I'm working on a 1980s solid state 12" video monitor. All pretty conventional stuff. But the LOPT sometimes shrieks at half linescan frequency. I've measured it by beating against an audio oscillator. It's a horrible noise. Goes away if I prod the LOPT. May well go away altogether if I tighten the mounting bolts to clamp the core a bit better.
But why HALF frequency? I know magnetostriction noises can happen on harmonics of the frequency, but subharmonics?
I'm looking at this monitor with a view to converting it to 405. Has reasonable prospects but I hope the LOPT isn't too loud. If I succeed I will replace the Prowest PM14/1 at the Vintage Wireless Museum which always seems to give trouble. Since I may have more than 1 of these monitors my own PM14/1, lovely pictures but not the most reliable, may well also be retired.
Maybe if some loose part has a natural freq of half
line scan freq., it gets a boost 'push' as it were every other scan freq
and so oscillates at the half line'
Don't know why it would shriek at half line frequency. But converting it to 405 sounds interesting .
Maybe after converting, because the line frequency will have changed it will no longer shriek.
Frank
Certainly heard this before. Common on old colour sets with delta gun tubes.
Line linearity coils and pin cushion correction transformers would do it. Sealing the windings with araldite worked if a replacement wasn't to hand. If you can get into the core of the transformer its worth a go. The Araldite needs to be warm and runny though so the slow setting stuff would be better.
Any core driven to saturation will make excess noise. The pincushion transductor made a horrible noise. It was being driven into saturation at both H and V frequencies. Hence a buzzy whistle. Line lienarity coils are also intended to saturate. LOPTs shouldn't saturate.
It's been suggested that S correction instroduces an H/2 component into the scan current.
Frank, I don't think you have a 405 monitor. Doing all the standards conversion work with a modulator and receiver seems far too hard. I may have another spare one of these 12" monitors available which you could have. Need to see how it works out on 405. Obviously for collection when you're next in England. NVCF?
Lopts do though "rattle" it's common enough. Had many over the years that do!
I know they rattle and squeak, that's commonplace, but why at HALF line rate?
I've had poorly-clamped 400Hz transformers singing at half (or lower) frequency. They sound dreadful.
Can't think of a mechanism why! It's like a 2-stroke engine four-stroking - but in this case there is a decent explanation why (the 'missed' stroke does a bit of extra scavenging, so the next charge does actually go pop with the spark).
I suspect chaos theory might provide an explanation.
If whatever is producing the sound is resonant at a frequency where the length of one cycle is longer than one line length but shorter than two, say, but the Q is low enough not to keep it ringing at the end of one cycle, then if one flyback pulse kicks it off, it is still part way through its cycle when the next appears, so is ignored. It then completes its cycle and waits for the next flyback pulse, and so on.
You say it squawks rather than sings, so could that be a pointer to its discontinuous sequence of individual pulses?
It's certainly not a totally smooth note, which is why I'm saying squawk. It's very sensitive to the LOPT being prodded with an insulated tool. When I get the chance I'll tighten the bolts which will probably solve the problem. But they are not easy to get at.