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| Fool for the 8656A Sig Gen |
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| Tony_G:
Quick clarification - I was referring to this U2: and not A10U2: Without looking further into the crowbar circuit, I think that the output of the of the comparator should be negative or zero (I think SS22 said that the crowbar throws when the output goes positive but I'd have to go have a look - Yes, that is the case - My quick read is that the current over R75 causes U2 to go positive, which then turns Q2 on and triggers Q1 which blows F1). It might be worth looking more closely at the TO-3 regulator as I can't see how the attenuator crowbar would affect the 5V rail and exhibit that with Pin 2 being connected/disconnected. TonyG |
| Swainster:
They are here on the 8656A/B, possibly covered by little plastic hats as the case is at the rail voltage on some of them. Actually, I was hoping that you could compare the current drawn on the 5V rail with the measurements I took a few posts back - the idea was to get an idea if the rail was collapsing due to overload or weak supply. Pic from google (scribbling from me): On my unit these are the part numbers (cross reference where necessary from various online HP references): Ideally, U2, U3 and U4 should have plastic covers on them as their case has some voltage with respect to ground. edit: huh, I guess that I still haven't worked out how to use in-line pics |
| Tony_G:
[updated a bit for better clarity] Great reminder there Swainster - You measured around 3.5A draw from the regulated 5V rail so JRH should have something similar I would expect - The part on the 8656A according to the OSM, is a LAS1905 (1826-0513) whereas your unit (or at least the one in the photo is using this) have an LM396K (1826-0966) - Don't see why that wouldn't work if the output spec for current matched. JRH - You won't get the same numbers due to voltage etc but if you're drawing "more or less" the same than that I would seriously consider U2 being the issue and bodging in a replacement to test. To clarify what we're talking about here - You would expect that the LAS1905 would drop the voltage to keep itself safe per (or go nova trying to supply it and die): If you're drawing closer to 10A then the problem is somewhere else I think. Thanks again Swainster - Appreciate the community here, even though it isn't my SigGen :) TonyG BTW I've never got hosted images to work inline - That is why I use Imgur and the linked BBCode instead - If you ever do get it to work, please let me know what you did. |
| jrharley:
Thanks for the U2 clarification Tony_G! I had mentally skimmed over the A10U2 symbol as being some kind of Op-Amp or something! These schematics are foreign to me “Ahn I will not lie.” But that makes things much clearer now. It’s also very interesting to me that it’s involved with the attenuator protection “crow-bar” circuit. A lot of things have pointed to this area since the beginning. Recall that I found F1 bridged with a wire when I opened her up initially. I’ve tacked in a 2A fuse at the F1 position for now, and it hasn’t blown yet. But I think you’re right in that the main regulator U2, might need more scrutiny. I’m hearin’ your hint swainster, thanks for the insight! I’m studying things and thinking about how I implement your suggestion and trying to keep up with you guys. In the meantime I’m occupied by stabilizing the “damage” I’ve done at this point, getting the removed IC pins and pads ready for re-install, improving solder joints, etc. Speaking of appreciating the community, it’s really interesting to me how things are moving back towards the main regulators and rectifier in the troubleshooting discussion. The reason being, is that “Buddy”, of the YT channel “The Radio Shop”, did a real solid troubleshooting job on the power section of an 8656A a while back. Link is here: I think I’m going to look forward to removing, cleaning, testing, and re-seating those TO-3’s. Peaceful work! Always appreciate the thoughts folks! I’ll report back soon. JRH |
| Tony_G:
JRH - There is a good point he showed in that video - If U2 is delivering too much current then it should also be getting pretty hot (you can look at the lines that I had in the previous image I included). Looking forward to hearing what you find. TonyG |
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