Coils….

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Simon_Carr
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Coils….

Post by Simon_Carr »

I have about 4-5 coils from Spectrums that seem to be fine when tested for the resistances when not in circuit (open between, minimal/zero within windings) , but when I solder them into place onto a board I lose the open circuit between windings. I have checked for sloppy soldering or splash, and have removed the coils and retested them, and they seem okay.

Other than an issue with the PCB (in this case it is a brand new 3b board I have been building from scratch), any thoughts on what the issue might be?

Regards,

Simon
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1024MAK
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Re: Coils….

Post by 1024MAK »

Are you testing with a multimeter on its resistance range?

I take it that with the coil not fitted to the board, both the coil and the board test okay?

See also this post.

Mark
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Simon_Carr
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Location: Lake District, Cumbria

Re: Coils….

Post by Simon_Carr »

Hi Mark

Yes, I’m testing using two multimeters (one manual, the other auto-ranging), and off the board the coils are all fine. They look good as well, and I can see no damage to the wires connecting to the pins. However, I have just tested the board, with the coil removed, and that is where the problem is! As the photos show, there is something not right about the ‘outer’ pair of solder pads. Checked again and again, and I can’t see anything amiss with the PCB or traces, so it looks like this is a bad board?

Cheers,

Simon
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Too many hobbies, too little time...
Simon_Carr
Posts: 64
Joined: Sun Nov 02, 2014 12:15 am
Location: Lake District, Cumbria

Re: Coils….

Post by Simon_Carr »

BTW it was my daughter holding the probes, with her yellow nail varnish!
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1024MAK
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Re: Coils….

Post by 1024MAK »

What do you get with TR4 removed from the board?

Mark
ZX81 Variations
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Simon_Carr
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Re: Coils….

Post by Simon_Carr »

Hi Mark,
Just removed TR4, and I get Open Circuit on all combinations. Occasionally I get a very high (5 MOhm) reading, but never consistent, and usually flashes straight to open. If it was working, what would removing TR4 result in?

Cheers,

Simon
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1024MAK
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Re: Coils….

Post by 1024MAK »

Without TR4 fitted, there should be no electrical connection (within reason) between the two windings on the coil (see note below). If human hands are touching the probes or the circuit board, you may get some results of a few MΩ.

The most common fault with the DC/DC converter circuitry, is the failure of TR4. Typically it self destructs and goes partially short circuit between all legs…

As the base (of TR4) connects to one coil connection and the collector (of TR4) connects to another coil connection, a faulty TR4 can connect both together!

However, keep in mind also that, as TR4 is a bipolar NPN transistor, the semiconductor junctions in the transistor can be turned on if the test voltage from a multimeter is high enough (about 0.5V or higher) when it’s switched to a resistance range. For readers that only have one multimeter, you can’t obviously test this though. Although the test voltage should be stated in the manual for the multimeter.

So if your multimeter(s) do turn on a junction in a semiconductor, test both ways (both polarities) round.

* note: if an earlier version of the circuitry/board revision is tested, and the multimeter test voltage turns on semiconductor junctions, there may be some current leakage via TR5.

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.
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