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-   -   MK14 schematic revisions (https://www.vintage-radio.net/forum/showthread.php?t=145663)

SiriusHardware 3rd Jun 2020 12:28 am

Re: MK14 schematic revisions
 
One more thing, I recall that Phil down in France made himself a 3D printed frame to go over the top of his 'Martin' overlay to replicate the appearance of the anodised aluminium frame used on original MK14s.

If Phil is going to be sending a package over anyway perhaps he would be kind enough to include one of those frames, as that would be a good reference template for the hole positions and key positions.

TonyDuell 3rd Jun 2020 3:42 pm

Re: MK14 schematic revisions
 
I should still have the original SoC metal frame that I took off my MK14 to fit the Maplin keyswitches.

If I can find it I would be happy to make any measurements you need from it.

SiriusHardware 4th Jun 2020 6:49 pm

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.

Slothie 4th Jun 2020 7:15 pm

Re: MK14 schematic revisions
 
The 78xx devices have a dropout of 2v so a 7805 should be good down to 7v. I was going to solder a couple of 5A diodes in series to drop the voltage off my 9v power supplyto about 7.5v .

SiriusHardware 4th Jun 2020 9:03 pm

Re: MK14 schematic revisions
 
1 Attachment(s)
Probably worth doing, the drop has made quite a difference to the running temperature of my regulator.

Another snippet in respect of your replica PCB, I've attached a sketch of the edge connector connections looking at it from edge-on.

Timbucus 5th Jun 2020 12:23 am

Re: MK14 schematic revisions
 
Fitted a 100uF cap to the Composite out on the VDU card as I noticed a tingling touching the video lead (always noticed the same with the PSU as well) - video still works fine and thought I had beaten the funny voltages....

Refitted the 8154 and they were gone, then they came back...

Well I spent quite a bit of time before my virtual people / real beers Thursday evening, de-soldering (well destroying with a rework heater and extracting each pin) my nice red socket as I thought I had an intermittent ground connection for the 8154 which could explain the odd 7v on SIN and SENSEB if a chip was being powered through unusual pins - and flexing the 8154 seemed to reduce it to 5v - got a similar effect with my eliminator board.

Wow was that hard to get out thought I had cleaned the holes and decided to put a nice turned pin - got that trapped half way in as some holes were not quite clear and had to break it to take it back out - now fitted a standard socket and the fault is back but flexing the chip in that area has the same effect. So I thought.

Use the scope to look at the lines - nothing - meter also shows normal things - hmmm - difference my scope has a good ground... chasing ghosts again... ??? Thinking about it flexing the chip near there I could have touched pin 20 (GND)

I can only think it is to do with ground as if I touch the shield on the Composite the voltages drop the same way.... This is analogue stuff, like TV, radios and things.... not my wheelhouse....

Anyway as with my SCRUMPI - hook the ground clip from the scope to the board and all is normal... except I have lost my nice red socket and burned my fingers...

Timbucus 5th Jun 2020 12:23 am

Re: MK14 schematic revisions
 
By the way to calm nerves it is a rock stead 5.04 volts from the regulator....

SiriusHardware 5th Jun 2020 12:43 am

Re: MK14 schematic revisions
 
Your 'tingling' is due to the fact that you are using a un-earthed switched-mode supply. Try this:

-Plug the mains PSU into the mains
-Don't plug the DC power plug into anything
-Set your meter on AC Volts, select manual 200VAC range if possible
-Measure the AC voltage between the outer contact of the DC plug and any known good mains earth, such as one of the metal screw heads on a mains socket.

What voltage do you see? I'm guessing ...~115VAC. That's why you feel a tingle when you touch the 0V areas, such as the modulator casing, on your project.

The 115VAC is coming through a high voltage, low value capacitor which connects the low voltage side of the power supply to the high voltage side in PSUs like this. When you connect the scope 0V to the project, you are earthing that low current AC through the scope's mains earth so that's why the problem disappears when you connect the scope.

Now you see why I never use anything but linear supplies with sensitive old stuff like this.

If you have to use an SMPSU, select one which has a metal earth pin and has the 0V side of the DC output connected to earth. Better still, find or make a linear regulated supply to power these projects.

Timbucus 5th Jun 2020 12:50 am

Re: MK14 schematic revisions
 
Hmm funny on 200V AC only 0.6 to both my scope earth and the screw on the socket - I am assuming they are the same potential anyway as I use it as a good earth.

SiriusHardware 5th Jun 2020 12:53 am

Re: MK14 schematic revisions
 
Now I'm really baffled. ??? You had the PSU only, plugged in, mains turned on, and no AC voltage difference between the DC output and mains earth?

Timbucus 5th Jun 2020 12:55 am

Re: MK14 schematic revisions
 
29v on the composite shield... which would explain the tingling and why my 100uF CAP is doing nothing....

Timbucus 5th Jun 2020 12:56 am

Re: MK14 schematic revisions
 
...except protecting the TV from the 7v that is not there....

Timbucus 5th Jun 2020 12:56 am

Re: MK14 schematic revisions
 
... and maybe giving the effect you were seeking...

SiriusHardware 5th Jun 2020 12:59 am

Re: MK14 schematic revisions
 
If so, can you try the same thing with the TV / display, disconnect it from everything else, power it on, measure the AC voltage between the outer ring of the aerial socket and a known mains earth?

SiriusHardware 5th Jun 2020 1:01 am

Re: MK14 schematic revisions
 
Ah, cross posted - yes, there was a possibility that your unwanted 'tingle' source was the TV / display rather than the PSU.

Timbucus 5th Jun 2020 1:03 am

Re: MK14 schematic revisions
 
At least we will sleep better tonight...

SiriusHardware 5th Jun 2020 1:16 am

Re: MK14 schematic revisions
 
Yes, may be worth looking in the mains plug of that TV (if the plug is not a moulded type) and just making sure the earth screw, if the TV is earthed, has not come loose. I used to be the official PAT tester among other duties at our place and it was amazing how often the earth screws in fitted mains plugs (as opposed to moulded plugs) were found rattling loose. If the live or neutral came loose, that stopped the appliance from working and it would normally get looked at and sorted out, but a loose earth didn't cause any obvious problem and wasn't normally noticed until we came along.

Timbucus 5th Jun 2020 1:37 am

Re: MK14 schematic revisions
 
Will do - in the morning now, as it is into an 8 way extension behind this big desk for all the screens and table lamp but, I did just check the earth in the plug that the extension lead is running from - that could be really bad... it was fine.

I just realised that the scope is on that extension as well so that was a pointless test....

philoupat83 5th Jun 2020 10:27 pm

Re: MK14 schematic revisions
 
1 Attachment(s)
hello
a scan of the mk14 martin stick I scanned.
Iíll print you one of the facades I made for mine.

Slothie 5th Jun 2020 10:50 pm

Re: MK14 schematic revisions
 
1 Attachment(s)
That's a great scan! Could you measure as accurately as possible the following measurements A - F:


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