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Re: THS710 Repair and (future) upgrade to THS720P
Hi all,
this involves the "upgrade" from being a THS710 and now being a THS720... I think because my scope has the older FW (V1.03) it will only be a THS720, not an "A", nor a THS720P, but the higher bandwidth could be nice t have. To make this upgrade happen I had to do the following: Remove R207 and R213, this allows for the higher bandwidth in the input stages; removing R204 tells the FW to behave as a THS720. In the album are two pictures showing the result: /g/TekScopes/album?id=41029 I have not found the article about using another (smaller) screen as a replacement for the LM32P10 or the LM32010 yet. Un saludo, Leo |
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Re: 422 AC/DC Version
I now have the copy from Tekwiki and it is the same as the other one which I think came from bama or that Polish server. But they are the same. I tthink I screwed up slightly, there is more current than that available for the K.
At the CRT, G1 is held at -1420 volts by V 829 which is pretty much a big Zener shunt regulator in effect. The resistors to K from ground are R 831, 832, 833 and 834. All current to K which is the source of the beam current must come through those resistors. However I notice now that they do operate in parallel effectively with R 837 and 838 even though they are tied to the G1. this serves to provide a less negative voltage to K which is shunted by R 837 and 838, with 837 pulling K and G1 voltages closer for less effective negative bias. Obviously R 837 closing increases K current. This still does not explain why it gets darker when R 837 is advanced past halfway. There are high value resistors involved which are more prone to failure than lower ones, but what would cause the intensity to reverse effect after halfway ? One of them going high in value ? I think I'll check them first ? They are still the only path for K current. One or more would have had to change value though because any of them being open would result in zero current. This is of course when I get it backup on the bench. I like to gather information first as I have limited bench space. Right now there is a bohungus LASER printer on there that is finally working and since it is mine I have to find or literally build a place for it. I'll have to use some of the lumber I have stashed in the attic for in case the house burns down. I can probably check them by voltage readings, saves taking them out. If I find nothing there, what does that leave other than the CRT ? Of course I could probably also shunt a resistor or something and see if it increases the intensity. I'll have to give it good look, but if for example I short R 831, with the intensity at max leaving only 20 K between G1 and K, if I don't get more intensity then it must be the CRT. Right? Q#1. No, the intensity just stops and then decreases. There is no blooming or change in focus at any setting. Q#2. The intensity is stable at any setting, even toward the top when the control seems to work in reverse. On this one, using the horizontal magnifier makes it too dark to use at all. Though it seems (at least Tek) scope CRT filaments are a bit different, from my TV days I would tend to increase heater voltage rather than rejuvination at first. Rejuvination is a one way process, stripping some of the coating off the cathode. And if boosting works first and then doesn't at some later date that leaves an option for then. Also, I have seen cathodes just get better running hotter for a time and actually don't need the boost anymore, though I won't hold my breath. I have also seen some, that after boosting they snap and rejuvinate themselves, but again I am not holding my breath. If I have to boost the CRT, I think it would be preferable to add a turn or so to the secondary rather than using caps on the rectifier to get a partial doubler effect. What say you ? Or is there yet another way ? I installed a wall wart in the 561A mainframe because of an arcing power transformer and it worked fine. That is not so easy here as the AC input is far from where it would be needed and then there would be wires where a plug makes all the connections now. Also, that would preclude DC operation unless I pick it up from the DC/DC convertor, I would rather not for a few reasons. It won't be long until I am back on it. I am going down there tonight to figure out what to do with my bodybuilder model printer, then I got an inkjet to clean the head on and it either works or it doesn't, so that won't take long. then up goes the Tek 422. If I get the intensity fixed I will try to put it together with the battery pack out and see what is out there in the way of more modern replacements. The only modification would be to increase the charging rate really and that should not be a big problem. I'm pretty sure the newer batteries would be so much smaller they don't even have to look like the old ones let alone conform to dimensions or anything, unless I can get a HUGE capacity out of the same form factor cells. I'll be baaack. Thanks so far. |
satbeginner
Jeff Urban