Author Topic: Fool for the 8656A Sig Gen  (Read 27387 times)

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Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #75 on: February 01, 2023, 06:00:32 pm »
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
« Last Edit: February 02, 2023, 04:34:30 pm by Tony_G »
 

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #76 on: February 03, 2023, 11:51:56 am »
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.

 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #77 on: February 03, 2023, 04:40:58 pm »
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

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #78 on: February 04, 2023, 09:02:39 pm »
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
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #79 on: February 05, 2023, 04:23:45 am »
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

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #80 on: February 05, 2023, 02:44:38 pm »
Well, things certainly got a lot more interesting last night!  :-//

I decided that the next step for me, would be to test voltages on the pins connected to each of the by-pass caps along the H string shown on the schematic on service sheet 14. So, I connected the pin2 wire back into circuit and powered the ‘muther up. Much to my surprise, the power up display was different! Obviously, something that was done in the process so far has awakened new elements.

The display read like this: Modulation window was blank, Frequency domain read 100 MHz and the Amplitude domain was blinking “1.8” along with all the adjacent LEDs. This was a big change from power up displays so far! I took a solid mental note of the display, but should have taken a picture. DOH! I was encouraged to see that the display was showing the default boot up value of 100 MHz.

Of course, I couldn’t leave well enough alone and pressed the Reset switch. The display went dark, but it was like it was before when pin 2 was in circuit. When I powered off, I heard an attenuator solenoid “clack” in! (Attenuator cable was in circuit). This was also a new development. Previously, no attenuator action was evident.

This beast is trying to come back to life! despite missing some limbs.

It continues to bother me intuitively that there is currently a big hole in the attenuator assembly. At least some portion of it is “open circuit”. I don’t have a good enough understanding of things to describe what is happening, but having an entire section of the attenuator disconnected just “don’t seem ‘natch’ll.”. At least it confirmed that the 24 V rail is working.

At this point, I haven’t been able to reproduce the different display again, it’s back to acting as it did before, except that now, I can here the attenuator trying to work when powering on and off. It doesn’t sound particularly “happy” though. Incidentally, all the voltages on the H string read around 1.6 to 1.7 V, all 13 of them.

I’ll do some more head scratchin’ and report back.

JRH
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #81 on: February 05, 2023, 08:16:33 pm »
Interesting - I wonder if there might have been a dry joint/grounding issue on the power supply and you addressed that by cleaning everything up and taking the caps out of circuit to check them? Probably a mystery that will never be solved...

Can you expand on what you mean about this when you get a chance?

It continues to bother me intuitively that there is currently a big hole in the attenuator assembly. At least some portion of it is “open circuit”. I don’t have a good enough understanding of things to describe what is happening, but having an entire section of the attenuator disconnected just “don’t seem ‘natch’ll.”. At least it confirmed that the 24 V rail is working.

TonyG

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #82 on: February 05, 2023, 08:40:51 pm »
Hey Tony_G,
Yeah, I think you're right. It would seem to be the only plausible explanation for the improvements in function. Hard to pinpoint precisely what it could have been, but a lot of stuff was done. Caps and a few other components were partially removed from the board an then re-soldered, switches, keyboard keys, connectors and pins were cleaned, I re-tensioned the pins on a few connectors back there, etc., etc....... I think the lesson I've taken away, is that there's no harm in giving these old kits some TLC as long as you're careful and meticulous in the re-assembly. We've all seen controls and connectors that get dodgy just from sitting around a long time. At any rate, something woke up!

On the text clip - Sure, it's kind of how this all started. You'll have to climb into the "way-back machine" and go look at the initial postings to get the context. When I first opened the 8656A up I found a bag of parts inside and an entire set of actuators missing from the attenuator assembly. All I have from them is the gold contacts.

Depending on the feedback I get, (I'm hoping b_dunham is following along), I'm thinking that I should secure things as they are and get after the task of having some actuator forks for the attenuator fabricated to see if I can repair that assembly.

JRH
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #83 on: February 06, 2023, 02:31:07 am »
Thanks - Went back and looked at that, so you're basically missing A9MP3 from here (you have the A9MP2 and A9MP1 parts):



Your best bet there is to recreate the part in a 3D modeling tool from an existing one you have and then see if you can find a place to print it (or do it yourself if you have a printer). The only other realistic approach is to buy a parts mule and then try and recoup the costs by parting it out after you've used all of the pieces you needed - I had to do that with an 8901B to get proper replacement parts for an 8902A that someone had dropped in shipping...

As for the way forward, the thing that is causing me to pause is that you're not getting a booting instrument with an abnormal indicator as described in BD1 - I think it would suck to spend all that effort to fix A9 mechanically to discover that there is a completely unrelated issue with regard to the rest of the instrument. That rail is a problem and I'd hate it to turn out to be some unobtanium HP custom IC that is causing the problem after all that work.

However, I don't have one of these so please take my feedback for what it is worth (clearly a few hundred bytes worth of electricity :D) and I defer to BDunham7 (who I think has one of these) as a better expert.

All that said, if you can get me a model, I do have a friend that can print some out for me so I'm happy to send the model to him and ask him to print half a dozen for you.

Let me know.

TonyG

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #84 on: February 06, 2023, 04:23:00 am »
The attenuator is one of the major differences between the 8656A and 8656B so I can't make any comments on it, however it sounds like your logic/MCU board is not yet functioning properly? (i.e. the board which is supplied by J2 pin 2). I'm with TonyG on this - if it were me I'd concentrate on getting the logic/control side fully working before worrying about the attentuator. I know that you started to tackle this around post #30 but I dont think that you have exhausted this avenue of investigation yet.

In this regard, have you measured the resistance to ground of what you refer to as the "H string"? Ideally you would use a 5 or 6 digit multimeter to see if there was any difference in resistance at the various caps, as the lowest measured resistance may indicate a nearby fault i.e. with the cap itself or the associated logic IC. Another option is the finger test - the faulty part may be dissipating extra power so touching the chip packages may be enough to identify the culprit, or at least narrow down the location.
 
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Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #85 on: February 06, 2023, 05:12:04 am »
For those following along at home, the A10 board delivers a 5V rail to the other boards in the SigGen:



The CPU board starts to get covered in Service Sheet 14 (the nomenclature being the H in the circle followed by the 14 to refer to the applicable Service Sheet).

JRH has said that the 5V rail only drops when he connects pin 2 of A10J2 up and that is the one that leads to the CPU board:



Now it goes to other places on A14 but there are a bunch of bypass caps on that rail and this is where I would start - Then I would go with Swainster's suggestion - If nothing else leaped out then I would probably begin to identify the relevant ICs touched by the 5V rail and work from there.

A somewhat slow slog if the unit is uncooperative in the fault-finding mission.

TonyG


Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #86 on: February 07, 2023, 10:31:13 pm »
Hey HP Sig Gen’rs,

Great feedback and insights as usual, really appreciated! It’s a little hard for this old Bodger to keep up with it all, but that’s not a complaint, it just takes me some time to go through things and take it all in.  :P

I hear everyone loud and clear on solving the 5V rail, and I concur, don’t get me wrong. I'm afraid that my ignorance of this kind of equipment and circuitry, has my pea brain struggling to comprehend how an electrical gap in the attenuator section couldn’t be contributing to the symptoms I’m seeing. But I’ve been studying the attenuator schematic and I think I’m starting to understand why I shouldn’t be concerned. (Any insights on that front would likely be useful though). I think I’ll just continue to try and sniff out the problem with the attenuator cable out of circuit entirely moving forward, and just get over it.

That said, fabricating the missing attenuator parts is going to be a real task, in and of itself. That’s why I’m going to start the process of finding a shop that can do it. I’m thinking not just any 3-D print set up can do this properly. A precision scan will have to be done, the right plastic will need determining, etc. In any case, I won’t pull the trigger on fabrication until the 5V rail mystery is solved, but there’s some leg work to do on this aspect of the repair.

Tony_G –  Thanks for the printing offer, much appreciated! We’ll see how it goes.
I think I’ve beat the components along the H string to death at this point, I’m convinced there’s no component level issue along there.  :horse: All of those IC pins are showing a positive voltage of approximately the same value, 1.6 to 1.7V. By the schematic, I think it should be 5V or thereabouts. I'm thinking the issue lies downstream of this section. I’ve been looking at service sheet 15 for the next place to investigate, giving some “side eye” to chips U1, U22, U24 and U32.

Great test suggestions swainster, thanks! As for the touch test, I’ve done a good bit of that and haven’t found anything finger blistering yet, even after prolonged power up, but it’s a thing I’ll be constantly doing. Also, I measured resistance across all of those caps statically a while back and the caps were consistent in their readings, giving me some confidence that they’re solid.

Anyway, I’m taking a brief sidetrack with a cute little Spectrum Analyzer I acquired recently for a couple of days. It needs some mechanical stabilizing before I can shelve it.

Then I’ll get back to the HP. I’ll report back with any new developments as they occur, and always love the comments folks! Thank you!

JRH
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #87 on: February 08, 2023, 02:54:19 am »
I don't think you need to go all "high spec" on those actuators initially - You could just use a micrometer/caliper and simple 3D modeling tools to create a model that would work - That would get you an "operating" attenuator - You can then get the unit all running and see if you need to step up the attenuator components to have something in spec later.

Good luck with the SpecAn - Look forward to hearing the next updates.

TonyG

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #88 on: February 10, 2023, 03:31:41 am »
Quick 8656A update to ponder here - Maybe someone can explain this observation:

When the unit is powered up now, with the A10J2 pin 2 CONNECTED,  The 5V rail reads 5.4V for a couple seconds, until the attenuator latches in, and then it drops back to 1.8V. (what part of the attenuator is switching in, I'm not sure). If I power up with pin 2 unconnected, it behaves as before.

I can't escape feeling like this is telling me something, these things are dependent on each other in some way guys.

If any one can prove to me why the attenuator can't be affecting the 5V rail, I'm all ears.

I may go dark for awhile following my gut here. Bear with me.

JRH
 

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #89 on: February 10, 2023, 07:55:00 am »
Sounds good! I think the point is that the priority should be the problem with the 5V rail rather than the attenuator (which is mostly just a bunch of 24V relays from the control point of view). Of course, if the 5V rail problem can be traced to the attenuator then that is a different matter. Did you try unplugging the attentuator control input (J3 on the power supply board) to see if that makes any difference? Then again, erhaps the reverse power protection circuit is involved somehow - that appears to use a 5V relay
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #90 on: February 10, 2023, 03:08:32 pm »
Swainster's suggestion is a good idea - The attenuator control section draws from the +24V Unregulated & +5V Regulated rails (as shown on SS7) and there could be a problem with the  ICs (like U6, U11, etc) that control the attenuator - Removing J3 might not solve the problem as the path to GND might be through the IC and not through the output of the IC and into the attenuator segment itself.

You might want to consider removing U6 & U10 as that should break the path from the 5V rail to GND completely - the signal/5V goes into U10 and then into the inputs of U11 & U12 before flowing through to A9.

Interested to see what you find.

BTW It looks like both those ICs are still available from Mouser et al

TonyG
« Last Edit: February 10, 2023, 03:10:44 pm by Tony_G »
 

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #91 on: February 12, 2023, 12:59:12 am »
Back to the HP here -
As always, thanks for weighing in on the project guys!

I've got the little Spectrum Analyzer squared away for later work now and I'm just back from "Hamcation" in Orlando FL. I won't lie, I was scanning for an old junker 8656A, but didn't see one.  ;) A good day though, got some supplies and got some fresh air. Now it's time to get back to the HP.

The J3 connector was actually unplugged when I first opened the unit swainster, and has been in and out of circuit a number of times during testing. You're spot on with it's importance. Having J3 in circuit, or not, does not seem to affect the 5V rail voltage discrepancy in this situation. Now though, if J3 is in circuit, I get some attenuator action. That wasn't happening before.

I'm just getting back into the SS7 schematic Tony_G, printed it out for study. That could take a bit.......

Thanks guys!

JRH
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #92 on: February 12, 2023, 02:39:32 am »
Great to hear that you had a good time and a bit of a break from the SigGen follies...

It's a pity that the design of the 8656A has the attenuator control circuits on the A10 board otherwise you could just take it out of the circuit and see what happens...

The ICs that directly control the attenuator are A10U7, U11 & U12 - These take a TTL control signal but will output the +24V to drive the attenuator.

The ICs that handle the control latching are A10U6 & U10 - The CPU will send control signals to these and they'll then work out what they need to send to the driver ICs to toggle the appropriate attenuator segments.

The A10U3 IC just enables the CPU to latch in the data before it is applied to the outputs of U6 & 10.

I would tend to think the issue, if it is in one of these ICs, would be in U6, 10, or 3 but it is certainly possible that U7, 11 & 12 could be drawing too much on the output of U6 & 10 that could be dragging the 5V rail low but I would expect that to cause U6 or 10 to go tango uniform.


These are all probably available at Arrow, AvNet, etc though I didn't check as I have accounts with Digikey, Mouser & Newark.

I happen to have a Hakko FR-301 so I'd probably just pull U6 & 10 and see what happens - If the rail is still low then I'd pull U3 and check again - If that didn't fix the problem then I'd look elsewhere. If the problem goes away by pulling U6 & 10 and they pass testing (breadboard it up and try applying power and see what happens) then it could still be because of U7, 11 & 12 so you could rinse & repeat that.

Look forward to hearing what you find.

TonyG
« Last Edit: February 12, 2023, 02:58:10 am by Tony_G »
 

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #93 on: February 12, 2023, 10:03:22 pm »
Thanks Tony_G!

I've been studying the SS7 schematic and your thoughts have helped me see the organization clearer, so, thank you for that. :-+

I'll see what happens pulling U6 and U10, maybe U3. It makes sense to start with those ICs to me too, since removing connector J3 hasn't seemed to have had any effect on the symptoms during testing. I have a decent de-solder gun, so removing those chips shouldn't be too bad. (Gotta' clean that sucker up though!)

In the meantime I noticed something while checking the passives associated with the chips. Capacitors C13 and C14 should be 1uF. They're both associated with U3. I lifted a leg to confirm, but they're both reading at 2.3uF with what seems like pretty bad ESR at around 4ohms. Since they're 10% parts, that doesn't look too good to me. I think I ought to replace those guys no matter what, make sense?

It's interesting to me to finally find a component that doesn't test very good on this beast, since nearly everything tested so far has been acceptable. The resistors in front of each of the caps tested pretty much spot on @ 132.5K Ohms.

 After I pause to reflect on things for a couple days, I'll start pulling some ICs and report back.

JRH
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #94 on: February 12, 2023, 11:34:09 pm »
They're the external timing caps for the IC - I'm not sure what would happen if you just left the legs lifted but you could certainly do that to see if they're the cause of the 5V Rail problem - You will probably have "undocumented" action from the attenuators but for the purposes of a short run to see if they impact the rail I don't think it would hurt.

TonyG

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #95 on: February 18, 2023, 02:21:35 am »
HP8656A update on a Friday night!

A10 C13 & C14 replaced with brand spankin' new Vishay Tantalums now, because it had to be done. It could just be me, but the unit seems to "cycle up" faster now. Not sure if that's real, or just my perception.

Anyway, I've removed the A10 U6 chip, no problems. De-soldering gun is doing the deed nicely. The chip just fell out and the pads are clean. Love it when that happens!

After those moves, the power up exhibits a dark display and a 1.8 V .....5 V power rail. I'll remove U10 next and see what happens.

JRH
 

Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #96 on: February 18, 2023, 08:18:53 pm »
Had a bad week so I'm looking forward to hearing some news from you - Thanks for making my day a little brighter

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #97 on: February 18, 2023, 11:31:00 pm »
DRAT! Sorry to hear it Tony_G.

For what it's worth, I'm sure if you wait a minute, I'll do something so head slappingly stupid with this unit, that it would bring a smile to ANY techs face. :)

At any rate, I pulled the A10 U10 chip this evening. Same results, which is honestly not unexpected. That's not to imply that I actually know what the heck I'm doing here, just that my gut is telling me that I'm chasing an unlikely component failure. Incidentally, I've restored the J2 connector to original status and I'm thinking I keep it that way, with pin 2 back in place. I'm thinking it doesn't really matter if taking pin 2 out of circuit allows the 5V rail to get up. It's still got the problem. It's an interesting clue, but I think I move forward testing the unit with all wires in place.

The question I'm pondering now is, do I put U6 and U10 back in before moving ahead? Or do I pull the next victim with them still out of circuit. Maybe U3 should be done before I get to that question. Not sure.

I'm Also wondering if there's a safe way to "bridge" the missing attenuator contacts, to at least put the unit into a defined state. Not sure if that makes any sense or not. It just makes my teeth itch that there's no connection across that section.

As always, the feedback I get from you and others, is invaluable, and I really appreciate your interest in this long shot project.

"Chin up" Brother, hope things smooth out for you.

JRH
 
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Offline Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #98 on: February 19, 2023, 07:07:52 pm »
Thanks - Appreciate it.

You're not chasing a "ghost" problem - You know that the 5V rail is being dragged down by something, what you are doing is methodically checking components that can cause the issue - So I wouldn't consider it wasted time.

One quick question - Did you check U2 (located somewhere in the unit, probably towards the rear)? Confirm that the unregulated voltage stays well above 5V and the regulated side drops when Pin 2 is reconnected on J2? It's the 5V regulator and has the part number LAS-1905 (should be 5V +/- 5%) - It's not unheard of for the failure mode to be dropped regulation under larger loads. There should be a vast array of TO-3 regulators that can be used in place - Parametric search time... I'd be inclined to bodge something in place that matches the headline specs to see if the same problem goes on - Maybe measure the current being drawn on the regulated side to see if it is a reasonable amount, that sort of thing. It should be capable of providing up to 5A (probably some more but spec'd only to 5A).

You've tested the caps on the rail per SS14 and the caps on A10 so you should be expanding on what might be causing the problem, again by methodically ruling out components that could contribute to the issue.

It really is "Lather, Rinse & Repeat" now. Sorry :(

TonyG

« Last Edit: February 19, 2023, 07:37:19 pm by Tony_G »
 

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #99 on: February 21, 2023, 02:59:50 am »
Thanks for the pep talk TG!

It's too funny that you mention U2! I was removing U3 last night, (no change by the way), when I happened to notice an IC labeled U2, down on the far corner of the A10 board. I hadn't noticed it before and I still haven't found it on the schematic, but it was real evident from the back of the board and obviously right in the power circuit. I dug into it a bit and it's an LM393 N, a voltage "comparator" apparently. (But still a regulator of sorts). I've always felt confident that the main regulator U2, located near the transformer, was fine, but I was unaware of this little guy.

So I buzzed out some voltages on it. The 2 outputs read 0 Volts basically. No output. Pin 8 read 23.4V. I'll check the inputs just for grins next, but it seems like the LM393 is knackered. This just might be a good thing, I'll report back when I discover something else.

JRH
 


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