Master 128 Volume

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guddler
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Master 128 Volume

Post by guddler » Sun Sep 16, 2018 1:44 am

After just blaring out the "Twisted" demo at nearly 2AM and thinking that I really should be able to do something about the ridiculous volume on the master, I searched this form only to find people saying that you can't. Stick a sock in it (literally!) and so on.

So I just grabbed the schematics and am wondering if there is anything wrong with pulling C10 which is a tantalum cap on the input of the LM386 amplifier chip and sticking a small 10k variable resistor in it's place, input on the input side of C10, wiper on the output side of C10 / pin 3 of the LM386 and output of the resistor to GND?

I might just go ahead and give it a whirl tomorrow but just wondering if anyone has already figured this out?

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1024MAK
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Re: Master 128 Volume

Post by 1024MAK » Sun Sep 16, 2018 5:11 am

Are you sure C10 is a tantalum capacitor? It may be another type. Normally film capacitors are used in audio circuitry.

Anyway, it's best if you include a 100nF capacitor between the wiper of your variable resistor / pot / preset and pin 3 (the LM386) input to seperate the different DC levels. And the third terminal of the variable resistor needs to be connected to a 0V/GND point, preferably the audio 0V/GND rather than any 0V/GND so it does not pick up switching noise from the digital circuitry.

I would suggest in fact, that after removing C10, that you fit a pin header. Then you can have a plug in wiring loom connected to the variable resistor / pot / preset of your choice (with 100nF capacitor of course).

Should you wish to remove the 'volume' control, a 100nF capacitor connected to a socket can then be easily placed on the pin header :wink:

Do let us know how you get on.

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Lardo Boffin
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Re: Master 128 Volume

Post by Lardo Boffin » Sun Sep 16, 2018 7:38 am

I have a SCART cable from Retroclinic which also has audio that plugs into the telly. I disconnected the internal speaker and now just get sound through the telly. Instant volume control with no messing around.
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Re: Master 128 Volume

Post by kieranhj » Sun Sep 16, 2018 7:43 am

I use an audio SCART cable too and disconnected the internal speaker in favour of the TV.

For POP Simon implemented a proper volume control which requires an extra bit of code for lookups in the music player but I’ll try to include it in future demos if feasible.
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geraldholdsworth
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Re: Master 128 Volume

Post by geraldholdsworth » Sun Sep 16, 2018 9:43 am

Lardo Boffin wrote:
Sun Sep 16, 2018 7:38 am
I have a SCART cable from Retroclinic which also has audio that plugs into the telly. I disconnected the internal speaker and now just get sound through the telly.
Couldn't the jack socket on the Master be replaced with one that has a switch, which then disconnects the internal speaker when something is plugged in?
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AndyF
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Re: Master 128 Volume

Post by AndyF » Sun Sep 16, 2018 9:53 am

geraldholdsworth wrote:
Sun Sep 16, 2018 9:43 am
Lardo Boffin wrote:
Sun Sep 16, 2018 7:38 am
I have a SCART cable from Retroclinic which also has audio that plugs into the telly. I disconnected the internal speaker and now just get sound through the telly.
Couldn't the jack socket on the Master be replaced with one that has a switch, which then disconnects the internal speaker when something is plugged in?
With mine (its gone now!) I had disconnected the internal speaker and just had the audio out feeding the monitor, thankfully it was a simple phono-phono connection, no fuss and I had a home made RGB - Scart lead already as the AKF monitor had that socket.

The idea of the auto disconnect is quite valid, using say a 3.5 or possibly better a 6mm jack plug, you can wire the speaker to the 'switch' contacts on it (not output lol) , only real issue is going to be mounting it properly, unless you can find one that will be a chassis mounted one to fit in the case that has switched contacts. Its not really any different in principle to a tape player that disconnects the internal speaker when you plug in an earphone/headphones into the 'audio out' socket. :) At least as I see it anyway.
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guddler
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Re: Master 128 Volume

Post by guddler » Sun Sep 16, 2018 10:14 am

I’m using a proper cub monitor so therefore not getting involved with the audio jack or scart plugs, besides, I want to maintain the use of the internal speaker. I like the fact it has its own speaker and is self contained, it’s just way too loud.

@Mark I’ll have a play and report back. And you’re right, it probably is a film cap :)

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Re: Master 128 Volume

Post by AndyF » Sun Sep 16, 2018 11:36 am

I've not tried the software in question but I'm assuming *CONF QUIET is too quiet (or ignored) yes ?

Regarding the switched socket, something like this I was thinking of but forgot to include in my post above:

https://www.ebay.co.uk/itm/400454771474/ , maybe cheaper elsewhere just picked one at random that looked half decent/suitable.

Maybe best of both worlds, as in you can mount to the case hole and still have the internal speaker wired (to the switch contacts on said socket) and have the audio out fed into the new socket 'connectors'

I only picked the 6 / 1quarter size simply figuring the case hole would be a bit too large for the 3.5mm ones without some untidy packing washers whereas the larger version may fit as is. Untried though, no M128 here to check for sizes.
Last edited by AndyF on Sun Sep 16, 2018 11:36 am, edited 1 time in total.
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guddler
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Re: Master 128 Volume

Post by guddler » Sun Sep 16, 2018 11:44 am

AndyF wrote:
Sun Sep 16, 2018 11:36 am
I've not tried the software in question but I'm assuming *CONF QUIET is too quiet (or ignored) yes ?
Yup, that config option is only for the system beep (bell) and is ignored by pretty much everything and everyone :lol:

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AndyF
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Re: Master 128 Volume

Post by AndyF » Sun Sep 16, 2018 12:04 pm

guddler wrote:
Sun Sep 16, 2018 11:44 am
Yup, that config option is only for the system beep (bell) and is ignored by pretty much everything and everyone :lol:
Oh its been ages since I had one to try/remember.

Perhaps it should of read *CONF VDU7/BEL :lol:

Understood! :)
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guddler
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Re: Master 128 Volume

Post by guddler » Sun Sep 16, 2018 12:10 pm

I'll have a tinker with the circuit later once I've done such duties as food shopping.

I need to take the master apart and work out what I've done wrong with the screws that are making it scratch the hell out of the desk that it's on every time I drag it out to get to the back of it :evil:

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Re: Master 128 Volume

Post by guddler » Sun Sep 16, 2018 7:16 pm

SO, had play about and pretty much achieved what I wanted. But it didn't all go entirely to plan.

First of all, on removing C10, it broke and I don't have any 100nF ceramics. Only mylar. I think i can use one of them but for the moment, I just removed the cap. Secondly, I didn't have a 10k trimmer, only a 50k but, meh, close enough :lol: Finally, yeah, it's a trimmer, not a nice externally mounted pot but I wanted to see how it turned out first, if I want to make it more permanent I'll order the parts in.

So yeah, it works fine and I can turn it down as much or as little as I want, I just need to take the lid off to do it, not the end of the world at the moment. The only thing that is very apparent is that as soon as you drop off of maximum volume you lose the treble from the sound. Not a desperate concern as far as i'm concerned for now but it would be interesting to work out why? I already have a mylar cap between pins 3 & 4 of the amp as part of a "reduce the noise from the screen updates (or is it disk updates?)" that was documented somewhere on here.

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