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General Category => Technical Support => Topic started by: gw0udm on 00:39, 08 May 21

Title: CPC 464 - very quiet speaker audio
Post by: gw0udm on 00:39, 08 May 21
Hi all


I've rescued what turns out be quite an an early 464 (keyswitch keyboard etc) which I've cleaned up and generally works.


However there is a problem with the audio - from the internal speaker there is a lot of noise and crackling and it's really quiet even when turned up full. However the actual sound itself is OK if you listen hard, and if I connect up external powered speakers it sounds fine.


I've had a look at the volume control and tried to spray some switch cleaner in, also remade the solder joints on the volume assembly but with no improvement.


It does load games from tape with no issue. I'm going to have a look at the speaker itself although it sounds a bit more like an amplifier / electronics type issue.


Any thoughts? I've read suggestions that this might be the AY chip at fault although if that were the case I would have thought that I'd not get proper sound at all?


Grateful for any suggestions


James



Title: Re: CPC 464 - very quiet speaker audio
Post by: Bryce on 11:19, 08 May 21
The capacitors in the amplifier might be failing.

Bryce.
Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 21:00, 09 May 21
Thanks - any particular likely culprits? There are a lot of small capacitors scattered around and they look OK but obviously difficult to conclude much from that.


I'm sure it's something with the electronics, I've checked the speaker and that looks fine but I resoldered everything for good measure.


James
Title: Re: CPC 464 - very quiet speaker audio
Post by: Bryce on 14:08, 10 May 21
Only the electrolytic capacitors will be an issue. Have you any measurement equipment?

Bryce.
Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 23:57, 10 May 21
Thanks


I've got a fairly basic digital multimeter but nothing else unfortunately. I probably should get something a bit more up to date.


James
Title: Re: CPC 464 - very quiet speaker audio
Post by: thomas on 15:31, 12 May 21
Measuring capacitors is difficult. I just swapped this one (C30X?) which looked suspicious, and internal sound went from not working at all to flawless :-)
Might have just been luck, but I suggest you'll give it a try.

- thomas

Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 18:59, 13 May 21
Thanks - that's a good tip. I'm going to replace a few of the capacitors around the amplifier as a first step
Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 00:20, 16 May 21
Right a quick update


Rummaging through the junk box I've found a couple of spare capacitors and changed them out (C309 and C311) but with no improvement (see pics, although I'd only done one when I took this).


I think the one that thomas mentiones in the post above is C308 (also circled on the pics). It's covered with what looks like a blob of glue. Unfortunately I don't have any of these in stock (look like a 22uF) so will get one in.


There is also C314 which is a non-polarised electrolytic, don't have any of those either so maybe worth replacing that too.


Any other thoughts? I will probably try and get hold of one of these cheap oscilloscopes and see if I can trace the path if I can't get anywhere.


James
Title: Re: CPC 464 - very quiet speaker audio
Post by: Audronic on 02:28, 16 May 21
@gw0udm (https://www.cpcwiki.eu/forum/index.php?action=profile;u=4246)


It is also possible that the Speaker Cone is Jammed and only moves a small amount ?
If you have an other speaker that you could use as a test that might prove if I am correct.
Or keep changing the Caps C301 is also a good one to change

Good luck
Keep Safe

Ray
Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 13:23, 16 May 21
Thanks. Yes I did wonder about the speaker, but the 'interference' noise is actually reasonably loud with the beeps hidden in the background somewhere so it does feel more like an amplifier issue. I'm going to order some more caps so will report back.


James
Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 11:42, 20 May 21
Right, I've changed those capacitors out and it's still the same.


I'm wondering if the amplifier IC itself is at fault but I'm a bit reluctant to start messing around with that.


I am going to have a closer look at the volume control pot and just satisfy myself it's not that, as that would be rather embarrassing!


James
Title: Re: CPC 464 - very quiet speaker audio
Post by: Bryce on 13:22, 20 May 21
It could be the op-amp, but they usually either work completely or not at all. A simple resistance measurement of the pot will tell you whether that ok or not. I thought you had done that already.

Bryce.
Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 12:23, 21 May 21
I thought I had too... but want to double check before changing anything else. Haven't had chance yet so that's a job for the weekend.
Title: Re: CPC 464 - very quiet speaker audio
Post by: gw0udm on 00:52, 23 May 21
Right further update


I've checked the volume pot again and it seems fine.


So in summary I've changed C301, C304, C308, C309 and C311


The only ones left now are C303 and C306 (which I don't have any spares so need to get some)


there is also C314 which is right at the start of the chain and seems a likely suspect. However this is labelled 'NP' which I'm assuming means 'non-polarised' and these seem difficult to find in this value but I'll persevere.


Losing the will to live somewhat now... I should probably give up soon and use external speakers instead but it's bugging me!


Any other thoughts welcomed


James