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Mac SE CRT ghosting/Bleeding to the right

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  • GiGaBiTe
    replied
    Originally posted by ivannudem View Post
    So I assume there must be something wrong on the analog board, But I didn't find any suspicious amplification circuit there?
    It's pretty obvious where the amplification circuitry is for both the horizontal and vertical drive by tracing back from the deflection drive outputs.

    The TDA1170N and the associated circuitry between it and the flyback is for the vertical drive, while the GBU406 and area around it are responsible for the horizontal drive.

    If you don't have access to an oscilloscope, do the old fashion method of testing components out of circuit. It's not difficult to test resistors, diodes and transistors with a multimeter. Capacitors need a capacitance meter or LCR meter.

    Leave a comment:


  • ivannudem
    replied
    Hi, everybody,
    Just an update on this issue. I spoke with a family member (who used to work in this field) to ask him to help pinpoint where the problem was, and sent him some photos of this smearing problems, even before I explained further, he immediately told me this was a bandwidth, frequency response problem in the amplifier circuit (I forgot the exact phrase, but like Hugo said.) So the cause I think is quite clear, something wrong with the amplifier circuit. Since I have the "good" FDHD, and that there's a NPN amplifier transistor on the neckboard/video board (the one connected to the neck), I did a video board swap, I installed the known good FDHD's video board onto the neck of this bad SE's CRT...Unfortunately It didn't solve the problem, there were still smearings to the right after a long black line..
    https://imgur.com/a/JvLwAMJ

    So I assume there must be something wrong on the analog board, But I didn't find any suspicious amplification circuit there?
    https://imgur.com/a/eBipQLz

    I do agree that a CRT defect is rare, and I hope it was not that which caused the problem...
    This family member lives in another city... and I don't think he still keeps a scope at his home... LoL
    Sigh...Facepalm...
    Last edited by ivannudem; June 6, 2021, 09:22 PM.

    Leave a comment:


  • ivannudem
    replied
    Originally posted by Hugo Holden View Post
    When there is a frequency response issue in the video amplifier/CRT gun driver, it will always seem less severe at low contrast and low brightness levels.

    It is pretty difficult to make any helpful suggestions, without the schematic of the CRT neck board and he circuit that feeds that from the main board.
    Hi Hugo, Thanks a lot for your help, I tried my best but didn't find an exact schematic of the Mac SE video board, but I found a general mac repair guide, which seemed to use the Mac Classic. Classic II's video board as example and stated: "The horizontal and video circuits are also virtually identical to those used in the SE and SE/30, but the vertical and power sup- ply circuits are completely different." attached in the following link:

    https://imgur.com/a/8MPXmeC

    It is really simple, it seems that the core part is just the NPN transistor.... but since this is the video board schematic of Mac Classic not SE, the number of diodes cannot correspond to the actual SE board (4th pic) which has 4 diodes, nor did I see the inductor on the actual SE video board (which is drawn on the schematic), but the core part 2N3904 was there.

    I did tried to find the photo of Mac Classic's neckboard and Mac SE's neck board, upon comparison of the two boards, they really look similar, even the positions of the components, what seems have changed are just the names of the components, eg. C1 -> CC1

    Apart from the previous schematic of the Classic found in the repair manual, I also found a copy of the classic's neck board from as follows:

    https://imgur.com/a/rcrAAXP

    (I'm sorry I have to use imgur, because this forum doesn't allow upoading attchment more than 256KB which will require me to compress the image to a blurring unreadable quality)

    Assuming that the statement of this manual is true: the SE and Classic video board are similar, Could you please help me identify in this circuit which part is most probably faulty that I should try to replace? Thanks
    Last edited by ivannudem; June 4, 2021, 05:58 PM.

    Leave a comment:


  • NeXT
    replied
    Schematics for macs is spotty. Some models got most of a complete set (the early compact macs especially) while later models have almost nothing. Most repairs are shotgun jobs or through years of other people shotgunning the same parts and building a relatively "accurate" list of parts known to fail and replace. That often is worthless when it's a fault that isn't a regular fault.

    Leave a comment:


  • Hugo Holden
    replied
    When there is a frequency response issue in the video amplifier/CRT gun driver, it will always seem less severe at low contrast and low brightness levels.

    It is pretty difficult to make any helpful suggestions, without the schematic of the CRT neck board and he circuit that feeds that from the main board.

    Leave a comment:


  • ivannudem
    replied
    Hi, Hugo, Thanks a lot for your help. I see, the problem is on the small board, (called video board/ neck board?) which is connected to the rear neck of the tube. I didn't find any related schematic online, but I did find a photo from 68kmla:
    https://imgur.com/a/etf4Xvo
    It seems its relatively simple, composed of several diodes, resistors, ceramic caps and one 2N3904BU NPN transistor...
    this photo was from the 68kmla post found here:https://68kmla.org/forums/topic/2381...ions-and-se30/

    It says that there are pretty common problems of the neckboard with this transistor which will result in the shown inclined scan lines problem... (I don't know how to describe it, see the above pic)...
    But my problem is the smearing to the right problem....should I suspect this NPN transistor or the ceramic caps and resistors on this small board?

    Interesting enough there's another post in this thread, where the person used to have the tilted scan line patterns and after changing the NPN transistor, the CRT finally displayed the desktop but he got the smearing to the right problem:
    https://68kmla.org/forums/topic/2381...comment-668071
    also in screenshot here:https://imgur.com/a/DcX1TlS
    So there must be some other components (ceramic caps, diodes?) failing on this video driver board causing this...

    Really big thanks for your advice, so the issue is now (at least with very high confidence) located on the small video/neck board.... upon checking the photo of the neck board, could you please tell me which components are most probably faulty (this transistor? Diodes? Or ceramic caps?)Thanks.

    PS. I just want to add something about the possibility of “overdriven Contrast”, this CRT seems to be old and dim, compared with the other FDHD I had. I mean there’s an internal pot controlling “cutoff/brightness” on the analog board. Both of the 2 machines have this pot set to about the same middle position, but on this bad SE, the front panel external “contrast/brightness” needs to be tuned to its highest/last 1/3 range to make the raster/image visible, whereas the good FDHD only needs the External contrast knob to stay at around middle to get descent bright pictures... but even tho the contrast of the bad se is set to the Highest 1/3 range, i didn’t see any retrace lines or faint overbrightened pale image, so I assume it is not overdriven that caused the smearing... But I did notice that if I turn the contrast knob down, to first 1/3-1/2, the smearing seemed disappear, but the screen got very very dim, just barely a little bit brighter than barely visible,unable to use it in that setting...
    Last edited by ivannudem; June 3, 2021, 09:41 PM.

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  • Hugo Holden
    replied
    Originally posted by ivannudem View Post

    I found a SE/ SE-30 Analog board Schematic online, could somebody help me point out which specific part/component is possibly causing this problem?
    https://imgur.com/a/eBipQLz
    I don't have a scope, just a multimeter, so what I can do is just shotgun changing components....
    The circuit you posted: https://imgur.com/a/eBipQLz is of the horizontal and vertical deflection (scan) stages and the EHT generator and CRT's grid voltage supply. The fault causing your problem is not in this part of the circuit.

    It is the circuit that provides the video drive to the CRT's (most likely cathode) that we need to see.

    As I mentioned in a previous post, an exception where the CRT can contribute to an abnormal frequency response, is in a case where the video drive to the CRT is at its cathode, and heater cathode leakage in the CRT's gun. It is relatively rare and if it does happen, there is a work around by running the CRT's heater from an isolated supply. But, most likely the fault is in the video amplifier driving the CRT's cathode and not the CRT.

    Unfortunately, for this type of issue, not having a scope to look at the video signal makes things very difficult.

    Most video amplifiers in VDU's are wide band amplifiers and should have a flat frequency response from a very low frequency, 30 Hz right up to over 5MHz, some better monitors up to 8 to 10MHz. (A typical analog TV usually only makes it to 5 or 6 MHz). In Computer VDU design there is often more attention to the video amplifier high frequency response, this is how they can display super fine detailed graphics in a monochrome VDU. In color CRT's the resolution becomes limited by the geometry of the shadow mask and phosphor dot pitch, rather than the bandwidth of the gun amplifiers.

    Leave a comment:


  • ivannudem
    replied
    Hello to every expert, Thanks for all of your suggestion. I do agree that the chance of a CRT failure is rare, but it did happen as in this post (https://68kmla.org/forums/topic/6064...-to-the-right/)
    as I quote:
    "Sadly recapping the Analog board didn't show any results, horizontal image still collapses weirdly on reset and there still is bleeding/ghosting to the right.
    All caps (Samyoung branded) measured reasonably within spec (all capacitance values are met higher then spec, ESR was a bit up but nothing that shocked me...)
    "

    "CRT swap didn't have the issue, so its either the CRT itself or the cables to it...
    Can anyone explain this behaviour of the CRT? I don't understand how the tube itself can cause this to happen"

    I have replaced all electrolytic capacitors on the analog board, as suggested in this diagram (https://imgur.com/JhQwNvK), but not the ceramic ones, nor any resistors, ICs, transistors...

    I found a SE/ SE-30 Analog board Schematic online, could somebody help me point out which specific part/component is possibly causing this problem?
    https://imgur.com/a/eBipQLz
    I don't have a scope, just a multimeter, so what I can do is just shotgun changing components....

    And as for the possibility of a bad grounding, I did check the continuity of the power cable to the analog board, every line is conntected, and the solder joints of the connector have been reflowed, the PSU's extra ground wire (not sure what it is called, the green extra wire extending from the PSU metal case) has also been firmly screwed to the metal chasis. the other FDHD which is in even poor cosmetic and electronical conditions plugged in the same power outlet without problem, its PSU cable has been badly dammaged, my friend saved it from a scrapping/recycle center, and some of the cable's wire skin has been shoved by knife, exposing the metal wires, I used some insulation tape to wrap them, they work OK. The cables in this "bad" SE are intact from visual inspection and the connectivity is fine, I didn't mesure the resistance of the wire comparing to known good one, since I don't have another set of PSU cables of this type...But I assume the grounding is OK.

    Thanks
    Last edited by ivannudem; June 3, 2021, 01:21 PM.

    Leave a comment:


  • Hugo Holden
    replied
    One great thing about a CRT, it does actually do a good job of showing what is wrong with the frequency response of the amplifiers that are driving it.

    This shows up, if there is a defect on a black to white signal transition, or a white to black transition, not as easily seen unless the CRT's brightness is elevated.

    The defect though, is there to be seen on the black to white transition, of the picture content.

    however, when the video signal transitions from white to black, if the signal overshoots, more into black, you cannot see it of course, because the CRT's beam is already cut off in most cases.

    To see the defects in the video amplifier processing system (and to isolate and repair them) requires that the video amplifier is driven with square waves and the frequency response (the quality of the square wave reproduced) is assessed on a good scope, like a Tek 464,465,466 or 2465B.

    Then you would see that the video amplifier frequency response of your monitor is abnormal, as there would be overshoots after the fast rising edge (or fast falling) of an applied square wave test video signal from a generator. Though some suitable computer graphics work just as well as a test signal.

    I have attached an image of part of one of those Imgur images posted (that my computer struggles with).

    If you look you will see on your image, that one scanning line, that if the video signal has been near black (or a low beam current value) for a while and then it transitions to white, there is a whiter than white overshoot, or a "smear" characteristic of a defect in the frequency response of the video amplifiers.

    Where is this defect ?

    Who can say at the moment, as I have not seen the information required to isolate it. This is the schematic, and scope recordings of the signals at points on the circuit, as the signal passes through the video amplifier stages. If I had the schematic, and a scope on the circuitry, it would be dead easy to show where the problem was occurring.
    Attached Files
    Last edited by Hugo Holden; June 3, 2021, 08:54 AM.

    Leave a comment:


  • evildragon
    replied
    Originally posted by Hugo Holden View Post
    I have been working with CRT's since the early 1970's, I have designed & built many video monitors and TV's with them and have nearly all the books relating to TV and scope CRT design going back to the 1920's, in my library, that I have thoroughly studied.

    I can say that not a lot of what has been posted on this thread makes any sense. The smearing effect to the right, in the direction of scan progression, is not generated by any kind of CRT defect. (perhaps, one exclusion see below under Heater-Cathode leakage topic).

    Generally, the fault of defective high frequency video response (smearing to the right) is caused by an abnormality in the frequency response of the amplifiers driving the CRT, not the CRT itself.Or a corruption of the frequency response where the video signal is formed initially.

    Video amplifiers drive either the grid or the cathode of the CRT according to the particular design of the VDU or TV.

    In the case where the cathode is being driven, if CRT heater to cathode leakage develops, this is about the only way a CRT defect in itself could produce the smearing effects discussed on this thread, and that is a moderately rare event. If it did happen, it is fixable by adding a CRT heater drive transformer with a low secondary capacitance.

    Blaming the CRT itself is for this fault, I think, is barking up the wrong tree most likely. But to confirm this was the case for sure, I would have to have one of these defective units on my bench in front of me and go through it using my test generators & the scope.
    I can agree with this. I didn't work on CRT's for very long (and am only 34 years old for that matter and didn't start working till 200, but this is true. Smearing to the right is generally not the CRT. People assume this from old transistorized TV's when the CRT goes weak because they crank up the contrast to make it brighter and it smears. That isn't from the CRT though. Yes, the CRT is weak and needs to be cranked up, but that's saturation in the transistor logic. Tube based TV's didn't do this, instead when a CRT gets weak, you see a weird posterization of the picture, like it's going silver and glimmering. (At least on black and white sets, which I fool around with for fun).

    In this case, it very much seems like a high frequency issue, or even possibly a bad ground. I've seen badly grounded VGA cables do this to LCD's as well, same effect smearing to the right.

    Leave a comment:


  • Hugo Holden
    replied
    I have been working with CRT's since the early 1970's, I have designed & built many video monitors and TV's with them and have nearly all the books relating to TV and scope CRT design going back to the 1920's, in my library, that I have thoroughly studied.

    I can say that not a lot of what has been posted on this thread makes any sense. The smearing effect to the right, in the direction of scan progression, is not generated by any kind of CRT defect. (perhaps, one exclusion see below under Heater-Cathode leakage topic).

    Generally, the fault of defective high frequency video response (smearing to the right) is caused by an abnormality in the frequency response of the amplifiers driving the CRT, not the CRT itself.Or a corruption of the frequency response where the video signal is formed initially.

    Video amplifiers drive either the grid or the cathode of the CRT according to the particular design of the VDU or TV.

    In the case where the cathode is being driven, if CRT heater to cathode leakage develops, this is about the only way a CRT defect in itself could produce the smearing effects discussed on this thread, and that is a moderately rare event. If it did happen, it is fixable by adding a CRT heater drive transformer with a low secondary capacitance.

    Blaming the CRT itself is for this fault, I think, is barking up the wrong tree most likely. But to confirm this was the case for sure, I would have to have one of these defective units on my bench in front of me and go through it using my test generators & the scope.
    Last edited by Hugo Holden; June 2, 2021, 02:38 AM.

    Leave a comment:


  • ivannudem
    replied
    Hi, NeXT,
    Thanks for your suggestions, I also suspect there was a problem of the tube, maybe it has very high hours of usage and had been worn out. Because comparing with my SE FDHD, when the brightness knob (on the front panel) was turned to the maximum, it was not very very bright, whereas the FDHD was really bright even shining to the eyes (both of the SE and SE FDHD’s brightness pots on the analogue board were at about the same position, around the middle...)

    ironically the FDHD has more/darker menu bar burn-in whereas this troublesome SE CRT has less burn-in on the top menu bar area.... maybe burn-in is not a good meter to measure the hours of usage LoL... I really hoped that CRT manufacturers installed some kind of mileage meter like cars, so that second hand buyers to assess its remaining life and value....

    also I may suspect the life of flyback Transformer, if it is too old, it may not generate enough voltage... but ironically the flyback’ sealing epoxy (the brownish thing, don’t know what it is called) was lighter than the good FDHD’s, so maybe the browness of the seal is not an indicator of flyback running hours either...
    so this dimmer SE CRT was caused by either the remaining filaments Itself or an aging flyback....

    I do not want to invest more on this troublesome SE, but I did spend time to clean the 2 floppy drives, recap its AB, I don’t want to throw it away, but the smearing is really annoying, and seems to appear on many of the compact macs (the SE30 I linked before, and other SEs I saw online) the best chance for me is to wait on local Craigslist to see if someone has a broken SE, Plus, early Classic as donner machine to sell me the tube LoL... (I know there’s a compatibility issue about the classic screen with this SE) buying from eBay +shipping is expensive ....

    Leave a comment:


  • NeXT
    replied
    The last time I've had to address horizontal smearing to the right it was a completely tube driven set. It can be an issue in the sync separator/AGC circuit or low anode voltage. I have also seen it in tubes with very, very high hours. This however is a solid state analog board, so I got no idea where to investigate.

    Leave a comment:


  • ivannudem
    replied
    I also find something strange about this troublesome CRT, when powered up (to the screen prompting inserting floppy disk (it doesn't have hard disk)) , the right edge of the screen is more dark/bluish than the left part, it's hard to catch by a camera because my phone seems to automatically adjust the exposure of the img.

    https://imgur.com/a/AzPfwNN

    But the good one (FDHD) i got seemed to be OK, the contrast of this FDHD's CRT is also different than the bad SE, it seemed more blackish than bluish, where can I tune the tone of the screen?

    https://imgur.com/a/Sdr9zn4

    I also noticed some other compact macs on sale on ebay which have this kind of bright streaking and black smearing problem, this might have been a common problem of this CRT,
    https://imgur.com/a/Rc2GNuE

    Apart from recapping the AB, reflowing suspicious solder joints (which have been performed), how can I improve the streaking problem of the SE without buying a new used CRT? Does this means that it has approached its end of life?

    Thanks

    Leave a comment:


  • ivannudem
    replied
    Hi, Just an update, I recapped the problematic Mac SE's analog board (changed all electrolytic caps bought from digikey), but the problem still persists.

    https://imgur.com/a/gJ4QRkP

    Recap the analog board doesn't make any changes, except making the max brightness a little bit brighter... But under the max brightness, I did find the horrible phenomenon that even the whole large area to the right of a vertical black line will bleed as gray color to the right, which is the symptom of the unfortunate SE 30 mentioned on this forum and 68kmla (where he found the problem was the CRT)
    https://68kmla.org/forums/topic/6064...-to-the-right/

    https://imgur.com/a/ROddjRf

    I don't know what to do, but recently I picked up another SE FDHD from a friend, the CRT was perfect without problem and after recap of the analog board with the same batch of new capacitors. So i'd assume that it was the problem of the CRT. But I'm just curious, how could a CRT itself has such a failure mode?

    I did find that the CRT's neck has a pinkish indicator which indicates that it has some leakage? After the same recap process, this troublesome CRT was dimmer than the FDHD's when both are at max brighness, so does this has anything to do with aging, i.e. less electrons resulting in the bleeding to the right?

    I also touched some old solder joints on the AB if I find that they are old and oxidized, but no apparent soder joint cracks, and used IPA to clean the tuning pots (of brightness and focus)

    Any suggestions?
    Thanks

    Leave a comment:

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