ASTEC UM1233 has no audio

The BBC Micro came with an on-board ASTEC UM1233, which has no audio input (or output). It is purely video only.

Astec UM1233 and UM1286 data sheet #1 Astec UM1233 and UM1286 data sheet #2

Solderless fix

On page 31 (or page 43 of the PDF) of Making music on the BBC computer1, by Waugh, there is mention of an audio mod (emphasis is mine):

There is yet another alternative, which consists of routing the sound through the UHF modulator into your television set. Unless you are an accomplished electronics constructor, it will be simpler and safer to buy a ready-made unit from a manufacturer. They cost in the region of £10 and some can be fitted without soldering. The sound is undoubtedly an improvement, but is subject to the constraints of the modulator and TV set.

I have had a google search but can see no reference at all to this magical component. I have also looked on eBay, and can see no such second-hand offering nor repro product.

Does anyone know what this solderless component could be? Is it a modified ASTEC, or something else?

As an aside, in this video, Another Way of getting Audio out of the BBC Micro Model B, on the Matthew North Music channel, Matthew also uses the ASTEC modulator but in a rather different way.

He modifies it by opening the ASTEC UM1233 up (lifting the top shield) and cutting the link from a resistor (internal to the ASTEC unit) to the output pin (thereby disabling the video out) and then soldering the audio output, from the speaker, directly to the output pin. Effectively, turning the RF video out into solely an audio phono out.

However, I would rather not use this destructive, yet effective, method. Nor do I want to swap the UM1233 for a UM1286, if I can help it.


1 Link to PDF on 8bs.com

  • So something like amazon.com/Modulator-Extender-Converter-Channel-Adjustable/dp/… ?
    – Jon Custer
    Commented Dec 24, 2022 at 15:50
  • 2
    @JonCuster no, I think he’s referring to the fact that the BBC has a modulator built in, but doesn’t route audio to it. Audio is played via an internal speaker, with all of its attendant ‘qualities’.
    – Tommy
    Commented Dec 24, 2022 at 18:10
  • I think you mis-understood the video. What the guy in the video does is not using the UHF modulator at all - He simply uses the (no longer used) RF output plug, he disconnects it from the video generation and uses it to feed the pure audio out of the computer without the need of a further hole to the case.
    – tofro
    Commented Dec 25, 2022 at 17:04
  • 1
    @tofro - Yeah I got that, hence why I said it was destructive. I've tried to clarify my question a bit. However, I only mentioned that as an aside. The real query is the thing referred to in the book. Commented Dec 25, 2022 at 19:27
  • @Tommy - yes, you are correct, that is what I was getting at. I've tried to clarify the question a bit. Commented Dec 25, 2022 at 19:37

2 Answers 2


With the information given, we can only speculate what "solderless solution" the book is referring to.

It could probably refer to an external RF modulator that has a separate audio input, something like this product, which would allow mixing the audio output of the BBC Micro with its video output signal and modulating it. The problem here is the BBC's composite output is only monochrome - You'd have to do some the colour composite mod which doesn't exactly make the solution solderless (but nearly: the colour composite mod consists of adding a simple solder bridge and maybe a capacitor to the mainboard).

This would by-pass the internal modulator completely, but otherwise doing what you want.

  • 1
    I am/was hoping that someone who had a Beeb in that era, and who had read the book and who had actually purchased one fo these gizmo, might see this question, and be able to answer it. That's a lot of conditions to be met, I know - but sometimes the designer of such devices surfaces here on Retrocomputing. Also, some of those ands could be ors... :-) Commented Mar 19 at 16:51
  • 2
    Well, from a technical point of view, it's definitly possible to modulate the audio signal to a carrier frequency of 5.75MHz (that's where it belongs in a TV signal) and mix it with the RF from the BBC (thus, not by-passing the Beeb's modulator) - I sincerely doubt, however, that you will find a commercial product that does that.
    – tofro
    Commented Mar 19 at 16:53

This query made it over to stardot, the forum for Acorn computer users. Its contributors typically have encyclopedic knowledge of all things Acorn. Their discussion appears to conclude that Waugh was describing a product which could have existed at the time but was never manufactured.

  • Very interesting. I haven't been over on stardot for over a year now, shortly after I found (and commented upon) the thread on the Waugh book. Seems like they picked up on this question shortly after my initial posting. Commented Mar 21 at 17:59

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