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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 1st Mar 2019, 12:42 am   #1
Semir_DE
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Default Simple System A Band I Modulator - (No Processors)

Dear All,

since I am now the proud owner of a 405 line TV set, a Sony TV9-90UB I decided to build a basic System A modulator that requires no processors or programming. The design uses a simple crystal as a frequency reference and the well known SA/NE612 mixer chip as a modulator. The design consist of the following blocks:

- Sync tip clamp for DC restoration.
- Vision carrier generator and modulator SA612
- Intermediate mixer to generate the audio carrier, this uses another SA612
- Sound modulator with a third SA612
- Low pass filter to eliminate 3rd and higher harmonics from output signal

The design requires only one crystal for the vision carrier. Luckily I have found a supplier that offers 45MHz crystals at €2.50 each, so the design is a channel B1 modulator. Since it is near impossible to find a 41,5MHz crystal for the audio carrier I have decided to generate the audio carrier by mixing the vision carrier with a 3.5MHz signal generated by an LC circuit. The original idea was to use a 3.5MHz crystal but so far I have not been able to source one. Alternatively a ceramic resonator could be used, but I have found using an LC circuit to generate the 3.5MHz offset to be stable enough.

Here is a brief description of the circuit, please refer to the attachments. Transistors Q1 and Q2 form the sync tip clamp together with D1. Residual carrier level can be adjusted by RV1.

IC U2 is the vision carrier oscillator and modulator Fi1 is needed to ensure the crystal Y1 oscillates at its 3rd harmonic and not its base frequency, it should be adjusted for maximum carrier level. Since the SA612 is very sensitive the input video signal level at pin #1 needs to be very small in the order of 100mV this is difficult to achieve by just decreasing the value of R13 as the DC component of the signal would become too low for U2 to operate properly. I overcame this problem by feeding the modulating video signal to both inputs at differing levels via R14. This common mode component reduces the input sensitivity while maintaining a proper DC level at these pins. This trick also allows me to add C11 and C14. In order to achieve a good clean modulation the input pins #1 and #2 need to have a low impedance with respect to ground at the RF carrier frequency. Due to the relatively high frequency components of the video signal present at these pins a simple capacitor of e.g. 1nF to ground can not be used in this position as it would impair the video frequency response. In this circuit C14 needs to be adjusted for flat video frequency response this is achieved when it is set to about 45pF. A 45pF capacitor connected to ground at 45MHz has an impedance of roughly 80Ω, enough to sufficiently attenuate the carrier signal. Without this measure there would be a higher level of the 3rd harmonic in the output of the modulator.

The output signal is transformed to an impedance of around 75Ω by RF transformer T2. This is a simple dual-aperture core with only a few turns as indicated in the diagram.

The audio carrier is generated by mixing the 45MHz vision carrier with a 3.5MHz signal generated by U3. Originally I had planned to use a 3.5MHz crystal but found that an LC circuit works very well in this position. The output at pin #5 of U3 contains both the sum and difference frequencies of 45MHz and 3.5MHz i.e. the desired 41.5 MHz and an unwanted 48.5MHz component. Resonant circuit Fi2/C15 is tuned to the desired 41.5 MHz and delivers the audio carrier to U1 via C12 and RV2. Fi2 needs to be adjusted for maximum level of the 41.5 MHz carrier. RV2 should be set for the desired 4:1 vision/audio signal level ratio.

The audio modulation is done with U1 yet another SA612. R9 unbalances this modulator to achieve a regular AM modulation with carrier. Without this resistor the output would be a dual side band (DSB) signal and would sound very distorted on a TV with normal envelope demodulation. In the audio section RF transformer T1 performs the impedance transformation to 75Ω for the modulated audio signal.


The resistor matrix consisting of R15/16/17 joins the modulated vision and audio signals into a single combined signal. This is then passed through a 5th order 75Mhz low pass filter consisting of L2/3 and C25-27. The output of the circuit is about 90dBµV and can thus feed several TVs via a passive distributor.

This circuit has worked quite well with my TV9-90UB. In the attached image you can see the prototype and the TV displaying a cross hatch from my generator while “Radio Jackie” from London is playing as an audio feed

I am planning to build a Band I modulator using MC145106 PLL chips based on this design. This modulator will cover channels B1-B5. Once I get it finished I will post the design here.

I am aware that there are dedicated modulator chips like the Motorola MC44… one used in the Hedghog and Aurora. My intention was, however, to build a project that does not require a processor or programming skills and is easy to build for someone with basic RF skills. I am planning to design a PCB for this project and also the upcoming PLL based version…

Semir
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File Type: pdf Sys-A_Modulator.pdf (58.9 KB, 69 views)
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Old 1st Mar 2019, 6:11 am   #2
TonyDuell
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Default Re: Simple System A Band I Modulator - (No Processors)

I love the 'dead bug' prototype
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Old 2nd Mar 2019, 1:51 am   #3
rambo1152
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Default Re: Simple System A Band I Modulator - (No Processors)

Quote:
Originally Posted by TonyDuell View Post
I love the 'dead bug' prototype

I hope you like mine as much.

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It's this design

http://www.earlytelevision.org/405_modulator.html

Two free-running oscillators at signal frequencies, and even with that rat's nest, drift is not a problem.

I didn't bother with the LPF.
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Old 2nd Mar 2019, 11:21 am   #4
Argus25
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Default Re: Simple System A Band I Modulator - (No Processors)

Quote:
Originally Posted by Semir_DE View Post
- Sync tip clamp for DC restoration.
Well done. This is very important and sometimes neglected. The DC axis of the video must be stabilized prior to the modulator, with clamping at the sync tip or the black level. If not there are serious implications for sets which DC couple the signal from their detector to the CRT. (In some designs of sets that re-insert the DC later at the CRT grid or cathode, often popular in American designs, it is not a problem).

For my modulators they are modified Aztec units for Xtal control, which is also essential I think and I organized the appropriate drive circuits with the video sync tip clamping.

But, the problem is not over there. Building proper modulators is only part of the problem. The RF outputs of the video and audio modulators need to be combined and then level controlled and metered (in my opinion to have a proper generator). This is also very handy as a tool to help service and test & verify the performance of the TV set. It is often neglected though.

I have attached an image of my mixer/combiner and level meter system and the schematic. It was built into a shielded enclosure with smc connectors and uses a hand made double sided pcb with a ground plane built into an old tuner casing. The level control device is a professional mini circuits RF attenuator.

The unit should ideally also have video and audio level controls & metering prior to the modulators too.

The article on the converter unit I assembled with my modulators in it is here:

http://worldphaco.com/uploads/RF_MOD...TELEVISION.pdf
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Old 2nd Mar 2019, 12:33 pm   #5
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Default Re: Simple System A Band I Modulator - (No Processors)

Hi,
I just checked and it looks like the early television foundation's modulator is out of stock.
( damn I should have bought one a year or two ago when the exchange rate was better ). I think both the programmable chip and the discrete approach have advantages..
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Old 6th Mar 2019, 2:58 am   #6
Semir_DE
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Default Re: Simple System A Band I Modulator - (No Processors)

Hi Everyone,

here is a follow up to this project. I made a few changes in order to unify component values. A switch to turn off the audio carrier was added and I made a PCB board for the project using SMD components. This allowed me to make a very small device that fits in a readily available aluminium housing.

The whole circuit runs on 5V so a regular mobile phone charger can be used to power the unit, therefore, I added a micro USB port as a power input. Also powering from a PC or laptop is possible. This might be of interest to those of you who use a PC to output 405 signals at shows or in the field.

The project was made in KiCAD. Please contact me for the project files. I used a dual layer PCB but the top layer is only ground. Since I can only do single sided PCBs at home I etched only the bottom side and soldered several wires through the two layers in order to connect the top an bottom ground planes. This worked very well and was easy to do.

Attached you can see the finished modulator in its box. I also included a scope image of a modulated staircase signal with the audio carrier switched off. This nicely shows the linearity of the signal. The singal level on peak white is about 150mV into 75Ω so it can drive many TVs...This test signal is a 625 signal but I am waiting for parts to build Frank C.s "Picgen" generator. One plan I have is to combine the Picgen and a modulator on one PCB to have a source for RF test signals in one small box powered from a USB charger.

Semir
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File Type: pdf Sys-A_Modulator_Rev02.pdf (67.0 KB, 45 views)
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Old 6th Mar 2019, 8:41 am   #7
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Default Re: Simple System A Band I Modulator - (No Processors)

Looks very good!

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Old 7th Mar 2019, 12:27 am   #8
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Default Re: Simple System A Band I Modulator - (No Processors)

Hi Semir
That has turned out very well. It is a nice small size and I like the idea of using a micro USB socket as a power socket. There are plenty of phone chargers etc. going spare to power projects like this.

Frank
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Old 7th Mar 2019, 2:03 am   #9
Argus25
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Default Re: Simple System A Band I Modulator - (No Processors)

That looks very good, well done !
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