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| Fool for the 8656A Sig Gen |
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| Tony_G:
As Bdunham7 said, they're in parallel so I would be inclined to remove them from the circuit and test - If you have enough space, you could just lift a lead and then test. It certainly could be an IC but I'd want to rule out the possibility of it being a cap on that rail. From memory, I think you said that the main 5V rail stays fine and it is just that pin 2 line that goes low so I would approach it along the lines of checking each cap and if they're all good then start removing the ICs that are related to each cap while monitoring the voltage. If you've removed all the ICs and still have a problem then you're looking for a different reason. Sort of a longish process but it would allow you to rule out the caps and ICs. Also if you don't have one I can heartily recommend the Hakko FR-301 as it makes desoldering much easier. TonyG |
| jrharley:
Hey guys, copy that! Thanks for weighing in bd7! I wasn’t putting much stock in the in circuit measurements either. Lifting a leg on them wouldn’t be too tough to do and it may come to that, we’ll see. Meanwhile, a little more accurate discovery on the caps in question has come to light: Once I found the right parts list, (DOH!), the cap descriptions made more sense. The bulk of the capacitors are, in fact, ceramic and they’re labeled so in the description. What I didn’t expect to see, since I’ve never seen any before, is that the other type of caps in this chain are axial Tantalum caps that look like regular axial electrolytics. But the parts list designates them with a “TA” in the description and they do exist. So, there you go, learn something every day. There are about 20 ceramic caps in this part of the circuit, most are .01uF. I decided to just test the resistance across them all, since I could do that from the top side easily. Maybe I could squeeze a little more information out of the board. All of them seemed to have pretty consistent and reasonable readings in the hundreds of ohms, the bulk of them being around 390 ohms. There are also 2 very small ceramics that are identical and specified as being 22pF. One measured about 700ohms and the other one seemed to measure open. Nothing. So, I cobbed a couple 60pF ceramics together in series and tacked them across the suspect cap “Shango style” to see what effect would occur, if any. Not unexpectedly, no change. At the moment, I’m thinking this is a low level power supply issue so I think I will run down a couple of other lines before sucking on IC pins. The pins 1 and 3 on the J2 connector where this Rabbit Hole originates, are tied together with pin 2 where it connects, on board A10. I think I’m going to follow 1 and 3 along for a bit, to see what I can learn. By the way TG, I found the programming switch you referred to a few comments back. It appears on service sheet 16. This one’s set to 5V, which seems to be correct. So that’s where I am - this is still a very active crime scene. More discovery later, and thanks again for the input guys, it's very much appreciated! JRH |
| Tony_G:
--- Quote from: jrharley on December 12, 2022, 11:06:44 pm ---There are about 20 ceramic caps in this part of the circuit, most are .01uF. I decided to just test the resistance across them all, since I could do that from the top side easily. Maybe I could squeeze a little more information out of the board. All of them seemed to have pretty consistent and reasonable readings in the hundreds of ohms, the bulk of them being around 390 ohms. There are also 2 very small ceramics that are identical and specified as being 22pF. One measured about 700ohms and the other one seemed to measure open. --- End quote --- Just thought I'd check-in and see if Santa brought you a working SigGen. Out of curiosity, are you measuring those caps using a proper LCR meter or just using the resistance mode on a DMM? They should all be "open" from a DMM resistance measurement and with an LCR meter, you'll see the ESR values. Just wondering. |
| jrharley:
Hey TonyG, Thanks for weighing in!, Happy New Year to you and all the HP fan boys on the EEVBlog forum. Nope, Santa’s delivered nothin’ but coal lumps so far. But learning has still been happening. On the cap testing, I hear you, I’d considered the same things. My resistance readings were with a DMM, but I also have a decent LCR meter that I’ve been using to test caps as well, both in and out, of circuit. What I learned from just slapping a DMM across those caps was that things were pretty consistent across the board. Call it a “qualitative measurement”. The circuit was definitely influencing the readings, even if I don’t really know how, precisely. It was just a data point, even if a marginally informative one. First impressions sure can fool you sometimes. I’ve realized that my initial conclusions about the power rails on this guy were a little bit “slap-dash”. This has screamed power problem from the very beginning, I just didn’t look close enough. The bottom line at this point, is that when all connections are hooked up as intended, the +5V rail becomes +1.8V, and it only does that when pin 2 of the J2 connector is connected into circuit. I’ve looked at the string on sheet 14 pretty closely at this point. I’ve tested the tantalum caps out of circuit, (they’re bang on), and really doubt any of the ceramics are the problem, so I’m moving onto sheet 15, following the 5V distribution. I’ve determined that I do not have the 5V at chips U29 and U33 per the schematic. Interestingly, with the wire from J2 disconnected, I got essentially no voltage on those chips, but with it connected, I got low voltage, 1.6V and 1.7V respectively. The interesting part is that's really close to the 1.8V faulty rail reading observed previously on board A10. Following other points on sheet 15 where the 5V should be, has led me to the only resistor network on the board, R9. There are 9 - 3.3Kohm resistors in the package. I’m currently doing some investigating of that package and pondering a way to test it. Not sure if I’ll find a diagram anywhere that shows how all those resistors are hooked up inside the package, but it certainly seems like a possible culprit. Great, I suspect that part will be unobtainable in its current form, but we’ll just have to see if it’s got a bum resistor in it somehow. The package has 10 pins. So, nothing insightful to report unfortunately. This Bodger is still just poking around in discovery mode. More later with any progress.... JRH |
| themadhippy:
--- Quote --- There are 9 - 3.3Kohm resistors in the package. --- End quote --- Wot like this? --- Quote ---Great, I suspect that part will be unobtainable --- End quote --- https://www.digikey.co.uk/en/products/detail/bourns-inc/4610X-101-332LF/1089196 |
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