MK14 bad regulator & PSU tip

[IanB_63]

I'm building a replica MK14 to go with my replica VDU card and I recently received an 8060 SC/MP and 8154 so I thought I'd test them in my original MK14.
I dusted that off after some time in storage and powered it up and it worked OK but I checked the power rail and it read 5.8V instead of 5V so thankfully I caught that before anything bad happened.

I've replaced the regulator and it's now back to 4.9V but one thing I started doing some time ago was powering anything that has it's own 5V regulator with a switched mode 7.5V power supply which is about the minimum you can use due to the voltage drop in the regulator.
Using a lower voltage like that results in lower heat and even if the regulator fails, most components might survive the higher voltage and maybe it saved my MK14 this time.

Of course you can modify equipment to run directly from a 5V switched mode psu but I have found that a lot of older stuff doesn't like the noise on the power rails put out by switched mode supplies.
e.g. My UK101 won't even boot when connected directly to a 5V switched mode PSU but will happily work on a 7.5V one via it's on board regulator which runs cool enough to touch compared with the original unregulated transformer.

[Slothie]

I use a 7.5v wall-wart on my MK14 too, to minimise heating on the regulator which still gets toasty with a 21deg/W heatsing I have on it. Mine is my Issue VI which has a lot more places for fitting decoupling caps so its more noise tolerant if you directly power from 5v, although my first prototype had the same decoupling as the SoC issue V and worked fine despite incredibly noisy power rails which surprised me somewhat - perhaps I was lucky with the component combinations I had.

I did some testing with 9v supply and that was about the limit without a bigger heatsink. The heatsink got so hot it made it hurt to touch with measured temperatures on the tab in the 80s. I doubt it would last too long being run like that.

[SiriusHardware]

I use an old 'Uniross' multi voltage LINEAR power supply which has 7.5V as one of the output voltage options, so that the onboard regulator runs as cool as possible.

There is a problem with nearly all un-earthed SMPUs in so far as they usually have a high voltage, low value capacitor between the low voltage output GND and one side of the mains input circuit as an EMC measure. If you measure (AC Volts) between true mains earth and the outer barrel of the DC power connector you will generally find between 80V-110V AC - very low current, but if you touch it you quite often feel an unpleasant tingle.

While harmless to humans this 'tingle' is still 80V-100V as far as antique digital ICs are concerned. I don't allow SMPSUs anywhere near any of my old digital stuff, and when interfacing between the MK14 and anything else which is independently powered, possibly by an SMPSU, I tend to use optocouplers so there is no direct connection between the two systems.

[Phil__G]

Most of my boards have a serial connection so are USB powered from either the laptop thats acting as a terminal or a phone powerbank for standalone... and although I have a few proper bench PSUs, for convenience I find you cant beat batteries.
At the retrofest all my stuff ran both days on 'drill pack' 18650 cells without charging, 2S is spot on for a 7805 input. Sometimes when the workshop is cold I'll bring the board I'm using into the house, with batteries its so easy to grab everything I need without pulling PSU power cords and carrying heavy PSUs around.
I know it sounds a naff solution but for me it really is so convenient. And no switching ripple!

[ortek_service]

Originally Posted by SiriusHardware

>>

There is a problem with nearly all un-earthed SMPUs in so far as they usually have a high voltage, low value capacitor between the low voltage output GND and one side of the mains input circuit as an EMC measure. If you measure (AC Volts) between true mains earth and the outer barrel of the DC power connector you will generally find between 80V-110V AC - very low current, but if you touch it you quite often feel an unpleasant tingle.

While harmless to humans this 'tingle' is still 80V-100V as far as antique digital ICs are concerned. I don't allow SMPSUs anywhere near any of my old digital stuff, and when interfacing between the MK14 and anything else which is independently powered, possibly by an SMPSU, I tend to use optocouplers so there is no direct connection between the two systems.

Yes, we encountered a problem with these 'Double-Isolated' Non-earthed Laptop PSU around 20years ago, where the 'half-mains' biased DC-side 'ground' resulted in that floating at a high voltage that could damage P-Ch MOSFET's in a test jig they were being use with. So they then only used Laptops with 3-pin power connector on the mains PSU input, although I'm sure some additional clamp diodes on the FET's would have protected them (I think they were BSS84's, without much gate-source protection).

More recently, I found on MS Surface Laptop PSU's that they seem to have biased the O/P 'Ground' to just one pin of its Fig.8 mains-input connector. As depending which way round you plug the mains-lead into it , results in whether you get an uncomfortable tingle from its (anodised metal!) case whilst using it.And I'd mainly been using it on internal battery, until I discovered that reversing the plug into PSU's mains-input socket stopped the problem.

[ortek_service]

Originally Posted by IanB_63

>>
I dusted that off after some time in storage and powered it up and it worked OK but I checked the power rail and it read 5.8V instead of 5V so thankfully I caught that before anything bad happened.
>>

Of course you can modify equipment to run directly from a 5V switched mode psu but I have found that a lot of older stuff doesn't like the noise on the power rails put out by switched mode supplies.
e.g. My UK101 won't even boot when connected directly to a 5V switched mode PSU but will happily work on a 7.5V one via it's on board regulator which runs cool enough to touch compared with the original unregulated transformer.

It's odd for a (Presumably) 7805 to fail like that, with voltage going a bit high rather than either having nothing from it or output at same voltage as input due to it going short-circuit In-to-Out (Luckily, not too common!).

I wouldn't have thought the amount of noise on most SMPSU's would be high enough to upset digital circuity that should be fairly-tolerant to even a few 100mV's of ripple etc.
And I can't see why an SMPSU wouldn't work in principle on a UK101. However, it could be an issue with the SMPSU you used, that either it wouldn't work at low-loads (Like many early PC PSU's, with the original AT having a dummy load resistor in a metal 'Fire' cage, if you didn't have the HDD fitted) or that it wasn't powerful enough and was tripping on initial start-up surge current etc.
- I assume you disconnected on-board regulator, to prevent feeding supply to its output that may damage these with no input voltage, although it won't usually sink any current from output-side so shouldn't be causing a PSU overload.

[IanB_63]

Originally Posted by ortek_service

It's odd for a (Presumably) 7805 to fail like that, with voltage going a bit high rather than either having nothing from it or output at same voltage as input due to it going short-circuit In-to-Out (Luckily, not too common!).

Yes, I've not seen that before either. I may do some tests with it to see how it behaves with other input voltages.

I did actually damage my MK14 not long after building in by shorting the input and output of the regulator and had to return it to SoC for repair.
The return paperwork indicated that only the PROMS and a couple of TTL chips had actually failed, not the CPU or RAM so most of my MK14 has already survived excess voltage.
I've attached a photo of the repair slip that came back with it.

Originally Posted by ortek_service

And I can't see why an SMPSU wouldn't work in principle on a UK101. However, it could be an issue with the SMPSU you used, that either it wouldn't work at low-loads (Like many early PC PSU's, with the original AT having a dummy load resistor in a metal 'Fire' cage, if you didn't have the HDD fitted) or that it wasn't powerful enough and was tripping on initial start-up surge current etc.

It was a Meanwell one from RS which was rated for the UK101 load.
However I had severely modified my UK101 back in the 80s with many trailing wires and it would not boot reliably from that psu so I assume my UK101 was too marginal due to all the mods and was more susceptible to the switched mode noise. (The voltage was stable at 5V but noise was visible on a scope)
See attached photos which show the mods and PSU (now removed).

Originally Posted by ortek_service

- I assume you disconnected on-board regulator, to prevent feeding supply to its output that may damage these with no input voltage, although it won't usually sink any current from output-side so shouldn't be causing a PSU overload.

I Injected it after the on board supply with the 5V fuse removed which isolated it (The PSU was not connected in the photo)

[Slothie]

That's a well loved and modified UK101!

I remember seeing them and trying to get my dad to buy one "for the family", or the similar Ohio Superboard. He was really unsure about us being able to assemble it and in the end he decided to fund the purchase of a Commodore PET if I paid him back for half - So I worked my school holidays and the first university break when he said I'd paid enough, about a quarter I recall. But back in 1980 that was a lot of money. He argued that the extra cost was worth not haviing to make a case, get a TV and make it into a monitor, and it looked a bit better sitting in the living room - which it did for at most a few months before being relocated to a certain persons bedroom....

I think the computers of that era in the UK had a huge impact on computing in the UK and I think they were the reason the British software industry became so infuential into the 90s despite arguably the USA taking the lead in the hardware front. A lot of US companies actually did their software development in Britain and Europe.

[IanB_63]

Originally Posted by Slothie

That's a well loved and modified UK101!

Yes I really enjoyed working on that machine, the mods included:

Switchable screen resolution (48x16 or 48x32)
Switchable monitor ROMs
Extra utility ROMs
Parallel port interface used for eprom programming
Creed 7b teleprinter interface including ROM based driver
Noise free display mod which I designed.

In common with some other early computers like the Superboard, Nascom, TRS80 Model 1 and Atom, the UK101 had noise on the screen whenever the screen RAM was written to by the CPU due to bus conflicts between the CPU and the screen DMA
I worked out a way to synchronise the CPU and screen accesses so that there wasn't any noise or slowdown which is the same technique used by other 6502 computers such as the Apple II and BBC micro.

The bizzare thing is that it didn't even require any extra chips, just a minor re-arrangement of the signals so it could have been like that from the start.

That mod got published in the December 1981 issue of PC World. See here on page 147:

https://worldradiohistory.com/UK/Per...1-12-S-OCR.pdf

Sadly the Creed 7B teleprinter went to the scrapyard many years ago but I'd like to get my hands on one again to resurrect that interface

[ortek_service]

Back in the early 80's, I recall seeing an Engineer I knew having a UK101 kit running, that he'd assembled and remember the large PCB this had. Not sure if it was before he got a TI 99/4a, but he did have a BBC Model B later on that my Brother bought from him and still has.

We've been using Meanwell SMPSU modules recently, and not found any problems with these. And the ones with a buily-in terminal block adaptor PCB-base are quite handy for bolting inside a larger chassis cases. Plus they do a range of power-ratings in the same size case, with the max power one only being a few pounds more.in the ones around 60-90W? (Were using these for a 12V etc. output at several Amps, but probably rather much-more powerful than would be needed for a 5V supply)

And if you are getting a static 5.0V from it on a DMM etc, then it would seem there's something happening further down the line on the UK101 OCB, so might have been worth 'scoping there or adding some large capacitance (especially if removing the 5V fuse isolates some on-board capacitance on o.p of its regulator) in case of surges and voltage-drops on the wires from the external PSU. I wouldn't expect there to be much noise radiated / conducted from the SMPSU to be picked-up that much on all the extra mods wiring etc