Disconnecting scan coils

[llama]

Is it safe to just disconnect line and field coils?

The reason I ask is my son wishes to send audio into the scan coils - it's similar to what I did years ago with a TV53 to improvise a large-screen 'scope.

My reservation concerns mainly the line scan coils as I would have thought the inductive loading would affect the tuning around the LOPT and in particular spoil the 5th harmonic tuning that should flat-top the flyback pulse.

Am I being over-cautious? Would a load resistor help/be just as good? Should I substitute an inductor and if so what value?

Any help much appreciated.
Graham

[Boom]

The first immediate danger that comes to mind is that there is a very real chance of screen burn.

[Peter.N.]

I would agree, you will need to keep the brightness down. You could use the frame output stage to amplify your audio but the line timebase is not so easy, depending on the circuit it may rely on the inductance of the coils to function correctly. You will need a timebase with variable speed to replace the line scan.

Peter

[Tractorfan]

Hi,
Coo, that takes me back!
At school I unplugged the scan coils of a Bush TV43 and connected the line coils to a doorbell transformer and the frame coils to the output of a Collaro record player amplifier which had CY31 & CL33 valves. It worked quite well and my teacher was suitably impressed at the Lissajous patterns thus displayed when certain notes were whistled into the microphone.
Both the TV and the amp were 'live chassis' and I knew nothing about mains isolation for the mic. Or any of the metalwork, come to that . How nobody got a shock, I will never know. No H&S back then! I probably had 13amp fuses in the plugs too.
Years later I tried the same trick on a more modern set and the line output stage just stopped. I assumed that this was a design feature to kill the EHT in the event of the coils going open circuit. Was that the case?
Cheers, Pete

[gingpeakin]

And me - it must have been in the early 70's I bought my 1st telly. I couldn't tell you what it was other than dual standard. When I was bored with it I disconnected both coils and connected them to (I think ) a Sinclair Z12. Fantastic lissajous figures ensued caused by the different inductances of the coils. Also the frame blanking produced a pleasing circular effect on sustained bass notes. Oh and yes it didn't take long turn burn a hole in the centre of the screen.

Ging

[Malcolm G6ANZ]

As above it can be done. When I did it to a set I put a low value hight wattage resistor in in place of the line coils to maintain continuity so the line stage would still work.
The best effect was from a stereo amp feeding one channel to the line and the other to the frame, as said keep the brightness down when there is no signal.

Malcolm

[Maarten]

Whether the coils can be disconnected safely, depends on the design of set they are in. Some chassis employ for example a vertical protection circuit which will switch off the set if no coils are present.

[Welsh Anorak]

Hi
Most modern sets' EHT generation would be far less efficient with the line scan coils disconnected. Perhaps you remember a 'Television' project to make an oscilloscope using a Bush TV125? As far as I remember a second set of scan coils (simply hidden away) were used so that the line output stage would operate at optimum efficiency.
Glyn

[alynG1TBL]

Some where in the back of my brain cell a voice tells me the scan coils are around 16 ohms imp. I built/cobbled an old 405 line set (late 60's) to do the same with the audio amplified through two 10watt brick amplifiers. Again no elf & saffty. 73's Alyn

[KeithsTV]

Originally Posted by Welsh Anorak

Hi
Perhaps you remember a 'Television' project to make an oscilloscope using a Bush TV125?

I remember that article, some time in 1975 around the time they changed to the larger format. I'll dig it out, scan and post it.

Keith

[llama]

Thanks all for the responses and advice. I've now tried this on a Teleton b/w portable and, as expected, the brightness needs to be kept low to avoid screen burn. I put a couple of k ww resistors where the scan coils were connected and they barely get warm - similarly all the other original components. I don't know if they are needed but it seems worth doing to damp things down a bit.

It's still left me wondering about the inductance of scan-coils - and how to measure it. I have a sig-gen and scope so I'm wondering if a potential divider made up of a variable R and the L in question should give 50% when the R equals the reactance.

Sorry for being late in replying.
Graham

[Restoration73]

I used my multimeter (also measures C and L) to get 20mH for the field and 5mH for line
coils (field = 2 in series, line = 2 in parallel) I turned an old KB Featherlite VC11 into a scope by connecting the field coils in place of the speaker and feeding audio to the volume pot. And of course turning the deflection yoke 90 degrees.
If you wish to replace the line scan coils with an inductor ensure it has a ferrite core (i.e. 15kHz)

Tony

[nigelr2000]

I had a couple of old B&W portables on my mobile disco with extra scan coils leaving the old ones in circuit. I used old car stereo amplifiers in each one powered from the TV PSU with a simple AGC and an electret microphone capsule connected to the inputs the outputs were connected to the new scan coils to make 2 stand alone pattern generators to stand on top of each speaker stack, it looked really good and impressed a lot of punters but that was the early 80's before all these pretty computer graphics came along. Oh and yes the tubes both had black sections in the middle from screen burn but as they were never going to used as a telly again it didn't matter.

[llama]

Thanks for the info.
Failing to sleep one night last week I engaged in a little retail therapy and spotted an LCR meter for about £15 so I'm now just waiting for that to arrive. I've got some ferrite cores so I might just try a few turns to get a feel for "stuff" vs L.
Graham