecting this "intercarrier" from U487 both picture and sound are still there.

Last edited by Kyle__B; 15th Jun 2020 at 5:54 am.
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Old 15th Jun 2020, 10:33 am   #11
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Default Re: Video Detector has a second function - Philips F6

Quote:
Originally Posted by Kyle__B View Post
That all sounds very technical but it doesn't give me any understanding at all of what's supposed to be going on here.

I have no idea what an intercarrier is or why you would want one. A complete audio signal is there at plug B3. After disconnecting this "intercarrier" from U487 both picture and sound are still there.
Have a look at this:

https://books.google.co.uk/books?id=...206mhz&f=false

It describes the intercarrier sound system, it's for 6MHz but your 5.5MHz will be the same system, once you have read the first couple of pages or so you should be able to understand why the FM sound signal is taken from the vision demodulator circuit, its route from there is slightly unconventional but hey, it's a Philips.....From the vision demodulator circuit it's connected to the audio amplifier output stage via the 5.5MHz take off transformer and the 100pF capacitor (C524) but remember at this stage the signal is still in the form of an FM signal, it's amplified by the audio output stage and fed to the intercarrier amplification stage via C510 (it's still an FM signal) after amplification it's just a matter of conventional FM demodulation which is done with a ratio detector, the output of which, is fed to the volume control circuit.

Hope that helps.

Lawrence.

Last edited by ms660; 15th Jun 2020 at 10:39 am. Reason: addition
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Old 15th Jun 2020, 4:43 pm   #12
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Default Re: Video Detector has a second function - Philips F6

You're saying that the sound FM is coming from the video demodulator, going through most of the sound amplifier, then going back out by traveling down the wrong way of a 280V power rail. (!)

What a stupid trick, all to save couple of quid. It leaves a mystery: Why can I still hear everything when C524 is taken out?

Until I started this thread I'd assumed FM was going from D446 to D440, but looking closer that line is marked DC going from 440 to 446... automatic gain control?
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Old 15th Jun 2020, 5:03 pm   #13
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Default Re: Video Detector has a second function - Philips F6

Quote:
Originally Posted by Kyle__B View Post
You're saying that the sound FM is coming from the video demodulator, going through most of the sound amplifier, then going back out by traveling down the wrong way of a 280V power rail. (!)
The inductor S462 will present itself as a high impedance anode load to the PCL805 at 5.5MHz.

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Old 15th Jun 2020, 8:55 pm   #14
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Default Re: Video Detector has a second function - Philips F6

A couple of quid is a saving in the 5 to 7 digits range when applied in mass production. Also less points of failure, less space on the pcb, etc. I think it's quite ingenious.
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Old 15th Jun 2020, 9:16 pm   #15
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Default Re: Video Detector has a second function - Philips F6

Anything ingenious about this chassis has been cancelled out by them putting the sound output valves about an inch from the yoke. It hums like mad unless you take the back off and fold it down.

And no don't tell me to check lead dressing i've been over that 50 times. I'm going to dummy out the loudspeaker with an 8watt resistor and use my hifi for sound, had enough of it.
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Old 15th Jun 2020, 10:26 pm   #16
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Default Re: Video Detector has a second function - Philips F6

I wonder about that, I'll have to look whether Philips ever did a service bulletin on that (certainly seems large enough an error for that) and possibly experiment. I must have an F6 in storage somewhere but not readily accessible. A valve can certainly pick up magnetic fields, B402 of which the pentode section is used in the output stage, seems to sit nearby the yoke.
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Old 16th Jun 2020, 12:32 am   #17
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Default Re: Video Detector has a second function - Philips F6

I've been experimenting with what angle the chassis is at before the noise starts, it's when its closer to vertical than horizontal.

What I'm tempted to do is extend the cabinet out further back another 15cm and move the brackets, then the whole board would permanently be further away. Dramatic!

Before I'd do that I'd want to build a temporary jig to put the chassis on to confirm it's distance to tube that's doing it, or purely about lying down/standing up. It could be the world's most obscure bad connection.

I'm ruling out the idea of trying to shield the valves from interference because there's no room in there.

Or there's also the idea of getting the 4 or 8 ohm equivalent of the loudspeaker, running that off a transistor amp and putting a dummy load on the TV's sound amp. It wouldn't spoil the look of the thing and there's precedent in that it's how they hush up buzzing vectrex machines.

All just ideas for now. I'm waiting for igor to come back from the shop.
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Old 16th Jun 2020, 2:49 am   #18
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Default Re: Video Detector has a second function - Philips F6

Shielding a valve can be as simple as shoving a metal sleeve over it. Indeed a good test.
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Old 16th Jun 2020, 3:47 am   #19
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Default Re: Video Detector has a second function - Philips F6

That wouldn't cause it to overheat?
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Old 16th Jun 2020, 7:38 am   #20
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Default Re: Video Detector has a second function - Philips F6

Philips were known for some, shall we say, more unusual ideas but they usually worked extremely well.
The loud speaker is fed by a shielded cable, is there a possibility of a fault there introducing the hum into the circuit.?
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