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-   -   David Robinson's System A modulator (https://www.vintage-radio.net/forum/showthread.php?t=129850)

FRANK.C 21st Sep 2016 5:24 pm

David Robinson's System A modulator
 
5 Attachment(s)
Hi All
This is my attempt at building David Robinson's System A modulator which covers all 13 Channels. The original threads are here and here

Both Audio and Vision modulators are the same as David's. The only change that I made was to simplify the audio input as I don't need the separation between left and right channels that David did. There was some debate about whether the 3.5 MHz filter was necessary or not. I decided to keep it, the coil the David used is no longer available, I used a 'Mini Mod' transformer in it's place and it works quite well.

I have made some changes to the Microcontroller side of things. I am using a 18F1330 Microcontroller. The I2C buses for the two modulators now share a common clock. The microcontroler produces a very basic test card and an audio tone of approximately 300Hz which are fed to the modulators via switched phono plugs. When a Video or Audio plug is inserted into a Phono socket the test card or audio tone is disconnected.

I made a PCB for it which made assembly of the SMD's much easier. The PCB is double sided with one side used solely as a ground plane. The PCB was produced using the laser printer method which left the copper pitted but it is still acceptable.

Frank

FRANK.C 21st Sep 2016 5:26 pm

Re: David Robinson's System A modulator
 
1 Attachment(s)
And this is the circuit.

HamishBoxer 22nd Sep 2016 10:23 am

Re: David Robinson's System A modulator
 
Very tidy job I note and looks good results on screen.

Dave Moll 22nd Sep 2016 3:17 pm

Re: David Robinson's System A modulator
 
2 Attachment(s)
Unfortunately, when I attempt to access the circuit PDF (using Acrobat Reader XI), it results in just the bit of image shown below, along with the error message also shown.

ntscuser 22nd Sep 2016 3:40 pm

Re: David Robinson's System A modulator
 
Quote:

Originally Posted by FRANK.C (Post 879569)
This is my attempt at building David Robinson's System A modulator which covers all 13 Channels.

Very neat, very professional looking. Well done! :thumbsup:

FRANK.C 22nd Sep 2016 6:26 pm

Re: David Robinson's System A modulator
 
3 Attachment(s)
Thanks for the comments.
It preforms well, David did a good job in designing it. Been able to switch between channels is handy particularly when you have both London and Birmingham sets.

Quote:

Unfortunately, when I attempt to access the circuit PDF (using Acrobat Reader XI), it results in just the bit of image shown below, along with the error message also shown.
Dave
I don't know why it wont open for you. I have just downloaded it and it opened OK, I am using Foxit Reader.
I printed it using "Foxit Reader PDF Printer".
Maybe Acrobat Reader don't like Foxit?

I have taken 3 screen grabs of the circuit and attached them.

Frank

Dave Moll 22nd Sep 2016 7:41 pm

Re: David Robinson's System A modulator
 
Thanks.

AC/HL 22nd Oct 2019 12:51 pm

Re: David Robinson's System A modulator
 
Reopened by request.

FRANK.C 22nd Oct 2019 12:59 pm

Re: David Robinson's System A modulator
 
4 Attachment(s)
Bill, thanks for reopening it.


I have built a second version of this modulator and have made some changes to it.
I have changed the test card to one in the style of a PM5544, I have moved the audio gain control to the front panel and have fitted toggle switches for selecting test card and test tone.
If anyone would like to build one all the files that are required can be downloaded here.

Frank


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