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CRT repair help

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MatthewH1919

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Sep 8, 2024
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Hi, I initially posted this in off topic and was told to post it here with better photos, also i should mention im not very knowledgable with analogue electronics.
a few months ago I picked up a jvc A101G video monitor. Just the other day while i was using it, it suddenly went out of focus, distorted, larger and the colours were more saturated looking. And after turning it on and off there is also a large black bar on the left side of the screen running from top to bottom (this is why the text is cut off in the picture). I recapped the entire monitor, (except for the higher voltage caps as I dont have any) but it didnt make any difference, nothing looks noticably burnt out. What is the likely cause of this, im worried it might be the tranformer failing, as people with similar issues said it was. I have the schematic for the monitor however its not much use to me as im not really able to decifer it very well. I can take some voltage readings from the transformer if someone would be able to let me know if they seem normal.
any help is much appretiated.
thanks, matthewunnamed (1).jpg20240916_140428.jpg
 

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To start with, you need to confirm that the power supply output voltages are correct, particularly the 115v to the flyback. Have you tried adjusting the focus control to see if it does anything?
 
As @andy has stated, I would also measure the output(s) from the power supply unit (to make sure that they are all stable and within specification). This can be done with a multimeter set to the correct range. Just be careful in there (due to the high voltages floating around)!

The issue does seem related to the horizontal output (in the first instance). Most people (incorrectly) jump to the conclusion that it is the high voltage transformer that is to blame!

The signal path for the HOUT is:

IC201 pin 4 (H.OUT) -> Q501 -> Q502 -> scan coils.

The technical manual contains the expected oscilloscope traces to be found at these points. However, I am guessing that you do not have an oscilloscope or the knowledge to use such a device?

Dave
 
the focus control did make it clearer but not a whole lot. I also suspect it was left outside for a while.
im pretty sure it was left outside for a while also as the rf shield has rust on the top.
the flyback is reading 114.7v dc between pin two of the flyback and ground.
in the power supply i am getting readings of 122v and 60v ac on the legs.

I actually just began an electrical engineering degree this week, so the small amount of knowledge i have know is just from dissasembling tape decks and repairing them.
id imagine in the next few weeks ill have access to an oscilloscope.
if I cant manage to get this thing working soon anyways i can always put it away until im more capable
 
If you have just started an Electrical Engineering degree then "learn a bit and do a bit"...

I started the same way (but many many years ago), repairing radios, record players, HiFi. Then moving on to electronic musical instruments, televisions and finally computers.

In doing so, the extent of the test equipment increased and went through upgrades as finances allowed. I started off with a £20 oscilloscope and went through at least 4 upgrades.

Dave
 
I'm NOT an EE, so most of the analog stuff confuses me. I found this book helpful. It gives general overview of the process. It's fairly accessible. If there's already a picture, I've usually fixed a CRT with limited knowledge.

https://www.amazon.com/Troubleshooting-Repairing-Computer-Electronics-Technician/dp/0070057346

You sound knowledgeable and are starting EE, so I'm sure you're aware those CRTs pack a punch on the flyback. Worst thing is you tend to move your hand away in shock and hit something else, like line voltage if you're unfortunate.
 
mR_Slug said:
I'm NOT an EE, so most of the analog stuff confuses me. I found this book helpful. It gives general overview of the process. It's fairly accessible. If there's already a picture, I've usually fixed a CRT with limited knowledge.

https://www.amazon.com/Troubleshooting-Repairing-Computer-Electronics-Technician/dp/0070057346

You sound knowledgeable and are starting EE, so I'm sure you're aware those CRTs pack a punch on the flyback. Worst thing is you tend to move your hand away in shock and hit something else, like line voltage if you're unfortunate.
Click to expand...
No the, flyback doesn't pack a punch. The current sourced from the EHT is very low, usually well under the 15 to 30mA danger threshold. Most EHT supplies only have to provide 1 or 2 mA of current. The beam current for a CRT is usually under 1mA.

It is more paranoia than anything else, because you can draw large scary looking sparks from the EHT or from discharging CRT's bulb capacitance. Much has been made of this by idiots on youtube. The stored energy in a CRT bulb capacitance is normally under 200mJ (unless a large color CRT), harmless, it simply gives people a fright, nothing more.

What is a worry though in CRT VDU's is the B+ voltage in the set, because any power supply that is over about 75V that can source more than 30mA is potentially lethal. So forget about worrying about the CRT's EHT, the flyback transformer and the CRT's harmless stored charge after turn off, instead worry about the VDU's DC power rails if they are over 75v. As you say, line voltage is more hazardous.
 
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And don't forget to power your setup from an earth leakage circuit breaker (or whatever they are called in your country).

One day, this could save your life - but it is still no substitute for being savvy!

Dave
 
Hugo Holden said:
No the, flyback doesn't pack a punch.
Click to expand...
I'm a bit confused, as you basically restate my point. In 240v-land a typical monitor will have 3 amps at the wire. Ok, usually it's more difficult to touch line voltage than any of the wires by accident (except in mac pluses etc, that switch with the back off nearly caught me once). By pack a punch I mean it hurts.

My intention is not to frighten, just point out, that reaching into adjust a pot, while in operation is something that presents multiple dangers. There's a tool that the name escapes me, its basically a long plastic pot adjuster. Plan ahead, like have a way to cut power without having to reach over it and pull a plug. If you need to reach round to adjust something, turn the monitor away from you and get a mirror, much much easier.

My safety record is pretty bad, but the shocks make me improve somewhat.
Testing_Electricity_Safety.JPG
...and the unfiltered Camel didn't even drop from his lips.
 
I also remember working in an environment where routinely electricians would test for live or dead with the back of their finger. Argh! They are dead now - either prematurely or of natural causes!

In the UK we would likely have a 3A fuse in the plug. This is designed to protect the cable and not the equipment, and is designed to go open circuit when a fault occurs that draws a current in excess of 3A. This will not protect the human in the loop if they get their fingers (or other appendages) in the way.

This is why I was suggesting the inclusion of an earth leakage circuit breaker (or RCBO) inline with the mains socket and the equipment. This is designed to trip at way less then 30 mA and under any condition that will result in a current flow to earth or a current imbalance in the Line and Neutral conductors. This will protect the human (if it works).

Dave
 
Thanks for the help everyone,
I live in ireland which has basically the same electrical system as the uk so it should be grand, my kettle plug has a 3 amp fuse also
 
Well, something is obviously causing the HV to be too low, and appears to be affecting the horizontal blanking, causing that black bar in the picture. Since the B+ voltage to the flyback is correct, and there's no direct HV regulation, it doesn't leave a lot of possibilities.

I see a signal called "FBP IN" (presumably flyback pulse in) coming from the collector of the HOT, and going to the signal board. It probably has something to do with blanking, or synchronizing the on screen display. I would be looking at the collector circuit of the HOT since a problem there could be causing both problems. Check for shorted diodes, and look at the ceramic, and film capacitors, even though they rarely fail.
 
I'd also vouch for a failing flyback transformer. Sudden out-of-focus picture and low HV causing increased raster, plus the blanking issue... Unfortunately, if that's true, it's not a part you can replace universally, the whole horizontal stage is built around it.

Hugo Holden said:
No the, flyback doesn't pack a punch. The current sourced from the EHT is very low, usually well under the 15 to 30mA danger threshold.
Click to expand...

Hand-to-hand shock from a DC flyback of a free-running horizontal stage feels like a strong cramp immediately followed by a punch to the chest, expletives afterwards. The actual injury comes from the resulting motion of getting away from it, taking whatever nearby with you, the current won't kill.
At line voltages such as 230 volts AC, you're at the mercy of the (initial) skin resistance between whatever made the contact. Below 10 milliamps of current you usually jerk free, damning the army of ants and the buzzing in your head. Above that, your muscles may not let go voluntarily, and every microsecond will feel like eternity. Having said that, no GFCI will save you from hazardous electric shocks where the current from the live wire returns through the neutral unhampered. The nearest household example of that would be a microwave oven... (But you can wire 6 of them transformers in a three phase configuration for an interesting show in the bathroom.)
 
deanimator said:
I'd also vouch for a failing flyback transformer. Sudden out-of-focus picture and low HV causing increased raster, plus the blanking issue... Unfortunately, if that's true, it's not a part you can replace universally, the whole horizontal stage is built around it.



Hand-to-hand shock from a DC flyback of a free-running horizontal stage feels like a strong cramp immediately followed by a punch to the chest, expletives afterwards. The actual injury comes from the resulting motion of getting away from it, taking whatever nearby with you, the current won't kill.
At line voltages such as 230 volts AC, you're at the mercy of the (initial) skin resistance between whatever made the contact. Below 10 milliamps of current you usually jerk free, damning the army of ants and the buzzing in your head. Above that, your muscles may not let go voluntarily, and every microsecond will feel like eternity. Having said that, no GFCI will save you from hazardous electric shocks where the current from the live wire returns through the neutral unhampered. The nearest household example of that would be a microwave oven... (But you can wire 6 of them transformers in a three phase configuration for an interesting show in the bathroom.)
Click to expand...
I can promise you that I likely have sustained more shocks from high voltage apparatus than anybody on the forum, over the last 52 years. I started playing around with high voltage supplies when I was 13 years old (now 66). I have had every manner of voltage and current source applied to my body, including a 30kV neon tube transformer (including an episode were it was applied to my face), being attached to a high power 400V DC supply for over 30 seconds, a 300J shock from a defibrillator and multiple shocks from VDU's, tube amplifiers and an 8J burst from a farmers electric fence when I was standing on wet ground with bare feet. Also numerous shocks from 230V line wiring. I know what a "punch" is and a shock from a CRT's EHT is certainly not one of them, but the defib and the farmer's electric fence certainly was. Charge a capacitor bank to over 600V with 5J to 10J of stored energy, connect yourself to that, then you will know what a punch is. It works very well for cattle and they seldom re-approach a fence after receiving one.
 
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Hugo Holden said:
I can promise you that I have sustained more shocks from high voltage apparatus than anybody on the forum, over the last 52 years. I started playing around with high voltage supplies when I was 13 years old (now 66). I have had every manner of voltage and current source applied to my body, including a 30kV neon tube transformer (including an episode were it was applied to my face), being attached to a high power 400V DC supply for over 30 seconds, a 300J shock from a defibrillator and multiple shocks from VDU's, tube amplifiers and an 8J burst from a farmers electric fence when I was standing on wet ground with bare feet. I know what a "punch" is and a shock from a CRT's EHT is certainly not one of them, but the defib and the farmer's electric fence certainly was. Charge a capacitor bank to over 600V with 5J to 10J of stored energy, connect yourself to that, then you will know what a punch is.
Click to expand...

I am a super chicken around power now... Most recent shock was from an old Amstrad monitor I mis-unplugged and touched the live section. ouch. At least we have RCDs in houses now. I had a sparky put one on every circuit first chance I got. I've had lots of mains shocks, including across the body. Also been struck by lightning but don't recall the event at all so couldn't mention the circumstances, and no scars or anything like that from it.

I try to stay away from shocks now. I figure electricity is out to get me.
 
Hugo Holden said:
started playing around with high voltage supplies when I was 13 years old (now 66) (...) Charge a capacitor bank to over 600V with 5J to 10J of stored energy, connect yourself to that
Click to expand...

And I thought wisdom comes with age. Perhaps there are exceptions to rules, who would have thunk.
 
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