Strange CRT effect

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Strange CRT effect

Hi everyone,

 

I just picked up a broken Apple IIe on craigslist. I am working on replacing the motherboard video rom chip and CPU but that is not what this post is about. I got a monitor with the computer, Taxam company. I tested the monitor with my IIGS. The monitor works but there are slow moving areas of horizontally compressed display that start from the bottom of the screen, slowly move to the top, then restart at the bottom. Only one compressed area exists at a time.

What can I do to fix this? I have included a picture below:

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My guess is that there is

My guess is that there is something wrong with the vertical deflection circuit.  Since it's only partial, my second guess is that it's an electrolytic capacitor that is in the process of going bad.  Assuming I am not feeding you bad info, you'll have to suss out which capacitor and replace it.  I've never had to work on my Monitor II (assuming that's what we're looking at), so I am not sure what to tell you beyond that.  :(  Hopefully someone on here can clue you in with more details.

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Prob a PS Capacitor

Usually this is caused by a capacitor in the power supply section of a monitor or tv which is supposed to filter out/smooth ripple a/c current supplied by the house mains once it passes through the rectifier portion of the supply which converts the a/c into dc by clipping off half the wave then smoothing out the ripple on the other side.  We used to call them ripple caps back in my tv repair days.   Once they start to dry out they no longer filter all the a/c pulse out of the rectifier circuitry and so the 60 Hz (or 50 in the EU) ripple kind of bleeds over into the other circuits rather than a stable "flat" DC.  Because the vertical circuit also coincidentally operates at 60 hertz, having this slight phase shifted a/c ripple in what would normally be a ripple-free DC voltage suppling the vertical circuitry, this slight phase shift makes the vertical circuitry slightly roll like this.  So...if you know what you're looking for and have parts or a scope or meter, I'd start by looking at any large electrolytic capacitors over near where the a/c power line comes in around the rectifier/diode area.  You might even be able to just visually see one that is bulged or has dried crusties coming out of it or underneath it around the board depending on if its radial or axially mounted.  Not familiar with this monitor at all so not sure what it looks like inside but most likely a radial capacitor where both leads go through the board and often as they start to dry out they leak electrolyte from underneath which dries into crusty stuff around the bottom.  Of course be careful, dont do anything with it plugged in, dont touch the big anode suction cup on the tube as it holds a charge for days...etc...dependong on your knowledge of electronics :-)  That's my educated guess tho...one of the large filter caps in the power supply area is drying up and needs replaced.  Probably the whole set needs recapped by this point.

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What is the exact model

What is the exact model number on the back of that Taxan?

 

I weould have expected a Taxan monitor to be PAL, as their CRT division were a UK-based company, Taxan, Ltd. 

 

Please photograph the monitor in the light, front, and rear, for identification. 

 

If it is what I think it is, then this should be the correct service manual for it. 

https://www.ebay.co.uk/itm/202956559917

 

It could also be a KG-12N: https://oldcrap.org/2020/03/07/taxan-kg-12n-monitor/

 

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Could be a PS issue like

Could be a PS issue like thellman said, although that would usually present with a darkened band also floating up through the video. And just for the record, it moves slowly up the screen because the vertical freq is 59.9 Hz in relation to the 60Hz power FREQUENCY (not phase).

 

Could also be as simple as a bad ground connection. If you're using the composite out from the Apple try wiggling or twisting the connector at each end or try a different cable.

 

 

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I believe you have identified

I believe you have identified the monitor correctly:

I apologize for the sideways orientation

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Thank you to everyone for the

Thank you to everyone for the information! I am going to take a look at the capacitors. I'll send a photo of board when I take it out. If the capacitors do not look bad, how can I test them? I have never had to test a capacitor before. I have a multimeter, but not a scope.

Also, I do not see a thin dark line with the horizontal distortion, just a wave of horizontal compression going up. The reason for it going up slowly makes sense with the slight difference from 60 Hz. I could also use some advise on how to safely work on a CRT, I know that the capacitors can be dangerous. Thanks!

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Before you go digging into

Before you go digging into the monitor (lots of danger there), have you tried feeding the monitor from a different source? Say another computer or VCR for example. That would tell you for sure if there really is an issue with the monitor.

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Steven Carpenter wrote:Thank
Steven Carpenter wrote:

Thank you to everyone for the information! I am going to take a look at the capacitors. I'll send a photo of board when I take it out. If the capacitors do not look bad, how can I test them? I have never had to test a capacitor before. I have a multimeter, but not a scope.

Also, I do not see a thin dark line with the horizontal distortion, just a wave of horizontal compression going up. T

 

Before you even open it up, buy a CRT discharge wand, a set of CRT plastic adjustment rods, safety goggles, electricians gloves, and have a second person with you. There is actual, lethal voltage inside.

 

When working on a CRT, prior to discharging the anode, always work with one arm behind your back, grasping your leg, if possible. This seems absurd, but it prevents accidentally arcing through your heart. CRT imposion is another risk involved, so you need to be meticulously careful around the CRT neck nipple. Always remove any yoke or collar/retainer attached to the tube neck before moving around anything wlse inside. 

 

You will also want a wooden workbench with at least approximately a square metre/yard of free space, and you want to erify that your power outlets are properly grounded--check every point of power, should you be using any sort of power strips or extensions from mains. You will want that space so that you can string out components and power them on, as needed (hence the wood), in a disassembled state.

 

To ensure that you do not burn the phosphour, never power on the tube without the coils also connected and powered. If you see a pinpric dot of light, or a straight line (raster collapse), immediately power it off. 

 

There is still a company who do full repair service on that monitor, which would likely represent a smaller investment than the tools and the risks involved, if you do not plan to regularly service CRTs. https://www.smhelectronics.com/taxan-products/

 

If you decide to buy that service manual, do be a dear and image it for us, as there is no PDF of it that I've seen.

 

 

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Good point. I tried it on the

Good point. I tried it on the busted Apple IIe motherboard as well. All I saw were green horizontal bars (thinking maybe video rom chip error) but I did also note the same behaviour with the distortion as I did on the IIGS.

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jeffmazur wrote:Before you go
jeffmazur wrote:

Before you go digging into the monitor (lots of danger there), have you tried feeding the monitor from a different source? Say another computer or VCR for example. That would tell you for sure if there really is an issue with the monitor.

 

The only way that this could not be a problem with the monitor is if the signal itself is getting enough noise to disrupt its sync/phase, or if it was the completely wrong signal/frequency. I doubt that any of those are possilble with the //gs, but I suppose eliminating the possibility can't hurt.

 

TBHT, the old slap test is probably more useful in determining if the monitor has issues. 

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Thank you for the safety

Thank you for the safety information. It would indeed be a great investment in learning and equipment to try to fix the CRT. I suppose that is part of the fun of this old equipment but I will consider having the monitor serviced if it will be reasonable conpared to purchasing a replacement. I have limited means right now.

I will try to do some more digging on this issue. I promise to scan to PDF the service manual if I can obtain one. Where would I upload this to?

Also, if anyone else has any suggestions of what they would recommend I do I will keep checking back here to get more insight.

I really appreciate the help from everyone! This community has a lot of knowledge and I appreciate that you all share it with beginners like myself.

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Just a quicky, anyone suggest

Just a quicky, anyone suggest just trying different leads to hook it up?

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What do you mean by different

What do you mean by different leads? A different RCA cable?

Also I have a little update: I opened the case and inspected the board. I did not see any sign of a bulging or blown capacitor but I did see that the power filtration seems to be handled in a metal cased in area. I do not yet see a good way of opening this area so I have left it be.

I also noticed that the distortion is actually both horizontal and vertical, but it moves as a long region from the bottom of the monitor to the top.

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Any video distortion that

Any video distortion that slowly moves up the screen is an indication that the 60Hz power signal is getting somewhere it should not be. This can be caused in one of three ways:

 

1. Power supply ripple. As previously mentioned this would probably be due to one or more faulty capacitors. This is the most likely issue (especially if the monitor has a pincushion correction circuit) but is also the hardest to troubleshoot without a scope and the necessary skills.

2. Electrical interference from outside the monitor. 60Hz signals coming in on the video cable or through the case due to a poor ground connection. Easy to check - just try a different RCA cable (like a cable with Yellow connectors meant for Video). Usually this will cause sync issues rather than that collapsed video, but it's worth a try.

3. Magnetic interference. This could come from a large transformer nearby (or on the other side of a wall - yes, I have seen that!) or from another CRT display very close by. Again, easy to check by simply moving the suspect monitor to a new location. This kind of interference is usually much more subtle (just a line or band of incorrect luminance) so probably does not apply in your case.

 

Bottom line: try a new cable and if that does not help, you probably will need to engage the services of a TV/monitor repair person.

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Steven Carpenter wrote:What
Steven Carpenter wrote:

What do you mean by different leads? A different RCA cable?

Also I have a little update: I opened the case and inspected the board. I did not see any sign of a bulging or blown capacitor but I did see that the power filtration seems to be handled in a metal cased in area. I do not yet see a good way of opening this area so I have left it be.

I also noticed that the distortion is act

 

That is what he means. If you photograph the enclosure at many angles, I could probably deduce how to open it, but without you owning the appropriate safety equipment, I am reluctant to give further information that could put you at risk.

 

I'm an old bugger, so whenever I open a CRT, it could be my time to go.

 

Have you tried the old CRT slap test? Applying slaps in gentle to firm pressure to the side of the entire unit to see if it changes the issue generally tells you if it is power board/capacitor or yoke/deflection. Hell, we used to slap 1701s on a regular basis to stop them rolling, as they have a sync issue when their caps or their power connections go south, and we ran a lot of them as test monitors. 

 

Overheating easily dries out old caps. You won't see physical damage but the actual capacitance of them will go south. You will need either a very good multimetre or a scope/probe to check each one, and agiain, if you are going anywhere near the flyback, transformer, yoke, deflection coil, or the anode, you need safety equipment, patience, and proper tools to check it all. 

 

The lowest budget solution is to buy a Monitor //c. Those should be common as dirt, and they used to be close to free, although now people want your right arm for anything with an Apple logo. :shrug:

 

Taxan monochrome CRTs were never popular outside of the UK, where they had large market control due to contracts with Dell, who were right next door (literally), and to a much lesser extent, Japan. That is the same Taxan who published NES games (most famous for publishing '8-Eyes') in the later 1980s and 1990s. 

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Hum Roll

I agree with all of the above.  I probably posted prematurely.  Phase/freq are similar in that a frequeny introduced into the B+ which is out of phase with the oscillator is a phase issue but also a frequency issue, so semantics more than anything here but I won't argue the point.  First try a new different cable...make sure nothing RF-interference-wise is nearby.  And yes, unless you are really knowledgeable with regards to safety, CRT's can be dangerous inside - there is a 18-20KV anode there ojn the tube which holds a charge for days, and the large caps can hold charge for hours on the board.  That said, my bet is on that large can capacitor in the power supply area..  This slowly moving hum-bar is a tell-tale sign..and once it gets really bad it does even turn into a black/gray bar that moves up or down (usually up), but with only a light drying can cause just a raster hum that moves.   With a monitor circa 1985, probably most of the can-electrolytic  capacitors are on their way out, especially the larger power supply caps.  But yes please be careful.  A Volt-Ohm-Meter isn't going to tell you much wirth regards to caps...but if you have a source to buy them cheaply or some old scrap bhoards from which to pull I would start with the large can cap in the supply and any others in that shielded PS area and rthen move from there. Make sure the voltaqge rating is => than the one you have..ie if it says 1000 mFD at 16 volts, a 1000 mFD at 25 volts will be fine, jusrt don't go less on voltage ratings.  If the cable doesn't help, that's where I would start anyway.  If not RF inte5rference, its ripple bleed into the supply and ripple bleed is almost always due to old drying caps.

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Isn't there some vertical

Isn't there some vertical collapse going on too though?  Could power supply ripple cause that too, or must there be something else wrong?

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Vertical

Hard to say based on the pics..but could be.  If the circuit was expecting say a 35v DC and was instead only  getting 20V DC with a 60 Hertz plus ripple,  then yes, the vertical deflection would be less PLUS a sync phase offset  based on any ripple atop that...    Without schematics and voltahes its only a guess, but  my bets are for a decreased B+ output, along with a top-side ripple, causing a decreased vertical output atop a ripple effect roving hum-bar resulting in the decreased vertical scan  and modulating wave. Just a guess.  But Id be looking to replace the PS caps and go from there.  

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Lee Adamson wrote:Isn't there
Lee Adamson wrote:

Isn't there some vertical collapse going on too though?  Could power supply ripple cause that too, or must there be something else wrong?

 

I figured this was simply due to not adjusting the raster height. (It could be raster collapse, but it could also be out of adjustment.)

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Thanks everyone for the

Thanks everyone for the advice! I will try the slap test and a new cable soon and post a brief update. I have one more question: I live in a very old building, may over a century old. We do not have grounded outlets here anywhere. I know this is a hazard, but I rent this place and did not realize it was not grounded because some of the outlets are 3-prong. Could this be a possible cause for this issue?

I tried to find the law in my state. I can't yet find if having an ungrounded 3-prong outlet is illegal, but that is beside the point.

(EDIT) I have realized that the monitor does not have a ground connection on the cord, so I am guessing that this is not related. I tried a brand new RCA cable to no avail. I will try the other suggestions. I will also think about getting a different monitor or replacing the crt in this montor with an lcd screen--I think that could look cool.

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grouded

I really doubt this is the cause.  The 3rd grounding prong is technically the same electrically as the larger-blade (in the US) of the "neutral" prong  of the plug (both are connected to earth ground usually but not always through 2 separate grounding rods or connections in the electrical box).  It is a redudant 'safety' ground but isn't actually used for anything during normal operation of any device.  The third prong exists as an extra metallic case ground in case something goes wrong in a power supply so that rather than you being the third ground to earth the third prong does it instead.  Inside a computer (or power supply, drill, fan, overhead lamp, etc) the third prong connects to the metallic case so that if the regular neutral connection/prong fails inside the device and/or shorts that conection to the case,  it will short through that prong and not your body.  Most old buildings never had them and even now they aren't technically used for anything but that extra safety factor.  

 

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monitor

You can (and should) try a different monitor.  Anything with a composite (yellow cable) input should work although some LCD's on an Apple II often have sync issues.  You can even run the output from the II through a VCR input and then the output to a TV - VCR's are great at cleaning up an Apple II signal for an LCD.  Definitely worth giving either a try.  I'm still willing to bet money on the monitor you have just needing the power supply and probably other cap(s) needing replaced.  

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Interesting turn of events: I

Interesting turn of events: I fixed the Apple IIe yesterday by putting in a new video ROM chip. I tried it with the monitor, at first I had no luck. The screen was still distorted for many reboots. After an afternoon using the monitor regardless of the distortion, I rebooted the computer again and the monitor has appeared to be fixed ever since. I do not know if this will last, but it has been working properly for a day now. Why might the monitor start working again? I didn't hit the monitor or change the cable.

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Monitor

Well there are a couple scenarios that could have happened, one is the cable connection in the monitor (or A2e)  could have had microscopic layers of crud/oxide/gunk buildup which finally after a few cable swaps finally has a good connection, or 'capacitor reforming' has occurred.  After a capacitor has been un-used for some time (years) an internal oxidation can occur on the metal sheets inside, which after re-use, slowly reverses the process and eventually re-forms itself and returns to somewhat normal functionality.    Also could be one of the potentiometers had an old/oxide connection between the swiper and the plate which has finally after some current flowing through it has restored a better connection.  Who knows. Keep and eye on it, but if its working fine now I wouldn't bother to replace anyhing unless it recurs. 

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Steven Carpenter wrote
Steven Carpenter wrote:

Interesting turn of events: I fixed the Apple IIe yesterday by putting in a new video ROM chip. I tried it with the monitor, at first I had no luck. The screen was still distorted for many reboots. After an afternoon using the monitor regardless of the distortion, I rebooted the computer again and the monitor has appeared to be fixed ever since. I do not know if this will last, but it has been

 

My term for this, has and always will be, CRT Pre-Warm. Essentially, lazy components are given enough time to build a capacitence charge, and stop being as lazy. If you leave it off for a few days, sometimes weeks, it will probably revert, but if you use it regularly, it may be fine for an indefinite time. 

 

This tells me that is is absolutely a capacitence issue, probably on the power board or in the vertical coil circuit. 

 

This same type of thing ccurs when you leave any CRT off for a very long period of time, and its components are getting old. One of my Monitor 100 displays exhibited some symptoms where the image was out of focus or the colours were horribly wrong after 20 years of storage, that regular use has at least, temporarily restored. Likewise, one of my Monitor /// displays had diagonally scrolling lines that went away after a few hours of use after 20 years of storage.

 

Have you been able to properly adjust the vertical size? 

 

My general tip here, is to use the system at least a few times a week, so that you keep components in a woken state, an you o not need to pre-warmup the CRT, until you can do a proper recap. Do not leave the monitor on when not in use, as this risks other possible failures. 

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These explanations make sense

These explanations make sense to me. It probably is a capacitor issue. I will keep using the system and hopefully I will not need to replace the capacitors for a while.

I am very happy for the time being!

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