- Amperex M24-302W equivalent? - 5 Updates
- ESR meter trap - 6 Updates
| frank <frank@invalid.net>: Feb 05 12:00PM Hi all, does anyone have any very old CRT cross-reference around? I probably need to replace an Amperex M24-302W 9" CRT on a vintage CBM PET 2001 and probably these CRTs are unobtanium now. Maybe some old monochrome TV CRT could be substituted? The tube has probably a cathode to heather short. Even with the cathode lead grounded, the screen is all white with brighter retrace lines. G1 sits at -32V so, it's not a matter of wrong bias or something similar. Cathode signal generated from the monitor CRT is positive and about 0V should cutoff the beam, according to the schematics. Might be a failure on the HV generator that gives too high voltage, I'll measure it later (shoud be 10 KV), but I think if it was that, it would kill the flyback transformer quickly, isn't it? Frank |
| tabbypurr@gmail.com: Feb 05 06:04AM -0800 On Monday, 5 February 2018 12:00:54 UTC, frank wrote: > measure it later (shoud be 10 KV), but I think if it was that, it would kill > the flyback transformer quickly, isn't it? > Frank An h-k short is easily sorted with a transformer. EHT failure wouldn't get you a white raster. Cathode v normally needs to be raised to cut a tube off, not lowered. NT |
| frank <frank@invalid.net>: Feb 05 03:15PM > An h-k short is easily sorted with a transformer. EHT failure wouldn't get > you a white raster. Cathode v normally needs to be raised to cut a tube off, > not lowered. Yeah, I feel so dumb. I've just figured it out myself hooking a variable power supply to the cathode. It cuts off at about +20V. So the CRT (thanks heavens) looks good. It must be some stupid fault on the video path. It has three stages only, two inverting amplifiers (single bjt each) and one emitter follower. First stage works, so I'll replace the last two transistors and check again. It's been a few years since I was "thinking" about cathode and grids. Frank |
| jurb6006@gmail.com: Feb 05 07:36AM -0800 Can you get a wire in between the flyback coil and core ? If so you just make your own winding that is totally floating. Start with one turn and work your way up until the filament is bright enough. It is better if you have measuring equipment. A true RMS meter can read the waveform at 6.3 volts RMS but not everyone has that. A scope should show about 20 volts peak to peak, that corresponds to about 6.3 volts RMS with that waveform which is usually a half sine resonated at 5 times the horizontal sweep frequency. Iff the monitor has fast retrace it might have to be a bit higher. In any case, use as little as possible to get a decent display. The less the better as it will result in the longest cathode life. The cathode has already been abused putting out more current than was ever intended. They used to make a small cathode transformer for insulation but finding one today rather than a piece of wire is not easy. If you can't get in between that core and coil on the flyback get back to us and I will see if I can think of another solution. |
| jurb6006@gmail.com: Feb 05 07:39AM -0800 >"It must be some stupid fault on the video path." Disconnect the cathode from the video output to make sure it is a shorted CRT. If it is not all white then it is in the video. If it goes black it is the CRT. |
| Chuck <ch@dejanews.net>: Feb 04 01:29PM -0600 On Thu, 1 Feb 2018 00:00:24 -0800 (PST), Phil Allison >.... Phil Grass Valley fiber cards had +15 -15 V power supplies where the 220uf axial caps checked good for ESR with the Bob Parker meter in circuit but were faulty. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
| John-Del <ohger1s@gmail.com>: Feb 04 01:32PM -0800 On Sunday, February 4, 2018 at 2:29:53 PM UTC-5, Chuck wrote: > Grass Valley fiber cards had +15 -15 V power supplies where the 220uf > axial caps checked good for ESR with the Bob Parker meter in circuit > but were faulty. How did they check out of circuit with the Parker meter? Earlier in this thread I described a 33uf electro reading less than an ohm in circuit, and 350 ohms out of the circuit. The culprit was a surface mount 1uf film cap with an ESR of <1 ohm right if you can believe that across the 33uf fooling the ESR meter. |
| Phil Allison <pallison49@gmail.com>: Feb 04 05:29PM -0800 Chuck wrote: -------------- > Grass Valley fiber cards had +15 -15 V power supplies where the 220uf > axial caps checked good for ESR with the Bob Parker meter in circuit > but were faulty. ** How uninformative. Bob's meter tests ESR, far and away the most common, age related failure mode of electro caps. It does not test for high leakage, being wrongly marked in the factory or installed backwards. .... Phil |
| jurb6006@gmail.com: Feb 05 02:54AM -0800 >"Grass Valley fiber cards had +15 -15 V power supplies where the 220uf axial caps checked good for ESR with the Bob Parker meter in circuit but were faulty. " this can happen if there arew good caps in parallel with the one under test. Like the supply itself might have a 220uF, but there could be a bunch of 1 uFs across the vcc/Vdd of a few ICs which will make it read good in circuit. Same with banks of lytics. i have found that on many powwer supplies you oly have to bridge one in a bank to test the unit. this is from the TV days when we got in a TV that is dead, to put a bunch of caps in the PS oly to find it has screen or Tco problems and is not worth repairing. So I started just taking out one and putting one cap in its place, and I didn't even cut the leads, I let it stand up. I had a stash of caps at the bench for this purpose and saved some money, which of course was part of my job, we weren't wasting a bunch of caps anymore. Once you cut the leads it is alot harder to install on a sometimes overcrowded PC board. Same thing applies to computer motherboards. just one will get it to work. They use alot of them because of ESR, ESL and ripple current. they might have five of them in there adding up to 10,000 uF, but a single 1,000 will get it working temporarily. One time I did that and it turns out the problem corrupted the BIOS and the system did "hang permanently" in the lingo of the manufacturer. Everything is so cheap now servicing is all about saving money. People don't care because you can get a new one so cheap, and they don't care that the quality is even worse. Some do, or are comfortable with their old equipment. People who finally learned the whole menu on their TV or whatever, or have older high end stereo equipment. Even then you can't waste money without good reason. Some people want ten buck caps and all thin film resistors in the audio path, change them all. But they are willing to pay, the average sheeple is not. And those cap values in uF are not that critical. And contrary to popular belief it is NOT good to up the voltage rating. the parameter that really matter are theesr and ripple current cpacity. You really think am SMPS running at 100 KHz needs 10,000 uF worht of filtering ? Shit, 10 uF will do it if it can handle the ripple current and has low ESR. and the two go hand in hand. higher ESR causes an AC voltage drop which produces heat by actual power dissipation in the cap. That destroys it prematurely. they got it timed so it lasts through the warranty, and they hope not much longer. Only a few companies build everything so if you need to buy a new one they benefit anyway. That is one of the main reasons I do not fix TVs anymore. Actually most newer equipment, fuck it. Built cheap and specifically hard to service. I got tired of fighting it. And they are winning beause they REALLY control the parts ow. You can get two TVs the same model and there could be five different variations on the chassis, and almost none of the parts are interchangeable. And then there is alot of software and firmware that gets corrupted, and bad caps can cause that. |
| jurb6006@gmail.com: Feb 05 02:57AM -0800 >"It does not test for high leakage, being wrongly marked in the factory or installed backwards. " I heard of lytics marked wrong. The shop that got them said they all blew up. Turs out when they paid a bit more attention the negative lead was longer than the positive lead. I remember a very long time ago in a small Sony TV the rectifiers were marked backwards, and so was the board ! That was a fun one. |
| tabbypurr@gmail.com: Feb 05 06:00AM -0800 > >"It does not test for high leakage, being wrongly marked in the factory or installed backwards. " > I heard of lytics marked wrong. The shop that got them said they all blew up. Turs out when they paid a bit more attention the negative lead was longer than the positive lead. > I remember a very long time ago in a small Sony TV the rectifiers were marked backwards, and so was the board ! That was a fun one. I once found a stuffing error I'd made many years before. A 16v lytic was in backwards on a 21v rail - had never caused any problem though. NT |
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