Replacing foam pads on top of heatsink in Spectrum+

Mousey

Inside a Spectrum+ that I'm working on, the 2 foam pads on top of the heatsink (see photo for where they were) had become a disintegrated mushy pile that kept sticking to the membrane tails (which I assume they were intended to protect from the heat). They were 20x6mm in size, and I'm guessing around 2 or 3mm high when new.

Can I just stick any new foam pads in their place, or will I need some kind of special heat resistant ones? Sorry if this sounds like a stupid question; I'm pretty new to all of this and just trying not to damage my Spectrum.

DEATH

any bit of foam can be used, there's nowt special about it.

Mousey

Good to know; thanks DEATH!

Mousey

Quick update - instead of attaching new foam pads, I ditched the heatsink altogether! Instead, I fitted a new voltage regulator, as detailed here. I have never desoldered before, or soldered anything so delicate, but - to my amazement - it went ok and works perfectly!

balford

Nicely done. Now replace those capacitors :)

B

1024MAK

@ Mousey, your board does not appear to have a electrolytic capacitor mounted across the +9V and 0V/ground connections near the DC power input connector :-?
I thought I saw that this was recommended in the service manual (but it's been a while...). What issue is your board ?

Mark

balford

Issue 6 Mark, by the looks of the PCF1306P beside the CPU :)

B

1024MAK

Ahh, me not being observant for once. Yes, the aforementioned capacitor would be laid out on the PCB as normal, rather than a bodge in last minute affair #-o

Right, back as you were, nothing to see here... Move along...

Mark

Mousey

balford wrote: »

Issue 6 Mark, by the looks of the PCF1306P beside the CPU :)

Yep, it's a 6A. As to replacing the capacitors... maybe one day if I continue to grow in knowledge and skill. But way too scary for now!

DEATH

Learn fast mate, replacing the electrolytic caps is pretty much a "must do" - considering those caps are 30 years old now, you're deffo on borrowed time.

Zorn has done some excellent videos on how to do this....

Mousey

DEATH wrote: »

Learn fast mate, replacing the electrolytic caps is pretty much a "must do" - considering those caps are 30 years old now, you're deffo on borrowed time.

Thanks for the link; what an excellent video! I know you're right, and although it looks really daunting, I think I could do it with a bit more practice and confidence. I need a smaller soldering tip, as the one I did the voltage regulator with was too big really, and made it a struggle to accurately solder the 3 pins. I should also get a desoldering pump; I used braid last time, but a pump seems more useful in this case.

1024MAK

Really big hint - cut the leads of the old capacitors. Then desolder one wire at a time.

If you hold the iron on for too long, you risk lifting the pad and the track. 3 seconds max unless it is a ground or power plane / wide track.

Mark

Mousey

1024MAK wrote: »

If you hold the iron on for too long, you risk lifting the pad and the track.

Yeah, that's the scary bit. Noob question alert - I'm assuming the pads are the small metal circles that the pins go through? I wasn't even aware they could be lifted up until I heard the warning in the video.

I don't know if I was in danger of doing that when I swapped the voltage regulator, as I'm not sure if that had pads?

I need to get a duff circuit board from something and just practice desoldering and soldering.

DEATH

As far as the Spectrum board goes, yes, pads are the circles of copper area which actually have solder on them. Just to confuse you, in the case of SMD boards (SMD = Surface Mount Device), the pads don't have holes and they're not necessarily round. In all cases though, pads are the areas of copper that you solder to.

Tracks, or traces is the name given to the copper wires bonded to the actual PCB material. Large areas of copper are knows as "Planes", holes that are coated/plated with copper and go through the board to join tracks together on both sides of the board are known as "Vias"

If you're going to get into soldering and stuff like this, get a decent soldering iron - many on here will tell you to get a temperature controlled beastie, and yes they have their merits, but that isn't necessary in the slightest - that iron used by Zorn in the video is either an Antex Model C or an Antex model M - either one will be perfectly ok for Spectrum circuit boards. Keep it clean and learn how to use it.

What lifts pads is excess heat and to a lesser extent, poor quality adhesives used during the manufacture of the board itself. It doesn't always follow that an iron that is too hot will cause the most damage. An iron that is too hot can cause damage, sure - but one that is too cold held in contact for too long can do much more. Trick is to get the balance right - solder should take 2 seconds or less to melt and form a good joint, any longer then your tip is either too small (in respect to "thermal mass"), or too cold. MANY people on here will help guide you, that's why we are here :) Oh, and whether they admit it or not, even those among us who have been soldering for 30 years or more will still lift a pad or two, so don't feel bad about it if you do. Boards are of various quality, and I find issue 6 boards are among the worse boards to work on with regard to pads lifting.

Get yourself some resin cored solder (AKA Rosin), a set of snips, needle nose pliers, some small screwdrivers, both slotted and cross point (AKA Pozidrive/Phillips) size 0 will do nicely. Last but not least, a multimeter - cheap as chips (literally) on ebay.

As you get more into the hobby, then you can spend out on more advanced tools - really, the world is your oyster!

For something to practice on, strip board and resistors are cheap, bits of wire etc - practice with those. Look at Bowood electronics, they will help you as much as is possible. https://www.bowood-electronics.co.uk/

Resistors are literally pennies, stripboard can be got for a quid.

What you waiting for? Get practicing :D

Mousey

Great post again DEATH; I really appreciate all your advice, and the website link.

I have got most of the tools that you mention, and was looking as ESR meters so I can test the existing capacitors. I liked the look of the one in a Mark Fixes Stuff video, a Peak Atlas ESR70, and thought I'd pick one of those up - until I found they are £85! The cheap ones on eBay don't seem to work with capacitors still on the circuit board. Not a big deal though, as I know it's best to replace all the capacitors anyway - I just wanted a cheap new toy. :)

Since my last post it occurred to me that I have a box of broken joysticks that I can practice on.

Mousey

One other quick question - if I should happen to lift a pad (or track) from the board, am I stuffed? Or is there a way back from that?

1024MAK

If you are going to be doing a lot of fault finding, on many boards, a ESR meter may be useful. But it's not worth buying for the occasional repair or servicing of one or two Spectrums. Just buy new electrolytic capacitors. Even when the old ones test okay, you still don't know how long they will last. They are known to degrade. That's why we keep banging on about replacing them.

Of course, I also have various Peak made test gear items (they are very good and it's a UK company) including a ESR meter from them. I have had electronics as a hobby for many, many years, but still hardly use the ESR meter. Heck, I also have a transistor tester and that only gets used infrequently.

Instead spend your money on new electrolytic capacitors and a good quality multimeter (although as Death says, a cheap one is fine for basic fault finding on low voltage boards like the Spectrum).

Mark

DEATH

There is a way back, of course, Get a length of stranded wire, strip it out and take a single strand of very fine (42 swg approx) tinned copper wire. 3A fuse wire will work too, just make sure it's tinned. Make a tight loop at one end and solder this around the component lead, flush with the board. Guide the length of wire along the same route that the stripped trace took, and solder that end of the wire to the trace where it broke - you may have to remove a bit of the solder mask. A drop of Cyano (superglue) to hold it in place AFTER you have soldered it, job done! If the trace is quite a thick one, like for example a power carrying trace, just twist two strands of the wire together, although one strand of that 42 SWG wire will carry over 2 Amps quite safely so long as you use a short length. Be neat when using this method so you don't get any accidental short circuits.

I assume you know what I mean by "tinned"? Bare copper is reddish, tinned will be silver in colour, because it's coated with tin/solder, Use this in preference to bare wire, it's easier to solder and won't tarnish so easily.

Don't apply heat to cyanoacrylate by the way, the fumes really ain't that good for ya ;) It'll burn your eyes, sting your throat and if you have asthma or something, you'll be reaching for the ventolin!