Monday March 22nd 2010
The office/workshop re-vamp is now showing positive results. One thing, one very simple thing, has made it much easier to free up precious table space even when existing projects are being worked on. I went to Tesco and bought 3 cork-faced boards which are 600mm X 400mm, each current project, including it’s power supply now sits on a board, and individual circuit boards are held securely in place with thumb-tacks. When I need space for a short while, the project board is simply lifted and moved! Obvious? Yes, it is now! (see photos at foot of post)
When going through my stash of broken/outmoded/unloved equipment I came across a little old Philips colour monitor, which had developed the annoying habit of video fading away to nothing after warming-up for about 10 minutes. In the (more-affluent) past, this sort of thing would have ended up in the dustbin double-quick, because my free time was too precious, and I would have just shelled out for a new monitor. The reason this particular monitor survived was that it had been used for a BBC Micro, and had a TTL-RGB 8-pin DIN socket and a SCART socket and sound, AND it had been hand-made!
Using some of my new-found space, I dismantled the unit and plugging in a video source from a Freeview receiver, was pleased to see picture and sound were both fine. I left it to ’soak’, and sure enough the picture dissappeared after 10 minutes or so. Aha! I let it cool down and repeated the soak test, this time watching Top Gear on it. After another 10 minutes I watched as the picture dimmed slowly, but with no loss of video content, colour or size. I checked the CRT gun and no heaters were visible. With a test-meter I followed the heater circuit lines back to the PCB and on to a winding on the LOPT (Line Output Transformer). This was a little surprising. In my previous life as a Radio/TV engineer I was used to seeing the final-anode recifier valve heater being run off the LOPT, but not the CRT heaters. I looked very closely at the substantial LOPT pins as they emerged from the PCB solder pads, and noted a needle-fine tell-tale ring just at the peak of the solder blob on 2 or 3 of the pins including the heater supply. I re-soldered all of the LOPT pins, switched on, and this time the picture stays on! A little curious, I checked the voltage across the heater and this was just over 5 volts, but naturally at the line frequency. (just over 15Khz)
So having moved the CRT heater supply from where it is normally taken – the power-supply, Philips designers have transferred it to the line-output stage – a stage that gets severely stressed anyway! A possible plus is that if the line output stage fails, then there is no point in heating the tube!
There is another down-side though. If for some reason (and our case is an example) the heater remains cold, whilst the final anode has it’s full potential applied, (about 22KV) a phenomenon often called ‘cathode-stripping’ is said to occur. I quote from an audio forum the following treatise:
‘The vacuum in a valve is not perfect, so there are copious stray gas molecules randomly floating between the anode and cathode. If an electron should be accelerated towards the anode from the cathode, there is always a chance that it will strike a gas molecule and have sufficient energy to remove an electron from that molecule, rendering it positively charged and attracted to a lower potential such as the cathode. The problem is that the ion is not merely a single electron, it’s a molecule with a nucleus composed of (heavy) neutrons having considerable momentum when it strikes the cathode emissive surface.
In general, that means that we shouldn’t allow the anode to become positive before the cathode is able to develop a space charge.’
Whether any of this is true or not, in my opinion this is a bad design.
Fresh with my success with the monitor, I solved another problem.
The desk which is normally behind me, has frequent use for PCB production, having the light-box, guillotine etc. It has never been much use for the running/development of projects because there simply isn’t space for a PC. Now, the only requirement any of my PIC projects have is access to a USB port, and ultimately to a directory on one of the development machines, where the final binary (hex file) is deposited.
My old Toshiba Satellite Pro 480CDT long since developed DPD (Dramatic Peripheral Death) and both it’s CD and floppy disk became unusable. However, it has Windows 98SE installed, and there is a solitary USB 1.1 port on the rear, and I fitted a PCMCIA 10Mbs Ethernet card to it back in 1998. I downloaded the Win98SE PNY Attache USB driver from PNY’s site, and after fitting an old Belkin USB 6-port hub, now have a 2Gb flash memory. This together with an FTDI-sourced Win98SE driver for their RS232-TTL USB lead, and access to my network, means that another test area can be brought rapidly into use.
Note that so far, I haven’t introduced anything new into my workspace, but what was junk, lying about unused, is now being put to good use.
Since my experiences last week when I had a day stolen from me, I had made up my mind to stop taking both the Morphine and Diazepam. However, I’ve been obliged to take a couple of the MST tablets some nights, otherwise I would not have slept. Despite the severe physical discomfort, my mood has been surprisingly upbeat. This is probably due in part to the mild euphoria that the small-dose morphine induces, and partly due to being rather pleased that skills which I developed in my late teens, can still be put to good use, in bringing old ’stuff’ back into working order. Whatever it is, I am glad for it, as there is still plenty to do in the workshop, including fixing an old dual-channel ’scope with an annoying intermittent fault on one channel.
Now, put down that cup of Horlicks, and get back in the saddle – you’ll like it!
JWB 22 March 2010