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| Fool for the 8656A Sig Gen |
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| Tony_G:
Good to hear. I'm sure that someone who does this all the time could chime in and comment but I've had regulators that will regulate correctly under no/light load but then sag under the expected load. The "proper way" to test them (in my view) is to use an Electronic DC Load (I have one that does up to 300W I think, would have to dig it out as it is in the "don't use often" pile of equipment) - You provide them the correct/designed input voltage and then vary the load up to the designed amount of current and ensure that the voltage doesn't sag. What I mean in this case is that the unregulated voltage coming in is +9.5V to +13.5V and U2 should kick out +5.19V to +5.61V but that might be the case when U2 is providing 0.1A but when asked to provide 3.5A the voltage might drop to +2V. This would mean that it would "pass" when simply checked with a DMM but "fail" in operation. Buddy's test was just the DMM version and not a load test - He did test whether or not it would stop regulating with higher voltage and he also pointed out that his U2 was drawing 21mA (from memory) which he, correctly, thought was strange - Where he sort of missed it was that the quiescent current of a LAS1905 expected is 10mA, so his was showing something different from expected but according to the datasheet the max was 25mA - So his U2 could have still be working correctly even though it showed something was beyond what would be expected. This is why I think you need to actually put the regulator under the expected load to really test whether it is working or not. All that said, if I didn't have either a DC Load or the desire to bodge up something to test that with the correct power dissipation, then I would just go ahead and replace it. Looking forward to your next update. TonyG |
| Swainster:
Yes, you are correct, of course - its a 5V regulator. I guess that what I am getting at is that if you disconnect it from its load i.e. board A10, you need to make some other arrangement for the virtual ground (which is basically the same as an adjustable regulator with a 5V reference voltage). In short, to measure the output of A12U2, assuming you have completely disconnected it from A10, then you need to at least ground pin 8 of A12J1, or jumper over a ground to the case of U2. As you say, a suitable dummy load would also be needed for a reliable test. If no electronic load is available then some HP power supplies have a "down programmer" which can sink some current. |
| jrharley:
All good points, well taken guys, appreciate the feedback. :-+ No doubt loading the components is the ultimate test. I've always had an electronic load on the "maybe someday" list, but I don't have one on my bench and acquiring one is unlikely in the near future. It's interesting to browse the Bay for them, they tend to be pricier than I would have expected. Anyway, to the point - the 5V regulator seems to support exactly the scenario you've described Tony_G. Presumably when pin J2 was connected, it loaded the regulator down. Since it tested poorly with the DMM test also, I'm going to replace that sucker. That's enough evidence for me. Both 15V rails tested good at the test points on the schematic and with the DMM test, so I think I'll just re-seat those guys. (Although I'm still poking around with the -15V regulator, its pin-out is different, so I'm still looking at that for curiosities sake.) As I've removed the TO-3's from their mounts, it's become clear that things were a little rough under there. It's turned out to be a good thing that I'm re-seating them with fresh thermal compound and cleaning the surfaces. I'll send a pic of the final install when I get a new LAS1905. My overall mindset on this unit is one of "reasonable restoration", so I'm inclined to repair and replace as I go, with the hope that I can bring it back into service, knowing full well that I could ultimately hit a dead end and have to scrap the effort. My fun comes from the journey and the learning. Thanks for helping with that! More to come I'm sure, JRH |
| Tony_G:
Just dug out my DC load (at least enough to see it) - http://www.maynuo.com/english/pro.asp?tid=97 - These guys I think are the OEM for BK Precision (or at least there was some back and forth about who did what/stole what) - Still not an inexpensive thing to have but useful if you're building your own supplies rather than using off the shelf converters. To add to the list of projects you have, you might want to look at what GreatScott did to build one: He does seem to have a newer one in his latest videos but my SearchFu failed me in trying to find anything on that. TonyG |
| Tony_G:
--- Quote from: Swainster on February 28, 2023, 02:43:27 am ---In short, to measure the output of A12U2, assuming you have completely disconnected it from A10, then you need to at least ground pin 8 of A12J1, or jumper over a ground to the case of U2. --- End quote --- Absolutely, my apologies if I wasn't being clear enough in my post - You're completely correct to point out that the "in place" test needs to do those things. Thanks again Swainster for the comments - Love hearing from you - Again, let me know if you need a more hi-res photo of that cam for your 461. TonyG |
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