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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 17th Apr 2019, 5:59 pm   #1
Semir_DE
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Default "PICGEN" pattern generator for 405 - SMD version

Hello fellow 405 enthusiasts,

I will split this post into two parts in order to be able to upload all associated images. This is part 1/2. Hint: the circuit diagram is in part 2/2.

When building the Hedghog converter I stumbled across another one of Franks nice projects, the “PICGEN” pattern generator. I found this to be a very interesting project and went on to build one.

I started with a simple universal board:

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Ever since building the Hedghog I had become a fan of SMD technology, so I decided to make an SMD version of the PICGEN generator I will call it “PICGEN-S”. I not only wanted to make it smaller, but I also changed some parts of the circuit to improve usability. In the following images you can see the (for now) final PCB layout with SMD components.

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Here is a list of changes I have made:

a) Added some low pass filtering to the output signal in order to limit its bandwidth. This will result in less steep rise and fall times of the signal pulses thus reducing the chance of vision buzz. By adding some variable resistors a precise setup of the sync and video amplitudes is possible. Here is an image of the horizontal sync interval as it appears at the output.

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b) Powering from a 5V USB port: By design the output level of this generator depends on the supply voltage. The original design was for a 5V supply. The used PIC processor is, however, specified to run on 3.3V as well, so I changed the design of the circuit to work on 3.3V. This allowed me to put in a 5V to 3.3V voltage regulator (MCP1700) which guarantees a fixed voltage even if the input 5V is slightly off which is often the case with power banks and chargers. I have measured anything from 5.2V down to 4.85V. This image shows the PICGEN and my channel 1 modulator running while being powered from a USB power bank.

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c) More outputs: Instead of the simple single transistor output I added the THS7314 output chip to the design. This is the same output driver that the Hedghog uses. This IC can run on a mere 3V of supply voltage and has 3 output drivers. I used two of them as video outputs and the third as a “Composite Trigger Pulse” (CTP) output. By altering a solder jumper the third output can also be turned into a video output.

To be continued in part 2/2.

Semir
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Old 17th Apr 2019, 6:25 pm   #2
Semir_DE
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Default Re: "PICGEN" pattern generator for 405 - SMD version

Hi again,

This is part 2/2 of my PICGEN-S project. I will continue with changes I have made to the original design:

d) Single scope trigger pulse for H and V: Instead of having separate horizontal and vertical sync outputs, I created a sort of composite sync mainly for scope triggering. This is what it looks like (top trace, scope trigger set for vertical):

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The pulse has two levels; 0-1V is negative going vertical sync, 1-2V is positive going horizontal sync. When this signal is applied to a scope it allows the user to select vertical or horizontal triggering by merely adjusting the trigger threshold of the scope. The output level of this pulse is 1Vpp into 75Ω.

Another application of this pulse is to set the free running frequency of the flywheel sync in a TV. Due to the negative going vertical sync this signal can be supplied to a modulator just like a pattern signal. As it does not have horizontal sync pulses the the TV can recognize, only the vertical sync will be locked while the horizontal will be free running. By adjusting the horizontal hold to show the positive going horizontal sync pulse as a jittery white vertical line the free running frequency can be set precisely on the TV set. I found this useful as the setting of the horizontal hold influences the picture position. By first setting the H-hold to the right frequency the positioning magnets on the CRT can be set for the correct horizontal picture position more accurately.

e) I had read that some users of the original PICGEN had experienced a hum bar rolling across the picture. While this is usually due to bad mains filtering in the TV set in question, the effect is most visible when the frame rate of the signal supplied to such a TV is not exactly 50Hz i.e. 20ms. I noticed that the PICGEN needs a clock frequency of 5.994Mc to deliver the exact 20ms. A 6Mc crystal can not be pulled low enough to oscillate at that frequency but a ceramic resonator can. I added a variable capacitor to the design to allow for precise adjustment of the frame rate in conjunction with a ceramic resonator. This will render all hum bars stationary on the screen which will make then less objectionable.

The board layout is made to accommodate using either a regular crystal, an SMD "MQ" crystal or a very small ceramic resonator in SMD format. A regular ceramic resonator can probably also be fitted instead of the crystal if a third hole is drilled in the middle between the two crystal holes. I have not tried that, however.

f) since the SMD version of the PIC processor has to be soldered to the board it can no longer be removed for (re-)programming. In order to allow in-circuit programming I have added an ICSP header to the board. The programmer simply hast to be connected to that header for programming the PIC. While programming, the small SPST switch should be set to the "OFF" position to separate the power circuit of the board from the processors power pin. The power LED will remain active and should light up during the programming process.

In these two posts I have introduced my version of the PICGEN. The original project was designed by Frank Cuffe and I would like to take this opportunity to thank him for creating this project and the software for the PIC processor! I could not have done that as my programming skills are very basic.

The PCB was made using KiCAD. Please PM me for the project data should you want to build one.

I am planning two versions of this project. One is the current generator only version which is very compact. The second which I will introduce shortly will be a combination of generator and modulator.

I already have a band I modulator working in my workshop which uses PLLs covering all 5 band I channels. My plan is to make the PICGEN board plug into the larger PCB of the modulator. A band III PLL modulator is also in the planning stages.

The circuit diagram can be found in the attachments.

To be continued…

Semir
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File Type: pdf PICGEN-S_Rev2.pdf (63.6 KB, 77 views)
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Old 17th Apr 2019, 11:03 pm   #3
FRANK.C
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Default Re: "PICGEN" pattern generator for 405 - SMD version

Hi Semir
What a great version of PICGEN you have developed there.
I like it built with SMD's. I too have become a fan of them.
Looking forward to seeing your version with the modulator.

Frank
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