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Fool for the 8656A Sig Gen

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Tony_G:
The SigGen is now 20% faster due to the lighter weight from dust removal...

Swainster - Did you by chance look at the ripple on your supply? How different is the supply circuit? Here is the relevant unregulated part of the power supply schematic:



Also if you happen to catch an example waveform of the inputs that couldn't hurt.

TonyG

Swainster:
Hi TonyG,

Although the boards have different part numbers, the unregulated part, and probably 90% of the regulator circuit design, appears to be identical in the 8656B. The only differences that I can spot are that the attenuator in the 8656A uses an unregulated filtered supply with over current crowbar, whereas the 8658B has a proper linear regulator (7815K), and the 8856A has a 110VAC fan, whereas the 8656B has a 24VDC fan running from the rectified 18VAC i.e. before any regulators.

Ripple across the 5V supply filter cap C30 is 1V pk-pk on top of about 10VDC. It looks like a pretty typical post rectifier 100Hz waveform (we are on 50Hz mains here).
Ripple on the 5V supply post regulator is tiny i.e. too small for my scope to pick up. One thing to note is that the 'ground' on the big filter cap is not the same as the regulator ground. There was significant AC ripple between the 2 'grounds' (maybe something like 0.3Vrms) so when measuring the post regulator voltages you cannot leave the scope ground clip or DMM lead on the filter cap - it needs to be moved to the ground test pin provided on the A10 board.

Tony_G:
Interesting - There is indeed a regulated GND test point that should be chassis GND on pin 1 of J6 but no mention of that in the textual part of the manual - The unregulated side has it's GND (tagged with a 1 on the schematic) and being connected to chassis GND via pin 1 of J5. In fact, I don't think there is a specific GND point mentioned for the TP measures of either side.

JRH - Where did you measure to?

TonyG

jrharley:
Hey Sig Gen folks, very interesting dialog, enjoying the thoughts!

Interesting comparisons swainster.
I'm generally measuring from the ground points on the respective boards, if I'm following you correctly TG.

I'm beginning to think I need to move on from the basic rails though, things have improved after all the poking, prodding, and re-soldering.

After completing the housekeeping back at the A12/A14 boards, I disconnected connector J4 to isolate test points 9,10 and 11. Voltages were good and ripple was modest, around 8mV. When I connected J4 back in and tested the other side test points, 13, 3 and 2, the ripple and voltages seemed to be in spec more or less, even on the 5.4V rail. Ripple was between .5 and 1.5 mV on that rail. I'm not thinking that's enough to cause a problem. For whatever reason, testing the three rails now, seems to be close enough to spec to eliminate this area from being the culprit. Connecting pin 2 of the J2 connector still causes the 5V rail to drop of course.

Could have been testing inconsistencies on my part, could have been dirty contacts, etc., but it seems to be good to me now. I'm currently taking a closer look at service sheet 14, following the J2 pin 2 wire through the circuit.

All the insights are terrific guys, really appreciate it! I'll report back with any developments after tonights adventure.

JRH

Tony_G:
Good to hear - The tightest requirement for ripple was 10mV so if they're all below that then you are in spec.

Looking forward to hearing how the rail exploration turns out.

TonyG

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