Amplifying generated sine wave problems for a CRT project - Page 1
Forum section: electronics / beginners

author: Cykar
author_rank: Contributor
timestamp: January 01, 2021, 06:26:21 pm
content: Hi everyone. I thought this would be a simple thing to do, but I just can't figure it out.The project I am working on is based around a small portable TV, a model from the 1980's which has a 5" black and white CRT, and AM/FM radio.Inside, there is a 10V dc power supply from a transformer, - there is no earth ground as the power connection is one of those figure-of-eight types, live and neutral only straight into a transformer.I have split the power supply with a voltage divider to provide dual supply for an MC33079 quad opamp for the oscillator which generates sine waves, something like 1khz to 20khz. From there I use an all pass filter to phase shift the sine wave, so I end up with sin and cos wave forms. These I then want to use through the horizontal and vertical coils on the CRT so that I get, in effect, a poor mans XY oscilloscope. In the end, the plan is to have 3 of these all at different frequencies, playing pretty patterns on the TV screen while the radio is playing, and possibly have some extra interaction between the audio and the wave form display - I thought that would be quite cool.So far, the sine wave is good, and with fiddling with the phase shift pot I can create different frequencies of sine wave, and adjust the phase shift accordingly so that I can get a nice round circle on my oscilloscope with battery power directly to the oscillator. Connecting up to the power from the TV, the waves forms are good, but very small. So I figured I'd amplify the signal using an LM386, but then things don't work so well. This is where I get confused.Firstly, I realise the oscillator is running between +5V and -5V and therefore has a floating ground. But I don't understand what effect this has when I pass the signal into the LM386 which is powered from the same TV 10V supply and the TV GND which is equivalent to the opamp's -5V supply. The effect I get is wildly distorted and not amplifying properly either. I clearly have something wrong but I cannot work it out. I have tried decoupling caps around both ics, and I have tried connecting the LM386 to the same floating ground, but I get similar poor results but with less amplitude I presume because it is running between 0v and 5V as far as the amp is concerned. I thought at one point the problem was that the pot that the signal passes through before it goes into the LM386 was connected to the TV GND rather than the floating ground, but changing that did not work. I was wondering if the the signal going into the LM386 is just too high a voltage, but with the pot connected to either ground doesn't work so now I'm just getting really confused.I should also say that if I feed a signal from the TVs radio, or my phone or computer etc., whether a sine wave or just audio playing, the LM386 works as I hoped it would when connected to the TV 10V and GND, amplifying the signal plenty enough to cover the whole screen of the CRT.So, I thought amplifying the signal would not be a problem, but I can't work it out.I have attached a quick schematic of my working circuit - apologies if it isn't clear, I'm not used to doing that.Also I should just say where I got the oscillating circuit from:https://sound-au.com/articles/sinewave.htm#s53which, as I said, works well for me and which I particularly liked because changing the frequency is done with a single potentiometer rather than having the need for a dual gang with good tracking or switching between capacitors.Thanks for reading and I hope someone can help

author: Benta
author_rank: Super Contributor
timestamp: January 01, 2021, 06:39:23 pm
content: It's unclear to me what you actually have.Is it a 5" B/W TV CRT with deflection coils? Or is it an oscilloscope-type CRT with capacitive deflection?If it's a normal TV CRT, your project is not really feasible.

author: Cykar
author_rank: Contributor
timestamp: January 01, 2021, 06:49:49 pm
content: Normal CRT.Why is it not possible? Amplifying a signal to create a basic oscilloscope look on the TV screen has been done by many many people. Or is the problem the way I am generating the sine waves?

author: Ian.M
author_rank: Super Contributor
timestamp: January 01, 2021, 07:25:19 pm
content: The existing field coil is optimized for 50 Hz scan, with approx 7% of the frame period used for flyback. The existing field output stage will be driving the coil hardest during flyback, the limit being the available supply voltage. If you use bridge tied load audio amp chip/module with the existing supply rails you wont be able to get full deflection at much over 300Hz without serious distortion. If you use a single ended driver the best you can hope for is a bit over 150Hz.Most CRT TV>'scope' conversions rotate the yoke so they can use the lower inductance faster line scan coil for the Y axis (and accept that it will crap out somewhere above 10KHz even with high drive voltage) , and the higher inductance slower field coil for the Y axis and accept sweep rates no faster than 0.5ms/div. This is complicated by the line output stage, flyback transformer providing the EHT and the line coils being all parts of a tuned system so if you don't want to have to fully redesign the flyback drive circuit and output stage you need to provide a ballast inductor to match the inductance and DC resistance of the line coils to replace them in the existing flyback driver circuit.TLDR: A magnetically deflected TV CRT + yoke makes a very poor analog 'scope', even for only audio frequencies, at an even worse cost/reward ratio.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 01, 2021, 07:49:06 pm
content: The problem is in the electromagnetic deflection system - it has a large inductance and this will greatly affect the deflection at different frequencies. A CRT-based oscilloscope does not display live beam movement. The beam moves in memory, and then the screen displays the contents of the memory at a fixed frequency. This is how all modern TFT oscilloscopes work.

author: Circlotron
author_rank: Super Contributor
timestamp: January 01, 2021, 09:02:21 pm
content: Quote from: S. Petrukhin on January 01, 2021, 07:49:06 pmThat is quite true for older DSOs that all had a magnetic deflection CRT. They scan at a fixed rate regardless of what signal is being displayed. Analog scopes with electrostatic deflection CRT display the waveform live or real time. Their scan rate varies with the time base setting and the signal being displayed.

author: Benta
author_rank: Super Contributor
timestamp: January 01, 2021, 09:03:23 pm
content: Quote from: Cykar on January 01, 2021, 06:49:49 pmYou need to understand how a TV or a CRT monitor works.It's laid out for raster scan, which means two sawtooth waves to control vertical (50/60 Hz) and horizontal (15625 Hz) deflection.Manipulating the vertical deflection with an outside signal is absolutely possible.Your problem begins with the horizontal deflection. The horizontal deflection circuit and transformer do not only provide the horizontal scan output, but is also the main power supply for the rest of the monitor/TV as well as being the source for the HV ladder.There's no way you can modify the horizontal scan without messing up everything else.What you CAN do, is to add your own power supply. It'll probably need 5 V, 12 V, plus a couple of other voltages for the circuit, and also a 5...8 kV voltage for the CRT itself.Then you're in a position to drive the horizontal deflection coil directly.EDIT! It's certainly possible if you replace the horizontal yoke inductance with a simple inductor. Then you can drive both vertical and horizontal yokes freely.

author: Circlotron
author_rank: Super Contributor
timestamp: January 01, 2021, 09:08:18 pm
content: Maybe just remove the deflection yoke from the tube but still leave the horizontal (and maybe vertical) coils connected to the deflection circuitry. Get another deflection yoke and drive those coils with the external signals. And drive them with a current output amplifier, not a voltage output amplifier.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 01, 2021, 09:25:27 pm
content: Quote from: Circlotron on January 01, 2021, 09:08:18 pmIt's not a bad idea to leave the feeding part of the line scan alone and not spoil it. But it will also be necessary to detach the beam blanking signal from the circuit for the reverse stroke. And, again, the electromagnetic yoke cannot work successfully beyond the frequencies it likes.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 01, 2021, 09:27:16 pm
content: Quote from: Circlotron on January 01, 2021, 09:02:21 pmYes, I meant tubes with electromagnetic deflection. I'm not very good at telling the difference between them and electrostatic deflection in english.

author: ledtester
author_rank: Super Contributor
timestamp: January 01, 2021, 09:35:03 pm
content: QuoteI suspect that your virtual ground is the culprit. I would buffer it with an op amp like shown here:https://electronics.stackexchange.com/a/306953It's not entirely clear what your set-up is, but in this video the X and Y deflection coils are disconnected from the TV's circuitry and controlled by external signals. Is that what you are doing?https://youtu.be/QWh4wcPNwgM

author: Cykar
author_rank: Contributor
timestamp: January 02, 2021, 07:33:43 am
content: @ledtestet yes, that's what I'm doing. I hadn't seen that particular video but that is exactly my aim.I feel the discussion got a bit distracted by CRTs, no doubt my original post was not clear about what i was asking for help with.I will try buffering the signal first before sending it to the LM386 and see of that helps. Thanks.

author: Cykar
author_rank: Contributor
timestamp: January 02, 2021, 09:19:14 am
content: Re reading your reply, i realise you meant buffering the virtual ground. I'll try that.

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2021, 09:30:03 am
content: Quote from: Cykar on January 02, 2021, 07:33:43 amIt wont. Your problem is you have a 5V DC offset between the 'ground' the OPAMPs are using and the ground (0V) the LM386 is using which upsets the bias voltage of its In+ on pin 3. You cannot run the LM386 referenced to your rail-splitter signal ground as even with buffering, the rail splitter cant handle the current. The cure would be to put a 10uF coupling cap in series with the right-hand end of POT3 to block the DC offset.However I also recommend adding a 100nf decoupling cap across the lower 1K resistor of the rail splitter and using the spare OPAMP channel as a unit gain buffer, to buffer your rail splitter for a cleaner more stable signal ground in the oscillator section.P.S. *PLEASE* make sure *ALL* parts on your schematics have unique designators. Multiple parts called 'R?' can only lead to confusion when we attempt to discuss circuit operation, or describe what changes you need to make.

author: fcb
author_rank: Super Contributor
timestamp: January 02, 2021, 11:45:45 am
content: As Ian M. pointed out the vertical scan coil is optimised for 50Hz+vertical flyback. You'll also typically find the horizontal scan (15.6KHz) coil is part of the EHT generator setup. The EHT circuit will often generate the focusing grid and control grids too.So if you can divorce the EHT generator from the horizontal scan coil (not that difficult), and you can cope with the different inductive limitations of the scan coils, then all you have to contend with is preventing your electron beam from burning a hole in your phosphor when you aren't scanning (development!).Also - is the LM386 beefy enough to drive a coil? - they are only 1W from memory. You might want to use transformer matching? Or test it with a hifi amp. Certinaly if you've only got 10V to play with you need to think about bridge config amplifers.

author: Cykar
author_rank: Contributor
timestamp: January 03, 2021, 02:37:50 pm
content: Thanks Ian.M and fcb. I did as suggested, created unity gain circuit for the virtual ground, and included a cap to block out the dc offset and it worked! Funny how I just couldn't work it out before, as now it seems quite obvious to me so thankyou!Quote from: fcb on January 02, 2021, 11:45:45 amI was wondering about this too. Testing with batteries, when I increase the available voltage, I get greater amplification from the LM386 before it starts to distort, about 3V peak to peak from a 10V supply as opposed to 2V peak to peak from a 7V supply. So i was thinking would a voltage doubler from the 10V TV supply to power the LM386 work, so as to get a larger range of clean signal to work with?