Ekco TS88 vision on sound issue

[high_vacuum_house]

Good evening,
I have been restoring this set recently one wax capacitor at a time and now have a very good stable picture. I have been having problems which I believe is vision on sound which I cannot get to the bottom of.

The sound is present though the buzz can be increased or decreased by turning up or down the contrast control. Brighter scenes also increase the buzz.

I have changed all of the problem capacitors around V1 and V2 though I cannot see what is causing it.

Does anyone know the best place to start looking for the issues? I have the vintage TV CD-ROM service sheet for this set though it is very basic.

Many thanks,
Christopher Capener

[Argus25]

I don't have the schematic for your set. However in general there is the opportunity for either vision signal getting into the sound channel, or the reverse, sound getting into the vision. And I'm not sure if your set a TRF or superhet either, but..

In the case of the the vision stages there are often sound traps to get the sound signal out of the vision channel. Early in the piece, the first RF amplifier, which might be common to vision & sound, the sound trap there is also used as a way of extracting the the sound carrier to the sound channel as well, and often a second trap in the vision channel kills and remaining sound carrier in the vision channel. These traps are tuned to the exact sound carrier frequency. Some cases the sound could be extracted from the second trap.

In the case of the sound channel though, there are no "vision traps" because the vision signal is a broad bandwidth signal and can't be trapped out at one discrete frequency. The sound channel (to ignore vision channel signals) relies on the fact it is a narrow bandwidth channel, only enough bandwidth to support the audio spectrum. Or at least it should be if the coils in the sound channel are peaked properly.

So it appears that your sound channel resonant circuits, and possibly the first sound trap, are out of adjustment and your sound channel is responding to vision carrier information, putting a buzz corresponding to picture modulation into the sound.

(However, depending on the design of the set, there are other possibilities with cross modulation in a common stage with overload, but maybe less likely since your test pattern looks so good, so I don't think any of the vision stages are likely overloaded).

So its most likely not a "faulty component" causing this sort of thing, its an adjustment issue. It might be necessary to subject your set to a vision and sound channel alignment which is best done with a sweep generator and scope.

Having said that you might get away with feeding the set with just a modulated sound carrier, peaking the sound channel for max audio signal and tweaking the sound trap/s for minimal signal in the video channel.

[Peter.N.]

Check that the CRT aquadag is earthed.

Peter

[Heatercathodeshort]

http://www.vintage-radio.net/forum/s...ad.php?t=55206

Hello Chris,
Over the years I have serviced large numbers of the TS88 based chassis. I suspect the fault is due to poor alignment usually caused by the tuning cores in the RF transformers becoming loose and winding themselves up or down their formers. It is worth checking the earthing of the push on screening cans.
I have actually had them rattling around in the cans! Being a TRF unit it is very easy to align. You may find this old posting of interest.
John.

[beery]

Hi Christopher,

I'm basing my comments on trader sheet 980. I have the TS188, the Birmingham model myself.

Note that there is a sound sensitivity control.
L 18, L19, L20, L21, L22 and L23 are straight tumred to 41.5MHz and so could a tuned for maximum output using the test tone from the Aurora. Refer to the sevice sheet for the correct order in which to adjust them.
The vision section is heavily damped and so is simply straight tuned, but you will need a signal generator to do this. Note that the set would have originally been tuned to 46.5MHz and working on the upper sideband. After Crystal Palace transmitter tok over from Alexandra Palace, the set would have to have been tuned down to the lower sideband. With the first and second RF stages tuned to 43.5MHz, vision on sound and vise versa could be a problem. You can easily check with an RF signal generator if this has been done to your set.
All of the modulator designs that you find in the standards converters that are available have double sideband modulators anyway, so having a set tuned to the upper sideband is not a problem anymore.

Cheers
Andy Beer

[high_vacuum_house]

Good afternoon,
Many thanks for your help. I have carefully adjusted the cores which helped but then I came across the real problem.

The RF input from the Aurora was much too great and the vision signal was so strong it came through the sound traps. Attenuating the input from the Aurora has completely cleared the vision on sound and even test card C now looks better.

I shall add a new U24 EHT rectifier (fell off its base) and I will be happy with the sets performance
Christopher Capener

[beery]

Hi Chris,
That's great.
These sets are really sensitive. Compare that with a TV22 which is quite a deaf set really.

The TS88/188 are reliable performers, if you find one with a good tube. Your set does indeed seem to have a good tube and you can apreciate just how bright and clear the picture looks on a Mazda tube running with relatively low EHT, compared with the more reliable Mullard tubes of the time which somehow just don't have the wow factor...

Cheers
Andy

[high_vacuum_house]

The tube is a good one. It has been regunned at some time. My heart dropped like a balloon when I first saw it as there was no getter material visible in the neck where the getters were, just some rainbow effects where the silvery deposit should have been. Hopefully it lasts a reasonable amount of time.

I thought I had a duff tube so after checking everything looked ok I ran it straight up on the variac and surprised me no end when an unfocussed blob appeared in the centre of the CRT. knowing the tube was going to give something I decided on restoring the set one capacitor at a time. Not too much has been changed really other than the usual waxies and some of the Hunts mouldseals in the receiver section. The 2 50 ohm resistors in the power supply were changed as the cement cover was crumbling away on both of them. And away it went!! shows that pulling it to pieces first thing isn't always necessary.

Christopher Capener