(Solved) Old CRT circuit repair - Page 1
Forum section: electronics / repair

author: SpidersWeb
author_rank: Unknown
timestamp: June 26, 2014, 05:53:26 am
content: New to the forum, and while I have electronics knowledge, I'm still a beginner in the big scheme of things - so I thought I'd post my thread here, rather than Technical, but if I got it wrong just let me know.As a hobby I maintain/collect computers from the 1980's. Generally if a PSU or CRT fails - I replace it - however in the future I'm going to need to be able to repair these properly, and doing that will take some learning piece by piece. I have 4 CRTs to begin with and I haven't bought myself an oscilloscope yet - but it's on my 'to get' list.So the first project I want to take up is a monochrome display from 1987 - and I was hoping you guys could either discuss the circuit that's at fault so I can understand how it operates better, and/or point me towards which components are most likely to be failing to cause the issue.So here is what I get:WhatI thinkis happening is the horizontal deflection yoke isn't getting enough voltage to pull the beam to the left, so it starts drawing near the center, gets a little bit right, then oscillates backwards. So any lines which contain more than 5 characters, start overwriting each other. "C:>_" should be hard left, not centered, and the content above that is text laid on top of text - until you just get a mess.Vertical is perfect.This is the circuit that is responsible for feeding the horizontal deflection yoke:The arrow is pointing to the trigger input - Hsync - which pulses at 18.4KHz. The line below the arrow is +12V.The horizontal deflection yoke can be seen up the top - labelled H-DY.Myunderstanding(I could be wrong, I'm new to this) is that when Hsync comes in, it should apply full voltage to the H-DY - pulling the beam to the left, then gradually reduce moving it towards the center, then start applying an inverse voltage moving it right, then the next Hsync should come in, and it should repeat for the next line.(I'm also aware of the dangers working inside these, I'm very careful, take my time, and make sure components are discharged before manhandling things)Any information or discussion on this circuit appreciated! Apologies if this isn't the place for this, just I'm quite interested and looking for people with more knowledge to clue me in.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2014, 07:33:03 am
content: Unusual, because failing horizontal drive generally means HV is going to crap as well. Which should cause the picture to dim and bloom (deflection sensitivity is way higher).Tim

author: Sparc
author_rank: Contributor
timestamp: June 26, 2014, 07:40:10 am
content: Its good to see someone looking after the old gear. I've only repaired a small number over the years, but none recently. CRTs are almost gone now. For some background theory about CRT operations and repair, see "Sam's REPAIR FAQ". Its a free document on web. Lots of good info there for repairing CRT TV and computer monitors, and many other things.As best I can see from the circuit: When Q503 is OFF, current flows into the H-DY and charges up C505. When Q503 is ON, current flows back out of the H-DY and C505 also discharges through the coil. Generally, the horizontal output transistor acts as a switch, either ON or OFF.For this monitor, yea, it looks like the H deflection coil isn't getting enough current to deflect the beam. First thing I'd check is for burned/discolored components. Check for bad solder joints with a magnifier. They can especially develop in the high current, high temperature areas of circuit boards. Check the power supply for correct voltage. Electrolytic caps can go bad.Check the components that drive the base of Q503,BU189. If Q503 doesn't get enough drive current it won't pull enough current through the coil and could overheat and fail. Check R507, C503 and everything backwards from it.Looks like BU189 is a darlington transistor. You might have to pull it from circuit and test it independently. Maybe check other components at the other end of H-DY, C504, C505, etc.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 26, 2014, 07:50:23 am
content: Since the HV seems more or less ok you should start by looking at the components that are just there for the horizontal deflection, like the deflection coil itself and the return circuit like C505 and the components around there.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 26, 2014, 07:53:08 am
content: It looks like you are getting EHT,so it would seem that the horizontal output is working to some extent.Two possibilities come to mind:-(1) C505 may be faulty.so that it places a high impedance between the junction of L503 & C504,& ground,reducing the picture width.(Note that this cap needs to be a low ESR type)(2) Even though you havesomeEHT,the voltage on Q503 collector may not be correct.Normally,at start up,that voltage will be about +12v or so,but as soon as the Horizontal output transformer is fed with signal,this voltage will rise to,typically,around +120v due to the action of D502,C506,which make up what we called in the vacuum tube days a "Boost HT" circuit.If either of those components are faulty,you will lose this voltage.I doubt that (2) is the case,though,as you still have full vertical scans----vert output also uses "Boost HT".

author: Andy Watson
author_rank: Super Contributor
timestamp: June 26, 2014, 09:13:51 am
content: Another vote for looking closely at C505. This cap is usually working under very stressed conditions - greatly accelerating its demise. They dry-out and become high impedance. This was a common fault leading to the classic shrinking screen on the old Apple Macs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2014, 10:02:47 am
content: Looking at the schematic, I don't see why it needs to be nonpolar anyway. It should have full DC boost on it. Are NP lytics more robust (higher ripple / AC voltage ratings)?An open somewhere in that path (also check continuity on L501-L503) would easily explain the symptom: in fact, I'd pretty reasonably guess the top end of L501 is still connected, because the switching edges are making it bounce -- it's still scanning a little bit, but very weakly, and only for a short period, pretty much going nowhere otherwise.Tim

author: What_NZ
author_rank: Regular Contributor
timestamp: June 26, 2014, 10:28:34 am
content: Quote from: T3sl4co1l on June 26, 2014, 10:02:47 amYes I agree, I would say L501 or L503 (or soldering of) is open circuit. the reason it is scanning at all is the circuit made by R506,C504 & C505.

author: SeanB
author_rank: Super Contributor
timestamp: June 26, 2014, 07:38:03 pm
content: Quote from: T3sl4co1l on June 26, 2014, 10:02:47 amIt needs to be a non polarised capacitor, as it is part of a resonant circuit and will swing to about the supply voltage below ground to drivve the coil to deflect to the left. These caps do die with age, so check them and the supply caps as well, and look for bulging ones. With these as well look for smaller electrolytics that dry out with age, and high value resistors that go high in value or open circuit.

author: RazSlack
author_rank: Contributor
timestamp: June 26, 2014, 07:53:10 pm
content: I've dug up my old "picture errors" CRT repair handbook. Here's what's in it:First off, a precaution: Don't keep the screen running like that for a long time, it might damage the CRT, causing a permanent burn-in.The book doesn't say any other useful stuff, so this is all my contribution I can give. Check the horizontal amp and the cabling, all it says. Bah.

author: SpidersWeb
author_rank: Unknown
timestamp: June 26, 2014, 08:05:27 pm
content: Thanks guys, this is exactly the kind of discussion I was looking for. I think I'll have to spend a lot more time here.I'll start by checking the continuity of the inductors/circuit in that section. If I don't find any issues, I'll start replacing the 5xx electrolytic caps one by one. C505 is an easy starter. I was unable to see any visibly damaged components or dark/burned areas.I also need to confirm that this circuit isn't modified from the diagram, logically it seems to match up with the diagram but there was a couple of open unlabelled spots on the PCB so I'll double check that.I may not get back to it until tomorrow, but here are a few pictures in the mean time.Neck PCBMissing inductor spot, C505, L504 etcNB: Black soot on resistor in last picture - it was in a factory controlling a welding machine for many years, it's just soot that wipes off, resistor isn't burned.Most of the circuit is here, Q503 attached with a heatsink, Q502 nearby.Bottom of main PCB:

author: SpidersWeb
author_rank: Unknown
timestamp: June 27, 2014, 11:59:49 am
content: Quote from: What_NZ on June 26, 2014, 10:28:34 amSo, if one connector to L501 was disconnected, that would still allow for a bit of scanning?I'm double checking because the other coils checked out OK (continuity), and then I checked L501.L501 actually does the vertical and horizontal, it's just clipped off the diagram I posted, it has 4 wires going in - one to each end of a coil.Two of the connections show 0 ohms and gradually increase to 7.9 ohms.The other two appear open circuit :/

author: What_NZ
author_rank: Regular Contributor
timestamp: June 27, 2014, 12:07:03 pm
content: Sorry my mistake. My post should have read L502 or L503......If L501 was open then you should get a thin vertical line.Although in saying that. I just thought, what would happen if L501 had a shorted turn?

author: SpidersWeb
author_rank: Unknown
timestamp: June 27, 2014, 12:23:21 pm
content: Ahh well L502 and L503 check out okay (and L504).L503 did give 1 ohm, the others 0.And now I'm getting continuity readings on both of the coils in L501 - so possibly a bit of a red herring - I did carefully adjust one of the coils leads but the solder joins looks good and I can't be sure I just didn't make some mistake when measuring. But L501 is OK.Edit: to check for continuity, I'm just using DC resistance, I haven't worked with inductors before so wanted to check that was correct.

author: What_NZ
author_rank: Regular Contributor
timestamp: June 27, 2014, 12:30:56 pm
content: Resistance is futile (Star Trek Borg joke) when measuring inductors for a shorted turn.You need a shorted turns tester or the like. An inductance meter may not show a problem.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: June 27, 2014, 06:00:02 pm
content: Have a gander here,CRT monitors-Part 1of 4Randy should help you identify the problem

author: zaoka
author_rank: Frequent Contributor
timestamp: June 27, 2014, 07:18:31 pm
content: You should replace C501, C502 and C503 without testing if you dont have ESR meter. Test Q501 and Q502, D502, C505, C506 and C508. Test transistors and diode with analog multimeter (1K range).C505 should be tested for both; capacity and ESR, you can use two 22uF 25V, connecte minuses together and use plus sides to solder in the board.Also if power supply is low you will get the same symptom, L504 is supplying that. Make sure it works correctly. It uses either 10.8V or 12V. Also make sure that filtering for this voltage is OK.L503 may be short, however, I dont this thats the problem, it usually change width only.Also post full schematic if you have one.Most critical parts are C505, C502, C503 and power supply.

author: sunnyhighway
author_rank: Frequent Contributor
timestamp: June 30, 2014, 09:48:55 am
content: Quote from: SpidersWeb on June 26, 2014, 08:05:27 pmCould be nothing, but R414 seems to be chipped a bit.

author: SpidersWeb
author_rank: Unknown
timestamp: July 02, 2014, 05:53:42 am
content: Thanks to everyone who replied, sorry for the slow updates, I still haven't got the caps to go in yet.Mr Smiley - great link! I haven't finished watching the video but I'll make more timezaoka - I'll be blindly replacing all the electrolytic caps in the 5xx section and see if that helps. C505 I'm thinking may have already been replaced. I went back to my old notes from before I got it, and the previous owner took it to a repair agent who said it was out of his knowledge range and was unable to get it running again - I think he may have replaced that cap. It's the only bright green one, higher voltage (100V) than on the diagram, and identical to the NP model sold by our local JayCar.D502 checked out ok, need to test, Q502 and Q501 still.Sunny - I'll check itI've attached the full diagram for anyone who is curious.

author: SpidersWeb
author_rank: Unknown
timestamp: July 02, 2014, 05:57:01 am
content: Also I've assumed that inductors in this circuit should be near 0 ohms.I get < 1 ohm from between the top of C505 to P503 (the plug for the yoke).

author: vk6zgo
author_rank: Super Contributor
timestamp: July 02, 2014, 06:46:16 am
content: Quote from: SpidersWeb on July 02, 2014, 05:53:42 amBe cautious with generic NP caps.I had one which was "obviously dead" in an Electrohome Monitor.Replaced it with a new (much smaller) NP,marvelling at how technology had advanced since the original had been made.The new one lasted 5 minutes then let the "magic smoke" out----the "big" one was low ESR,& the new small one wasn't.Mega-Oops!!1 minute's closer perusal would have shown me that the replacement would be inadequate.As it happened,I had to make a "Christmas tree" of poly Greencaps to replace it,as that capacitance, voltage rating ,& ESR value.was not locally available in an NP.Two polarised electros of suitable rating connected in "reverse series" was even bigger than the poly stack!

author: SpidersWeb
author_rank: Unknown
timestamp: July 02, 2014, 08:10:02 pm
content: I'll check the labeling on it to make sure it has a low ESR rating (and check that it is actually new and not a 1988 factory substitute). This guy likely wont see power again until I've either found the faulty component and/or replaced the caps in that section and put it back inside it's case. This is also because I accidently pulled a +12 line when I moved the board and need to solder it back onLast night I went through the diodes in that section and Q501-Q503 - allappearedfine (using a diode test while in circuit at least - showing 0.45 to 0.68V with correct polarity). Was going through the resistors and then got interrupted by dinner - but each one tested so far shows it's inline with the diagram. I'll post the list and recorded values when I finish (actually recording them this time).If I don't see a specific issue, and caps don't help, I'll put it to one side until I get an oscilloscope running. I did grab a 10Mhz dual channel from a recycling center with the plug cut off - but no idea if it works yet - always planned to get one of those affordable Rigol models - and some probes with clips because I'm not a fan of poking around in a 25-30 year old CRT while it's on.Others I need to look at:- Amstrad PC-MD - brand new in box from 1987 - first power up was last week and it's got multiple issues (probably just needs a recap) plus I need to hack it slightly to make it useful (it has a full PC PSU inside and that's more of a problem than a help).- Sprite RGB display - sync was crazy when powered up, I thought it was the source but then pow - thinking cap went bad then it took out the switching transistor - upside here is finding the fault should be easy - it'll be clearly marked!- Taxan RGB - works fine for 5 minutes, then powers off, tapping it gets you a bonus 30 seconds.

author: zaoka
author_rank: Frequent Contributor
timestamp: July 03, 2014, 01:59:57 am
content: Did you measure 12V on L504?Also make sure you are getting proper input signal, horizontal should be arount 15kHz and vertical around 50-60hz. Without correct input signal this thing wont work.

author: SpidersWeb
author_rank: Unknown
timestamp: July 03, 2014, 05:05:45 am
content: Not yet, I need to resolder the +12 line as I broke it off (genius) when removing the PCB and I was avoiding power ups until I'd put in the new caps - partly because I'm comfortable it's safely discharged at the moment. (Apologies for being a bit slow).But I will check it when I get it powered up. I wasn't too suspicious of voltage levels because the other functions (which run off the same supply) seem to be 100%.Edit: had to educate myself a little bit on inductors - now I get it it a bit more - will certainly testI've tested the source with another MDA display (IBM 5151). Spec is a little bit higher the a normal old style CRT ~18.432Khz horizontal (720x350@50).Edit2: Quick Update, unsoldered C505 - 1uF! That's at least a sign it's stuffed, others could be similar. It is identical to the one sold at JayCar though - MDL 100V 10uF NP.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: July 04, 2014, 07:11:08 am
content: Any chance it's been replaced before? I was looking at the glue around it a few days ago which seemed to suggest it was. That is to say it almost looks like the factory glue outlined a different diameter capacitor. The soldering looked factory so I didn't mention it, but the fact that there's an identical one at Jaycar gives pause.

author: SpidersWeb
author_rank: Unknown
timestamp: July 04, 2014, 08:23:00 am
content: Quote from: Paul Moir on July 04, 2014, 07:11:08 amI think it has been replaced, what I'm unsure of is if it was installed faulty or got damaged in circuit (poking around the web suggests to me it's not exactly a quality part).There isn't much point grabbing that exact model again tomorrow and installing it if it doesn't fit the bill.I know it was mentioned I can use a pair of 22uF Low ESR caps joined at the hip, but since I can get a 10uF greencap (poly?) - does anyone know if that would be ideal?

author: mij59
author_rank: Frequent Contributor
timestamp: July 04, 2014, 09:02:33 am
content: I know it was mentioned I can use a pair of 22uF Low ESR caps joined at the hip, but since I can get a 10uF greencap (poly?) - does anyone know if that would be ideal?The capacitor has to withstand high frequency an high current, the capacitors will start to conduct, heat up, and blow to pieces.You might try a low loss metal film capacitor.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 04, 2014, 11:37:46 am
content: A Greencap should do the job OK.I used a "stack" of them in a "Boost HT" circuit to replace an NP Electro.---maybe a bit less current,but only a "bit"!Greencaps are used extensively throughout Industry,& they are pretty tough!

author: SpidersWeb
author_rank: Unknown
timestamp: July 05, 2014, 07:03:05 am
content: Cheers guys.The 10uF 'greencap' I found was $11 in store, and enormous, so I did the normal thing and grabbed a 10uF NP electrolytic - the model in store was of a different brand to the one I just pulled (C505) so I thought it was worth a shot. I did grab a couple of 22uF Low ESR caps just in case someone thought it was wise to try out (something like 50c each) if the NP didn't pan out.If none of them can survive, I'll order a better part and wait.I compared another mono monitor I have open, found it's equivilent C505. It uses a 4.7uF NP electrolytic (I know it's original becasue I unboxed it two weeks ago) - made in the same year, and it's 4.7uF cap is as big as the circle drawn on my PCB. I'm not sure if this is just "technology improving" or if it was larger for an engineering reason (like heat). Further confirms C505 was replaced - will be interesting to see how it's similarly sized replacement deals with it.For the 2.2uF caps, I found the originals were marked +/- 5% and still in their value range, so I plan to reuse them to start with because I think the replacements I grabbed are possibly worse (and I could just be creating a new issue by fitting them).Turns out my $1 scope works (Advance Instruments OS250TV - 10Mhz dual channel CRO), so grabbed a set of 1x/10x probes - calibrated it using the built in square wave - and I should be able to post more interesting/informative results if replacing C505 doesn't do the trick. (absolutely amazed it works)Still no ESR meter, but I'll take care of that soon.Busy weekend but will try and make time to get it powered up tonight/tomorrow.

author: SpidersWeb
author_rank: Unknown
timestamp: July 07, 2014, 09:01:48 am
content: Woo! I just powered up to a perfect display. The display is bright, steady and in focus.C505 was the only component that tested as faulty. The replacement looks tiny, and I'm not sure how long it will last, but the fault has been foundEdit: of course after posting this, I went back to use it again, and now the power supply wont start upGot a dull green light, then it shutdown, now it does nothing. Resistance shows 0 ohms initially but increases to 12 ohms (I guess as caps etc charge up). Will check fuse next, then test PSU on it's own.Edit2: was the fuse

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2014, 12:58:04 pm
content: If the replacement is tiny,it will last around 30minutes before the magic smoke comes out.Even back in the mid '90s,the lousy,lossy NPs were much smaller than the low ESR ones.Have a look at my posting (#20).

author: SpidersWeb
author_rank: Unknown
timestamp: July 07, 2014, 07:52:05 pm
content: Ah yes your post #20 covers it. Thanks, it's weird that I missed that point.Jaycar only stock one model by the looks, the tiny one, ESR not advertised. The other place I grab components locally is RS NZ - but for the life of me I can't see how I find NP caps on their site. Mouser etc charge a large shipping fee, but I'll work it out and get something sorted. It's a lot more promising now I've seen the clear display.

author: SpidersWeb
author_rank: Unknown
timestamp: July 23, 2014, 09:30:33 am
content: So I decided to wait for delivery, and order a part specifically rated for this purpose. I got two sets from different suppliers, and just as well, because one set was 21uF despite the 10uF label (I tested 4 of them). The lower quality models I got with +/- 20% tolerance however, were a nice 10.45uF. I fitted one along with a new fuse, and tada:New man-sized capacitor - designed for "horizontal deflection circuit" and apparently 9A of ripple current. Has been running for over an hour now with no explosions and/or fire storms.Thanks to everyone who posted. I learned a LOT about how these monitors work in the process.

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2014, 09:39:49 am
content: Nice screenshot. Well done.