ZX Spectrum issue 3 speaker issue

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pintglass
Posts: 2
Joined: Sat Oct 15, 2022 12:19 am

ZX Spectrum issue 3 speaker issue

Post by pintglass »

Hi All
I'm trying to fix a non working speaker on my Zx spectrum issue 3 board, I have changed TR7, diode D9 and replaced the speaker its self but none of those made any difference.
I do get sound if I use the mic socket so assume the ULA is OK.
Whilst I've been checking for a possible fault I've noticed that the Ear/Mic circuit on my board has a resistor of (~1182 ohm's) where according to the schematic it should be a 10nf capacitor which is C35, has anyone else seen this on a issue 3 board or any other revision?
I know this may not be why my speaker is non working but thought I'd check before I continue trying to find a fault.
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1024MAK
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Re: ZX Spectrum issue 3 speaker issue

Post by 1024MAK »

Hello and welcome pintglass :D

On the subject of TR7, keep in mind that the collector connects to the +9V (nominal) line, NOT the +5V as shown in most schematic circuit diagrams.

Did you use the same type (ZTX450) or a different type?

Diode D13(1N4148) sometimes fails and clips the audio signal, so it’s worthwhile renewing this.

It’s possible that the ULA may have developed a fault, but before even thinking about changing it, when LOADing, do you get the sound from the speaker? You should do, so if you don’t, then it’s not likely to be the ULA. Oh, and you don’t have to use an actual cassette tape of Spectrum software, any (reasonable) audio source fed in via the Ear socket should result in the sound being reproduced by the speaker.
pintglass wrote: Sat Oct 15, 2022 5:13 pm Whilst I've been checking for a possible fault I've noticed that the Ear/Mic circuit on my board has a resistor of (~1182 ohm's) where according to the schematic it should be a 10nf capacitor which is C35, has anyone else seen this on a issue 3 board or any other revision?
I don’t have a issue 3 board to hand to check, but I don’t remember any resistors replacing C35. I would be tempted to lift one end of this resistor.

Mark
pintglass
Posts: 2
Joined: Sat Oct 15, 2022 12:19 am

Re: ZX Spectrum issue 3 speaker issue

Post by pintglass »

Thanks for the reply mark
1024MAK wrote: Sat Oct 15, 2022 5:55 pm Did you use the same type (ZTX450) or a different type?
I replaced the ZTX450 with the same transistor
1024MAK wrote: Sat Oct 15, 2022 5:55 pm Diode D13(1N4148) sometimes fails and clips the audio signal, so it’s worthwhile renewing this.
I have replaced this, it didn't make any difference.
1024MAK wrote: Sat Oct 15, 2022 5:55 pm It’s possible that the ULA may have developed a fault, but before even thinking about changing it, when LOADing, do you get the sound from the speaker? You should do, so if you don’t, then it’s not likely to be the ULA. Oh, and you don’t have to use an actual cassette tape of Spectrum software, any (reasonable) audio source fed in via the Ear socket should result in the sound being reproduced by the speaker.
I do get sound from the speaker when I load a game.

Sorry I was mistaken C35 was a capacitor not a resistor as I thought.

As I said before I'm getting about 1180 ohm's resistance to ground on pin 28 of the ULA is this correct?
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