Spectrum issue 6A - faulty coil?

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hatman72
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Joined: Fri Feb 12, 2021 12:59 am

Spectrum issue 6A - faulty coil?

Post by hatman72 »

Hi

I picked up a Spectrum+ locally and it turned out to be an issue 6A, but it clearly has problems :?
I plugged it in very briefly and there was a grey screen with flecks, and that was it. I immediately removed the power and did some checks using a YouTube video by JoulsperCoulomb as a guide. The obvious issue seems to be the coil - one side of the winding is returning 0 ohms resistance as expected but the other side (the one nearest the edge of the board) is not - it's showing open circuit.

For good measure I replaced TR4 and TR5 but this made no difference. I have another Spectrum and this shows 0 ohms on both sides as I would expect.

So, do I go ahead and get a new coil, or is there anything else that could be making the coil behave like it has failed? I'm clearly not keen on plugging it back in again until I have both sides of the coil behaving the same.

Thanks!
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1024MAK
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Re: Spectrum issue 6A - faulty coil?

Post by 1024MAK »

If one of the windings is open circuit, then first check for dry solder joints. If this does not help, carefully remove it from the board.

Then either rewind the two windings, or buy a replacement.

You may find that you have other failed components, as it is rare for the coil to fail in this way on it’s own. The prime cause of failure is either edge-connector abuse or one or more failed ‘lower DRAM’ (4116 or equivalent) chips, as well as the transistors (that were) fitted in TR4 and TR5 positions having been damaged.

Mark
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hatman72
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Re: Spectrum issue 6A - faulty coil?

Post by hatman72 »

Thanks Mark, I was going to ask about how best to remove it as it seems to be quite a large component.
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1024MAK
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Re: Spectrum issue 6A - faulty coil?

Post by 1024MAK »

You need a good soldering iron, a good solder sucker/ desoldering pump and a good light. Also liquid flux and IPA (not the drink!). If you no longer have young eyes, also a magnifying glass…

First, clean the area of the bottom of the board with IPA and a toothbrush (not the one you use for your own teeth). Put the board at a suitable angle so the IPA runs off it. Then after the board has dried, brush some flux over the existing solder pads/joints. Then add some (a little) new solder to them. The point of this is to try to dislodge the oxidation on the surface of the old solder.

Now get the pump ready. Heat up the joint until the solder is fully molten, wait another couple of seconds to ensure the heat has flowed through to the other side of the board and hence all the solder has melted through the plated through-hole. Then immediately push the nozzle of the pump over the joint leaving no or little air gap and push the button to activate it.

Carefully inspect the pad and the pin. Use a small flat bladed screwdriver to see if you can wiggle the pin from side to side in both directions. Is there still any solder in the hole? Does the pin move or is it still stuck with solder to the side of the through-hole plating?

If it’s free, move on to the next pin.

If it’s still stuck, still move on to the next pin, but you will have to return to this pin and try again. Before trying to desolder it again, add some new solder so that it flows into the hole. Otherwise not enough heat will flow into the solder that is left in the hole.

Note, be very careful not to overheat the pads for too long, otherwise the copper pad will lift from the PCB. And never use any force to try to remove a component, otherwise you risk lifting the pad and ripping the through-hole plating out.

If you have not done desoldering before, I definitely recommend practicing on an old scrap double sided board with plated through-holes first.

Mark
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hatman72
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Re: Spectrum issue 6A - faulty coil?

Post by hatman72 »

Thanks Mark - I'll get both types of IPA in - I can drown my sorrows if it all goes wrong! :lol:

Thanks for the details. I have desoldered a few memory chips and transistors, but with those you have the luxury of being able to snip the legs off if necessary. The difference between those chips and the coil is the size of the legs and the amount of solder used to attach them. There also appears to be some sort of "clip" poking through the solder in the coil's middle pins.

Cheers
Richard
hatman72
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Re: Spectrum issue 6A - faulty coil?

Post by hatman72 »

So, in an interesting turn of events I was checking the continuity of the circuit board to ensure that the power circuit was all ok and, still suspecting the coil was the culprit, I poked the tip of my multi-meter into the windings to find that one of them was not properly attached - it just lifted out! The end of it was covered in green ooze, so I don't know if it had simply corroded?

So as a test I did a hasty bodge - I used a short loop of wire to connect the base of the coil to the end or the wire and I got continuity on the coil. All the other tests looked good, so I turned it on and got the Sinclair copyright message :D

So that proves it was the coil, but now I have a fairly flaky fix in place. The question is, is there a better way to repair this coil without having to lift it out? I'm keen to avoid doing that if possible, but I can't get at the point at which the wire would normally attach to as it is under the plastic base.
30B482EA-1B44-43C6-B978-207D8ABC83D7.jpeg
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1024MAK
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Re: Spectrum issue 6A - faulty coil?

Post by 1024MAK »

Not having repaired an issue 6A board, all I can say is either revolve the coil from the board or live with your bodge…

The older issue boards use a coil mounted on an insulated SRBP/Paxolin base. The copper wire for the windings on these are also terminated on the underside on the pins. These have five straightforward pins soldered into the PCB.

Copper, when it is left in damp or humid conditions will corrode and change to a green oxide, often referred to as Verdigris.

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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