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CPC6128 Volume low

Started by minwah, 17:41, 30 April 16

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minwah

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!

TFM

Maybe it's worth to exchange the PSG (and give it a socket this way). Good luck!  :)
TFM of FutureSoft
Also visit the CPC and Plus users favorite OS: FutureOS - The Revolution on CPC6128 and 6128Plus

CanonMan

Quote from: TFM on 18:53, 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.

TFM

Probably a good idea, thought it goes out directly (from memory).  :)
TFM of FutureSoft
Also visit the CPC and Plus users favorite OS: FutureOS - The Revolution on CPC6128 and 6128Plus

Audronic

@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
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

CanonMan

Quote from: Audronic on 23:47, 30 April 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!

Audronic


@CanonMan

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


Still try the headphone socket.


Good Luck


Ray


Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

minwah

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.

Kris

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

Bryce

Could be a capacitor or the Op-Amp, you'll need to trace the signal with a scope.

Bryce.

minwah

Quote from: Kris on 09:27, 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

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

minwah

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.

Bryce

That would suggest you have a "low resistance to earth fault" in the amplifier. Definitely something wrong there.

Bryce.

minwah

Quote from: Bryce on 12:35, 05 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...

minwah

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.


Bryce

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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