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| Fool for the 8656A Sig Gen |
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| Tony_G:
That poor SigGen has had a rough life... I wonder if it may have been dropped at some point and the chassis has twisted a bit - These things weight at 2/3rds of a hernia so it wouldn't surprise me. JRH - The GNDs for A12 are all direct chassis connections so while you're cleaning that up you might want to check that it is well grounded - I suspect just putting nuts and washers onto the posts will do that but worth checking in passing I think. TonyG |
| jrharley:
I know right? It's amazing to me that a piece of equipment that cost as much as a new car back in the day, could be abused in so many ways. I don't think the chassis has flexed though. It's a solid sand casting I think, very rigid Aluminum. Which is not to say that it hasn't been dropped. It weighs 37 pounds. Anyway, the compartment is cleaning up nicely. I've swabbed all the grunge out of it, cleaned all the connectors and pins, and treated them with a light brushing of D-100. I've also discovered how the A12 board is mounted and it makes sense with the schematic diagram now. The "nutless" screws through the A12 board don't need nuts. Closer examination revealed that the holes in the board have a threaded insert in them and the screws are directly coming from the TO-3 package regulators on the other side of the chassis wall. The A12 board is directly coupled to the regulator mounting screws. Changing any caps on A12 would mean pulling those regulators. Right now, I'm thinking I don't need to change any. Once I disconnected the A12 board from the circuit, I got decent capacitance readings on them all. No visual signs of distress either. Grounding all seems solid, a single grounding point at the copper shield mounting bracket binds the ground to the chassis well. The bottom of the shield bracket which covers the A12 board, is uncomfortably close to the top of A12 when installed. I may glue a square fish paper sheet to the bottom of the shield bracket for good measure, we'll see. I think I'll button this up and move on with some of the testing ideas that have been offered up next. I'll send a photo of the compartment right before button up. Thanks for everything TG! JRH |
| jrharley:
HP Sig Gen fans, We're moving on to the next dead end here! Back cavity containing the A14 and A12 boards is back together. See attached photos. At this point, I think we've eliminated some variables, so all good there. Still, the original issue remains. The good news is, no harm done, the unit powers up exactly as before. Ready to follow up on the backlog of great testing suggestions! I'll post up any new developments. JRH |
| jrharley:
whooops, looks like a couple photos didn't attach....... |
| Swainster:
Just to check, my understanding is that the current status is: 1) 5V supply is ok without the MCU board connected 2) 5V supply goes down to 1.8V when MCU board is connected 3) It is unknown if the fault is due to excess load from the MCU board or a problem with the supply. If this is correct then perhaps a few comparisons will help - I made some current measurements of the 5V supply on my 8656B, measured at the 6A fuse: * Plugged in but 'powered off' -> 64mA * Powered on in default state -> 3.425A Measuring just the MCU board current (hopefully I identified the correct pin on J2 as it's a bit different on the 'B' version), powered on -> 761mA Lastly, when the MCU board in disconnected from the 5V supply and I power up the instrument, then the front panel displays all '0's and the total 5V current is 2.807A |
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