Noisy Audio on Taiwanese +2

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Morpheus
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Noisy Audio on Taiwanese +2

Post by Morpheus »

Hi
I have bought a +2 and I went with the Grey +2 rather than the +2a because I heard about the corrupted audio on +2a's and +3's. I seem to have backed the wrong horse here.
The Audio is overridden by a humming noise that drowns everything out it was worse using the old power brick but it improved when I used a more modern one.

I am wondering if a re-Cap would help but I am not sure at this point.

The audio is awful there is a lot of noise on the audio I have just fitted a Byte Delight Internal Mod and the noise is present there too. The only way I can get some nice clear AY sound is by plugging in my Simple AY Interface in the back and using that instead but then I loose the Beeper. I kinda feel like I've wasted money on this thing now.

My last resort today is to try disconnect in the Cassette deck and see if that is the problem. If I do that I loose the ability to use my TZXDuino as well as It has an external tape socket as well. I had fitted a ground wire across the two points on the cassette PCB and the didn't make any difference and right now I feel like I made a mistake buying this machine.
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Morpheus
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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

Well I disconnected the Tape and there is no Improvement so the problem is on the main PCB some circuit is generating some serious interference on the Audio.
It's almost like you can hear high pitched whine as the Spectrum processes something and then goes back to noise once it is finished.
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Ast A. Moore
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Re: Noisy Audio on Taiwanese +2

Post by Ast A. Moore »

How is it hooked up to the TV? RF or RGB + MIC socket?
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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

I had an RGB Lead from Retrocomputershack connected for the picture and first some headphones and then the powered mini PC speakers in the 3.5 Stereo Jack of the Internal mod.

I have it connected to pins 1,4,5,6 ( A,B,C & Gnd ) and the flylead connected to pins 34,35 of the ULA as per the destructions.

The noise is present on all output including the built-in Mono but I was barking up the wrong tree I think as disconnecting the cassette made no difference to the audio interference.
It is just noisy humming but when a screen or menu or program runs it's almost like you can hear the machine working as if the Audio is so sensitive it's picking up the machines inner processes.

It's very odd.
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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

Update:

I disconnected the flylead from Pins 34,35 of the ULA and the noise is gone but so is the Beeper sound now. I need to find a way to filter that signal or tap in somewhere else.
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Re: Noisy Audio on Taiwanese +2

Post by Ast A. Moore »

Morpheus wrote: Wed Sep 19, 2018 5:21 pm I had an RGB Lead from Retrocomputershack connected for the picture and first some headphones and then the powered mini PC speakers in the 3.5 Stereo Jack of the Internal mod.

I have it connected to pins 1,4,5,6 ( A,B,C & Gnd ) and the flylead connected to pins 34,35 of the ULA as per the destructions.
Destructions, indeed.

What sort of internal mod does this machine have? Hooking up anything directly to the ULA like that is a bad idea, especially for audio input. The Spectrum ULA generally doesn’t like the input signal going negative.

For audio output, just use the original 3.5 mm socket. The only mod that is necessary involves changing the values of two resistors to balance the volume from the AY and the ULA. R37 should be replaced with a 2.2 kΩ resistor, and R45 with a 33 kΩ one. That’s all.

Image

I don’t know if this Taiwanese model is that much different from any other +2, but I don’t think it is.
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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

Hi
I have replaced the two resistors as listed and the audio is more equal coming out of the mono socket. The noise is still there though and drowns out any Spectrum audio at normal levels. The audio off the little Module fitted to pins 1,4,5,6 of the AY chip has nice clear audio so it would seem to be something on the main PCB generating this interference, I have replaced 80% of the electrolytic caps so far and that has had no impact.

Any ideas what could cause some much noise that the audio is this bad???

Thanks.
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Ast A. Moore
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Re: Noisy Audio on Taiwanese +2

Post by Ast A. Moore »

Morpheus wrote: Thu Sep 20, 2018 2:02 pm Any ideas what could cause some much noise that the audio is this bad???
Revert the AY mod. More generally, revert the machine back to the original state, other than the two resistors I mentioned in my post and see if that makes any difference.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
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Re: Noisy Audio on Taiwanese +2

Post by 1024MAK »

This could be tricky to fix. The trouble with computers is that electrically they are very noisy. Just what you don’t want with audio circuitry. But in a computer like the Spectrum, the audio circuitry is surrounded by digital circuitry.

Can you estimate what frequency the noise is? Is it a low frequency, say below 150Hz. Is it mid-range? Is it a high frequency? Or is it a mix of frequencies?

Noise appearing in a design where other examples of the same device are fine, often comes down to:-
  • The smoothing capacitors in the PSU being in good condition. If a modern PSU is being used, is it a switch mode type (SMPSU) which does not have sufficient filtering.
  • Failed or open circuit (including dry joints and broken tracks) decoupling capacitor(s) allowing a greater than normal amount of noise to appear on the power supply or 0V/GND network.
  • A capacitor failure near a 78xx series voltage regulator allowing the regulator to oscillate.
  • A partly faulty 78xx series voltage regulator.
  • The Sinclair DC/DC inverter/converter operating outside of it’s normal frequency range (is the +12V supply okay?)
  • A PCB track, or wire in the audio circuit broken or unterminated (component missing or broken, or a dry joint) and so the track or wire is acting as an aerial and picking up noise from the digital circuitry. Can also occur if unscreened cables are used for audio signals.
  • An open circuit 0V/GND connection, track or wire, causing currents to flow via another route.
  • A faulty, or partly faulty component, causing noise.
  • Contamination of the PCB allowing currents to flow (but not enough to be classsed as a short circuit) where there should be insulated separations between tracks/components. If contamination is found, clean the PCB.
Note that this list is not exhaustive...

And a final thought, has this unit always suffered from this problem, or has the problem developed?
If the computer has always suffered from this problem, it could be due to a manufacturing fault, or a short circuit between tracks / circuits somewhere.

Mark
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Re: Noisy Audio on Taiwanese +2

Post by MatGubbins »

My +2 Grey has always had a loud hum to the sound. I've owned it since the release of 1986.

As you stated, it is possible to hear it working as the screen is being drawn in a BASIC listing, the high pitched whine when enter is pressed and changing colours can make it musical.

Code: Select all

5 LET A=20
10 BORDER 0 : PAPER 0 : CLS : PAUSE A : BORDER 7 : PAPER 7 : CLS : PAUSE A : GOTO 10
and the hum will change frequency when the colour alters. This is through both the RF/TV and the sound out port.
I altered the resistors (as shown above) a few years ago to make the AY louder, but the hum is present.

I always thought that all my Spectrums and Amigas were the same and had that working hum to them regardless of the TVs or monitors that they were connected to.
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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

MatGubbins wrote: Thu Sep 20, 2018 11:31 pm My +2 Grey has always had a loud hum to the sound. I've owned it since the release of 1986.

As you stated, it is possible to hear it working as the screen is being drawn in a BASIC listing, the high pitched whine when enter is pressed and changing colours can make it musical.

Code: Select all

5 LET A=20
10 BORDER 0 : PAPER 0 : CLS : PAUSE A : BORDER 7 : PAPER 7 : CLS : PAUSE A : GOTO 10
and the hum will change frequency when the colour alters. This is through both the RF/TV and the sound out port.
I altered the resistors (as shown above) a few years ago to make the AY louder, but the hum is present.

I always thought that all my Spectrums and Amigas were the same and had that working hum to them regardless of the TVs or monitors that they were connected to.
Thanks for the advice Guys
I have only had this a few weeks and In my haste sold my 48K+ to pay for this one. It seemed like a good Idea as most modern games have a 128K requirement or mode.

I also reverted it back to as it arrived and the fault is there anyway so it arrived with it.

Yeah what you are describing is more indicative of what I seem to have and this +2 is from '86 as well. It seems like one of the "quirks" of using real hardware. The noise is a bit less obtrusive when I choose 48K mode as well so I will just work through the list checking the joints and maybe looking at the decoupling capacitors as I have plenty in my box.

Do you think the Transistors that are the wrong way round could cause this problem???
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Re: Noisy Audio on Taiwanese +2

Post by 1024MAK »

Morpheus wrote: Fri Sep 21, 2018 7:28 am Do you think the Transistors that are the wrong way round could cause this problem???
I suspect they will not be the primary cause. But a transistor operated “in reverse” will generate more noise.
It’s still a good idea to replace them though.

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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

I have been having a go at this +2 this morning. I have replaced some of the decoupling capacitors on the power circuit and fitted a new 7805CV Voltage Regulator and the noise has diminished to the point where I can hear the Beeper sound/music on normal volume levels....

Maybe it was the VR after all, I know the thermal adhesive had deteriorated as the VR was not stuck to the heatsink when I unscrewed it.
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Re: Noisy Audio on Taiwanese +2

Post by MatGubbins »

+2 Grey issue 3 Z70500 - 8639 date.
Should note that I've also done the transistor mod for composite (unsoldered and replaced TR4, TR5 and TR7 with new from Maplins) and the C31 mod that removes the sound noise from the composite signal. I've not yet done a TEA2000 mod as I don't have the correct parts.
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Re: Noisy Audio on Taiwanese +2

Post by Ast A. Moore »

What PSU are you using with your +2?
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Re: Noisy Audio on Taiwanese +2

Post by Ast A. Moore »

MatGubbins wrote: Fri Sep 21, 2018 12:24 pm I've not yet done a TEA2000 mod as I don't have the correct parts.
You mean the RL filter that removes the yellow/blue tint/shift over composite/RF signal? I decided against it on my Toast Rack. I primarily use the RGB output anyway, but can switch to composite on the TV. This lets me see the “worst-case scenario” for testing purposes. Besides, it’s fun to look at things like this, which take advantage of that particular fault.
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and zasm Z80 Assembler syntax highlighter.
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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

Ast A. Moore wrote: Fri Sep 21, 2018 12:26 pm What PSU are you using with your +2?
I have the original power supply and the noise using that is very bad. I have a new 9V 2A Power Supply and it doesn't look a cheap and nasty one but I could not tell really it's made by someone called Akuru.
I normally use Goodleads as the ones they ship have a ferrite choke fitted to the lead which they reckon helps the noise.

Might get one and send this one back....
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Re: Noisy Audio on Taiwanese +2

Post by Ast A. Moore »

So, if you use a newer, regulated PSU, does it make a difference noise-wise?
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and zasm Z80 Assembler syntax highlighter.
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Re: Noisy Audio on Taiwanese +2

Post by Morpheus »

Ast A. Moore wrote: Fri Sep 21, 2018 3:50 pm So, if you use a newer, regulated PSU, does it make a difference noise-wise?
Yes noise is improved by using modern power supply compared to the old Sinclair/Amstrad one.

I changed some of the ceramic capacitors around the the power in and that helped a little bit as well and also the new 7805 Regulator too.

It's getting better incrementally

BTW this board is marked 86/28 on the back....

There is a Mod under the TEAC2000 with a capacitor and a cut track is that normal ???
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Re: Noisy Audio on Taiwanese +2

Post by Ast A. Moore »

Morpheus wrote: Fri Sep 21, 2018 4:23 pm BTW this board is marked 86/28 on the back
Early July, then.
Morpheus wrote: Fri Sep 21, 2018 4:23 pmThere is a Mod under the TEAC2000 with a capacitor and a cut track is that normal ???
Could be. I think some of the redesigns were incorporated into the main PCB layout around 86/37 and later boards.
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and zasm Z80 Assembler syntax highlighter.
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