On 24 Sep 2008 at 20:16, Gordon JC Pearce MM3YEQ wrote: I'd be sore tempted just to "eyeball" the adjustments, myself. I imagine a photometer is essential if you care about things such as color accuracy, but perhaps just checking white balance may be close enough. Cheers, Chuck

Well, that's the problem. According to one of the HP manuals, misadjustment can cause damage. Of course that damage might not be too serious so it's probably worth giving it a go. -tony

Yes, that was my thought. Excessive beam current could damage the CRT, the flyback transformer or bits on the video board. But I wonder just what 'excessive' is. I suspect I would notice it was far too bright long before anything was damaged. Now that I don;t know. I doubt the previous owner did anything, but I don't know about before that. Equally, I don't know if these 3 resistors were ever the right values, it's possible they failed in service and nothing was adjusted (in which case the adjustments will be very nearly correct if I put good resistors in), or if the board was twraked when the user complained about poor colour performance or something. Or maybe the board as originally set up for the incorrect-value resistors that are now there. Equally, I don;t know if the CRT has lost emission over time, it may well be that I could safely turn things up a bit. -tony

Gordon JC Pearce MM3YEQ wrote: Why bother with the Mk. 1 Eyeball if one has available a Mk. 2 Eyeball? Peace... Sridhar

Tony Duell wrote: I haven't seen the circuit, but I'm guessing you'll find some handy resistor around the cathode to sample the voltage across. Obviously this will be somewhat high due to the grid currents, but my gut feeling is that they won't be significant compared to the anode current. Gordon

Well, I certainly wouldn't use any form of electronic meter (digital, amplified analogue, etc) for this. I was thinking of using a plain moving coil meter, possibly with a shunt resistor. Hmm, possible I suppose. Maybe I chould encase it in a metal guard electrode, connected to a 25kV supply. Or maybe not... More seriously, I haev come across moving coil meters being used at quite high voltages wrt ground (not as high as this, but 10kV or so). They were mounted several inches behind a thick perspex window -- this was not a 'lash up'. I would be prepared to try it, given a fairly inexpensive meter movement. For those wondering what the current state of this monitor is, I've cleaned up all the case parts and put it back together. It still works (so I didn't do any damage taking it apart), although there are still red fringes to the right of (say) white objects. I am not sure if this is a convergence problem or due to the change in video amplifier response due to that open-circuit 12.4k resistor which I've not replaced yet (I wanted to make sure the monitor still worked after I'd finished pulling it about before changing anything). -tony

Exactly.I've not looked carefully enough to see this (I think a magnifier is needed), but I don't think the complementary colour fringes are there, which implies to me that it's an amplifier problem. That's exactly what I intend to do. The video board was clearly intended to ahve 3 indetical amplifier circuits on it, the resistors in question should all be 12.4k. My experience is that if you find something wrong, it's best to correct it even if it appears to make things worse. Sure. -tony

The red fringing is noticeable without a magnifier. Black-on-white text isparticularly bad -- a 'l' would almost appear as red on white. But I don;t nitce any cyan fringing in such text. I would not normally bother to correct a problem that was only visible with a magnifier, but it's useful to use one to attempt to identify what the problem really is. [...] Nrmally I'd agree with you (adter all 'colour temperature' was a user adjustment on a lot of PC monitors). It becomes critical if you need accurate colour reproduction (e.g. for TV studio work). But what worries me is a comment in the HP service manual. It basically warns you not to twiddle these adjustments (the same sort of message is printed on the metal cover over the monitor chassis). And that misadjustment can cause various problems : Visible flyback lines Missing colours Incorrect colours (compared to another 9836C) Significantly decreased reliability Now, the first 2 are very ovbious, and I'll not have problems with that. The third doesn't bother me at all, since I only have one HP9836C. But the last does worry me.. Particularly if the part that fails is the flyback transformer. The CRT seems to be a standard one (I can find no data on it, but some CRT testers/boosters list it as one of the CRTs they can handle). I would have a chance of finding a replacement. The transistors on the video board are all standard, so I could find replacements. But HP never supported component-level repair on these machines, and the flyback transformer was only avaialbe as the complete deflection PCB. And that's long unobtainable. I don't if any other HP monitor used the same transformer -- the horizontal scan rate is unusual -- about 25kHz -- but other HPs used the same sort of rates. -tony

Alas, yes.... Very few HP classic desktop computer service manuals contain scheamtics. You might, if you are lucky, get PSU schemaitcs, but not for the processor, etc, sections. I am, of course, trying to remedy this... As an aside, I feel it should be illegal to sell a 'service manual' for an electronic device that doesn't contain full scheamtics. Those are essential for servicing/repair. Of courwe I'd have no objection to companies selling 'boardswapper guides' provided they don't misrepresent them by calling them 'service manuals'. Anyway, the HP9836 'service manual' is a boardswapper guide, and contains very little information that's not obvious from 10 minutes of looking at the machine. There's a large section on running the diagnostics, with what to do for each error, and alas the latter is 'repalce the <foo> board'. And very often those error messages give a lot more information once you've learnt to interpret them The only really useful section is the pinouyts of all the motherboard connectors (this series of machines has amotherboard in the bottom of the case which contains the keyboard interface and HPIB circuitry. The keyboard connects to an edge connector on the side of the motherboard, the CPU, floppy controller and expansion backplane plug into edge connector sockets on the motherboard. The video system consists of 2 PCBs -- in the 9826 the text board plugs into the motherboard, the graphics board plugs onto the top edge of the text board (and extends forwards over the internal monitor), in 9836s the 2 video boards connect to ribbon cables on the front of the motherboard). Anyway, know the location of things like the address bus was a great help! Be warned there are some errors in those pinouts, though. The main one that I've found relates to the 'video coax' in the 9836C. In all 9836s (monochome and colour) there's a special ribbon cable consisting of 4 (75 ohm?) coax cables bonded together. It's terminated by 8 pin sockets at each end. One end goes onto a header plug at the rear of the motberboard (which is connected to the adjacent DA15 connector for the monitor), the other end plugs into the text video board under the disk drives. On the colour model, one of the cables isn't used, the other 3 are R, G, B. Anyway, it's clear that one row of 4 pins is the screens, the other row is the centre conductors, and it's equally clear that the latter carry the video signals. But the pinout in the manual shows the colour signals on 3 pins at one end of the connector, which would put one of them on a screen. It's obvious how this has happeend, it's the difference between numbering the pins 1 2 3 4 5 6 7 8 and 1 5 2 6 3 7 4 8 The dismantling procedures in that manual are a little odd too. Anyone who can follow the procedure for removing and replacing the internal CRT of the 9826 and not end up dropping it or using choice language is a lot cleverer than I am. You're supposed to hold the CRT against the front bezel (from the inside) And fit 4 screws (2 of which are almost impossible to get to as components on the monitor PCB get in the way, as does some chassis metalwork) with a founding spring between 2 of them. Not thanks!. My method may take a little longer, but it's not stressful. Here it is (basically) Remov top cover, board hold-down strip, PSU PCB, CPU PCB, floppy controller and expansion backplane. Remove graphics board (on top of monitor cover -- 3 screws) and text video board. Remove the internal floppy drive (6 screws + LED and power cable). Remove metal plate under floppy drive. Remove keyboard (5 screws on underside of machine, unplug ribbon cable and unscrew earth wire at keyboard end. Unplug ribbon cable and motherboard end and remove from machine. Unscrew power switch assembly from case, unclip and open cover, then release actuator from switch and remove the actuator/cover parts. Remove monitor cover (4 quarter-turn fasteners), remove monitor to motehrboard ribbon cable jumper, discharge final anode to CRT ground (faston tab on monitor PCB), then disconnect anode cap. Unplug yoke, CRT base wiring and earth wire carefully tilt it backwards (mind that CRT!) and pull it fowards to free the 3 tabs. Place the bezel on the bench fase down, remove the CRT socket, undo the 4 screws (note how the earthing spring is fitted) and take off the CRT moutning frame. Lift out the CRT -- NOT by the neck. There's one curious thing about that servic manual. It contains no schematics at all. Not even of the mains side of the PSU, which is trivially field-repairable. Juat about the only electronic tests you are asked to make are the PSU outputs at labelled testpoints on the PSU board. It doesn't, for example, describe the use of the testpoints [1] and LEDs on the disk controller board. And yet it contains this complex procedure for setting up the grey scale tracking involving an instrument -- the photometer -- that few people are likely to own (many fewer than would on the equipment to do proper electrical tests. [1] I think some of those are mentioned in the CE manual, which is similar to the service mnaul but contains a couple more useful bits of info. I _think_ the only misadjustment that will cause this is the A1 ('screen grid') adjustment on the flyback transformer. Maybe setting the clamps way too low will do it. Or if you have the clamps turned up too high, the electron cguns will be cut off for low, but non-zero input to the DACs. Als I doubt it. IT was not a common machine, so I doubt anyone bothered to make a nrw flyback for it. Who would buy one? If you follow the HP manual, all you'll do is replace the complete deflection PCB, there are no tests to do to check votlages from the flyback, etc. So unless you're clueful you won;'t even know you need a new flyback transformer. [...] Sure. There is the obvious resonating capacitor connected to it. This monitor is a little unconventional in that there are 2 output stages. The horizontal sync output from the computer goes through a couple of monostables (one of which is set by the horizotnal centring control), one of these monostables is held reset if the monitor's internal SMPSU outputs are incorrect, thus shutting down the EHT side. The output of that circuit drives the first power transistor. This has the horizontal yoke as its load (but not the flyback transformer). The flyback pulse from the yoke is detected and used to indicate that the yoke is doing something. This signal, along with a similar feedback signal from the vertical defleciton circuit are used to (a) turn on a green LED indicating that the deflection circuit is working and (b) enable the drive from the output of the monostable section to the second power transistor, which drives the flyback transformer. Now the power to that stage comes from a TIP122 transistor which is contolled by an op-amp circuit. One of the inputs to that comes from a potential divider connected to the EHT (25kV) supply, another comes from a voltqge reference circuit. This ,of course, is the EHT regulator circuit. A divider block on the side of the flyback transformer provides the A1 and focus supplies, other windings (brought out on the PCB) provide a -ve bias for the CRT control grid and a supply for the CRT heater. Of course this information is not in the HP manual... I think it's very liekly they did this (or rather got some company who made flyback transoformers to make one to their specification). My experience with working on monitors, terminals, etc, is that the flyback transformer is almost always specifci to that model. I say 'almost always' because Philips sold a few more general-purpose ones and those do turn up, e.g. in KME monitors (as used on PERQs, Whitechapel MG1s and so on). -tony

Waht does? The fact that HP desktop computer service manuals are anything but, or the fact that I am trying to do soemthing about it? THe HP ones are pretty specific. For example, this machine had a graphics board problem when I got it, the error would be reported as 'graphics failed Meory failed at <hex number> W : <hex number> R : <hex number>' It was a good bet those 3 hex numbers were an address, the value written and the value read. THe only problem was that the W anmd R numbers were 32 bits each (8 hex digits), the 68010 processor used in this machine [1] has a 16 bit data bus. Worse than that, the graphics memory uses the low nybble of each byte only -- this is to allow writes to individual pixels, there being 4 bits per pixel. So even knowing which bits had failed it wasn't immediately obvious which memory chip to change. In fact, after much testing, I discovered that 4 of the hex digits are always 0 (these correspond to the nybbles that don't actually exist), the other 4 nybbles are either all parts of high bytes or parts of low bytes, and these 4 nybbles correspod to different RAM chips on the PCB (there are 16 4416 RAMs on the graphics board...) having figured that out, I desoldered the chip I thought it was complaining about, replaced it (with one raided from an old EGA grapghics card) and was pleased to see that the graphics now passed diagnoistics And no, none of that is in the HP service manual either. If they'd explained how to decode the error message it would have saved me an hour or so's work [1] Yes, I know everyone claims it's a 68000 processor. This machine has a 68010 and an MMU circuit. It's the latest (AFAIK) version of the CPU board. Yes, I don't really trust diagnostics either. After all, the device producing the report (the computer you're working on) is probably not working correctly (otherwise you'd not be repairing it) so how can you trust the diagnostic program output. I prefer to use a 'scope or logic analyser whenever possible (and when finding the memory problem I've just desicribed, I did look at the RAS/ CAS/ etc signals first...) [...] The craziest one I've ever seen is on the PERQ. It uses a DB25 connector for a GPIB port (I am told this is an IEC625 interface or soemthing). The problem is that it simply maps the pin numbers pin-pin so pins 1-24 are used and pin 25 in a N/C. This puts one of the date lines on the top row of the connector after the shield pin (pin 12 is shield, pin 13 is a data line), it also means you can't simply crib an DB25 plug and a 24 pin microribbon socket onto a piece of 24 wire cabel to make an adapter. ARGH! YEs, but I tend to reserve it for when it's really needed. Like when a 30kV supply flasehs over to my hand, or I drop the power transformer on my foot. Exactly... For information, the PSU is bascially as follows. Mains comes in on a filtered IEC plug, goes through the mains fuse and then to the power switch under the keyboard. From there it goes to a little PCB screwed ot the (inide of the) back panel of the machine which contains a pair of ovltage sleector swithces. THese link to the primaries of the mains transformer (2 windings each 0-100-120V, the switches let you configure the machine for any of the normal mains voltages). There is a single centre-tapped secodnary on the transformer, this goes back ot the PCB which contains a pair of small diodes used to provide a power-fail indication to the optional battery backup unit and a large double-diode unit (TO3 can) for the main supply. The output of this is smoothed by what a frield of mine would call a CFE (Capacitor F-ing Enormous) to provide about 30V DC. This is fused (15A) and fed to an edge connector socket screwed to the bottom of the case. One set of fingers on the PSU regualtor PCB plug in there -- this PCB contains step-down switching regulators providing +5V, +12V and -12V. The outputs of those go to more edge figners which plug into a connector on the motherboard. The battery backup option was an 18V (?) NiCd (?) which fitted into a cavity on the bottom of the machine and which provided an input to the regulator board. There was an extra control PCB that munted under the (RH) disk drive, I know little about this apart from the fact that it appears to have contained an 8041 microcontroller. The battery unit is one of the rew options I don;t have for this series of machines. Yes. In fact this CRT has separate cathode pins for each colour, but for all the other electrodes, corresponding ones are linked inside the CRT and brought out to one pin. Yes, ben there, done that. On this monitor, the 'clamp' presets on the video PCB prerform essentially the same function I cna't be. All it has on it is an HP part number (which is not a lot of help). I have no idea who actually made it, whether it was used in any other HP product, or whether it was actually a standard one that was re-labelled (although I think that is unlikely). THis is HP we're talking about... I would not be at all suprised if it was a custom part. And even if you could find who'd actaully made it, they may well be under contract to to release any details. Of course now it has no real commerical importance, but trying to find somebody at HP who (a) realises this and (b) is prepared publically to say so is going to ba lot more work that designing and making a flyback from scratch. [Description of monitor horizontal deflction system snipped] Now, given that I've had this machine in many bits over the last few months what do you think? -tony

What's the difference between an "edge connector" and, say, the connector used for a PCI card or a modern peecee memory stick? (Tony, I realize you may not be familiar with either one, but just wander into a mass-market computer store and I'm sure you can find plenty of each, probably with some examples in see-through packages.) The only difference I can see is how the "socket" side of the connection is usually mounted (so the socket and board edge mate perpendicular to the baord the socket is connected to, rather than parallel). Oh, and the size of the contact fingers, I suppose, but _that_ surely doesn't count! /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B

Okay, so I havne't misunderstood anything there. Ah yes, I'd forgotten about that. DIMMs don't do that, at least in my experience; they are wiping contacts, very much like PCI or ISA slots. (I speculate that this is because while, like SIMMs, they have connection pads on both sides, they also, unlike SIMMs, connect different things to different sides' pads - and the pressure connectors SIMMs use aren't very well suited to that sort of pinout.) /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B

OK. In which case I'd class a DIMM socket as a particular type of edge connector (admittedly if I wanted to buy one, I'd ask for a 'DIMM socket' and not 'An edge connector with <foo> number-of-pins at <bar> pitch'). -tony

On Tue, 7 Oct 2008, der Mouse wrote: And they are far from being cheap or low-cost. I've recently ordered many different card edge connectors from DigiKey, the most needed were the 48 pin 0.156" ones used for the HP2100/HP1000. They cost 7,30 EUR each *plus* customs and VAT. And DigiKey is actually the only one that will sell you these, so I wouldn't say (apart from DIMM sockets) that they are still very common. Christian

Well, sometimes I can be a pedant, and one thing that irritates _me_ is the fact that English is not a precise language ... [Graphics PCB RAM chip allocation] Yes. The circuitry is a little odd -- there are 16 4416 RAM chips, and the data pins are not liked -- effectively there's a 64 bit databus on that card. Each RAM chip can be written individually from the approprate 4 68K data lines -- there are a couple of '138s which select the appropriate chip (or pair of chips for a word access -- 2 pixels at a time) for wriing. I forget how the data is handles for reading, I think there re 8 off 8 input multiplexer chips which bring the 64 data lines down to 8, these then go to the appropriate 68K data lines again ('appropriate' here means the low nybble of each byte, so likes 0-3 and 8-11). The decoders and muxes are controlled by the low 3 bits (A1-A3) of the 68K address bus. The data also endes up in 4 16 bit shift registers (a total of 8 '166s I think), which send it a a 4 bit serial pixel stream to the colour lookup table and thence to the DACs. I guess this was the easiest way to get the necessary pixel rate from slow-ish DRAMs. Having worked all that out, it was still not obvious what diagnostic error was telling me. Since it occured at the first RAM location in the graphics RAM (there's a pattial memory map in HP's CE handbook for this machine), I first looked to see eich RAMs were being written to. When I found it was only 4 of them, all on the high half of th word. I guessed the diagnostic stopped as soon as it found an error, and that this was a simple read/write test. Then, by carefully grounding RAM data pins (rememebr everything was buffered, so this wouldn't crash the processor) and seeing how the R: value changed when I re-ran the diagnostic, I figured it out. Then it was just a matter of desoldering the RAM chip and fitting a replacement. Indeed yes!. I love it when a dead machine, or one with a reported fault, works perfectly again. [..] Over 10 years ago I bought an HP LogicDart. It was expensive, but I have never regretted it. I've got other instrumetns that do all it will do and more, but that's a useful handheld tool that will find 99% of digital faults in classic computers, if used with that most important piece of diagnostic equipment -- the thing inside your head. This is the modern equivalent of an EZ80 or whatever you call it :-). 2 diodes in one (To3) package with a common cathode connection (broung out to tthe case) and separate anodes (to the 2 pins). It's used for the classic biphase rectifier circuit with a centre-tapped transformer. I think I've seen them in DEC machines too. [...] Did any companies produce catalogues of flyback transformers giving the winding connections to the pins and the inductance/number of turns of the windings? If so, it would be possible to see if any of them matched up at all. The 3rd-party suppliers over here will sell you a flyback for TV model <foo> or monitor <bar> but they don't give any more details than that. I am not so sure. HP used a lot of custom parts in their machines. The earlier desktop machines mostly had custom-designed processors, for example : 9100 : Custom, and very strange discrete-transistor thing. No ALU, even... 98x0 : Bit-serial 16 bit processor built from TTL chips. 9805 : Arithmetic/Registers and Control/Timing HP custom chips (as in handhelds), I think 9815 : Motoroal 6800 (OK, standard) 9825/9845/9835 : HP BPC processor custom hybrid module 8x series : HP custom 8 bit procesor (Capricorn?) 9000/200 series (the ones wr're talking about) : 68000, 68010 I would not be at all suprised if HP made/had made custom flyback transformers. Ok, OK... Well, if I get information from other sources I do try to give the reference (to a manual, a website, the person who told me, etc). So in this case I claim I figured it out myself, given some help from the HP service manual (connector pinouts), the CE manual (memory map) and plenty of databooks. -tony

[HP's odd designs] That reminds me, in passing, of the HP98x0. The 'service manuals' are boardswapper guides with PSU schematics only. But HP patented the machines, and the patents (I have a list of the numbers, I think they're also on Eric Smith's site) are several hundred pages each, and include such interesting things as schematics (albeit of pre-production machines), interface schematics, the machine language instruction set, ROM sources (sysrem firmware and many of the expansion ROMs, etc). In other words what _should_ have been in the service manual... [...] I will emphasise that again. The best tools/test equipment/CAD system/whatever are no use at all unless you think about what you are doing and use them intellegently. They're not generic, in that they don't fit more than 1 types of TV/monitor, but they're not really OEM parts, in that I suspect the original flyback came from a different supplier. I suspect in some cases the original was reverse-engineered and said third-party company came up with a functional equivalent. It's only worth their while to do this if they are likely to sell many of them, of course, so the HP9836C is not going to be on the list... I must admit I've never had any success with these third-party flybacks. I had an Amstrad VGA monitor with flyback trouble (the voltage divider block for the focus and A1 supply was reaking down), I ordered the so-called replacement and fortunately I checked out the winding connections before fitting it (that is, I checked which pins were connected by windings, which sets were totally isolated). I say fortunately, because if I'd fitted it, the 100V or so output from the monitor's SMPUS would have been directly connected to the CRT heater!. In the end Iought the genuine Amstrad part (yes, it was available, it was also a lot more expensive) and had no prolems. -tony

Oh yes. With explanations of what's going on too ! Oh don't get me started on that. If we go back to the machine that started this thread, the HP9836C. One of the faults in mine was that it reported 'Floppy Disk Missing' in the power-on test. In other words it hadn't found the floppy controller card. Now, the normal way that add-ons are detected in these machines is by seeing if said device assets DTACK/ or whether there's a bus timout, but the DTACK/ for the floppy controller is producted on one of the other boards (basically all intenral I/O gets the DTACK/ produced there) So it wasn't that. In fact it didn[t take me long to rrealise that it detected a floppy controller iff it could do a read/write to the first byte (I think) of the 256 byte sector buffer on the card. As soon as I spotted this was implemented with 2114s [1]s, I knew what to do. I remplaced them, and put the card back in, not conencted to any drives, and tired again. A great improvement. It then reported 'Disk drive not responding' or something like that. Which made sense as there were no drives. So I cabled up drive 0 [2] and tried agqin. I then got a 'Floppy Drive Failed' error.. After much testing, putting the drive on an exerciser, looking at signals round the FDC chip, etc, I realised that being a 9836 (and not a 9826), it was expecting 2 drives. If you gave it 2 drives, it correctly reported '2 Floppy Drives'. If yoy gave it 1, you didn't get '1 Floppy Drive' you got 'Floppy Drive Failed'. And this isn't in the boardswapper guide either! [2] In this machine, the drives are jumpered to be permanently enabled, and have separate cables back to the controller board [3]. This means the controller board can continually monitor the Wprot/ line to detect when disks are changed. [3] The first version of the controller, which I've never seen 'in the flesh' had one 34 pin cable soldered to it, and could only be used with one drive. Thus this controller is used in the 9826 only. The second version has a 34 pin cable soldred to it for drive 0 (in 9826s and 9836s) and a 34 pin header plug, into which a second cable is pluged for drive 1 in the 9836. You would ahve liked the way I had the machine set up for this. I wanted to be able to work on the PCBs easily, which essentially meant not having any case parts in the way. So what I had was : The monitor on the bench, with the computer's motherboard on top of it. Monitor cable plugged into the back of the motherboard. Video boards (and brightness control cabled up in the normal way. CPU board in its normal slot. Floppy controller in its normal slot. 1M memory board in the slot for the expansion backplane (I didn't need any more slots, the pinputs are similar enough -- jsut 2 signals, not used on memory boards different -- for this to work Keyboard connected by normal ribbon cable and placed between the edge of the monitor and the top of a bench PSU (see later). PSU PCB in the normal motherboard slot, but not connected to the HP transoformer/rectifier bits. Instead I wired a spare edge connector to take power from that bench PSU to the regulator board input. [...] Actually not. It was just that the picture got ever brighter and more out of focus as the monitor warmed up. Checking the CRT pin voltages revealed the obvious (A1 going up, and I think focus coming down, but I wouldn't bet on that).Further tests showed the problems was alsmost certainly the flyback and not leakage in the CRT, replacing the flyback proved it. -tony

Incidentally, in another thread you described the method of setting the purite on a delta-gun CRT by using both ring magnets on the back of the yoke and moving the yoke longtitudinally on the CRT neck. It's been a long time since I did this, but yes, you're right, the Barco manual I've been looking in describes just that procedure. And I think I understand how it works When the CRT is made, the shadowmask and phosphor postions determine the 'deflection centres' of each beam. In fact I've read somewhere that the phosphor dots were created by coating the inside of the faceplate unformly with a given colour of phosphor and a resist, putting the shadowmask in place and then using a point sourcce of (UV?) light at the appropriate defleciton centre position to cure the resist of thoat colour phosphor dots (shining thogu hte shadowmask holes, of course), after which the remaining unwanted phosphor was washed off. Anyway, back to the purity adjustmets. Basically (and there is some interaction), the rign magnets get the electron beams to pass through those deflection centre ppints, moving the yoke gets the actual deflecitons centres produced by that yoke to align with the ones used to position the phosphors. -tony

There is a resisotor in series with each cathode, but it's hard to get to, and I suspect connecting a meter across it would have an effect on the (20MHz+) signals that exist around that area. Measuring the final anode current owuld be easier in that respect since the anode is part of th esmoothing capacitor anyway, so there are no really high frequency signals around. On the other hand the charging currenct for said capacitor comes from a flyback-like transoformer (not strictly a flyback transformer, there are separate output transsitors for the deflection yoke and the transformer, the latter is only driven if there are currents in both sets of deflection coils, thus preventing a line being burnt on the screen if something fails). And it's going to be a fairly 'peaky' waveform.. Maybe an extra smoothing capacitor on the 'input' side of the meter -- the transofrmer side -- would be a good idea. It only has to be 100pF or so, but of course rated at 25kV or more. I think the first and second anode currents are very small compared to the final anode current. Any electrons collected on the first/second anode are effectively wasted (they don't contribute to the light output of the CRT), so it would be a good idea to keep them to a minimum ;-) -tony

On Wednesday 24 September 2008 16:38, Ian King wrote: For dynamic convergence, yes, but not for grayscale adjustments. -- Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters" - Information is more dangerous than cannon to a society ruled by lies. --James M Dakin

On Thu, 25 Sep 2008, Tony Duell wrote: Well, apparently only in Great Britain since in Germany, we call that 'screen grid', too. Only the final anode is called anode. All other electrodes in a normal tube besides the cathode are called 'control grid' (or just grid), 'screen grid', 'supressor grid' and so on. There are tubes with several control and screen grids, though, like hexodes or octodes. But there are tubes with e.g. helper anodes as found in thyratrons. Christian

Are you sure you two aren't talking at cross purposes? The biggest difference between UK and US English here is probably Tube (US) = Valve (UK). The usual German word for a thermionic valve, die Rohre, literally means "reed", and I think it is also used for "tube" or "pipe" in a lot of other contexts. So I suspect that second paragraph translates as "All other electrodes in a normal valve besides the cathode are called..." In the UK, when we call a valve a "tube", we almost always mean specifically a CRT, and not any old valve. (On the other hand, the German manufacturers call the huge thyristor arrays that form high power solid state switches for transmission systems "valves" in their English brochures. I'm not sure what German word they use.) Philip.

On Fri, 26 Sep 2008, Tony Duell wrote: Yes. Yes, the same here. Or, as we say, cathode, grid 1 (or Wehnelt), grid 2 (or screen grid), grid 3, grid 4 and anode. Eh, no ;-) But it's called grid 4. Grid 3 however has the same potential as the anode. Christian