Issue 4B upper ram PCB tracks

sP1d3r
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Re: Issue 4B upper ram PCB tracks

Post by sP1d3r »

1024MAK wrote: Tue Jul 19, 2022 11:11 am Issues with no sound are often TR7 failed, D9 or D13 failed, or the speaker.
The DC resistance of the speaker (as measured using a multimeter) should be less than 45 ohms.
Thanks for the 'autopsy' Mark, the speaker is open circuit and as the LCR tester shows, the ZTX450 TR7 transistor has hFE of 170, (100-300 spec).

I think that I blew 2 speakers not heatsinking the speaker terminal, I'll get another speaker and use piggybacked croc clips to soak up some heat next time.
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1024MAK
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Re: Issue 4B upper ram PCB tracks

Post by 1024MAK »

You could try a speaker out of a cheap set of 32 ohm headphones…

Mark
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sP1d3r
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Re: Issue 4B upper ram PCB tracks

Post by sP1d3r »

1024MAK wrote: Wed Jul 20, 2022 1:30 am You could try a speaker out of a cheap set of 32 ohm headphones…
Because Mutant Caterpillar have a new 40 Ohm speaker available cheaper than an OEM speaker from eBay, it's no bother to get one. :D

The two blown speakers had coils that were intact but the solder joint between the pin connector and the coil wire was the problem :x
sP1d3r
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Re: Issue 4B upper ram PCB tracks

Post by sP1d3r »

The 4B motherboard now has a new speaker and is fully functional.
Here's a shot of it with a working issue 4S motherboard which I was also able to repair, it came in a Spectrum + and had a crazy CPU replacement which damaged the through holes and stopped it from booting up.
Spectrum 48K iss. 4S & 4B
Spectrum 48K iss. 4S & 4B
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1024MAK
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Re: Issue 4B upper ram PCB tracks

Post by 1024MAK »

Good news!

Glad to hear that it’s all fixed and working :D

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: Issue 4B upper ram PCB tracks

Post by sP1d3r »

1024MAK wrote: Mon Oct 10, 2022 11:35 am
Glad to hear that it’s all fixed and working :D
I was able to solder the speakers by attaching piggybacked croc clips to the pin to be soldered, on the component side.
Heatsink for speaker pins
Heatsink for speaker pins
This must be done or the speaker won't work. :o
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1024MAK
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Re: Issue 4B upper ram PCB tracks

Post by 1024MAK »

I’ve never seen piggybacked croc clips used like that for temporary heatsinking!

I normally use one of the croc clips that is part of a helping hand… (like this link).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: Issue 4B upper ram PCB tracks

Post by sP1d3r »

Also useful for holding through-hole components prior to soldering.
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