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Thread: composite monitor retrace issues

  1. #1

    Default composite monitor retrace issues

    Hi all,
    Not sure where to ask about this, so thought I should ask here first.

    I have an old green screen Mitsubishi sv-1290x that has some issues. It's been sitting waiting for repair for a number of years now, so I finally thought I should give it a go. When I got the monitor it worked, but would sometimes flicker showing the retrace issue but over time it got worse and then finally one day it would not show any image, just the retrace lines. The screen is very bright, and adjusting the brightness doesn't dim the screen much.

    I have been able to find the schematic, but just not sure where or how I should start diagnosing the problem. After doing a bit of research I thought maybe something in the blanking circuit might be the issue??? The fact that it died over time also makes me suspect a leaky or out of spec capacitor...but it's all guess work for me at this point.

    I also know the inherent dangers involved with working on monitors which has me a little timid to just poke around randomly.

    Hope someone can help.

    Philip

    monitor_sv1290x_schematics.jpg

    IMG_7212.JPG
    retro computing at: www.neoncluster.com ....is dead

  2. #2

    Default

    Quote Originally Posted by Nama View Post
    Hi all,
    Not sure where to ask about this, so thought I should ask here first.

    I have an old green screen Mitsubishi sv-1290x that has some issues. It's been sitting waiting for repair for a number of years now, so I finally thought I should give it a go. When I got the monitor it worked, but would sometimes flicker showing the retrace issue but over time it got worse and then finally one day it would not show any image, just the retrace lines. The screen is very bright, and adjusting the brightness doesn't dim the screen much.

    I have been able to find the schematic, but just not sure where or how I should start diagnosing the problem. After doing a bit of research I thought maybe something in the blanking circuit might be the issue??? The fact that it died over time also makes me suspect a leaky or out of spec capacitor...but it's all guess work for me at this point.

    I also know the inherent dangers involved with working on monitors which has me a little timid to just poke around randomly.

    Hope someone can help.

    Philip
    First of all, you would need some tools, like at the very least a multimeter, better if you have an oscilloscope, an ESR meter and a high voltage divider probe.
    Having said that, with the monitor off and unplugged, inspect the solder joints, discharge the electrolytic capacitors and then test with the multimeter (in diode test mode) all diodes and bipolar transistors (I assume you know how to do it).
    Then it would be advisable to test all electrolytics for short-circuit and high ESR.
    A similar monitor I've fixed had a broken stage on the grid driver circuit, it needed a new transistor, but the fault was obvious only once I started to probe signal with the oscilloscope.

    Frank

  3. #3

    Default

    Hi,

    I could help better if there was a higher resolution image of the schematic.

    The design looks good, it has a blanking amplifier and input signal DC clamp. The video output stage is a cascode design and drives the CRT's cathode. A negative voltage supply supplies the CRT's grid(control grid) and that supply is derived from the secondary of the line output transformer with a rectifier and filter cap. If that supply was low(not negative enough) the brightness would be very high and not controllable. Generally the CRT grid should be more negative that the cathode.

    The retrace blanking is introduced into the emitter circuit via a diode, of the lower transistor in the cascode video output stage. There could be a number of possibilities for the cause of the failure you are seeing.

    If you at least had a DVM and if I could see a clear schematic, I could suggest some places to test voltages and it might be possible to repair it without the need for a scope.

    Hugo.

  4. #4

    Default

    Ok, Thank you for sending a higher resolution image of the schematic. And its is good you have a scope and meters.

    Initially the first test would be using the meter to check the voltges A,B,C,D, see attached images. Voltage D should just represent the position of the potentiometer and C should be -60V.

    Also, if you can, try to get a voltage measurement on pin 1 of the CRT socket assembly, to check the actual grid No.1 voltage of the CRT.

    Please advise what you get with these measurements and we can go from there.

    When measuring, clip the negative lead of the meter to ground/chassis, then you only have to worry about the meter probe you are probing with. Be wary of avoiding higher voltage points as you don't want to damage your meter.
    Attached Images Attached Images
    Last edited by Hugo Holden; September 14th, 2018 at 11:03 PM.

  5. #5

    Default

    Oh Man...that was too easy!!!!

    So I started to take readings:

    A: 52V
    B: 51.4V
    C: -84.6V
    D: 1mV ???? What!!!!

    Ok something weird here, So I gave the 'CRT BIAS' pot a couple of turns and the screen suddenly came right!
    I guess the contacts had oxidised and weren't making contact. Now the picture seems nice a clear. Need to test further, but so far it's all looking good.

    Much thanks to Hugo, for without whom the fix would not have been so easy.
    And also to z8dwf for his input and support.

    I'm happy.

    Cheers & thanks

    Philip

    IMG_7217.JPG
    retro computing at: www.neoncluster.com ....is dead

  6. #6

    Default

    I'm glad it worked out.

    Open circuit or noisy preset pots on pcb's can be a major headache. Many types are not well sealed from atmospheric dust and moisture. Tek scopes are notorious for this especially 1980's vintage ones, they used two types of presets, some sealed and some not. It is the unsealed ones that give the trouble. The sealed type usually give no trouble at all.

    The spring metal contact which runs against the carbon track is often, but not always silver plated. After a while oxides develop (which are insulators) between the metal contact and the carbon track, or between the metal parts of the rotating contact itself. Often a few turns back and forth will wear it off and the connection is re-established. The tension in the spring arm is also a factor.

    Over many years I have found it better to spray them with a cleaning product first (such as CRC's CO contact cleaner) rotate them a few times. Then there is the controversial question of lubricating them. After decades of trying different techniques, I have found it is far better to add lubrication and corrosion inhibitors, a small amount of WD-40 is perfect for the job. It drastically extends the time frame to when they become intermittent again.

  7. #7
    Join Date
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    Default

    Quote Originally Posted by Hugo Holden View Post
    a small amount of WD-40 is perfect for the job. It drastically extends the time frame to when they become intermittent again.
    WD-40 is not a lubricant, it's a penetrating oil and water displacer. When used as a lubricant, it will evaporate and leave behind a gummy residue.

    I would recommend dielectric grease or 3-n-1 oil after using contact cleaner.

  8. #8

    Default

    I disagree.

    Its not a "gummy" residue, it is just a higher molecular weight hydrocarbon (Oil) component that remains when the lower molecular weight part of the solution evaporates. Ultimately what you call the "gummy" part also evaporates too, with enough time. Higher MW oils simply have a lower vapor pressure.

    3n1 oil is far too viscous for the application.

    I would caution though, that if WD-40 is being applied the surrounding area is protected with a cloth. Nearly every pcb component, bar 1 is ok with WD-40. Can you guess the one component that is not ?

    It is small adjustable ceramic or vane mylar trimmer capacitors. If WD-40 gets into them , because of its high dielectric constant, it drastically increases the capacitance. This is why WD-40 should not be used anywhere near attenuator circuits in oscilloscopes.

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