Early 80s industrial CRT wiring - what am I doing wrong? - Page 1
Forum section: electronics / beginners

author: moothefish
author_rank: Contributor
timestamp: July 24, 2021, 08:34:49 am
content: Hi all, i'm currently working on a somewhat ambitious project for my level of electronics knowledge. Basically, I acquired a nonlinear video tape editing control deck from 1983, and i'd like to use the original CRT and buttons and maybe hook up a Pi or something to it and make a faux "battlestation" if you will. I'd like to use as much of the original parts as possible so that it could potentially be put back to its original form.The CRT assembly is an off the shelf module similar to the ones you might see in old CNC machine controllers. It's made by Dotronix, model BHD500. Seems like they're still around but I have not been able to find a datasheet for this unit. The issue i'm having is that I don't know what kind of video signal this monitor is expecting. I looked at specs for other similar models of monitor and it seems to indicate that it would either be standard composite or some kind of TTL signal. The monitor board has an edge connector with 6 pins in use, one pin has two of the same green wires running to it. Red and black are the + and - 12v in (i assume 12v, seems to power up fine on my ultra high end lab grade ATX power supply....). Yellow, orange, and one of the greens run to a potentiometer mounted outside the chassis. The other green wire and white wire run out of the monitor and I assume are the signal inputs.At this point, I have been able to get the monitor powered up, can hear HV and can see the heaters glowing in the tube. However I have not been able to get a raster or any picture. I've tried feeding it a composite signal since there are only two wires leaving the board that aren't power, but this only results in the pitch of the HV changing and no image. I have gone through and tested all electrolytic caps and found one with high ESR so changed it, the others all tested within spec. This did not have any effect. But without knowing if i'm even feeding it a signal it likes i don't really know what to do next...Some more context on the machine: This control unit seems to be designed to connect to a rack of equipment which would supply power and other signals. The unit has a single dsub style connector mounted on an interface card which connects the jog wheels and control buttons. The 4 wires from the monitor connect to this board but look to be passed straight through to the dsub connector. I have bypassed this board at the moment as I don't think it has anything to do with controlling the monitor, but I could be wrong. I've attached photos of everything for context.Any help is appreciated as, I really don't know what i'm doing at this point (I am familiar with working around CRTs and safety precautions etc but not the ins and outs of video signals).

author: tggzzz
author_rank: Super Contributor
timestamp: July 24, 2021, 10:46:43 am
content: You should not be able tohearHV.I would wait until it is thoroughly discharged (several hours since the precise circuit is unknown), then clean all the black deposits off the wires, CRT and PCB.I presume you have checked the PSU voltages as well as the caps.

author: moothefish
author_rank: Contributor
timestamp: July 24, 2021, 11:12:26 am
content: To clarify, I can hear the high pitched whine that you get from any CRT in normal operation, not arcing/etc. As stated I have checked caps and voltages are fine as far as I can tell

author: strawberry
author_rank: Super Contributor
timestamp: July 24, 2021, 05:09:52 pm
content: sounds good, video got to sync IC. probably dead video amplifier , could adjust screen if possiblewhat IC on board?

author: moothefish
author_rank: Contributor
timestamp: July 25, 2021, 08:31:34 am
content: Quote from: strawberry on July 24, 2021, 05:09:52 pmThere's only two ICs afaik, the main one is a TDA1170S alongside an LM1391N.

author: Renate
author_rank: Super Contributor
timestamp: July 25, 2021, 04:44:59 pm
content: Forget about video, you need a raster first.This is not like some modern LCD that won't glow unless it gets a valid video signal.@Strawberry was right, try adjusting the "screen voltage" also "G2" or "sub brightness" to see if you can get a little glow somewhere.Do you actually have high voltage? Sometimes you can feel it on the front of the screen (outside the enclosure).

author: moothefish
author_rank: Contributor
timestamp: July 26, 2021, 04:11:49 am
content: Quote from: Renate on July 25, 2021, 04:44:59 pmThanks for that. Well, just as i thought, I was being dumb. Tried feeling the front of the tube and couldn't feel any static though there is a plastic lens bonded to the front of this particular one. I also tried discharging the tube right after it had been running and got no reaction at all, so either this thing has very effective discharge resistors, or we have no HV after all. At least that's what it looks like to me. I also had a fiddle with some of the adjustment pots (none are labelled on this) but there was no change.I always thought the typical high pitched CRT whining noise was from resonance in the flyback, maybe i've been hearing the deflection coils this whole time...

author: Renate
author_rank: Super Contributor
timestamp: July 26, 2021, 10:39:11 am
content: It's still hard to say whether you have HV.I used to have a nice HV probe for my DVM.Most of the unapproved ways to check without a probe I'd be hesitant to recommend.I seem to remember this device; I recollect that the screen was never too bright, at least on ours.What is the name of it?

author: moothefish
author_rank: Contributor
timestamp: July 26, 2021, 03:18:29 pm
content: Quote from: Renate on July 26, 2021, 10:39:11 amCheers, i'll look into confirming the HV situation. The device itself is a CMX "The Edge" editing controller. Doesn't really say much more than that. Would love to have seen it in operation with it's original equipment, looks to be an interesting device for sure.

author: Renate
author_rank: Super Contributor
timestamp: July 26, 2021, 03:24:46 pm
content: Quote from: moothefish on July 26, 2021, 03:18:29 pmOh, yeah! We had one about 1983.

author: strawberry
author_rank: Super Contributor
timestamp: July 26, 2021, 07:03:51 pm
content: usually filament should be fed from flyback transformervideo amplifier transistor ~TO126 or TO92L, there should be ~150..200V supply generated by flyback transformercheck allen bradley carbon composite resistors

author: fcb
author_rank: Super Contributor
timestamp: July 27, 2021, 02:43:12 pm
content: So the LOPT (line output transformer) will run at line rate (15-16KHz typically), a secondary coil on the LOPT will typically connect (through a passive network) to the horizontal scan coil. Done for cost reasons and also prevents the electron gun burning a hole in the phosphor (can't generate EHT if you aren't scanning).The LOPT will also generate the focus voltage, either internally (some LOPT's have an actual focus voltage control on the side) or more likely in this unit via a diode/capacitor near the base of the LOPT. Also the LOPT will generate the grid voltage (usually fixed) which maybe used by the video driver.Contrast and brightness both will be controlled by pots (contrast controls gain of video amplifier, brightness controls DC level of output) - probably just afew transistors. The LM1391 is the sync stripper and will feed into the TDA1170 (vertical scan coil driver) a slow signal (50Hz?) and also drive a transistor somewhere in the video amplifier to clamp to do the black level clamping (15-16KHz).TDA1170 tends to run hot and I've lost count of the number I've replaced (I still have a stock somewhere).