CRT Repair Help - No HV from anode - Page 1
Forum section: electronics / repair

author: Fusion916
author_rank: Regular Contributor
timestamp: December 21, 2016, 05:40:23 am
content: Hi allI'm in the process of attempting to repair a NEC XM29 PVM CRT Monitor. Everything powers up however there is no HV on the anode of the flyback transformer. I traced it back to for some reason there being no horizontal output. Actually to be precise, when the monitor is first turned on the horizontal driver is working for about 500ms then it shuts off.Now, with that said, I actually am just learning about the inner workings of a CRT monitor. I know there is a horizontal drive and a vertical drive for the yoke deflection, however why is the horizontal drive actually driving the primary of the flyback? I would of thought that would be a separate drive. Anyhow, I have narrowed down the issue here:The horizontal output drive is pin 13. When you first turn on the monitor, the horizontal drive is around ~11Vpp, however at around the 500ms mark, it drops down to ~1.5-2Vpp. It doesn't seem any conditions on the IC are changing (from what I have checked, relating to the horizontal drive). The horizontal output is driving a single NPN (Q9068) and a push-pull stage (Q5030 and Q5031) into a transformer (that drives other HV output to drive the horizontal deflection coil).Here are the schematics for the loads mentioned above.Here is the oscilloscope output for (ch11) the horizontal drive directly from the IC and (ch2) the emitter of the NPN Q9068. This drives Q9067 at the bottom of schematic 2, which eventually drives (after the opamp IC) another push pull stage that drives a transformer to drive another transistors that drives the primary winding of the flyback transformer.I am at a loss on how to further trace why the horizontal drive of the IC drops and is unable to drive the rest of the circuit and drive the primary. My first thought is to remove R9052 (second schematic) that will isolate the IC from everything else in the circuit. If the horizontal drive stays at the same voltage then there is loading issue down the line. Then i suppose I need to start removing other components. Any other suggestions in debug before I remove R9052? I have not remove any components from the board yet. If you would like me to capture anything else on the scope let me know.

author: helius
author_rank: Super Contributor
timestamp: December 21, 2016, 06:09:34 am
content: The EHV in a CRT is generated by a resonant oscillator, similar to a Royer converter, and the horizontal output transistor (the "HOT") drives it. Seehttps://en.wikipedia.org/wiki/Flyback_transformer.When the HOT is shut down by its protection circuit, you lose EHV.Try to read all the notes athttps://www.repairfaq.org/REPAIR/F_monfaq.htmlbefore diving in to HV monitor circuits, there is a lot of lore there.

author: Fusion916
author_rank: Regular Contributor
timestamp: December 21, 2016, 09:09:08 pm
content: Quote from: helius on December 21, 2016, 06:09:34 amBy EHV I assume you mean the anode from the flyback transformer. Also, yes maybe in most CRTs, but it seems from the schematics that the primary side of the flyback transformer is actually driven from the horizontal output from the IC I have in screenshot 1.I'll try reading those forums but this CRT chassis is very complicated compared to most other CRT chassis schematics I have seen. I was even looking at some last night where the primary of the flyback was driven from some simple 555 timer circuit. Anyway if you dont mind can you take a look at the actual schematics I have posted and let me know if you see anything interested or anythingI missed.

author: Fusion916
author_rank: Regular Contributor
timestamp: December 21, 2016, 09:21:06 pm
content: Let me address this one first:Quote from: aquamon on December 21, 2016, 08:58:49 amThis is no longer an issue and I have solved this issue. It had to do with the switch and the fan connections. Everything now powers up just fine now. All voltages are correct and stable.Quote from: aquamon on December 21, 2016, 08:58:49 amThe heater voltage is fine and the neck of the CRT is powering up and glowing correctly.Quote from: aquamon on December 21, 2016, 08:58:49 amThat is exactly what I posed in the OP, the horizontal drive directly from the IC. I didnt probe at the base of the HOT because even the pre-driver from the IC is dying after ~500ms. Please look at the oscilloscope captures I posted. However, a bigger problem is that the anode is dead also due to this horizontal drive since it drives the primary of the flyback transformer.Quote from: aquamon on December 21, 2016, 08:58:49 amAll power supply voltages, including B+, is ok. The only problem I am seeing is the HV anode, which is root caused to a lack of drive to the primary of the flyback transformer.Quote from: aquamon on December 21, 2016, 08:58:49 amCan you explain what you mean here? The horizontal output is there the first 500ms then diagnose, thats what I'm trying to diagnose. Even if the transistor is bad, the HOT is on a different board and is disconnected for this debug. If you see schematic 3 at the bottom right there is a connector labed "DH", that goes to a separate board which has the HOT and some other components. It's completely disconnected right now.Quote from: aquamon on December 21, 2016, 08:58:49 amI'm assuming its going into some shutdown state after the initial 500ms but I'm trying to diagnose why. Do check the internal cap do you usually need to take flybacks apart?

author: Fusion916
author_rank: Regular Contributor
timestamp: December 22, 2016, 09:54:03 pm
content: Quote from: aquamon on December 22, 2016, 11:35:52 amI already know for sure there is no HV from the anode. I tested it by disconnecting the anode and (attempted) arching it to ground. No spark. Also, like I said, there is no signal to the push-pull driver after the 500ms. I could probably arch the anode on startup as the push-pull driver is getting driven then but it quits after the 500ms I have shown on the oscilloscope. It all comes down to that horizontal drive IC.I will look at that xray protection tonight but it doesnt seem like its used. It's just grounded. perhaps the pins somehow developed a cold solder and are no longer grounded?

author: Fusion916
author_rank: Regular Contributor
timestamp: December 22, 2016, 10:59:30 pm
content: I can measure the cap, but what makes you think the flyback is blown? Physically there is no bulge coming from it.Are you saying it starts up from the horizontal drive, but continues running from elsewhere? I traced the circuit and can see no other flyback driver. I'll look over the schematic again tonight and see if I missed something.Quote from: aquamon on December 22, 2016, 10:51:41 pm

author: teddux
author_rank: Newbie
timestamp: December 25, 2016, 08:29:13 am
content: Hi,Pin 11 on that first picture posted.Pin 13 is the power pin for the Horizontal Oscillator.Could you post the first 1000mS with these two pins under your scope?

author: Fusion916
author_rank: Regular Contributor
timestamp: December 30, 2016, 01:58:14 am
content: Hi!Sorry I have been busy with family and the holidays over the past week or so so I haven't had time to go back in the lab and debug this issue. Ive been reading the replies though and it seems a lot of them disappeared. Any clue what happened?Anyhow, this is the capture of both the horizontal drive and the horizontal power supply. It doesn't seem like anything is wrong there. Any other ideas of what I can look at next? It seems obviously something does happen to the horizontal drive at around 500ms but its not the supply. It could be some load turning on or something, which is why I suggested by next step to remove the resistor R9052.Quote from: teddux on December 25, 2016, 08:29:13 am

author: TheoB
author_rank: Regular Contributor
timestamp: December 30, 2016, 06:56:33 pm
content: The driver times out as it does not receive proper feedback. My guess is that the diodes in the transformer are toast. If the primary drive is present on the transformer but no high voltage develops you do not have to search further.

author: Rog520
author_rank: Regular Contributor
timestamp: December 30, 2016, 09:01:22 pm
content: Sounds like a classic case of HV shutdown. If your B+ is OK then the flyback is almost certainly at fault. You could probably probe for shutdown (looks like there's a test point in the schematic you attached), but you'd need to know what to expect there.

author: Fusion916
author_rank: Regular Contributor
timestamp: December 30, 2016, 09:58:35 pm
content: If there is a HV shutdown event, what pin do I need to look at on the horizontal drive IC? One of those pins should be getting feedback to shut down the horizontal drive but it is not obvious which pin this would be.

author: Fusion916
author_rank: Regular Contributor
timestamp: December 30, 2016, 10:06:01 pm
content: Also, a general question for the CRT guru's.Why on this model the HV is driven from the horizontal drive anyway? I've looked at other CRT schematics (way more simple ones) and the HV drive has it's own oscillator. Here is an example, the HV drive in this one is driven by a 555.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 31, 2016, 02:49:18 am
content: Quote from: Fusion916 on December 30, 2016, 10:06:01 pmThe Monitor in the above link doesn't look like a raster display,it is probably a vector display as quite a few arcade game Monitors are.No continuous Horizontal deflection waveform means the drive for the EHT has to come from somewhere else.The vast majority of analog CRT type TV sets & Monitors obtain EHT*from the Horizontal Deflection Circuitry,for quite logical reasons.(1) Magnetic deflection CRTs require quite a lot of power to scan,so you already have an appropriate stage in the TV.(2) The Horizontal output Transformer is a necessary part of the deflection circuitry.I'm not going to give you a rundown on all the reasons it is needed,but the information is available inrealbooks!---the Internet sucks!!Hint:- Think about where the electron beam would point if there was no deflection.As it is there,it makes sense to wind an additional EHT winding on it.In fact,most TVs & Monitors haveanotherwinding,(which used to be called the "boost HT" winding in the vacuum tube days,but is still used with Solid State) which provides a higher supply voltage for the Vertical & Sound outputs than that available from the normal power supply board.(3) Any interference caused by the EHT circuit is in phase with the video,so is much less visible than a "free-running" supply.Looking at the schematics in your original posting,the third one looks to be of a different unit altogether--the component numbers don't match!From #1 & #2 it seems that Horizontal Drive also goes to IC9015 ( the "Chopper Drive" IC)via Q9058.As "Chopper" sounds a lot like something to do with the main SMPS,it appears that the switching of that unit is synchronised with the Horizontal signal------again to reduce interference.It is common with this sort of circuit to have a temporary "start up" power supply which supplies a reduced supply voltage to the Horizontal Oscillator circuit (part of IC9001 in this case)so it starts,feeding H drive to the Chopper Drive IC,which in turn starts the SMPS.It would seem that the SMPS is not starting,& that the "start up" supply has some sort of "timeout" circuit,so it stops after 500ms.*( That is what it is most commonly called by people familiar with such equipment --EHV may be more Politically Correct for some reason)

author: Fusion916
author_rank: Regular Contributor
timestamp: December 31, 2016, 03:18:34 am
content: Hi vk6zgo! I very much appreciate you taking out the time to reply!I have no idea if the previous schematic for the WG monitor is raster or vector, so maybe thats why it's driven by the 555 instead of the horizontal drive. I am just learning about CRT displays so I would of thought normally the primary side of the flyback would have its own drive but I guess that is wrong, my bad.I still don't understand why the flyback would be driven by the horizontal drive instead of it's own driver. You say books are better than trying to research the internet. Do you have any good book recommendation for CRT circuitry? Don't worry about how advanced it is, I'm an analog designer (but obviously grew up after the era of CRTs).Also, all of the schematics I've posted is from the same device, it just has multiple boards. I wish i could post the full PDF but it's 28MB. Do you have somewhere I can upload it? I would love to share the entire PDF in this thread for everyone to see.One thing I can guarantee is that there is nothing wrong with the main SMPS, that is the first place I looked. The startup is good, every voltage checks out (based off the schematics). All of the other boards receive all of their voltages correctly.I also want to point out again, and if my assumptions are wrong please correct me, there is something happening in either:1) Something is happening at one of the inputs at the horizontal IC (IC9001)2) Something is happening on the load side of the horizontal outputQuote from: vk6zgo on December 31, 2016, 02:49:18 am

author: Rog520
author_rank: Regular Contributor
timestamp: December 31, 2016, 03:34:11 am
content: It appears that the monitor you're working on uses separate drives and outputs for deflection and for the FBT/HV. Both appear to derive their signal from IC9001, however. IC2001 and TP5001 would be points of interest for troubleshooting shutdown. It's hard to say for sure because I have a hard time seeing/tracing the pieces of schematic. But this would be common for higher-end monitors with a multitude of scan and refresh rates.

author: teddux
author_rank: Newbie
timestamp: December 31, 2016, 04:47:56 pm
content: IC8701 pin 10Which is TP8701Quote from: Rog520 on December 31, 2016, 03:34:11 amI agree that this monitor uses separate deflection circuits (larger monitors).Some even use two Horz. Output Transistors.The reason the flyback is needed for this to start/run has to do with Synchronization.This changes with resolution (display mode) changes (frequency rate).It will not continue to run if the "jungle" cannot "sync"; hence, shutdown mode.I believe the sync circuitry is also in this same jungle.The reason I asked for the picture is if you doubt that there is a problem with the "high-voltage" area (common failure), is this:As the jungle contains its own oscillator, you can try freeze spray or heating it with a heat gun/blow dryer and see if the period increases/decreases.I have seen oscillators simply go dead from age.If you feel at ease replacing this IC before the larger investment of a flyback, then do so.But, please remember to reconnect your flyback 'n its stuff. INCLUDING a video source.EDIT:So, It's really your fault that you didn't supply alink to the full schematic, I would say.Now, scope pin's 5, 8 thru 10.Or just skip that and go to IC 9004...The others in here can continue this entertainment.To make things a lot faster for you:IC8701pin 10 (if nothing, pin 5).Which is TP8701 on RGB Board which is why you must have video input.What happened to aquamon's posts?He was banned; he followed the forum rules - deleted his posts as mentioned in these same rules.Not because of you, but as his signature states. I was following him and not once did he make any personal comments; yet, the biased moderators banned him.And so, good luck and goodbye.Funny thing is, they ended up exactly where he left them - 2 weeks before!Maybe I am Aquamon, maybe I am not, stupid forum doesn't follow there own rules anyway.GOODBYE!

author: xavier60
author_rank: Super Contributor
timestamp: January 02, 2017, 06:14:41 am
content: Maybe the H-drive is being clamped by a protection circuit. Latching transistor pairs are commonly used,https://upload.wikimedia.org/wikipedia/commons/thumb/e/ee/Latchup_ckt.png/170px-Latchup_ckt.png