Products > Test Equipment
Fool for the 8656A Sig Gen
jrharley:
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
Tony_G:
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?
--- Quote from: jrharley on February 05, 2023, 02:44:38 pm ---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.
--- End quote ---
TonyG
jrharley:
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
Tony_G:
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
Swainster:
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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