Using a 5.5 MHz IF transformer to build a transmitter

Hi,

I purchase a few 5.5 MHz coils from **broken link removed** in order to attempt building a PAL B/G TV transmitter following the instruction at this website **broken link removed**. The transmitter diagram is below:



The exact item which I purchased is "5.5 MHz IF coil, green, design G, square 7 mm". The website offers various designs for various transformer frequencies. However for 5.5 MHz, design G is the only available design:



I completed soldering the circuit on a stripboard and get the video section work without major issues. Well, the picture always rolls (vertical hold issues?) no matter how I adjust the trimmer for video modulation but I believe it is due to the incomplete circuit. The transmission is also very sensitive to the antenna position but I believe I can always adjust it later.

However, when I started to solder the IF transformer for the audio part (5.5 MHz is the audio subcarrier frequency for PAL B/G), I realized that my transformer is of design G (two pins on each side (primary/secondary) with a tuning capacitor on the primary side), while I need design A (three pins on primary side, 2 pins on secondary side with capacitor on primary side) in order to build this TV transmitter.

Any ideas how I can use the IF can I bought to complete this circuit, perhaps with some circuit modification?

Thanks

The oscillator type requires a tapped primary, so you can't use the transformer you have.

There is an oscillator type that uses two capacitors in series, to create a 'tap' that works in the same way - but you would need to redesign the circuit, and remove the existing capacitor in the IF transformer.

However, it's only for sound, so won't be affecting your picture, and there seems little point messing about with the sound until you get the main part working properly - if you connect R4 to ground, then that totally removes the audio part, so get the design working like that first - building it on stripboard means it might not work, unless your layout is suitable, the capacitance of the stripboard is substantial at the frequencies in question (although FAR, FAR better than attempting it on breadboard).

Hi Nigel,

Thanks for your early reply. Let me try again using your suggestion to get the video stable.

Do you have any ideas where I can purchase a suitable IF transformer for this circuit online? I have searched eBay to no avail. I tried salvaging them from an old VCR but the pins were too short to be of any use.

As far as building a TV transmitter, I have previously tried another circuit using a single transistor https://www.translocal.jp/microtv/howtotvtx.html. Although without sound, the picture was stable and the transmission distance was good too. I have built audioguru FM transmitter several times with success (great distance) so I hope my experience is sufficient to layout the components and make this circuit work properly on a stripboard

mdanh2002 said:

Do you have any ideas where I can purchase a suitable IF transformer for this circuit online?

Click to expand...

Sorry no, but you're in the right part of the world for finding them

Nigel Goodwin said:

Sorry no, but you're in the right part of the world for finding them

Click to expand...

Haha Nigel, digital TV broadcasting in Singapore is planned to start in 2016, with at least 3 years for local household to upgrade to digital TV before analog transmission would finally be switched off. That means analog transmission will be alive in Singapore until 2020. Good news for the local hobbyists here. I love analog TV as much as I love the PC parallel and serial ports - they are very easy to experiment and play with, compared with newer stuff like digital transmission and USB.

Hi,
Have a look at **broken link removed** which gives you the pin numbering used in the Toko 10K series of coils. Comparing this diagram to the schematic in **broken link removed** we find that we need turns between pins 1 & 2, pins 2 & 3, and between pins 4 & 6. Furthermore, we need an internal capacitor (although none is shown in the schematic). Perhaps the largest disributor of Toko coils in the UK is BEC distribution. The relevant document you need for your application is:
http://www.bec.co.uk/downloads/10K Series.pdf. The coil which will satisfy your requirements is the last one on page 3, the BKANK2819M. This has 13 turns each between pins 1 & 2 and between pins 2 & 3, as well as 25 turns between pins 4 & 6 (so rough parity between 1 & 3 and 4 & 6). The nominal frequency for this coil is 4.43 MHz. The fact that this is different from your target frequency of 5.5 MHz is not critical, since the capacitor-less BKANK2819M can be fitted (by BEC) with a suitable capacitor which will resonate it at 5.5 MHz. Alternatively, BEC may suggest an even better alternative. I did a quick stock-check for the BKANK2819M coil: http://www.bec.co.uk/new-toko-products/10K/
which shows current inventory for this part (Stock No. 363819) at 680 pieces.

As Nigel said in post #4, you are in the right part of the world for getting parts for your application. A major manufacturer of audio/video & only video modulators for TV (which were all the rage in the UK during 1980's, as the means by which the home computers - eg Sinclair ZX Spectrum and computer games consoles of the time, used domestic PAL TVs as their VDUs) was a Hong Kong firm called ASTEC.
More here: http://en.wikipedia.org/wiki/Birmingham_Sound_Reproducers

I have no idea if ASTEC are still making such modulators, but here is one suitable for your application: http://www.madrigaldesign.it/creativemu/gfx/compositemod/um1285-8.pdf

Hi Ramuna,

Thank you very much for the very detailed and informative reply.

I had a look at the links to the BEC stock list you provided. For item BKAN-K2819XM (stock number 362819, 680 pieces left), the nominal frequency is 4.43 MHz (I believe this is meant for the PAL color subcarrier frequency), you said it can be tuned to 5.5MHz by using a suitable capacitor. Can you tell me the capacitor value and how would I go about measuring the resonance frequency?

There is another similar item in the stock list #361769 (KACAK1769HM) that has a resonance frequency of 5.5 MHz. However the number of turns between 1-2, 2-3 and 4-6 is not listed. I believe this is not suitable.

Yesterday, I salvaged what looks like an IF transformer from an old VCR. Using a multimeter to measure resistance I was able to identify the primary and secondary sides, with 3 pins on each side. I follow the advice I get here https://forum.allaboutcircuits.com/showthread.php?t=37161 to measure the output impedance of the transformer at different frequency:

1. Feed a sine wave of reasonable frequency and peak-to-peak amplitude to the VCR (I used 0.25-15MHz, 2V peak-to-peak from my function generator)
2. With no load connected, measure the no-load output voltage (Vo)
3. With a reasonable load connected (< 100 Ohm, I used 47 Ohm), measure the output voltage with load (VL)

The output impedance is: Zo = RL * (Vo/VL - 1)

I then plotted the graph of output impedance vs. frequency in Excel and notice the frequency at which the impedance is the largest. After some trials and errors I was able to determine 2 sets of pin combinations on the transformer, which will result in a peak frequency of 5.5 MHz and 6.5 MHz respectively. See my graph below:



This makes sense (to me) since the transformer which I salvage was from the RF box which has a switch to toggle between PAL B/G and PAL D/K (which uses 5.5 MHz and 6.5 MHz audio subcarrier frequency respectively). Is this the correct way to determine the frequency, or is there a better way?

Thanks for link to the RF modulator module on ASTEC. I played around with a few RF modulator modules, salvaged from old VCR, Nintendo and other old video game machine. If connected to a good antenna, the broadcasting power is only good enough for a few meters. I am still hoping to find a suitable amplifier to make this good enough for broadcasting (other than building a transmitter)

mandanh2002 said:
I had a look at the links to the BEC stock list you provided. For item BKAN-K2819XM (stock number 362819, 680 pieces left), the nominal frequency is 4.43 MHz (I believe this is meant for the PAL color subcarrier frequency), you said it can be tuned to 5.5MHz by using a suitable capacitor. Can you tell me the capacitor value and how would I go about measuring the resonance frequency?

I cannot tell you the capacitor value since no info re the coil inductances is provided in that link. However, if you email BEC, with details of your requirement, I'm sure that they can advise you. Incidentally, it can even be tuned using a suitable external capacitor in parallel with pins 1 & 3, instead of having a specific Toko internal capacitor. As I said in my previous post, BEC may even be able to suggest a better alternative than the BKAN2819XM (for example from the Toko 7P range of coils, https://www.bec.co.uk/downloads/7p series.pdf ).

As regards measuring the resonance frequency, with reference to the Celestin circuit schematic: (a) short connector J1, so that the free end of C1 is at ground potential (b) connect one terminal of your scope or frequency counter between the junction of T1's secondary & R4 and the other terminal to ground (c) turn the inductor slug of T1 with the appropriate Toko coil adjustment tool (BEC should have this too, see under ceramic trimtools), until you zero in on 5.5 MHz.

ramuna said:

mandanh2002 said:
I had a look at the links to the BEC stock list you provided. For item BKAN-K2819XM (stock number 362819, 680 pieces left), the nominal frequency is 4.43 MHz (I believe this is meant for the PAL color subcarrier frequency), you said it can be tuned to 5.5MHz by using a suitable capacitor. Can you tell me the capacitor value and how would I go about measuring the resonance frequency?

I cannot tell you the capacitor value since no info re the coil inductances is provided in that link. However, if you email BEC, with details of your requirement, I'm sure that they can advise you. Incidentally, it can even be tuned using a suitable external capacitor in parallel with pins 1 & 3, instead of having a specific Toko internal capacitor. As I said in my previous post, BEC may even be able to suggest a better alternative than the BKAN2819XM (for example from the Toko 7P range of coils, https://www.bec.co.uk/downloads/7p series.pdf ).

As regards measuring the resonance frequency, with reference to the Celestin circuit schematic: (a) short connector J1, so that the free end of C1 is at ground potential (b) connect one terminal of your scope or frequency counter between the junction of T1's secondary & R4 and the other terminal to ground (c) turn the inductor slug of T1 with the appropriate Toko coil adjustment tool (BEC should have this too, see under ceramic trimtools), until you zero in on 5.5 MHz.

Click to expand...

Hi ramuna,

Thank you for the fast reply. Let me email BEC and see what they suggest.

Meanwhile what do you think about the method that I use to find out the resonance frequency by finding the output impedance? Is it correct? The result makes sense to me, but just need some confirmation from some experts just to be sure

Because of the detuning effects of transistor (and pcb layout) parasitic capacitances and to a lesser extent, inductances, it is best not to measure the resonance frequency of a tank circuit in isolation. This is why I suggested that you adjust for resonance in-circuit, with all the parasitics in place.

This circuit for TV transmitter is a poor quality one. There is one good circuit i can post you if you are interested. you can use an FM modulator tuned to 5.5 Mhz and mix with video signal before AM modulation so that you can porevent using this IF transformer. Also synch issue may be due to distrored negetive sync pule of your video input stage.

mbarazeen said:

This circuit for TV transmitter is a poor quality one. There is one good circuit i can post you if you are interested. you can use an FM modulator tuned to 5.5 Mhz and mix with video signal before AM modulation so that you can porevent using this IF transformer. Also synch issue may be due to distrored negetive sync pule of your video input stage.

Click to expand...

Hi mbarazeen,

Thanks. Can you provide me the link to the other video transmitter circuit? I will attempt it. My ultimate hope is to get a functional TV transmitter - perhaps not as good as audioguru's FM transmitter, but should be good enough for me to broadcast from the living room to the bedroom (~10m) with walls.

For this circuit, I have tried with different video sources (VCR, DVD player and from a CCTV camera) - all have rolling issues despite grounding R4 as per Nigel's suggestion. In my experiment, removing capacitor C10 470uF at the video input stage or replacing it with a smaller capacitor and the video *may* stop rolling, but the picture becomes black and white.

Hi,
These links may be of interest:
https://www.toolstation.com/shop/p8...=26799373823&gclid=CPPQlp2rlb8CFVIPtAodl0MAfA

**broken link removed**

https://cpc.farnell.com/rf-modulators-tv-tuners

https://ariessebayang.blogspot.co.uk/2008/03/pemancar-tv-100-mwatt.html

https://petlibrary.tripod.com/rfmod.htm

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&cad=rja&uact=8&ved=0CGcQFjAI&url=https://www.qrz.ru/reference/micro/analog/mc1374.pdf&ei=JeqqU8bfKq_H7Ab-uoCIDQ&usg=AFQjCNFALSQLuGzvvQRj3C3BpksXz5tJ7w&sig2=OIi_r4QJIRk_p1PSYljTGA

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=15&cad=rja&uact=8&ved=0CIwBEBYwDg&url=https://www.cedmagic.com/tech-info/data/lm2889.pdf&ei=JeqqU8bfKq_H7Ab-uoCIDQ&usg=AFQjCNGRsrG-nhASHGFulf4PtTUdTXLkBQ&sig2=Ki0ApMI623nW4jqE-C-VSA

https://www.ti.com/lit/an/snoa678/snoa678.pdf

mdanh2002 said:

Hi mbarazeen,

Thanks. Can you provide me the link to the other video transmitter circuit? I will attempt it. My ultimate hope is to get a functional TV transmitter - perhaps not as good as audioguru's FM transmitter, but should be good enough for me to broadcast from the living room to the bedroom (~10m) with walls.

For this circuit, I have tried with different video sources (VCR, DVD player and from a CCTV camera) - all have rolling issues despite grounding R4 as per Nigel's suggestion. In my experiment, removing capacitor C10 470uF at the video input stage or replacing it with a smaller capacitor and the video *may* stop rolling, but the picture becomes black and white.

Click to expand...

I have attached a circuit that is for 440Mhz , you can get the idea how to modulate sound on FM and mix with video and modulate carier frequency. I made one working transmitter long before and it gave good sound and picture quality. you can use part of your circuit (only oscilator & an amplifier) from what you have and use part of this (for audio in video) to make a working one.

There is another way to make transmitters by using only video and sound on FM sperated at 5.5 MHz for PAL. say if your frequency is 200MHz video them you have to have a normal FM modulator at 205.5 MHz and transmit using seperate antenna. I have seen some commercial transmitters having this concept.

mbarazeen said:

I have attached a circuit that is for 440Mhz , you can get the idea how to modulate sound on FM and mix with video and modulate carier frequency. I made one working transmitter long before and it gave good sound and picture quality. you can use part of your circuit (only oscilator & an amplifier) from what you have and use part of this (for audio in video) to make a working one.

There is another way to make transmitters by using only video and sound on FM sperated at 5.5 MHz for PAL. say if your frequency is 200MHz video them you have to have a normal FM modulator at 205.5 MHz and transmit using seperate antenna. I have seen some commercial transmitters having this concept.

Click to expand...

Thanks for the circuit diagram. Let me spend some time studying it.

As for using separate transmitters (video only + FM for audio), I have tried it before and successfully transmitted on Channel 3 VHF with audio and video. Well, sort of, any way. The issue is that the audio signal must be at exactly 5.5 MHz from the video signal. When I attempted it 2 years ago, this was impossible to achieve with an FM transmitter that uses an RC oscillator - the exact frequency cannot be set using the trimmer capacitor. I replaced the trimmer with a tuning capacitor from an old radio and achieved better results. However, there is always a background white noise mixed with the intended audio signal when watched on my old manual-tuning television set. Never figured out what exactly the problem was - but I will try again.