BBC Model B. Low sound

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Mamejay
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BBC Model B. Low sound

Post by Mamejay » Tue Jun 25, 2019 1:12 pm

Hi everyone,
Finally got my first Model B.
I got it all up and running and replaced the 2 X2 caps in the power supply.
The audio is a bit low and was wondering if replacing caps on the motherboard may fix this. If so which caps should I concentrate on.
The potentiometer is turned all the way up
Thanks

cmorley
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Re: BBC Model B. Low sound

Post by cmorley » Tue Jun 25, 2019 1:50 pm

Does the volume change at all with VR1?
Last edited by cmorley on Tue Jun 25, 2019 1:51 pm, edited 1 time in total.

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tricky
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Re: BBC Model B. Low sound

Post by tricky » Tue Jun 25, 2019 5:33 pm

That's the block with a screw driver slot under the keyboard near the speaker cable connector.

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1024MAK
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Re: BBC Model B. Low sound

Post by 1024MAK » Tue Jun 25, 2019 7:33 pm

Do you have a digital multimeter?

What voltage is on the main board power connector that connects to the PSU via the purple wire?
Test between a 0V/GND point (black wire from the PSU) and the purple wire connector, with your positive (red) meter lead on the 0V/GND point.

Mark

Mamejay
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Re: BBC Model B. Low sound

Post by Mamejay » Tue Jun 25, 2019 8:04 pm

Yes volume goes down with vr1 when I turn it.

I will check the -5v and see that it is ok.
I read about the lm324 possibly being an issue. I might just replace that as they are like $2 here.

Mamejay
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Re: BBC Model B. Low sound

Post by Mamejay » Thu Jun 27, 2019 12:29 am

Hi Mark,
I am getting -4.55V which seems a little low. Should I recap the complete power supply?

1024MAK wrote:
Tue Jun 25, 2019 7:33 pm
Do you have a digital multimeter?

What voltage is on the main board power connector that connects to the PSU via the purple wire?
Test between a 0V/GND point (black wire from the PSU) and the purple wire connector, with your positive (red) meter lead on the 0V/GND point.

Mark

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1024MAK
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Re: BBC Model B. Low sound

Post by 1024MAK » Thu Jun 27, 2019 2:08 am

Mamejay wrote:
Thu Jun 27, 2019 12:29 am
Hi Mark,
I am getting -4.55V which seems a little low. Should I recap the complete power supply?
That’s just within the 10% tolerance (-4.5V to -5.5V) so it’s okay, if not great.

You could renew the capacitors on the PSU output section, but I’m not convinced it will solve your problem. If you use the correct replacements and are good at soldering, it won’t do any harm either.

What, if any, other test gear do you have? Ideally an oscilloscope is the tool to use to further investigate this problem. But if you don’t have one, and can’t find one to borrow, we can try some voltage checks with your meter.

Mark
Last edited by 1024MAK on Thu Jun 27, 2019 2:09 am, edited 1 time in total.

Mamejay
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Re: BBC Model B. Low sound

Post by Mamejay » Sun Jun 30, 2019 5:18 am

Replaced the LM324 and the audio seems to be normal now.

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