Does it exist ? Generic Monochrome CRT Driver PCB ? - Page 1
Forum section: electronics / repair

author: snoozer
author_rank: Contributor
timestamp: August 19, 2021, 09:08:50 am
content: Hello,I have an old Siemens PG 685 portable computer. They have been used to work on old Siemens S5 PLC systems and its running a version CP/M. I had the computer in storage for a few years and some components on the CRT driver PCB did not like that apparently. I had the unit open when I powered it up to cut power immediately should I see magic smoke ans so it did. I have seen smoke coming for a resistor on the CRT PCB. It looks like some resistors are discolored and look overheated. The resistor is right next to what looks to be R20 and between C55 and C47. I can no longer read the color code and my attempts to find a schematic for that PCB was unsuccessful. The signal coming from the computer is a monochrome video signal which can be displayed on any screen with a composite input. The portable has a coax connector at the back for that. If I hook up a LCD with composite input I can see the screen content. Obviously I would prefer the original CRT to work so I am wondering is there a common way to drive CRTs of a seemingly simple monochrome display, are there possibly aftermarket PCB or kits available that can be adapted ? Has anybody done something similar maybe on other equipment ? The CRT is approx 20cm in diagonal. The original CRT PCB works from the 12V DC rail on that portable.The original has the following markings:C79453A3032B811U7 069159-A310-C74-03-85Here the original post in the Siemens forum:https://support.industry.siemens.com/tf/kz/en/posts/pg-685-screen-pcb-schematic/266372/?page=0&pageSize=10Kind RegardsJan P.PS, I can post pictures of the original if that helps, just did not feel like posting them for no reason. Also if there is someone out there in the EU that has the skills to repair such a device without having a schematic please get in touch, maybe we can work out some way to get it fixed.

author: snoozer
author_rank: Contributor
timestamp: August 20, 2021, 03:57:27 pm
content: Hi,I have removed the CRT assembly from the portable computer now and looked at the actual tube numbers. It is Siemens branded and the the following number:M23-108 PW 10 AR D 12M933486The diagonale measures approx 9 inch or 23cm.Attached a few pictures of the entire assembly and tube.RegardsJan P.

author: Renate
author_rank: Super Contributor
timestamp: August 20, 2021, 06:54:34 pm
content: I think that you need to bite the bullet and do a bit of tracing.In some ways this is easy, it's telling you where the problem is.I see a TO-92 transitor off that, but I can't tell if that's a power bus or something local to the transistor.

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2021, 07:07:07 pm
content: You're about 20 years too late to expect to find a generic monitor chassis, let alone a monochrome one. Fortunately this sort of monitor is very simple, as long as the flyback (LOPT) hasn't failed it shouldn't be too hard to fix it. The tube number is irrelevant, about 95% of small CRTs are electrically compatible, the variations are just in the mounting scheme and phosphor color. The deflection yoke is critical but it is sourced separately and matched to the electronics driving it, it is not part of the tube.Be very careful with the connector on the end of the CRT. In the middle between the pins is a glass evacuation pip, it is easy to crack that and then you will hear the hiss of air rushing into the tube and it is finished

author: snoozer
author_rank: Contributor
timestamp: August 20, 2021, 07:27:29 pm
content: Hi Renate,I am looking at that possibility. I am just not confident my skill level in understanding circuits is sufficient. I will have to desolder the resistor to really see where the traces are going to, its not easy to see with the component in place. How would one go about trying to find out what value resistor that was, the colors are burned up, can't identify. I suppose I will also remove that TO-92 transistor and see if thats OK.James, you are probably right re being "late". I do have a LCD screen with composite-in but that looks just unreadable or its at least very hard to read. The original screen was very clear and nice to look at. If memory serves me right I think it was green.Well, I check on the Siemens forums again, maybe someone can at least post a high res picture of a PCB with that resistor not burned up and I also keep an eye out on the usual market places for parts or devices.Thank youJan P.

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2021, 07:39:19 pm
content: A photo of an intact PCB would be ideal, or you can remove the burned resistor and measure it, it's possible it will still be intact and close enough to the original value to take a guess. Another option is to reverse engineer that part of the circuit, seeing a schematic it's often possible to tell what that function a part has and guess at the correct value. You are right to be hesitant if you do not know what you're doing, it's good to know your limitations. I cringe every time I see someone fumbling around with a repair attempt and doing further damage to an irreplaceable vintage item. Still with a bit of care and assistance from people here you may be able to fix this yourself. I would advise looking at schematics for other monochrome monitors, most of them are very similar in design and it will probably help to make sense of this one. I really like CRT displays, there's nothing else quite like them. The viewing angle and contrast ratio is far better than any LCD.

author: snoozer
author_rank: Contributor
timestamp: August 20, 2021, 07:51:42 pm
content: Hi James,I am under no time pressure so I can give it more time to investigate. I will have a look for schematics with the same vertical deflection IC TDA1770A and TV Horizontal Processor IC TDA1180P. I posted again on the Siemens forum to see if anybody might have a picture of the PCB. If I get to a point where I would actually do stuff I would connect the CRT assembly to a bench supply and limit current. When it started smoking it was powered by the computers supply. I wonder, would it be beneficial to disconnect the socket at neck of the tube to avoid any possible damage inside the tube ?RegardsJan P.

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2021, 07:58:03 pm
content: It's unlikely to damage anything electrically, the tube heater will almost certainly be 12V and probably comes directly from the power supply. If you do disconnect that socket be sure to pull it straight off so you don't crack the pip, and then put some kind of cap over the neck of the tube to protect the pins from accidental damage. I typically use the cardboard sleeve from a roll of toilet paper, cut it along one edge and roll it into a smaller tube that will slip over the neck of the CRT and then tape it.

author: Renate
author_rank: Super Contributor
timestamp: August 20, 2021, 08:43:32 pm
content: Well, the first bit is: Does one end of that resistor only connect to that TO-92 transistor?You can remove the burnt resistor and see if that transistor is dead (without removing the transistor).Then look where the other end of the resistor goes. Does it feed that TO-220 power transistor to the left?Is that any good?

author: snoozer
author_rank: Contributor
timestamp: August 20, 2021, 08:52:59 pm
content: Renate, the upper leg of the resistor goes to the collector of that BC547B, the other end is joined with lower leg of the resistor to the right and then the trace goes towards the TO-220 BUT passes without connecting to it and disappears below the inductor just next to the TO-220 (which is a Thyristor). I have also found an image now of the CRT PCB but it must be a different version. The TO-220 is not populated and also the capacitor just above the resistor is not populated. The resistor that is charred on my PCB is visible but I can realli make out the colors along all the dirt and grime.I am still looking through PDFs and Siemens documentation in the hope I might find a schematic somewhere.RegardsJan P.PS: Now that I see that picture here in the forum post, the resistor to the left of the ICT sticker/label I would read as yellow, purple, brown, yellow and red

author: Renate
author_rank: Super Contributor
timestamp: August 21, 2021, 12:31:08 am
content: Maybe yellow-violet-gold = 4.7 ohms?Does that collector of the TO-92 go to other things?That's a thick trace, I don't think it's really the TO-92 that concerns us.It appears to be a power bus.When you power this up, what happens?Can you measure that collector end of the resistor?It has stopped burning, hasn't it?If not, remove the resistor.I'd think that a low resistance, low power resistor would be feeding more filtered power to something.It might be a cap shorted to ground.Check the other end of the resistor for the resistance to ground.

author: factory
author_rank: Super Contributor
timestamp: August 21, 2021, 12:50:40 pm
content: At least check the tantalum capacitors aren't shorted, they are encased in those black plastic blocks and contain the usual bead type.David

author: snoozer
author_rank: Contributor
timestamp: August 21, 2021, 12:55:58 pm
content: David, thatnks for that tip, I was not sure what those black plastic blocks are. I did google the numbers but nothing came up. Is that 22μF 16V ?Thank you !Jan P.

author: factory
author_rank: Super Contributor
timestamp: August 21, 2021, 01:05:16 pm
content: Yes capacitance value in uF, then voltage rating, they are common in stuff I've repaired at work, not had many short circuit failures due to the equipment being in constant use, also had low level leakage when used in timing circuits. You can still buy them in the block package, just not as easy to find as the more common bead type.David

author: snoozer
author_rank: Contributor
timestamp: August 21, 2021, 01:08:28 pm
content: Thanks David, I have seen a lot on YT re those caps even exploding badly at times. Is there a important reason to replace them for tantalum types again or can I use electrolytic caps instead ?ThanksJan P.

author: factory
author_rank: Super Contributor
timestamp: August 21, 2021, 02:06:56 pm
content: Check if it is actually shorted first, if it is then find out what voltage across it would normally be, to determine if they choose adequate rated parts or lower cost. They tend to get a bad reputation from failures, particularly when the voltage rating isonlyjust above the supply rail.Stuff at work gets like-for-like replacement, but my own stuff I either use a new tantalum (maybe slightly higher voltage rating if it will fit), or fit a normal electrolytic of slightly higher capacitance.The capacitors used for timing needed to be tantalum, as certain electrolytic's were found to drift too much due to slowing drying out over time.David

author: CaptDon
author_rank: Super Contributor
timestamp: August 21, 2021, 03:28:35 pm
content: B.T.W., The crt filament voltage is likely to be 6.3v and not 12v.

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2021, 05:23:31 pm
content: Quote from: CaptDon on August 21, 2021, 03:28:35 pmThere are two neck sizes in common use. The large neck tubes have a 6.3V filament and the small neck tubes have a 12V filament, at least in my experience. There is no universal rule here but this has been true of every tube I can recall measuring. From the pictures this looks like a small neck tube, usually with these small monochrome monitors the monitor chassis runs off 12V and that is also used to power the heater directly. Obviously if you are trying to light up an unknown tube you would want to start with a lower voltage first but since you have the entire monitor here this isn't really something you need to worry about.

author: CaptDon
author_rank: Super Contributor
timestamp: August 23, 2021, 02:15:09 am
content: I sadly toasted a rare Tek CRT years ago. It was in a little battery operated scope,something like a 212 but I forget the exact model. I was expecting 6.3v for thefilament and I saw (6) on the schematic. It was a poorly copied schematic and itturned out to be .6!!! Well I heard the 'dink' as soon as 6.3v was applied!!! I founda replacement and actually even have a spare 154-0696-00 is the number.

author: james_s
author_rank: Super Contributor
timestamp: August 23, 2021, 05:09:52 am
content: Scope tubes and really small stuff is a whole different ball of wax. The "lollipop" tubes used in Sony Watchman portable TVs are similarly low voltage, about 0.8V IIRC. Standard magnetic deflection tubes from about 4" and up are electrically very similar to one another.

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 23, 2021, 06:40:52 pm
content: I'd guess 0.6 or 0.8 volts would probably be a direct heated cathode for max efficiency. I've seen other odd voltages too, I have little 12" all transistor B&W TV that uses 11 something on the heater for whatever reason, even though the whole set runs on a regulated 12.5 volt supply.As to OP's problem, the usual procedure needs to be carried out, sketch out a partial schematic (pencil and paper, DMM in continuity mode) as it radiates out from the burnt resistor, then start identifying and testing probable shorted parts (transistors, tantalum caps, etc.), show your work so others can review for anything you may have overlooked. You'll hopefully find the cause near the symptom and not have to reverse engineer the whole board.

author: snoozer
author_rank: Contributor
timestamp: August 26, 2021, 06:05:00 am
content: I have finally gotten around to remove that Tantalum Capacitor. It is showing 8 ohm resistance constantly and no capacitance. I am going to order new Tantalum Caps for the CRT PCB and replace them all to start with. I go for Vishay with a voltage rating higher than was is currently on the PCB. Its only 6 of them as far as I can see. Since I have no schematic and I can't see myself reverse engineering the PCB I can't judge which of them could be replaced by Aluminium Electrolytic Caps and which need to be Tantalum so I stick with it.Kind RegardsJan P.

author: 1audio
author_rank: Frequent Contributor
timestamp: August 27, 2021, 05:19:20 am
content: At some point you can replace it with a generic RGB LCD display. At its vintage you will either have composite video or a vert/horz/intensity signals. Those can either drive a vga or composite input. There are car PC displays that are pretty cheap that can do this. Fixing a CRT setup was not hard 25 years ago, but its a lost art today and not likely to become needed again.

author: james_s
author_rank: Super Contributor
timestamp: August 27, 2021, 05:54:15 am
content: LCD displays hacked into vintage computer gear look like shit, it's just totally wrong. Fixing CRT displays is not hard and it's not lost at all, there are quite a few of us around still that know how to do it. They're not complicated, far simpler than a LCD monitor. As long as the tube and flyback transformer are good the monitor can be fixed.

author: snoozer
author_rank: Contributor
timestamp: August 27, 2021, 06:35:09 am
content: I agree with James here, I prefer to get the CRT back on track and not use a LCD for that old computer.Jan P.

author: snoozer
author_rank: Contributor
timestamp: September 10, 2021, 04:51:00 pm
content: Some success !! I have trouble finding the tantalum capacitors so I have substituted the defective cap with an aluminium electrolytic and it works. I had a good look around and there are a lot of tantalum caps, I will try find suitable new tantalum caps and replace all that I can find. Below a couple of "screen shots".Thanks for your help with this, very much appreciated !! Should I ever find schematics I will share here.Kind RegardsJan P.

author: james_s
author_rank: Super Contributor
timestamp: September 10, 2021, 06:52:04 pm
content: Nice job, looks great! Glad you were able to persevere and keep another CRT going.