Magnavox Model# CM8702 074G Brightness Question - Page 1
Forum section: electronics / repair

author: bostonman
author_rank: Super Contributor
timestamp: November 30, 2021, 05:32:33 am
content: I've been using my CM8702 monitor with my C64 and it's very dim (especially compared to a TV when I connect it via RF - the TV is bright and crisp).At a previous job I repaired computer monitors, and, if I remember correctly, we had only one of three methods for getting them brighter: turning up the flyback voltage, using some sort of rejuvenation gadget, or replacing the CRT.I'm not planning to use this extensively, but would like it displaying correctly for future use; and without the risk of damaging the color guns. If I remember correctly, using the rejuvenation gadget was a last attempt and thought I remember hearing that it's a short lived solution. Since I don't have this gadget, it basically eliminates this option.Does anyone have an opinion on whether rejuvenating the color guns is safe and effective (assuming I can get my hands on some cheap gadget), or just turn up the brightness on the flyback?

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 30, 2021, 09:22:39 pm
content: I didn't do much rejuvenating in the day, from my limited experience it depended very much on the CRT model.Rejuvenating had close to no effect on some models, on rare occasions it would make things drastically worse on others it brought a substantial and quite long lasting improvement.When they were really at the end I'd add a turn on the flyback ferrite to put more power into the cathode heater filaments.Of course before fooling with the CRT it's worth checking supply voltages and electrolytics on the video path.

author: james_s
author_rank: Super Contributor
timestamp: November 30, 2021, 09:40:39 pm
content: The voltage you're referring to is the G2 or screen grid, usually there's a pot on or near the flyback and it adjusts the master brightness. If the tube is weak, a classic symptom is the focus starts to look a bit soft and gets worse when you turn up the brightness. You could start by checking the heater voltage to confirm that it is correct, usually 6.3V. Keep in mind that many monitors use high frequency AC directly from a winding on the flyback for the heater so you need to use a true RMS meter. I have found rejuvenating to work very well on some tubes but in other cases it does nothing or makes it worse, occasionally it completely ruins it. Something I have learned is if you rejuve it and get some improvement you should quit while you're ahead because hitting it again will often set you back worse than when you started. Unfortunately there are no CRT rebuilders left because that is the only way to really get back to like-new performance.

author: bostonman
author_rank: Super Contributor
timestamp: December 01, 2021, 04:12:40 am
content: I'll say that after an hour or so of keeping this powered and/or having actual video on the screen, it seems brighter than initially.With the exception of maybe 15min total about five-years-ago, and another 15min many years prior, I hadn't turned on the monitor in about two-decades. At this point I'm so use to high resolution monitors/phones/TVs, that maybe this monitor is actually normal.The brightness knob on the front is turned up all the way though. Video games seem decent, but the main text screen seems dim. In any case, it's going back into the storage box and plan to use a TV in the future.Ah yes, the old G2 voltage. I forgot much of the terminology, however, monitor repair was a very short lived career for me, so I didn't have time to absorb much except the flyback voltage can sneak under the boot and zap your wrist when turning the yoke after installing a new tube.QuoteYou basically hit the nail on the head from the experiences I remember. Rejuvenating a CRT was taught as common practice until someone approached me saying it's either not effective and/or short lived.As for measuring voltages, that's good input. I immediately went with an aged CRT due to the age of the monitor. For all I know, this probably has leaking caps too, so maybe either now or sometime in the future I'll take time to remove the cover. For now, I'm just happy to have one more project behind me (repairing my old C64).

author: floobydust
author_rank: Super Contributor
timestamp: December 01, 2021, 04:30:47 am
content: Usually a CRT with low emission will also be blurry, I think their bandwidth goes down with age but that depends on the monitor's circuit i.e. cathode drive or grid drive.First thing to set are the screen and drives, does it have a Service switch? If they won't set up then look for a power supply issue the voltages might be off or an old electrolytic capacitor has low value.edit: the blurriness is like smear- which is different than raw blur from poor focus. Smear is horizontal.

author: james_s
author_rank: Super Contributor
timestamp: December 01, 2021, 07:57:14 am
content: I remember discussing this about 20 years ago with an engineer who actually designed CRT monitors for one of the major companies at the time. I'm a little hazy on the details but I think the conclusion was that they get blurry because the center of the cathode is used more and wears out faster that the edges which leaves a larger effective spot size and you get streaking to the right because the video amplifiers start clipping while trying to drive sufficient brightness out of the worn cathodes.Rejuvenating was never standard practice in the sense of something you do when repairing displays. It was a last resort, when you know the TV is shot and the customer wants to try to squeeze a few more weeks/months out of it before having a rebuilt tube installed or buying a new TV. Some tubes, particularly the ones made during the peak of CRT technology in the late 70s-mid 80s had really beefy cathodes that would respond very well to rejuvenating. Others rarely did well at all, in particular I remember Sony tubes which were fantastic when in good shape, were really shot when they started to get weak. I don't think I ever saw a rejuve on one of those last more than a few hours. Now the CRT analyzers that can do rejuvenation have other features too, first and foremost they work well to assess the condition of the tube, checking emission, cutoff and shorts between elements. The Sencore analyzer I have even has a feature that reduces the heater voltage when you press a button and uses the resulting emission to estimate the remaining life. In general I've found that a visual assessment of the phosphor is a good estimation of condition in a lot of cases. A really well used tube with a lot of hours on it will take on a brownish appearance, or in the case of arcade monitors and data displays, often it will have images burned into it dark enough to clearly see what they are with the power off. If it's a nice clean gray with no obvious screen burn then usually it's in decent shape.

author: bostonman
author_rank: Super Contributor
timestamp: December 02, 2021, 02:23:01 am
content: QuoteThis is interesting.When I was repairing monitors, either I interpreted the advice of rejuvenating a tube incorrectly or the company didn't practice much long term effectiveness of it.Although I didn't plan to purchase a rejuvenator, I'll avoid trying it on this monitor to avoid damaging the CRT. The difference between brightness and crispness is night and day when compared to RF into a TV, but, as you pointed out, maybe a voltage is sagging and it's not the CRT.For now, I'll keep the monitor as is because it's rare I care to drag out the monitor and use the C64 on a 13" screen, however, the nostalgic feeling of actually having the original setup is very nice.

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2021, 02:53:58 am
content: Rejuvenating blasts a layer off of the coating on the cathode to expose fresh surface. It's like using rubbing compound to buff off oxidized paint on a car, every time you do it makes the coating thinner and eventually you'll rub all the way through to bare metal. This is why you don't use rubbing compound to polish your car every time you wash it and you don't try to rejuvenate a CRT unless it really needs it. Typically you'll only get a few rejuvenation cycles at best before the tube is done.

author: bostonman
author_rank: Super Contributor
timestamp: December 02, 2021, 03:01:08 am
content: I also know, or at least believe, that the glass on old CRTs can become weaker with age. So tinkering with the neck by removing the socket is a risk.I remember some CRTs had a neck board with a clamp to hold it on. Some cheaper ones like the Wyse 60 was just a plastic socket with wires coming off the main board (assuming I remember correctly).

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2021, 03:04:42 am
content: I've never heard of the glass becoming weak, glass is pretty robust stuff, though the plastic cap over the exhaust pip can become brittle. The tubes I remember having a clamp were high resolution monitors that had the video amplifiers in a shielded box right on the end of the tube, often with heatsinks for the driver transistors, those used a clamp to support the weight of that assembly rather than just hanging it off the pins. The Wyse terminals and other small monochrome monitors just had a little bare PCB to hold the CRT socket and maybe a single transistor video amp and some passives, there was no need for a clamp.

author: bostonman
author_rank: Super Contributor
timestamp: December 02, 2021, 03:12:29 am
content: I could be wrong. I remember cracking a few necks, maybe due to twisting the neck board too much. I always thought due to heating/cooling, the glass became weaker and cracked. At that time, I was so new to troubleshooting that I didn't know what I was doing.I still have a color bars file from an old job. As you know, in those days you had to adjust the colors with gray scale and colors. Not to mention, replacing a CRT required turning the yoke, setting it at the correct distance, turning the convergence rings, and adding additional strip magnets.After I got hit with the flyback voltage on my thumb, I was too scared to ever go near the CRT.

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2021, 05:24:01 am
content: I cracked a monochrome one once, broke the unprotected exhaust pip off by accident and that was that, after that happened I was always very careful with the CRT socket. I've done a lot of tube swaps in arcade monitors which requires a full alignment, I've been zapped a few times by the HV but never stopped doing it. The high voltage zap hurts but it's not nearly as dangerous as the lower voltages at high current that you find in other places and also switching power supplies. 340V with multiple amps behind it can knock you dead.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 02, 2021, 07:58:49 am
content: I can't remember ever cracking a CRT unwantedly and even on the large (at the time) 63cm models I'd remove the deflection yoke, rest the TV face-down on a cloth and lift the CRT out upright grasping it at the neck's base.

author: bostonman
author_rank: Super Contributor
timestamp: December 02, 2021, 02:52:42 pm
content: I still did CRT replacements and stuff, but with lots of fear.You are correct, it's low current.I put the monitor along with my C64 away, but with notes based off the feedback from this thread so in the future I'll know what to do should I desire making it brighter.

author: mykrowyre
author_rank: Newbie
timestamp: December 30, 2021, 10:03:06 am
content: Turning up the master brightness on the flyback mentioned above will help but not so far that you see the retrace.