C46 Capacitor Marking

dreamscape

Hi all

I've just watched this part of this video: http://www.youtube.com/watch?v=TeCVt2T-YSo&t=17m0s

And he says the marking is incorrect? Is this actually true? I'm not questioning his knowledge but currently my new capacitor is fitted the way the board says.. if what he says is true i will have to swap it around.

Can anyone confirm this please? :-?

Zorn

The silkscreen polarity marking for C46 has been incorrect in every issue 2 Spectrum I've seen. If you want to be certain, refer to the circuit diagram and make continuity checks to verify what each side of C46 is connected to.

If you've had the Spectrum on with an electrolytic capacitor installed the wrong way around, you ought to replace it; it will quite likely have been damaged.

dreamscape

Hi Zorn,

I've not turned it on yet i've just installed all the capacitors and I was waiting for confirmation on this before i tried it (although i still need to replace D13 so it won't be turning on for awhile yet still).

I shall swap mine around then as it must be wrong it's an issue 2. Luckily i have a number of spare 22uF caps left. :smile:

This might sound silly but what am i looking for when i test for continuity? I need to test all of my caps to make sure they are all ok i have my multi meter ready but to be honest i'm slightly unsure what I'm looking for..?

<--- n00b

Zorn

Look at the (issue 2 Spectrum) circuit diagram. For example, the positive terminal of C46 is connected to the collector of TR4 and one of the coil windings. The negative terminal is connected to one side of D11 and R55. By checking for continuity between C46's PCB pads and those other locations, you can work out which way around it should be installed without having to rely on the silkscreened labels.

P.S. C46 should be 1uF, not 22uF.

dreamscape

Sorry yes i meant 1uF

I went to remove it and I think i've just destroyed the whole board... :sad::cry:



the track came up with the cap.. :sad:

That's the end of that then i guess... I'm beginning to give up on Spectrums :sad:

1024MAK

If your multimeter has a continuity range, use this. It will beep when the resistance between the probes is low (meaning there is a low resistance conductive path between the two points).

If your meter does not have such a range/setting, use a low resistance (200 ohm) range. Again, a low reading (less than 2 ohms) means the points between the probes are connected.

Just a single "1" in the left hand part of the display on a typical meter means over range. For resistance, this means the value is greater than the meter can display on the range it us set to.

Always double check that the polarity of polarised components like electrolytic capacitors is correct. Electrolytic capacitors sometime explode if they are connected up the wrong way round.

Mark

Zorn

It's easy to do, but with a bit more care and experience it'll become a rare event.

It looks as if the lifted track is complete, so it can be stuck back down. I suggest using tiny amounts of superglue applied with a pointy stick. If there is a track on the rear leading from the pad the broken track connects to, feed a fine copper wire through the hole and solder it in place with the component and solder it down to the track on the rear. Make sure the broken track on the top connects to the pad either with a bridge of solder or the above mentioned wire.

If the track is damaged beyond reuse, you can glue down a thin wire along its original path to replace it or you can connect an insulated jumper wire (probably on the rear) between appropriate points to re-establish the broken connection.

dreamscape

Sadly it's not complete the end which was on the capacitor came apart when i removed it. I applied the iron to the solder to remove it with a pump and it just fell apart.

I think this board is a bin job... such a shame. I've had enough of 48k spectrums thats 2 I've had now which have both driven me mad.

:sad:

Zorn

Don't throw it away. It can be repaired.

dreamscape

How? I really can't see me being able to repair that track.

balford

All you need to do is to use fine point to point wire to essentially replace the conductive path provided by the broken track. Don't give up, it's a really easy fix :)

B

1024MAK

dreamscape wrote: »

I went to remove it and I think i've just destroyed the whole board... :sad::cry:

the track came up with the cap.. :sad:

That's the end of that then i guess... I'm beginning to give up on Spectrums :sad:

STOP
This is not a show stopper. Use a section of wire to replace the damaged PCB track. Thin tinned solid core copper wire is what you want. If the length is short and there is no chance of short circuits, you don't need to used insulated wire. For anything else use insulated wire. If the track is also damaged on the underside, make the end of any wire used into a very small coil to hold the leg and to give the solder something to bind to. Use very small round nose pliers for this.

Never give up, never surrender. All Spectrums can be fixed (it's just a matter of time, money and effort).
Oh, and now you are learning that when desoldering / removing components, you have to be very careful or the tracks may be damaged. Alway ensure that either all the solder has been removed and the component leg is free / loose, or if removing with a hot iron, that all the solder is fully molten and that the tracks are okay. Never apply heat for more than around FOUR to FIVE seconds (except where there are large tracks that eat heat).

Personally, I prefer to cut the legs off a component before removing it. Then you can do one leg at a time.

Mark

Any spelling errors or wrong works are due the the smartphone guessing and inserting the gone worts...

Zorn

>If the track is damaged beyond reuse, you can glue down a thin wire along
>its original path to replace it or you can connect an insulated jumper wire
>(probably on the rear) between appropriate points to re-establish the
>broken connection.

If you can't face that and want to send it to me, I'll repair it and make a video about the process.

dreamscape

Thanks guys for the positive feedback. Just feel very deflated after all this I thought i had done a pretty good job and then BAM!

Zorn, I will send you a PM.

1024MAK

You should have seen the PCB's of some of the 1980's and 1990's TV's after faults with the HT or LOPT sections. Wires had to be run to route the circuits around the burnt tracks and sometime the holes where the board had been burnt through...

Mark

balford

I've found it's particularly easy to damage the pads on the Amstrad produced Spectrums.

A temperature controlled iron is of great help, as is a decent sized tip - the smaller ones just don't have the thermal capacity needed to heat the joint quickly, and are only of use when soldering SMD's.

300 degrees is plenty for normal work, and snip the leads of any components you're not going to be reusing to reduce the stress on the pad.

B

dreamscape

and snip the leads of any components you're not going to be reusing to reduce the stress on the pad.

Really should of done that... Lesson learnt.