Amstrad GT65 CRT vertical collapse.

Discussion in 'Retro & Arcade' started by qwertylesh, Mar 31, 2020.

  1. qwertylesh

    qwertylesh Member

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    Hello OCAUers,

    One of my old school tubes has suffered a full vertical collapse since the last time i ran it (back in 2014 :Paranoid: ).

    I was able to find the electrical diagram for this unit, but since i've never repaired CRTs, and realize the caps and flyback hold high voltage, im weary on starting any repairs on the mainboard (like swapping out the vhold IC)

    This model does have a vhold pot on the front, the wheel seems undamaged and i doubt its just a bad pot.

    With all this pandemic business going on I'm unsure if i could get anyone local to repair it easily. Not sure even where to start to seek pro repair or whether i should learn the safe discharge practices and give it a crack myself.

    pics of it back when it worked:
    [​IMG]

    pic of it in its current sorry state:
    [​IMG]

    I can tell its only a fault with the screen itself, since the unit appears to function. There's a fray in the main data cable to the unit that causes some of the alignment to go out intermittently (h-hold) when the cable is moved slightly, but this collapse appears unrelated and more to do directly with the vhold circuit or caps, since the fray does not alter the collapse at all.

    Anyway, i'm expecting the usual safety warnings like dont open it up and try discharging it etc. which is fair enough, and

    Alternatively I could use the unit with a video out adapter, it would cost me about $45 to get it running to HDMI and ditch the CRT, but that doesnt seem the purist approach even tho its probably more cost effective than a repair, and a complete working replacement CRT would set me back about $180 from europe.

    bring on the advice boyssss! :D
     
    Last edited: Mar 31, 2020
  2. mkusanagi

    mkusanagi Member

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    Do you have the model of the vertical deflection IC? There was a common one (TDA xxxx) that would commonly fail and was pretty popular with many monochrome displays. If it is, and you can post the model I can see if I have any spare as I have a few variants in stock for when I get jobs come in.
     
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  3. OP
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    qwertylesh

    qwertylesh Member

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    Nice, I can open it up safely and get the IC code

    If the PCB is same as the schematic then it's either Q502 or Q501

    IMG_20200331_205130.jpg
     
    Last edited: Mar 31, 2020
  4. mkusanagi

    mkusanagi Member

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    If you have the schematics it should be on there. No need to open up.. yet.

     
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    qwertylesh

    qwertylesh Member

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    Yeah for sure I don't know which of the two ic's are V or H, but those are it :)

    Edit:
    Sorry those appear to be transistors. :/
    IMG_20200331_210002.jpg
     
    Last edited: Mar 31, 2020
  6. mkusanagi

    mkusanagi Member

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    The information is there under ICs’ . I’ll cross reference and check tomorrow to see if I have the right parts.
     
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  7. OP
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    qwertylesh

    qwertylesh Member

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    I cleaned up the schematic as much as i could.

    ic701 appears to do the main video handling. I think thats functional tho since its specifically vertical only thats defective.
    I cant make heads or tails as to which one out of ic501 and ic502 is vertical.

    GT65 Schematic.jpg


    I believe some of these old designs use certain resistors on the circuit as fuses, if one has blown it could also be the cause of the collapse.
     
    Last edited: Mar 31, 2020
  8. OP
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    qwertylesh

    qwertylesh Member

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    Got it worked out.
    It is the IC701.

    the 7 pin is the Vertical Deflector IC :D

    LA1385 or a UPC1031H2 is the correct part.

    like this one - https://www.ebay.com.au/itm/LA1385-Original-New-Sanyo-Integrated-Circuit/402027163507 ( or https://www.ebay.com.au/itm/UPC1031H2-Vertical-Deflection-IC-1031-IC-1-pcs/132920241079 )
    it is similar to (but not exactly the same) as - https://www.ebay.com.au/itm/TDA3656B-Philips-Vertical-Deflection-Circuit-SIL-9-/202319342130

    replacing the 7 pin ic is doable, but I have not had to safely discharge a CRT before, and as dangerous and scrubby as this next question might be, i'll need to know the answer, can you discharge the high farrad & voltage caps without disconnecting the flyback from the tube?

    once its all discharged, how difficult is it removing the fly back, i presume it should be uninstalled to make working on the PCB easier, and when reinstalling it are there any caveats to know of.
     
    Last edited: Mar 31, 2020
  9. Flamin Joe

    Flamin Joe Member

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    I know some people have the opinion that if it's not broke not fix it, but if you're going to open it up and attempt a fix yourself I would recommend you do a full recap of the electrolytic caps as well. Judging by the age of the CRT I would say most are at their expected end of life so it is a good preventative measure to replace them now while you have it open and not have to do this again later. The caps may look good and have no signs of leaking or bulging or anything like that, but I've had instances where a cap has looked fine but while desoldering it to replace it anyway one of the legs has completely come off due to hidden corrision. Seeing this happen a couple of times has confirmed to me this is a prudent measure to take.

    Whenever I set myself to work on a CRT the first thing I do is to leave it unplugged for at least a week and I use that time to gather the components I need. Much like a capacitor a tube will slowly discharge with no power source so by the time I get around to working on it there *should be* very little if any charge in it at all.

    Second thing I do is remove the flyback of which I have two reasons for doing so with one of course being safety. While the tube *should* be discharged by this time (which has been my experience so far) it's still worth doing just in case as you just can never be too safe when it comes to high voltage. Second reason is maintenance as I like to replace the dielectric grease under the flyback anode cup which by this time has already dried up. Of course you will have to do this anyway if you are going to replace the flyback.

    You shouldn't have to remove it for greater access as most PCB's should just slide completely out after you've removed any screws, grounding wires or anything else which is attached to it. I've had no issues replacing caps, transistors or other components after doing this.
     
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    qwertylesh

    qwertylesh Member

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    im going to just order the part of ebay is cheap enough, I realise leaving the tube unplugged for many weeks isnt enough to safely discharge it. so im going to presume shorting the biggest caps safely while the flyback is still attached to the tube will be ok to neutralize anything left.
     
  11. elvis

    elvis Old school old fool

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    You can relatively safely short caps with a big fat bleed resistor attached to a ground point (I use an alligator clip to the grounded frame).

    I go around and touch all of the big fat caps for a few seconds each to discharge them slowly.
     
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  12. OP
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    qwertylesh

    qwertylesh Member

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    Heyo long time no post here.

    I ordered a vertical deflector off ebay back when this topic was active and it finally arrived by slow boat.
    So I'm going to take a crack at replacing the deflector. even after leaving the tube unplugged for a few months one of the caps had a charge. (the ol' screwdriver short, dirty method worked fine for it)
    I've measured the caps now and various parts of the board/ cables and there doesn't appear to be any more residual power.

    [​IMG]

    I noticed some crusty stuff on the pcb under whatever that module the flyback runs out from to the tube, so I've given the board a good isopro tooth brushing and it cleaned right off. all the rails seem to look fine too no real corrosion or anything I'm fairly sure the crusty stuff was 80's flux.

    ---

    Thread goals:

    1) Don't die.
    2) Hopefully fix the vertical collapse by replacing the deflector.

    wish me luck :leet::Paranoid::Pirate:
     
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  13. OP
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    qwertylesh

    qwertylesh Member

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    Conclusion time.


    It was a partial success kind of.

    For starters, I lived, unfortunately the vertical collapse lived too.

    [​IMG]
    (new part installed)

    Now, the part is different yet the previous research claimed these parts were interchangeable, the collapse does now partially change when the vhold pot on the front of the unit is adjusted, just not enough and only on one half.

    [​IMG]

    Either a different component is faulty or i need to order and swap out the NEC i just put in for an original LA1385, but at this stage i really have no idea if that would really achieve the result i'm trying to go for.

    [​IMG]

    scratchy soldering workmanship, voltar would be ashamed :(

    [​IMG]

    ohwell for the $4 it cost to get the deflector, it was worth a try.
     
    Last edited: Jun 13, 2020
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  14. elvis

    elvis Old school old fool

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    From here I'd trace the whole vertical part of the board. Test every component in the path to make sure nothing's blown out shorted, check all the low voltage points.

    It'll probably be one shitty little component somewhere.
     
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  15. sean0118

    sean0118 Member

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    If your multimeter has a low impedance mode (LoZ) you can use that to discharge capacitors. Using a screwdriver can damage the capacitor because the discharge current is very high when shorted. Probably unlikely to damage it, but it is possible.

    A screwdriver with a lead to earth is a good way to discharge the CRT, don't use a multimeter to discharge that.


    I can't really help much with the vertical collapse issue though. You could try replacing the electrolytic caps directly connected to the IC. Being ebay it's also possible the NEC you were sold is fake. :(


    edit: If you replace the electrolytic capacitors replace those tantalum ones as well.
    edit2: I think it's probably better to discharge capacitors across there terminals rather than one terminal to earth. In a lot of modern PSUs the larger capacitors are referenced to neutral, I'm not sure if discharging them to the chassis would actually work for these?
     
    Last edited: Jun 13, 2020
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  16. OP
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    qwertylesh

    qwertylesh Member

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    yeah this is a bit beyond me.
    i was surprised that he v-hold became semi functional to a degree after changing the deflector out, maybe if i take a heat gun to the majority of the underside and give all the pots a good iso'proing, i could see some change, but since that involves re discharging and re disassembling it, its going on the back burner

    yeah Joe suggested the same, to do a full recap, and i understand why, it could be the fix, but again, i was willing to blind swap the deflector out given the relative ease of work, it only took about 40 mins and 4 buckaroos to try.
     
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  17. sean0118

    sean0118 Member

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    Oh yeah, I think that's worth doing too. But I guess there's a risk in spending money on it if it can't be fixed. Should be pretty cheap to get the smaller capacitors that are around the IC701 from RS or LCSC.

    So the vertical deflection IC produces a sawtooth signal which sweeps the beam up and down the CRT. It looks like your sawtooth is present now, but too small, so it's not reaching the top and bottom of the screen. I think a faulty capacitor or maybe the adjustment trimpot / resistors could cause this. But I'm really only guessing. ;)

    edit: You should also check that the little board which plugs into the back of the CRT neck looks okay and is attached properly. Be careful though, this is also 100's volts.
     
    Last edited: Jun 13, 2020
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  18. OP
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    qwertylesh

    qwertylesh Member

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    i gave that board that connects to the glass electron gun a good clean. all those parts look ok, if im going to take a blind stab again at resolving it, i think the cap near the vhold pot may be the one worth replacing out, if i give the pot a thorough clean and retest.


    vhold pot.PNG
     
  19. Vanne

    Vanne Member

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    love learning about CRT's.. please be careful, glad you achieved your thread goal #1.

    :)
     
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  20. OP
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    qwertylesh

    qwertylesh Member

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    thanks Vanne, there's an overwhelming amount to learn to do anything properly, truthfully im just taking blind shots in the dark, and bushing up on my soldering at the same time.
     

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