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| Fool for the 8656A Sig Gen |
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| jrharley:
Nope, I do plenty of time wasting inquiry all on my own! Busy poking around the thing the last couple of days. I eventually got the hint and tested the test points in table I with a scope. (Pin 2, J2 still out of circuit). The voltages were as outlined previously, but the ripple was all over the shop. Another good data point, thanks! Importantly I think, it was the worst on the 5V rail. (Worst delta that is, it was actually more or less the same unstable value for all of the test points, varying from 800mV to 1.2V p-p). I suppose I'm going to be lifting some cap legs soon. Is it possible that the regulator for the 5V rail is just doing its job poorly? The ripple was 1.2V p-p. Yeah, I think the table is saying it should be 0, so not so good. I guess a voltage ripple like that could cause the logic to fritz out downstream? We'll keep at it. Thanks Tony_G, you're a Wizard! I feel like I'm still making progress. JRH |
| Tony_G:
Good to hear that I didn't cost you any time. TPs 9,10 & 11 are unregulated and if they have excessive ripple on them then I would check out C17, C18 & C19 - You tend to find ripple in unregulated rails from the main caps and those tend to be large value caps. You might also want to check out A14 C11, C14 & C15 - At least at the start, if all those capacitors test correctly then you can start on the smaller value ones like A10 C15, etc. Dave did a good video on bypass caps if you want to get more background on why there may be multiple bypass caps on a rail. I'd work to get those TPs into spec first as the other ones are further down the chain (specifically the other 3 TPs from table 1). When you start looking at the regulated TPs then you should probably check the ripple if not in spec then C23, C24 & C25 - I wouldn't bother with A12 U1, U2 & U3 until you can confirm that the unregulated ripple is correct. Look forward to hearing the next thing you find. TonyG BTW It isn't impossible that the diodes doing the rectification are bad but I'd absolutely rule out the capacitors first. As an aside, getting a good LCR meter, like a DER EE DE-5000 is something to add to the toolkit as well but really you can get quite servicable results with some of those very inexpensive eBay devices. |
| jrharley:
Really great points to run down Sir! Much appreciated. I'll look at the schematic with those comments in mind tonight. Meantime, I've attached a photo from the project "journal". It's one of the photos taken randomly through the process. I'm testing a small cap in circuit with the LCR meter. :D Report back soon, JRH |
| jrharley:
Brief 8656A update here. I guess I'm just going to have to do the work. :P With this kind of issue in the supply rails, I've got to get a pretty solid test on the caps, bottom line, and that means lifting a leg on C17,18 and 19. I can quickly flay this board open and test these guys, that's not a problem, but here's where my mind is going - Let's face it, these caps are 40 years old. Good caps originally for sure, and in circuit, they're not raising too much concern. But, yeah, they're 40 years old and there's ripple out of spec. on the rails they smooth. I'm trying to get this old beast in a place to be a solid contributor on the bench. It seems inevitable that these guys are going to be replaced. Unfortunately, that "thought thread", runs headlong into the form factor problem. I've got suitable axial replacements in my stash that could go in here. They are about a 3rd the size though. I'm going to have to come up with an appropriate mounting strategy for modern equivalents. The original zap-strap approach from the factory, won't work in the current arrangement for modern caps. Not saying it can't be neatly done, but I'm just.............sayin'. So, I'll test 'em, get good data, and strategize about how to get a replacement solution that's appropriate. I have to admit that I found the mounting solution by HP on these guys to be a bit "how ya' doin'" right out of the factory. I'll ponder for a bit. Thanks again for the feedback. JRH |
| jrharley:
OK, So all of the big filter caps have been removed from circuit and tested with the trusty DER LCR meter. They are all out of spec to the high side on capacitance value - not horrible, but significantly. If this was an old all American five, I’d still call ‘em good. ESR on every one was strong, about .03 ohms. I’ll take that value. Still, they’re out of spec. I’m unclear on how higher capacitance values would affect this circuit, and the ripple I’m seeing on it. Replacing them, any way you look at it, is going to be a nosebleed. But it’s probably unavoidable in the long run. I’m currently thinking that the most “dignified” way to replace these guys, is to re-stuff the originals. I think it could be done by making new cores from radial units rigged in parallel to hit the value. Then I could install the caps back in, just like they came from the factory. I’ll have to do a deep dive on cap dimensions and values, but it’s probably the only cost effective solution. The job will always be obvious to an informed eye, but it just seems to be the least invasive fix. The million dollar question for me is this: Is this significant to the fault I’m chasing? My gut tells me no. But hey, I’m “Analog Man”, so I may be clueless. Calling all circuit Guru’s ! – should I tack these back in and move on for now? Or should I stop and do the deed to get the big filters into spec? Any feedback is much appreciated. JRH |
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