Capacitors

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
Post Reply
User avatar
MonkZy
Manic Miner
Posts: 278
Joined: Thu Feb 08, 2018 1:01 pm

Capacitors

Post by MonkZy »

I have just brought a 48k spectrum back to life. So far all I have done is removed the RF circuit and fitted a capacitor to give AV out and replaced the crumbling keyboard membrane. Oh, I also gave its casing and keys a soapy bath. Much fun has been had playing old games, loading them using .wav files on my PC. It seems rock solid. My soldering skills are not great. What are the risks of running this machine using the original 1980's capacitors? If I invest 60-70 quid on an IDE/MMC device, do the old capacitors pose a real risk to damaging the add-on?
Nomad
Manic Miner
Posts: 600
Joined: Thu Dec 28, 2017 12:38 pm

Re: Capacitors

Post by Nomad »

Recapping a vintage electronic device is probably a good idea, gotta think if its an early model that thing has been around a long time. The caps will degrade over time especially if they have been in a device that was stored incorrectly, or just heavily used back in the day.

For some devices, you need to recap it immediately or face the prospect of magic smoke pouring out of the widget :lol:. Combine elderly caps with a dodgy power supply and that is a sure plan for tears before bedtime. Don't buy expensive caps, just get them within the right spec and you will be in a happy place.

Unlike some of the hi-fi weenies there is not the stigma of recapping the widgets, lol - no golden sound to be had from crap components from the 80s. :lol: If its a really early board there might be other issues with the machine. But that is all fun to come.

Decent power supply + recapping the widget. For stuff like C64 the power supply (depending on what one came with the machine) could brick your machine given half a chance today because they were so badly designed.
User avatar
Ast A. Moore
Rick Dangerous
Posts: 2640
Joined: Mon Nov 13, 2017 3:16 pm

Re: Capacitors

Post by Ast A. Moore »

MonkZy wrote: Wed Feb 14, 2018 11:14 pm What are the risks of running this machine using the original 1980's capacitors? If I invest 60-70 quid on an IDE/MMC device, do the old capacitors pose a real risk to damaging the add-on?
The main risk that an old capacitor might develop a short, which in turn might ruin a few ICs. It’s generally a good idea to replace old and dodgy electrolytic capacitors, however, I personally rarely go all gung-ho on it.

If your soldering skills leave a lot to be desired, you risk messing up the tracks and vias, and creating bridges with solder splashes. This may pose a greater risk to ruining a perfectly working machine, than leaving the original capacitors in place. With cheap older PCBs—such as the ones used in early Spectrums—even experienced electronics engineers might experiences difficulties desoldering the components.

As for the use of peripherals, you might benefit from replacing just the capacitors that sit on the power rails going to the edge connector. And yes, you’ll be better off using a high quality regulated power supply instead of the Spectrum’s stock linear (transformer) PSU.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
User avatar
MonkZy
Manic Miner
Posts: 278
Joined: Thu Feb 08, 2018 1:01 pm

Re: Capacitors

Post by MonkZy »

Cheers for the advice.

For power I am using a D-SUN DC-DC step down module set to 9V which is powered by a modern (gamecube) 12v supply. The old PSU is long gone.

My soldering skills are most likely below average. I really do not want to kill the board with a ham-fisted attempt at removing the old capacitors. I think I will order a set of capacitors and have a little soldering practice, I have many old PCB's in the attic that I can test my skills on.

For now I will risk it, I am not sure if I will invest in a DivIDE yet. I don't mind waiting a short while for things to load and OTLA is incredible for speeding up this process. I enjoy using emulators for programming, but you cannot beat the real hardware for running games.

Thanks again.
Nomad
Manic Miner
Posts: 600
Joined: Thu Dec 28, 2017 12:38 pm

Re: Capacitors

Post by Nomad »

I can understand you not wanting to mess up a machine when you have limited supply of replacements.

One option that has not been considered or you are leaving off the table -

socially engineer a comp-head (electronics nerd) to do the job for you. Pitch it as a learning experience and a chance to work on some obscure British hardware with historical value. Put the case forward that this might be one of the few working spectrums in your place and you want to make sure it stays that way. Bribe with a few beers (important - give the beers after the job is done :lol: ).

To be safe just make sure the capacitors are not leaking worse than a bus full of senior citizens in a traffic jam.. check for bulges. Like what has been said by Mr Moore, focus on the power rail, that is where the failure will start. It would be weird for 30 year old caps not to need some love.

At least your power supply sounds like it was made with some love and attention :lol: Japanese engineering and all that. So that is one problem sorted already.
User avatar
rnhunt
Drutt
Posts: 10
Joined: Fri Jan 05, 2018 6:02 pm

Re: Capacitors

Post by rnhunt »

You also need the appropriate soldering iron for the job.

With any soldering if you need to heat for more than a second or so to get the solder to flow it is underpowered / too small a tip. The biggest issues are with the ground side of components. These are usually connected to a large area of copper connecting all the grounds, which also conducts heat away quickly. Hence the need for a more powerful iron here. The longer you are heating something the more it is conducting into the component and elsewhere damaging things. I get tetchy if holding heat on something for more than two seconds!

I haven't seen a speccy PCB in a while, just passing on some advice from part of my day job :-)
User avatar
1024MAK
Bugaboo
Posts: 3104
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Capacitors

Post by 1024MAK »

Ast A. Moore wrote: Thu Feb 15, 2018 9:23 am
MonkZy wrote: Wed Feb 14, 2018 11:14 pm What are the risks of running this machine using the original 1980's capacitors? If I invest 60-70 quid on an IDE/MMC device, do the old capacitors pose a real risk to damaging the add-on?
The biggest risk with a ZX Spectrum 16k, 48k (or Plus), is an electrolytic capacitor in the DC/DC converter failing (part of the circuitry inside the Spectrum) and causing the loss of the -5V supply rail. This can cause the RAM chips that make up the 'lower' RAM (4116 or equivalent DRAM chips) to self destruct :-(

If you are not very good at soldering, it's better to ask someone who is an expert to do the work. In any case, to remove the old components, cut the leads/legs off, don't try to desolder them and remove the old components whole. This way, you can heat up the lead (or leg) and gently pull it free of the hole with long nose pliers while ensuring the solder is hot and molten. Then use a solder sucker to clear the hole of solder.
Ast A. Moore wrote: Thu Feb 15, 2018 9:23 am And yes, you’ll be better off using a high quality regulated power supply instead of the Spectrum’s stock linear (transformer) PSU.
There is nothing linear about the Sinclair PSU. It's a mains transformer protected with a thermal fuse, followed by a full bridge rectifier, and one or two electrolytic 'smoothing' capacitors.
The advantage of using a high quality regulated power supply (with an output of 8V to 9V) is that the 7805 voltage regulator inside the ZX Spectrum will run much cooler ;-)

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
User avatar
Ast A. Moore
Rick Dangerous
Posts: 2640
Joined: Mon Nov 13, 2017 3:16 pm

Re: Capacitors

Post by Ast A. Moore »

1024MAK wrote: Thu Feb 15, 2018 7:57 pm There is nothing linear about the Sinclair PSU.
Technically, you’re right. Back in the day, I always either installed a voltage regulator inside a transformer PSU, or assumed that it will be present in the circuit of a piece of equipment. When switched-mode PSUs came along, I referred to the traditional PSUs as “linear.”
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
User avatar
RMartins
Manic Miner
Posts: 776
Joined: Thu Nov 16, 2017 3:26 pm

Re: Capacitors

Post by RMartins »

MonkZy wrote: Thu Feb 15, 2018 10:16 am For power I am using a D-SUN DC-DC step down module set to 9V which is powered by a modern (gamecube) 12v supply. The old PSU is long gone.
You do not need 9V, when using a regulated power supply, since it will make up for the extra power when it is requested.

The required minimum voltage for the 7805 to work is 7.5 Volts.
Everything above 7.5V is usually dissipated as heat, so not useful.

If you do not have any extra peripherals, chewing up power from the expansion connector, 7.5V at 1Amp is more than enough for a 48K.

If you have something pulling power from the expansion port, just to be sure you have enough juice you can go up to 8 or 9V.
But as I said before, most of it will be disposed of in the form of heat.

You really do not need more then 7.5V or 8V, with a modern regulated power supply, since it will make up for the extra power requirements by providing more current while keeping the voltage stable.

More voltage, is only relevant, if your power source, has a small capacitor or if power consumption has large spikes.
I have not found any interface to behave like that, except maybe the ZX printer :)
User avatar
1024MAK
Bugaboo
Posts: 3104
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Capacitors

Post by 1024MAK »

RMartins wrote: Thu Feb 15, 2018 9:02 pmYou do not need 9V, when using a regulated power supply, since it will make up for the extra power when it is requested.

The required minimum voltage for the 7805 to work is 7.5 Volts.
Everything above 7.5V is usually dissipated as heat, so not useful.

If you do not have any extra peripherals, chewing up power from the expansion connector, 7.5V at 1Amp is more than enough for a 48K.
Although the drop out voltage of most 7805 regulators is 2.5V (giving a minimum input of 7.5V), it is still wise to allow a bit of headroom for the losses via the cables and connectors. Plus, the actual output voltage of some regulated PSU is not always bang on the stated voltage. If the PSU has an output that has a 3% tolerance, a 7.5V output could be between 7.275V and 7.725V. So at 7.275V plus say 0.15V losses due to cable and connector losses, this will result in the 7805 in the ZX Spectrum getting 7.125V which is below the recommended minimum input. Hence why I suggested a minimum of 8V. In practice, there are not that many 8V regulated PSUs available. So often it is easier to find and use a 9V regulated PSU.

The other consideration with the input supply voltage, is the minimum voltage that the DC/DC converter (inside the Spectrum) needs.

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
User avatar
RMartins
Manic Miner
Posts: 776
Joined: Thu Nov 16, 2017 3:26 pm

Re: Capacitors

Post by RMartins »

1024Mak, I was not replying to your message :)
In fact, I only saw it after I postted mine.
I replied to a message on the previous page.

I agree, with everything you said.

I use 7.5 because my regulated power supply gives me exactly that plus a tiny bit (7.58v), and guarantees the lowest power dissipation as heat.

As you say, regulated 8V is hard to find, so 9V is a good option.
User avatar
1024MAK
Bugaboo
Posts: 3104
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Capacitors

Post by 1024MAK »

No worries :D

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
User avatar
MonkZy
Manic Miner
Posts: 278
Joined: Thu Feb 08, 2018 1:01 pm

Re: Capacitors

Post by MonkZy »

Many thanks for all the great technical advice.

I tested my iron on some scrap PCB's and it is woefully inadequate. It takes way longer than the two second melting threshold, on PCBs with thick tracks it is more like 10 seconds it seems. It is a simple 18w Antex iron, without any temperature control. Right now I do not know any techs who have the soldering skills needed.

The spectrum initially seemed very reliable during an initial 3-4 hour session loading games using .wav files. The next session was not so good. The machine suffered 'lock-ups', the screen had no screen corruption/artifacts but there was no keyboard input on any keys. The new membrane is working and connected perfectly, so I fear some other issue. For now I have stopped powering the old machine up. No capacitors have swelled or leaked. After a cool-down I did a swift power-up/keyboard check and things seem fine. No acrid smells or magic smoke, fortunately.

Can anyone offer comment on these two £60 solder stations from Maplin?

https://www.maplin.co.uk/p/60w-mains-lc ... tion-a55kj

https://www.maplin.co.uk/p/48w-mains-so ... tion-n34fb

Would these be good for general PCB work? The next price band is around £200, which is a bit too much for me. I have watched several speccy repair videos on You Tube and I am confident with a suitable iron I would go ahead and do a re-cap.
User avatar
1024MAK
Bugaboo
Posts: 3104
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Capacitors

Post by 1024MAK »

What type of bit are you using on your iron? A chisel tip is what I recommend. Is the tip clean? Did you "tin" it?

For normal tracks/pads, an 18W Antex should be okay. But is not very useful for the thicker tracks, like those used for the power supply or ground (0V) circuits.

I use a Antex TCS 50W iron. The one I have is now over 20 years old and is discontinued. But Antex have a new version.

Details of my old TCS (now discontinued) follow link

Hackaday review of the new TCS iron
CPC sells them
link
but they are also available elsewhere. Antex will also sell them direct, but for a lot more money :shock:

Or you can go for a Chinese made soldering station. If you do, fit a suitable bit, not the pointy type that are often supplied. Also do a safety check, specifically, check that the mains cable, plug and fuse, and that the earthing inside is okay.

I'll have a look at the links you posted later...

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
User avatar
Ast A. Moore
Rick Dangerous
Posts: 2640
Joined: Mon Nov 13, 2017 3:16 pm

Re: Capacitors

Post by Ast A. Moore »

In Bill Lumbergh’s voice, “Yeah . . . Um . . . I’m gonna have to go ahead and sort of disagree with you there, Mark.”

An 18-watt soldering iron is not going to be enough on its own, especially on thicker tracks/leads. With good technique, I’d opt for a higher-wattage iron—it melts the solder faster, so there’s less linger time.

As for these Maplins, well, they look a little dodgy. Then again, they might work all right. The advantage of any (decent) soldering station is that it can maintain the tip temperature. As the joint sucks heat away from the iron, the station will up the heat to the tip accordingly. Other than that, you can do almost any soldering job with a simple unregulated soldering iron. My main one is about 40 years old. It has a bare simple copper tip with no plating. I just tin it well and try to keep it clean. Ahem, okay, maybe not that clean, as this picture demonstrates:

Image

If you really need a decent soldering station, the Hakko FX-888D is a good choice. You can get it for around £100, although it comes with a pointed tip, which I personally never cared for. But you can get any tip you want for it.

Oh, and another tool you should definitely consider is a solder sucker.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
User avatar
1024MAK
Bugaboo
Posts: 3104
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Capacitors

Post by 1024MAK »

Yeah, I never said that the 18W iron was ideal for repair work :lol:

Both those irons available from Maplin are I believe, Chinese irons. Maplin has been selling them for many years. Not having tried one of these, I can't say how good they are. However, it has been found that some so called temperature controlled soldering stations from China are not quite what they appear to be. Instead of showing the temperature of the bit, some only display the temperature that you have set it to. Or they show the tip temperature while it is getting up to the set temperature, but as soon as it goes past the set temperature, they switch to showing the set temperature and not the bit temperature. Why does this matter? Well, with the heating element switched on, the tip temperature will overshoot the set point. The heating element will switch off. Then the tip temperature will start to fall. If you then use the iron, the tip may cool quite a lot before the element switches on again. But the temperature display will be rock steady still showing the set temperature!

The most important design feature for a soldering iron is not the Wattage. But instead it's ability to transfer heat from the bit to the joint. I have a Chinese 60W "temperature controlled soldering station" that is okay, but when I want to desolder items on PCBs that have large tracks and pads, I fit a large chisel tip to my Antex TCS iron. This does the job a lot quicker than the Chinese "temperature controlled soldering station" iron (admittedly I don't have such a large chisel tip for it).

For desoldering on the cheap, there is this desoldering iron. They are also on eBay etc. I have not however tried one.

Here you can see the difference between a small bit I use for fine work, and the large chisel tip I use when a lot if heat is needed:
Image

Here is my good old Antex TCS upgainst a younger Chinese 60W iron:-
Image

And here are two Antex tips next to the tip from the Chinese 60W iron:-
Image

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
User avatar
MonkZy
Manic Miner
Posts: 278
Joined: Thu Feb 08, 2018 1:01 pm

Re: Capacitors

Post by MonkZy »

I will check my local Maplin as they apparently have some TCS 50w irons in stock. I am not sure if they are the old type or the new push button models. I like the size of the iron and I already have a good old Antex iron stand, the other stations on offer are far more bulky.
User avatar
Guesser
Manic Miner
Posts: 639
Joined: Wed Nov 15, 2017 2:35 pm
Contact:

Re: Capacitors

Post by Guesser »

I have one of these which isn't perfect (had to replace some wiring inside after my dad left it turned on all day and burnt out the iron), but works fine for everything I've thrown at it.
https://images-na.ssl-images-amazon.com ... L1024_.jpg

It's been discontinued though and even if it hadn't I wouldn't buy anything from Maplins now.
Post Reply