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Astec UM1286 UHF modulator

 
mfd70
(@mfd70)
Trusted V-Ratter Registered

Does anyone have any experience on using these modulators for TV use ? They were widely used in home computers, prestel units and simple pattern generators, channel 36 only and UM1286 does sound as well. As I had a couple of these hanging about, I thought I'd make up an RF output for a Freeview box I picked up for 50p. I couldn't find a full datasheet for it anywhere, so I made up something like the circuit from the BBC Micro, it seems to work OK for vision but tuning for sound is critical and caption buzz is also an issue. I suspect this is something to do with the fine tune input which sets the sound spacing (I think). I wondered if anyone has had success with these, here is a picture to show you what I mean, there were also RS versions too;

UM1286
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Topic starter Posted : 02/06/2019 8:37 pm
Nuvistor
(@nuvistor)
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PDF of the data sheet at this link.

** link removed as its dead **

Frank

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Posted : 02/06/2019 9:02 pm
Terry
(@terrykc)
Famed V-Ratter Rest in Peace

Frank, that link brings up a message stating that the website is temporarily unavailable and has been like that all day  but if I just go to alldatasheet.com, it works normally but doesn't return any results for UM1286 (although it does have several for an ADUM1286 IC but, as these are '3 kV RMS Dual Channel Digital Isolators', they aren't much use!)

Can you post the schematic here?

When all else fails, read the instructions

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Posted : 03/06/2019 6:26 pm
Nuvistor
(@nuvistor)
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It did the same to me after I had posted the link last night, wasn’t me honest.

I downloaded the two page document and virus scanned it but it’s a PDF so cannot post it in the forum.  It isn’t a schematic just the technical specification and pin out, it’s also very poor quality but it is legible.

I can upload the file to Chris but not sure if Chris would want to have another sites work, or I can email it to anyone who wants it.

Frank

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Posted : 03/06/2019 7:36 pm
mfd70
(@mfd70)
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The link is still down, I had tried various google searches for UM1286 and usually nothing of much relevance turns up. I used to have a paper RS datasheet but I can't find it anywhere. its 5746 in the index.

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Topic starter Posted : 03/06/2019 9:27 pm
Terry
(@terrykc)
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Ah, if it doesn't contain a schematic, is it possibly like this?

https://www.vintage-radio.net/forum/showthread.php?t=119278

When all else fails, read the instructions

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Posted : 04/06/2019 9:19 am
Nuvistor
(@nuvistor)
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Yes like that, that scan is clearer than the one I downloaded but it’s not crinkled.

I emailed mfd70 a copy so he can look at the one in the link you provided and get the best from both.

Frank

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Posted : 04/06/2019 11:44 am
Terry
(@terrykc)
Famed V-Ratter Rest in Peace

Is this any better?

I've concentrated mainly on the drawings.

If there had been a schematic included it would have been worth spending more time on it.

UM1286

When all else fails, read the instructions

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Posted : 04/06/2019 6:01 pm
crustytv
(@crustytv)
Vrat Founder Admin

I had a play in photoshop as well

um1286

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Posted : 04/06/2019 6:49 pm
mfd70
(@mfd70)
Trusted V-Ratter Registered

Thanks for all the efforts, the datasheet is just what I needed. I think the relevant information is that the fine tune input needs to be set at 3.2V to ensure sound carrier appears at the correct 6Mhz spacing. I've used a resistor, but might try a diode clamp to see if it provides any improvement to the audio.

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Topic starter Posted : 04/06/2019 10:08 pm
Terry
(@terrykc)
Famed V-Ratter Rest in Peace
Posted by: crustytv

I had a play in photoshop as well

um1286

Wow! That's excellent, Chris!

I don't have Photoshop and just used Paint.net to adjust brightness and contrast with some additional tweaking in MS Paint, a technique I've used to to tidy up several schematics in the past but the level of grey in this was a step to far for my simple methods!

When all else fails, read the instructions

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Posted : 05/06/2019 8:51 am