Author Topic: CPC6128 Volume low  (Read 3921 times)

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Offline minwah

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CPC6128 Volume low
« on: 19:41, 30 April 16 »
I have a bit of a problem with my 6128 whereby the volume out of the internal speaker is very quiet, even at full volume.

I have compared to another machine which is several times louder. I have also tried the speaker and volume pot from the 'loud' machine to rule those parts out - it made no noticable difference.

The sound is correct, just quiet. Seems like there is an issue with amplification...just wondering if anyone else has encountered this?

Thanks!
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Offline TFM

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Re: CPC6128 Volume low
« Reply #1 on: 20:53, 30 April 16 »
Maybe it's worth to exchange the PSG (and give it a socket this way). Good luck!  :)
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Offline CanonMan

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Re: CPC6128 Volume low
« Reply #2 on: 23:41, 30 April 16 »
Maybe it's worth to exchange the PSG (and give it a socket this way). Good luck!  :)

I'd be tempted to check around/replace the audio amplifier chip first,  before condemning the AY to the dustbin.
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Offline TFM

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Re: CPC6128 Volume low
« Reply #3 on: 01:28, 01 May 16 »
Probably a good idea, thought it goes out directly (from memory).  :)
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Offline Audronic

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Re: CPC6128 Volume low
« Reply #4 on: 01:47, 01 May 16 »
@minwah


Try headphones on both machines.
If the sound level is the same look at the Speaker amplifier in the Tape Circuit


Good luck


Ray
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Offline CanonMan

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Re: CPC6128 Volume low
« Reply #5 on: 13:07, 01 May 16 »
@minwah


Try headphones on both machines.
If the sound level is the same look at the Speaker amplifier in the Tape Circuit


Good luck


Ray

It's a 6128, so there is no tape circuit as such. The audio amp is in the top left hand corner of the PCB, surrounded by lots of electrolytic caps,   and we all know what kind of problems they can cause if they dry out or go electrically leaky!
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Offline Audronic

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Re: CPC6128 Volume low
« Reply #6 on: 13:20, 01 May 16 »

@CanonMan

Oops i meant the Tape/Sound Schematic on Page 24 in the amendment service manual.


Still try the headphone socket.


Good Luck


Ray


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Offline minwah

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Re: CPC6128 Volume low
« Reply #7 on: 16:08, 01 May 16 »
Many thanks for your replies!

I have checked the output level from the stereo out on both machines, and they are the same (as far as I can tell). I don't think there is any problem with the AY.

So...I think I'll replace the electrolytic capacitors first and see if that helps. Failing that, try replacing the LA4140.

Thanks again.
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Offline Kris

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Re: CPC6128 Volume low
« Reply #8 on: 11:27, 02 May 16 »
Already got this issue on mine and it was "simply" due to a defective wire on the potentiometer....but i assume that you have already check this  ;D
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Offline Bryce

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Re: CPC6128 Volume low
« Reply #9 on: 11:46, 02 May 16 »
Could be a capacitor or the Op-Amp, you'll need to trace the signal with a scope.

Bryce.
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Offline minwah

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Re: CPC6128 Volume low
« Reply #10 on: 14:45, 04 May 16 »
Already got this issue on mine and it was "simply" due to a defective wire on the potentiometer....but i assume that you have already check this  ;D

Yeah I tried swapping the pot/cable and no difference...
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Offline minwah

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Re: CPC6128 Volume low
« Reply #11 on: 13:07, 05 May 16 »
Quick update...

I replaced all of the electrolytic capacitors - no difference unfortunately.

I did try to desolder the LA4140 while in there and put a socket in. My measley skills with solder sucker and braid couldn't get all of the pins desoldered properly though, so gave up on that. Now I know it is probably that chip at fault, I will revisit at some point...

I also noticed something else with the computer on, which is that the video picture 'twitches' (up/down) when the CPC makes sound. It is not apaprent with the volume control low, but gets more apparent as the volume is increased. Don't recall noticing this back in the day, so don't think it is normal...but having said that, I do have the volume cranked since the sound is so quiet.
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Offline Bryce

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Re: CPC6128 Volume low
« Reply #12 on: 14:35, 05 May 16 »
That would suggest you have a "low resistance to earth fault" in the amplifier. Definitely something wrong there.

Bryce.
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Offline minwah

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Re: CPC6128 Volume low
« Reply #13 on: 16:12, 09 May 16 »
That would suggest you have a "low resistance to earth fault" in the amplifier. Definitely something wrong there.

Bryce.

Thanks for your help. I will report back once I've had another look at it...
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Offline minwah

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Re: CPC6128 Volume low
« Reply #14 on: 13:47, 11 May 16 »
I haven't tried fixing this yet but having played around with the CPC a little bit I have noticed more related issues. Just making a note of all of the issues here for future ref:

1. Volume from internal speaker is low.
2. Sounds from internal speaker is distored when turned more than about half way up (not speaker distortion, volume is still low).
3. Video image twitches up and down when CPC makes sound. More apparent the more the volume is turned up.
4. Pressing keys sometimes affects the sound volume (jumps up or down).
5. Sometimes 'noise' can be heard from the speaker when the CPC is not producing any sounds. When the CPC makes a sound, this noise is temporarily reduced, and then tends to come back after a few seconds.
6. Tapes via the tape input do not appear to load...nothing on-screen after 'press play on tape'. I've only tried a few tapes so will try some more to confirm.

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Offline Bryce

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Re: CPC6128 Volume low
« Reply #15 on: 14:36, 11 May 16 »
These all point at a problem within the amplifier. Either internally in the LA or one of the supporting components.

Bryce.
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