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| Thread ID: 24384 | 2002-09-09 21:25:00 | Monitor Problems | B.M. (505) | Press F1 |
| Post ID | Timestamp | Content | User | ||
| 77962 | 2002-09-10 07:54:00 | Sorry God It's not a switched wave, that's only there if it is operating and even then the supply voltage will be measurable at the input side to the transformer primary. It is a DC supply somewhere between 100 and 200 volts. What we are trying to diagnose here is whether or not we are looking at an AC short (output transformer/tripler or any other AC load) which will only be present if the line output stage (scanning and high voltage circuits) are still operating or trying to operate; or a DC short which will stay there regardless (i.e. a shorted transistor or capacitor etc). The latter you can measure with your trusty AVO (I've got one too!) and it wouldn't be a bad idea to check this first because the dim light suggests a solid load somewhere. Have a look around the circuit near the output tansistor and transformer and you should see a high wattage resistor. This would be in series with the output stage and if you can access both ends you can see if it is dropping those missing volts. Some modern supplies won't work if there is no load (what we used to call "chase me- ***k me" circuits) so no volts with no load are not uncommon. The voltage "folds back" if there is insufficient load to hold it down within its ability to self regulate. So.....if you can't measure volts on the transistor but the bulb lights when connected,........try this BM, but carefully! Cconnect the bulb as before, attach the meter to monitor the HT voltage and short circuit base to emitter of the output transistor. If the transistor is hard on (and I doubt it) the volts should come up to normal. This is a brief test, either the light will flash up or it won't. If it doesn't, try shorting base emitter on the driver transistor if you can locate it. This will turn it off and stop any drive if the output stage is trying to run into an AC short. If the bulb lights up brightly you have probably lost the output transformer. If you decide it is a DC short you should be able to measure this with your meter but the situation is now getting past the scope of Press F1 remote diagnosis. Let us know how you get on. Cheers Billy 8-{) |
Billy T (70) | ||
| 77963 | 2002-09-10 08:05:00 | My reference book only goes up to a 2SC5025, but these 'HOTs' are usually rated at 8 or 10 amps, so a collector to emitter short is quite dramatic, and other bits will probably have gone down too. As Godfather indicated if the LOPT has gone short circuit it wont be worth replacing, even if you could get one, they are about $70 or so when I bought one for a Philips a few years ago. LOPTs can be tested for shorts with a "ringing" test. The Proview is only a "budget" monitor anyway so if it's outside warranty it wouldnt be worth pursuing except for the interest of locating the faults, and you may be lucky. |
Terry Porritt (14) | ||
| 77964 | 2002-09-10 09:16:00 | Yes Terry I understand where your coming from. In reality it probably isn't worth the trouble. However, it's the challenge, so I'll whack away until I fix it, or fix it so no one else can. At the moment I'm just sorting out Ohm's law and Murphy's Law. I'll follow your instruction Billy and get right back providing I survive! I had a laugh at that site I mentioned in the earlier post where a guy suggested the way to tell if the "HOT" was overheating was to spit on it. If it sizzled, it was too hot and would eventually self destroy! Not exactly high tech but apparently effective? OK, I'll print out Billy's instruction a see what happens. For what it's worth I tried an insulation test from CRT anode cap to earth and got infinity. (mind you I wasn't using 20kv) Get back shortly with the results Cheers Bob |
B.M. (505) | ||
| 77965 | 2002-09-10 10:25:00 | Billy I haven't done all you suggested yet but thought I would run the following past you. Collector voltage without 75 watt light, 67v DC, 150v AC, measured with AVO. Digital multimeter confirmed DC, but couldn't read AC volts. Collector voltage with 75 watt light glowing 62v DC, 140v AC, measured with AVO. Again Digital multimeter confirmed DC voltage but couldn't read AC. Just thought I'd run this past you before engaging in emitter base shorting. Cheers Bob |
B.M. (505) | ||
| 77966 | 2002-09-10 10:35:00 | Hi Bob B-E shorting is quite safe and unlikely to cause any failures because all it does is turn off the transistor. I must say it sounds to me like the lopt has shorted turns. That makes the B-E short test particularly useful because the bulb will light up as the load somes off if it is. Don't believe the AVO on AC, it was never designed for that kind of duty. Stick to DC measurents. It's fun playing when you've nothing to lose isn't it! Cheers Billy 8-{) |
Billy T (70) | ||
| 77967 | 2002-09-12 21:37:00 | Sorry about the delay replying Billy but “She who must be obeyed” noticed me enjoying myself and found a list of much more mundane tasks to attend to! Anyway, back to the monitor. I did as you suggested and shorted Emitter – Base on the HOT with no effect on the light. I think you expected this would more than likely be the case. I then went looking for the Driver & High Wattage resistor but can’t find either? 1 watt would be the biggest resistor I can find and it’s 0.67 ohms and connected to the base of the HOT? I’ll get my magnifying glass and try to trace the printed circuit to find the driver transistor. (Later today if there are no interruptions.) I did try another “High Tech” test whereby I removed the Anode Cap from the CRT, placed my right index finger upon connector and turned on! Dead as a dodo! Not even the slightest tingle, talk about disappointed! Well that’s the update, I’ll report back when I find the driver etc and let you know what happens. Cheers Bob |
B.M. (505) | ||
| 77968 | 2002-09-12 23:18:00 | Hi Bob Although it is a bit too power hungry in my opinion, the driver could reside within a chip, though it would probably need a heatsink . I must admit I'm not overly famiiar with variations on monitor output stage drivers . If you can't find a power resistor in series with the LOPT, go back to the power supply itself and find its HT output and work forward from there . If you can identify the main HT line, see if you can cut or disconnect it from the rest of ths circuits and see if the psu will deliver power to the load (75W bulb) . If it does, a DC resistance check on the disconnected side of the circuit should tell you if there is a short . You will probably read a short one way but with the meter reversed it should be a relatively high resistance of several K at least . If nothing else you should be able to identify the bridge rectivfier and main filter cap . Keep in mind that the power supply and line output cann be combined within the one circuit so you may not identify discrete PSU and Lopt stages . Then you should have a good idea of whether you are looking at a power supply problem, a DC short on the PSU output or an AC short such as the LOPT, scan coils, or raster geometry correction component . For more info on the monitor you are struggling with, try a Google saerch on the name and model number, or even any other number you can find that could be the OEM manufacturer's generic model designation . There may be sites around with information on monitor repairs . You could also look at this site (http://www . monitor . co . nz) and they might even give you limited help on the phone or by email . You could also put a question to E . Ric ( . paradise . net . nz/erichard/" target="_blank">homepages . paradise . net . nz) as he seems to know something about monitors . Cheers Billy 8-{) :D |
Billy T (70) | ||
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