On Mon, 9 May 2016, Adrian Graham wrote: No problem, I've often noticed a couple of things you should know: 1. Many times the pots are plastic. Be careful when you first "break" them loose, I've ruined a few in Trinitrons which were stuck so hard the thing kind of fell apart when twisting it. 2. Make darn sure you discharge the tube before you work on it. I use a flathead screwdriver with a 12g wire attached to it that runs down to grounded pipe. They will zap the living snot out of you if you don't. Just tie off the wire to a grounded source then stick the screwdriver underneath what looks like a little suction pad on the tube. You'll usually hear it discharge with a POP! If you already knew this, sorry, but I wanted to be sure you didn't get shocked. Messing with the CRT logic board is a great way to take a few thousand volts. :-) 3. Those CRT logic boards are usually replaceable all as a piece. Also, as someone already stated, they often have caps on them which impact the vertical sweep. If those go out, adjusting the pots does nothing or makes things look ruined/jittery. Some people can re-cap them, I've only done that once with another smarter guy watching me do it. I just usually replace the board if possible. Maybe you just hadn't needed to before. I used to work on monitors a long time ago, but I was never any good with electronics because I was never trained or self-motivated enough to learn digital theory or fight a logic analyzer or Verilog with any acumen. It's another hobby I'll have another go at one of these days. Today, I can mostly fix my guitar amps (old and analog), so that's good enough (for now). I learned those skills from doing electronics kit projects when I was a pre-teen and some extra analog tricks with a scope that I learned from an old hippie. I'll probably use the same sort of methods (but maybe start with some Arduino gear instead) the next time I try to pick up some more EE skills. It's hard not to be intimidated by all these guys on the list with mad electronics chops (Maciej, Eric, Fred and others are incredible; they know soooo much). -Swift

On 09/05/2016 22:21, "Mouse" <mouse at Rodents-Montreal.ORG> wrote: Not correct so much as to say what's in the official DEC docs for working on these things where they show a line drawing of a standard screwdriver with a ground wire attached. My own discharge tool is a big plastic handled flat blade with ground wire and croc clip, long may it make me nervous when using it. -- Adrian/Witchy Binary Dinosaurs creator/curator Www.binarydinosaurs.co.uk - the UK's biggest private home computer collection?

On 2016-May-09, at 5:20 PM, Tapley, Mark wrote: Resistors to limit the initial discharge current are a good practice for all those reasons, I would additionally suggest the place to put the 'ground' clip is on the ground straps over the CRT bell, or the ground wire coming from those straps, so the discharge currents are localised to the CRT. The CRT doesn't discharge to 'earth' or 'ground' (earth/ground is not some inherent vast expanse of charge sink), it discharges 'to' the outside of the bell. The bell forms a capacitor: one plate is a coating on the inside, one plate a coating on the outside, and the glass the dielectric. (Pedantically, at discharge, electrons flow from the outside plate of the bell to the inside plate.) Another good practice is to use several carbon-composition resistors in series, to increase the physical path length to avoid arcing across/within resistor(s). 1 or 2W resistors are all the better, not for power-handling but for the greater distance between the leads. Common film resistors with an internal spiral resistive element present a very short path for arcs to jump over.

Actually pencil lead is suprisingly low resistance. Try sharpening a pencil at both ends and measuring it. When I was younger, I carefully cut and split a pencil apart lengthways. This left me with the lead in a trough of wood. A wire wrapped firmly round one end and a wire wrapped round that I could slide along made a reasonable varialble resistor. I can remember using it with a battery (probably about 4.5V to 6V) and a torch (flashlight) bulb as a dimmer, so the resistance can't be that high. The resistance does depend on the grade of pencil. The more 'black' it is. the lower the resistance. So a 6B has low restance, a 6H high, Of course we are all talking about rods of graphite, not the metal lead, right? -tony

[VT101 with vertical linearity problems] In general it is a bad idea to cure faults by adjustments. Unless somebody has been twiddling, these adjustments only drift if some other component is failing. They are there to initially set the circuit up for the components that are used, or in some cases, particularly in delta-gun colour CRTs, to correct for external conditions, such as magnetic fields. My guess is a capacitor is failing. So far you are still within the range of the adjusment, but it may well get worse. I don't know which video board you have (there are at least 3 different ones), but if it's the one on pages 54-58 of the VT100 printset on bitsavers (the VT100 and VT101 use the same video board), then check C318 and C319 for a start. -tony

[Your VT101 video board] VT1xx's and any could appear in any model terminal. -tony

Forgot to mention... The keyboard itself is the same on the VT100, VT101, VT102, etc. The interface circuit on the main logic board is much the same too, so the info in the technical manual is of relevance. If you can stick one of my hand-drawn diagrams, I have reverse-enegineered the VT101 logic board and PSU board. Keyboard and video you can get from the VT100 printset, and I couldn't find a mains input bracket to trace out (maybe sometime) but that is pretty simple anyway. If you want the VT101 board diagrams just ask me nicely. -tony

On Tue, May 10, 2016 at 3:26 PM, Adrian Graham <witchy at binarydinosaurs.co.uk> wrote: Yeah... I had that exact problem last year with a VT100. I'd say that of several DEC terminals I've repaired, the two most common things were the EIA receivers (static/lightning, most likely) and the power switch. Distant third is video problems. -ethan