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Old 28th Jun 2005, 10:50 pm   #1
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Unhappy Vision on sound Bush TV22 etc

Don't know if I have been unlucky but after restoration of Bush TV sets (TV22/TV34/TV1/TV63) I invariably find that there is a fair amount of vision on sound - typically it's related to white-level - in other words on bright/contrasty subjects the audio output buzzes like mad but a low contrast/brightness screen brings some relief!
The problem is by far the worst on my Bush TV1. Looking at the diagram the sound circuit looks to be quite straightforward with a take-off via a tuned transformer in the anode of V2 which then leads to V10, V11, V12 and the o/p valve V9.
The Domino coverter I am using allows me to uncouple the audio from the tuner and so I have verified that with no audio input, the buzz is clearly still there, coming from the video feed.
I have limited expertise (and that's probably an overstatement!!) with signal generators/wobbultors so am wondering whether anyone can suggest a few actions I might take which could lead to an improvement.
Thanks, Trevor.
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Old 30th Jun 2005, 8:12 am   #2
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Thumbs up Re: Vision on sound.

Good morning Trevor,

there is definatly no problem with vision on sound at the TV22.
Put an instrument (DMM) on C 25 (sound AGC) and adjust the
fine tuning for maximum reading.

Good luck
Darius
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Old 30th Jun 2005, 8:18 am   #3
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Default Re: Vision on sound.

If the problem occurs on all your sets it could be crosstalk between the vision and sound modulators. I don't know whose design you are using but AFAIK the modulator in Domino is OK. Some of those built to my design using modified Rediffusion HF modulators tend to suffer, mainly to due inadequate decoupling to isolate the power feeds to vision and sound sections.
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Old 30th Jun 2005, 3:53 pm   #4
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Default Re: Vision on sound.

Thanks Darius and Jeffrey:
The problem is only evident in a couple (actually 3) of my sets - and these are all Bush...the TV1, TV34 and TV63.
It's by far the worst on the TV1 - which I believe has TRF vision but superhet sound. (I am using a Malcolm Everiss Domino)
Darius - I assume DMM stands for digital multimeter?? You suggest measuring the AGC line - I assume this will be a dc volts measure? And when you suggest I tune for maximum - do you mean tuning the IF cores from aerial to sound take-off?

Thanks
Trevor.
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Old 30th Jun 2005, 5:33 pm   #5
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Default Re: Vision on sound.

Darius - I assume DMM stands for digital multimeter??
Yes
You suggest measuring the AGC line - I assume this will be a dc volts measure?
Yes
And when you suggest I tune for maximum - do you mean tuning the IF cores from aerial to sound take-off?
No !!!
I mean the local oscillator.
To tune the sound IF you need a sound IF signal generator.
Please don't tune the other cores without the right equipment and
know how.

Kind regards
Darius
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Old 1st Jul 2005, 9:30 am   #6
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Exclamation Re: Vision on sound.

Hi Trevor,

I looked at the Domino website. The RF output level is 95dBuV.
Try an attenuator 20-30dB to make sure that the RF stages are
not overloded.

With 65dBuV the vision sensivity of my PYE's is adjusted to minimum.
The contrast in the left position.
I think 55-60dBuV is a good level. I changed the attenulators in my
modulators to get this level.

Kind regards
Darius
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Old 1st Jul 2005, 10:09 am   #7
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Default Re: Vision on sound.

I can confirm that the Domino has a very high RF output that can easily overload a TV. Domino is normally supplied with an attenuator (18dB I think) which should be OK.
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Old 2nd Jul 2005, 10:25 am   #8
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Default Re: Vision on sound.

Thanks guys for continuing assistance. I have found that 10db of damping improves the situation somewhat - the problem is that it's something of a balancing act because although greater damping produces a continuing reduction in buzz, the picture strength is reduced to unacceptable levels. (obviously with these sets the CRTs are well below their original best).
I am still wondering/trying to remember whether these sets had this sort of problem when they were new....and I seem to recall that some did.

Idle speculation leads me to ask whether in these days of advanced electronics, a filter could be designed to improve matters??
atb
Trevor.
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Old 2nd Jul 2005, 11:03 am   #9
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Default Re: Vision on sound.

18dB of attenuation (damping has a rather different meaning) at the output of a Domino should still leave ample signal for any set in reasonable condition. If the picture is weak under those conditions the problem is likely to be lack of gain anywhere in the vision channel (from the tuner to the video output) rather than a weak CRT.
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Old 9th Jul 2005, 11:27 pm   #10
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Default Re: Vision on sound.

Jeffrey - I took your advice and spent a happy hour checking the Bush TV1 vision strip valves (EF50s) on a Taylor valve tester - it turned out that one of them (V3) had next to zero emission - after that was replaced the contrast control could be turned down from maximum to about half-way - and the buzz somewhat reduced.
However it is still there to a greater extent than I find comfortable.... so I also checked the sound section valves (ECH35, EF39 and EBC33)....these are all also rather "tired" - on average their MCs being about 30% below spec......but before I splash out on some new ones - do you feel their replacement would help this particular problem?
..and by the way, is this problem what I have seen referred to elsewhere as "intercarrier buzz"
Thanks,
Trevor.
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Old 10th Jul 2005, 8:16 am   #11
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Exclamation Re: Vision on sound.

Good morning Trevor,

intercarrier buzz cames up if you put the sound IF from the vision
signal. This sound IF is called DF.
In england it is not possible because the vision carrier goes to zero (at 405!).
Some modulator IC's uses this. But I am sure the Domino has a separate
sonund and vision transmitter.

Be carefull with the EF50. In my LV20 few were gasy and looks very good
on the valve tester. (NOS too!)

Changing the valves in the RF unit often causes a new agliement.
TV sets works only with the valve cap (Röhrenkapazität)in a tuned circuit
to get a high L/C ratio.

Kind regards
Darius

Last edited by oldeurope; 10th Jul 2005 at 8:25 am. Reason: smily
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Old 10th Jul 2005, 8:58 am   #12
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Default Re: Vision on sound.

Darius's explanation of intercarrier sound is correct but some of his terminology may not be familiar to UK readers. Here is a fuller explanation with some of the background.

In most (all?) 625 UK receivers there is no separate sound IF. Instead the vision IF has sufficiently wide bandwidth to pass the sound carrier which is 6MHz away from the vision carrier. The vision detector output has this 6MHz beat frequency present which is then amplified and detected. This is the basis of intercarrier sound.

There is no absolute need to utilise it in a receiver. Many years ago David Read published a design for a very high quality TV tuner which had a separate sound IF. The main reasons for using it are economy and reduction of problems due to local oscillator drift. A very small drift in the UHF local oscillator could badly affect the sound. AFC was found to be essential for UHF reception. Because the 6MHz beat frequency does not depend on the LO, the sound worked on early designs without AFC.

To use intercarrier sound at the receiver the system must use negative vision modulation. With positive modulation the vision carrier drops to zero during syncs and so would the intercarrier beat signal. Also the sound must be FM. The vision signal is present as amplitude modulation on the 6MHz intercarrier signal. This is the source of intercarrier buzz. An FM detector can be designed to be insensitive to AM. If the sound signal was AM then there would always be too much buzz.

The PAL countries adopted system I (UK, Eire) or B/G (most of western Europe with 5.5MHz sound spacing) with FM sound and -ve modulation. Hence intercarrier sound was possible. The SECAM countries (France, Eastern Europe) mostly used +ve modulation and/or AM sound so it was not possible to use intercarrier sound.
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Old 24th Jul 2005, 11:43 pm   #13
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Smile Re: Vision on sound.

Hopefully the final part of this particular saga....the replacement of the EF50 brought about a considerable improvement but still some hum persisted. At the suggestion of a pal who is very experienced, I checked out the earth/screening associated with the two chassis of the TV1. All seemed in order but just for the hell of it I tried earthing the loudspeaker to the power-chassis....and that was it - no more hum!!
Can't really see what the l/s was picking up...although it is quite close to the mains transformer which itself is not that well shrouded. Anyway, I made up an earthing strap and installed it and am now very pleased with a near perfect result!
Moral must be to always check your earthing!!

Thanks to all for assistance given,
Trevor .


PS - I did try removing the earthing strap from between power and video/audio/timebase chassis and that increased the hum level slightly - but not by as much as the reduction acheved by earthing the loudspeaker...but an interesting experiment!
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