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SiriusHardware · 4th Jun 2020, 6:49 pm

Tony, thanks for that offer, we'll see if Slothie would like to take that up at some point. There could be no better reference than one of the original keypad frames.

A couple of updates regarding the current version (revision 1.2) of Slothie's issue VI PCB - a few posts ago I said mine wasn't starting too reliably after I had fitted a (known working) 8154 RAM/IO IC, however I have now fitted a decent snug-fitting power socket and taken the value of the supply input capacitor down from 4700uF to just 470uF. After these changes the machine is back to coming up with the proper '0000 00' prompt every single time I power it up. In my case, the 14 positions for decoupling capacitors are still fully populated with 0.1uF parts.

Regarding Tim's alarming statement that he appears to have around 7V on some of the pins of the 8154 when the VDU is connected, I have just run through the default / after reset voltages on all of the pins on the edge connector on mine and I see nothing exceeding 5V. There is regulated +5V coming out on my 'VCC' connection on the underside of the PCB, as there should be.

The only ways I can imagine having 7V on the VDU are:-

-VDU being powered from 8V through separate regulator, which has lost its GND connection or is faulty. As far as I know Tim is just powering it from the MK14.

-It may be that the whole MK14 is running on 7V due to some problem with the main regulator. I really hope not.

It's worth pointing out that the LM340 is a 'family' of regulators which are available in a range of fixed voltages just like the 78xx series - they always have the 'LM340' part number but that part number alone does not signify a 5V regulator, there are further markings on the device to say which fixed voltage it actually is. That's why I generally stick with the 78xx series, the voltage is part of the 'big' main IC number.

The regulator in my PCB is an ST-branded 7805CV - the PSU I'm using as input is an old Uniross 'Uni 1200R' which is a 1.2A regulated multi-voltage linear supply. Originally I had the MK14 running on 9V from this supply and the heatsink was getting uncomfortably warm so I tried dropping to the 7.5V output (which is actually 7.6V off load) and to my surprise the output from the regulator is still 5.06V. I've had it running for a while now and it seems no worse than fairly warm.

4th Jun 2020, 6:49 pm	#503
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,586	Re: MK14 schematic revisions Tony, thanks for that offer, we'll see if Slothie would like to take that up at some point. There could be no better reference than one of the original keypad frames. A couple of updates regarding the current version (revision 1.2) of Slothie's issue VI PCB - a few posts ago I said mine wasn't starting too reliably after I had fitted a (known working) 8154 RAM/IO IC, however I have now fitted a decent snug-fitting power socket and taken the value of the supply input capacitor down from 4700uF to just 470uF. After these changes the machine is back to coming up with the proper '0000 00' prompt every single time I power it up. In my case, the 14 positions for decoupling capacitors are still fully populated with 0.1uF parts. Regarding Tim's alarming statement that he appears to have around 7V on some of the pins of the 8154 when the VDU is connected, I have just run through the default / after reset voltages on all of the pins on the edge connector on mine and I see nothing exceeding 5V. There is regulated +5V coming out on my 'VCC' connection on the underside of the PCB, as there should be. The only ways I can imagine having 7V on the VDU are:- -VDU being powered from 8V through separate regulator, which has lost its GND connection or is faulty. As far as I know Tim is just powering it from the MK14. -It may be that the whole MK14 is running on 7V due to some problem with the main regulator. I really hope not. It's worth pointing out that the LM340 is a 'family' of regulators which are available in a range of fixed voltages just like the 78xx series - they always have the 'LM340' part number but that part number alone does not signify a 5V regulator, there are further markings on the device to say which fixed voltage it actually is. That's why I generally stick with the 78xx series, the voltage is part of the 'big' main IC number. The regulator in my PCB is an ST-branded 7805CV - the PSU I'm using as input is an old Uniross 'Uni 1200R' which is a 1.2A regulated multi-voltage linear supply. Originally I had the MK14 running on 9V from this supply and the heatsink was getting uncomfortably warm so I tried dropping to the 7.5V output (which is actually 7.6V off load) and to my surprise the output from the regulator is still 5.06V. I've had it running for a while now and it seems no worse than fairly warm.