Category Archives: Projects

Vacuum Tube Stash

Today I received a delivery of a box of vacuum tubes, all primarily TV valves from the 1960s & 1970s. Was hoping for some radio valves, but no joy. At least I have these for future projects or I will sell them on eBay over the coming months; or maybe I’ll use a few of these in a couple of experiments. What to you think, anyone got some ideas?


  • EF9 – RF Pentode
  • EL36 – Pentode Power Output
  • UCH21 x 2 – Triode Heptode
  • N308 – ??
  • PL500 – TV Pentode Line Output
  • PL36 – TV Output Pentode
  • EV87 – ??
  • ELB21 – ??

Radio Haltron

  • U25 – ??
  • X78 – ??


  • U191 x 3 TV Efficiency Line Diode


  • 6SA7GT – Pentode Power Output


  • 35Z5GT – Half-Wave Rectifier

KT (Electron)

  • KY4 – ??

KR Tubes

  • ISSK7 – ??

I suppose the first step is to try to find out something about the ones indicated with “??”. Some of these are models that I haven’t heard of before, so there is a little bit of homework to do over the next week or so. 

Electronic Repair, we need to change.

When thinking about electronics, what do you think about? Is it your laptop, your mobile phone, your fitness tracker or a home appliance? What happens when it breaks, what do you do? Do you bring it to a shop that can repair it? Can you find a shop that is willing to repair your device, or do you get told, it’s not designed to be repaired?

Have you had any of these questions?What are your options? Well, there was a time when on almost every street, there were repair shops where you could bring your faulty item in for repair. While, yes it could be expensive to get the item repaired, it could be repaired. The same is true today, but as a society we have become lazy and impatient. We want our stuff fixed instantly and we want it cheap! Oh yes, we cannot be without our beloved gadgets or appliances; we feel broken or incomplete. When we look into this further we have to ask the question, why? Why have we become so dependent on these devices? What is the social reasoning behind this?

One reason behind this is a technical addiction; yes the gadget may get us online for our social fix or endorphin rush when looking at the latest YouTube video, but at what cost?

I’m not going to discuss the pros and cons of being online, that’s a topic for others far more smarter than me to discuss. If our lives are going to be so dependent on being online, what are we going to do to ensure we’re able to get our fix, or make one gadget talk to another gadget? More importantly what are we going to do when it breaks? How important is it to us, really? If it’s that important, have we considered a backup plan? What to do when our primary gadget is no longer working?

If it’s that important that we have a backup, who will repair our beloved gadget? Can it be repaired and at what costs?

Personally I believe all items can be repaired to some level. This depends on the manufacture of the product, some vendors make it easy, some not so easy. While others will deliberately go out of their way to ensure the item cannot be repaired.

If you look at your car or your washing machine, both of these can be considered as gadgets and both can be repaired. Why is this so? The vendors understand that these gadgets will need to be maintained to keep operating. They make parts available for services centers to repair these gadgets. They also provide training to humans with a specific skill set, so they can perform these repairs.

Back in the early days of radio and TV it was very common for a family to purchase a kit form of one of these appliances, take it home, assemble it and get hundreds or sometimes thousands of hours of use before it needed a small repair; after which it would keep going until the next service or repair. This was very common up until the age of consumerism, where we were told we needed the latest gadget and needed it now and our previous gadget was obsolete. Or was it?

Yes, technology does evolve and move on. We have seen the disappearance of analogue TV, and very few new radio stations start broadcasting anymore. We now see digital TV, digital Radio, Internet Streaming of these services. These devices although more complicated than their earlier brethren can be repaired.

While repairing a vintage radio circa 1960’s, I discovered that the vendor had provided all the repair information inside a door on the radio. Why did they do this? It was because they knew, try as they may, some part would eventually fail and they wanted to give the customer the choice to perform the repair themselves. Yes, we were given a choice, repair it or take it to a shop who would repair it for us. As consumers we understood the value of these repairs and we had an idea how long it should take.

What about computers, I hear you say? Well recently I repaired a vintage BBC B Micro circa 1980’s, and again the vendor was able to provide the repair manual to allow these repairs to take place. This was common for most computers made in the ‘80s and ‘90s. There are still some computers that are repaired today for specialized industries.

In the 1990s, with the advancement in computers, they became modular, so you could swap out faulty components for working ones. Your faulty components could be sold/donated to companies who would perform these repairs. If you look at the more modern video card of today, which can cost thousands of euro, these are considered repairable by some companies.

Nowadays when our mobile phone breaks, we are told that it is not repairable. If you take five minutes and search on YouTube, I’m sure you will find someone who has performed a repair for the same fault that you identified with your gadget. So what’s wrong here?

As a society we have lost the skills to repair what we own. Some of these skills make take time to develop, but the more you do it, the better you become.

All I can say is to start small and work upwards, don’t try to repair your sister’s iPhone as your first repair attempt, unless you feel comfortable.

Let’s get back to being able to perform some basic repairs, instead of throwing that laptop or mobile phone in the bin. There may be someone out there who is prepared to put their time into repaiting that gadget for a charity. If you don’t want it, do the right thing and find it a new home. Let’s also think about the environment, just because your laptop doesn’t play high definition movies, it might be just right for someone learning about computers, who isn’t interested in multi-media.

If you can keep the gadget in circulation its not going into landfill; I’ve seen many laptops being thrown out with their batteries, containing lead and lithium going into landfill. What happens after, is that semi-state authorities are spending tens of thousands trying to remove these harmful materials from the waste. Where are they getting the money from? Us, we are the ones paying for this through our taxes; but if we all take a little bit of responsibility and think about what we are disposing, separate the harmful from the safe materials and the recyclable from the non-recyclable materials, we will be doing our little bit to help ourselves, our children and the environment.

And what of time and money? Your time and your money…ask yourself, are you one who is willing to spend cash on letting someone else perform the repair, and if so, how much is their time worth to them; if they spend four hours repairing your device are you willing to cover that cost? Do you value the service that this person is doing, or do you just want it cheap and don’t care about the cost? Ask yourself the same question if the situation was reversed. How would you feel?

Being a person who performs select repairs, I am choosy with my clients, as I have been on the receiving side of customers who would not pay for repairs, although I had spent many hours and purchased parts for their repair. It’s very disheartening as a technician, who takes pride in their work, not to be respected. It’s one reason why so many repair shops just close up and move on to other type of work. Yes, I do accept there are some cowboys out there, but in my experience they are only in the low percentages. It’s the consumers who have to step up and take responsibility for their actions, and value the work.

One way that consumers can take responsibility is to perform some basic repairs themselves. This, I have found, gives them a better understanding of what can go wrong with their gadget. This helps in two ways: firstly, when they take it to the shop, they will quickly identify the bullshit or when they are told mistruths about the repair; secondly, they will gain Insight into the effort required to repair their gadget.

One piece of advice I can give consumers when handing in their gadget for repair, is to be truthful with the technician. Tell them everything that happened, no matter how small, as the smallest piece of information may be all that is needed to change the repair time from six hours to one hour.

Rant over, hopefully this has given you some food for thought. It’s not everyone who wants to repair, but at least try to replace a plug on the next gadget you buy. Take the old plug off and keep it for next one (if not molded).

DIY Isolation Transformer

One piece of test equipment that I had been missing for a while was a bench isolation transformer. This would have allowed me to perform tests and repairs that would protect me and the equipment being tested.

After looking around, I found it very difficult to find a device that met my requirements. Everything I found was too expensive or did not have what I wanted – a device with both voltage and current meters, with fuses on both the primary and secondary and an auto current trip on the input.

In the end I decided to build my own unit.

The core of the system was two identical transformers, wired back to back, to provide the isolation. These were sourced from some faulty UPSs, sourced on eBay. They were missing their batteries, but were working. I scrapped the UPSs and removed the parts that were worth keeping.

I recovered the current trip switch from an old Intermec 3400 Barcode Printer with a damaged chassis.

I was able to get a combination voltage/current/power meter on eBay, with a 6 week delivery from China.

The last few parts were a case and some IEC power connectors for both the input and the output, as well as fuses and fuse holders that I had in stock for many years.

The secondaries of the transformers were wired back to back – this provided dual isolation via the windings on both transformers. So when I tested it and connected the mains to the primary of the first transformer, I measured 230v AC on the primary of the second transformer. This proved that my theory worked, yay!

The meter gave me an indication when the device under test drew too much current, as well as showing the power draw. You may be thinking why did it show the current and voltage when the device was connected to the mains?

Well, I planned to connect the primary side of the isolation transformer into the output stage of a variac. A variac is a transformer with a variable output.

This allowed me to slowly increase the voltage to the device under test. I could see if the current draw was excessive and identify faulty equipment without having to connect the device to the mains and possibly blowing components, causing even  more damage.

Note:Variacs are very handy devices that can be used to recondition old power supplies that may not have been used for a long period of time.

These two units coupled together provided me with a good environment to test devices for repair, while protecting myself and the customers device being repaired.

A female IEC connecter is used for the power output. This allowed me to have a selection of different leads that could plug in here.

Here is a photo of the Isolation Transformer connected to a Frequency Inverter. You can see that the meter is showing the output voltage, current that was being drawn as well as the power consumed.

The back of the unit just has a power input connection and an input fuse for protection.

Retro Computer Build Part 6 – PLD or Not 2 PLD

Having decided that I was going to use an Atmel ATF22LV10C PLD to manage the address decoding for this project, I purchased a couple of these IC’s to try out. I was visited by Murphy’s Law. I found out that the new programmer wouldn’t program these ICs (although it listed these as supported ICs). After searching some forums, I found that I wasn’t the only person having a problem programming these ICs on this model of programmer. It turned out that the problem was down to the flash memory in the Atmel PLD. It required some special timing to program the ICs, which this programmer did not support.

So, my second attempt into this problem was to purchase some National GAL22V10 ICs (which my programmer said it also supported), but did I believe them? After testing them on my programmer and finding that they also didn’t work, I was getting very frustrated (serves me right for buying a cheap Chinese one). I really didn’t  want to purchase a new programmer at that point, considering that it did work for other ICs.

Just to prove to myself that I was not losing it and that my grey matter was not shutting down, I tried some Lattice and National GAL16V8 ICs that I had in stock, and these programmed with no issue. I needed to rethink the address decoding, as I could use the  working parts that I had to hand.

I was not happy to add two additional ICs into the project where I wanted to use one. Could I remove another IC to keep the chip count the same as before? After looking at the schematic it was simple to see that the /RE & /WE signal lines could be incorporated into one of the GALs.

If the first GAL contained the logic for these decoder lines (/RE & /WE) as well as the address decoding for the memory map, the second GAL could be used for the address decoding of the I/O.

This all seemed like a good plan, but NO, there was one sneaky problem that had now crept back in. The current consumption then increased dramatically in the system. Before we even measured this, I estimated that these GALs would burn about 40-80mA each (80 – 160mA in total). This blew the power budget that I wanted as a limit for the system.

As low power consumption was a key prerequisite for this project and as I then could not use the PLDs, I had to look back to using basic logic devices for managing the address decoding. It looked as if we would have to use some 74HC138s and 74HC139s. In the end the chip count increased.

That was a month’s work and planning that I wasn’t getting back.

Now to refocus my attention to the address decoding, using standard logic ICs. 

All the memory devices require an active low control line and A15 will be high when the CPU is reset/starts. We can just use an inverter to get an active low signal from the A15 line which is high when the CPU starts. This is done by feeding A15 into a NAND gate 74HC00 (U4A), in an inverter configuration.

This active low A15 signal with A14 and A13, feed into a two to four line decoder 74HC139 (U5A), which allows us to cut the upper 32K memory space into 4 x 8K pages. Three of these pages can be used for our ROM memory which stores our software. The last 8K page can be used for future expansion, video memory and I/O.

This I/O control line can be fead into another NAND gate 74HC00 (U4B), with address line A9. This gives us a memory space of 512 bytes – it make better use of the memory space. We can feed this control line with A8 and A7 through A5, into a three to eight line decoder – 74HC138 (U10). This gives us 8 x 32 byte address blocks, which can be used for I/O devices such as RTC, UARTS, PIAs.

As we can see there are some unused memory spaces in the revised memory map. These can be used by other interfaces in the future.