Author Topic: Fool for the 8656A Sig Gen  (Read 27381 times)

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Online Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #100 on: February 21, 2023, 03:32:35 am »
Quick clarification - I was referring to this U2:



and not A10U2:



Without looking further into the crowbar circuit, I think that the output of the of the comparator should be negative or zero (I think SS22 said that the crowbar throws when the output goes positive but I'd have to go have a look - Yes, that is the case - My quick read is that the current over R75 causes U2 to go positive, which then turns Q2 on and triggers Q1 which blows F1).

It might be worth looking more closely at the TO-3 regulator as I can't see how the attenuator crowbar would affect the 5V rail and exhibit that with Pin 2 being connected/disconnected.

TonyG
« Last Edit: February 21, 2023, 03:37:55 am by Tony_G »
 

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #101 on: February 21, 2023, 05:25:45 am »
They are here on the 8656A/B, possibly covered by little plastic hats as the case is at the rail voltage on some of them. Actually, I was hoping that you could compare the current drawn on the 5V rail with the measurements I took a few posts back - the idea was to get an idea if the rail was collapsing due to overload or weak supply.
Pic from google (scribbling from me):


On my unit these are the part numbers (cross reference where necessary from various online HP references):


Ideally, U2, U3 and U4 should have plastic covers on them as their case has some voltage with respect to ground.

edit: huh, I guess that I still haven't worked out how to use in-line pics
 
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Online Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #102 on: February 21, 2023, 05:50:42 am »
[updated a bit for better clarity]

Great reminder there Swainster - You measured around 3.5A draw from the regulated 5V rail so JRH should have something similar I would expect - The part on the 8656A according to the OSM, is a LAS1905 (1826-0513) whereas your unit (or at least the one in the photo is using this) have an LM396K (1826-0966) - Don't see why that wouldn't work if the output spec for current matched.

JRH - You won't get the same numbers due to voltage etc but if you're drawing "more or less" the same than that I would seriously consider U2 being the issue and bodging in a replacement to test. To clarify what we're talking about here - You would expect that the LAS1905 would drop the voltage to keep itself safe per (or go nova trying to supply it and die):



If you're drawing closer to 10A then the problem is somewhere else I think.

Thanks again Swainster - Appreciate the community here, even though it isn't my SigGen :)

TonyG

BTW I've never got hosted images to work inline - That is why I use Imgur and the linked BBCode instead - If you ever do get it to work, please let me know what you did.
« Last Edit: February 21, 2023, 10:31:01 pm by Tony_G »
 

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #103 on: February 23, 2023, 01:21:57 am »
Thanks for the U2 clarification Tony_G!

I had mentally skimmed over the A10U2 symbol as being some kind of Op-Amp or something! These schematics are foreign to me  “Ahn I will not lie.”
But that makes things much clearer now. It’s also very interesting to me that it’s involved with the attenuator protection “crow-bar” circuit. A lot of things have pointed to this area since the beginning. Recall that I found F1 bridged with a wire when I opened her up initially. I’ve tacked in a 2A fuse at the F1 position for now, and it hasn’t blown yet. But I think you’re right in that the main regulator U2, might need more scrutiny.

I’m hearin’ your hint swainster, thanks for the insight! I’m studying things and thinking about how I implement your suggestion and trying to keep up with you guys. In the meantime I’m occupied by stabilizing the “damage” I’ve done at this point, getting the removed IC pins and pads ready for re-install, improving solder joints, etc.

Speaking of appreciating the community, it’s really interesting to me how things are moving back towards the main regulators and rectifier in the troubleshooting discussion. The reason being, is that “Buddy”, of the YT channel “The Radio Shop”, did a real solid troubleshooting job on the power section of an 8656A a while back. Link is here:



I think I’m going to look forward to removing, cleaning, testing, and re-seating those TO-3’s. Peaceful work!

Always appreciate the thoughts folks! I’ll report back soon.

JRH
 

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Re: Fool for the 8656A Sig Gen
« Reply #104 on: February 24, 2023, 07:18:40 pm »
JRH - There is a good point he showed in that video - If U2 is delivering too much current then it should also be getting pretty hot (you can look at the lines that I had in the previous image I included).

Looking forward to hearing what you find.

TonyG

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #105 on: February 24, 2023, 09:49:56 pm »
I was thinking the same thing TG.  I realized I've never looked at the thermal condition of those TO-3's, they're under a shroud. After reviewing Buddy's video, it dawned on me as well.
Hey, it's Friday night! so I'll have a little more time to go at it. I'm also interested to test U2 with a power supply to see if it gets wonky with some over voltage.

Almost finished getting things in a state where I can move forward so, lot's to do and uncover. Me take pictures........

JRH
 

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #106 on: February 26, 2023, 04:17:36 pm »
Quick 8656A update here, work is on going.

Testing U2 with a power supply, current limited to .25A seems to say that she aint regulatin'. Once I climbed past 5 V on the input, the output tracked the input all the way up to 8 or 9 volts, when I shut it down, nothing else to see there. Seems to me it should hold the 5V pretty close.

I checked my stash of regulators for a test bodge, but don't have any 5V units, and definitely no 5 Amp beasts, but I do have a pile of adjustable LM317T's that I suppose I could work up on a piece of perf board to output 5 V. They're 1 Amp components, but maybe good for a quickie, not sure. The original LAS1905 TO-3 packages seem to be available, albeit from varied sources, nothing at the big parts houses that I could find.

I'll ponder it for a bit.........

JRH
 

Online Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #107 on: February 26, 2023, 04:42:19 pm »
Not sure that I understand the test setup - Sometimes these regulators, and I've never used the LAS1905 so I don't really know, is that they don't like being used without some form of load and some capacitance between the Vin, Vout and GND to stop the device oscillating.

This may not be the case here but I would go implement the example circuit in the datasheet:



And then change Rload to increase the current draw and measure both the current through and voltage across that load.

If that is what you've done and it isn't regulating then as you said, time to replace it - NTE (which I seem to find has very good availability in local stores) has a part that should work for a bodged in test repair - https://www.ntepartsdirect.com/ENG/PRODUCT/NTE931 - Those shops might even have a proper replacement spec'd to 5A.

TonyG

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Re: Fool for the 8656A Sig Gen
« Reply #108 on: February 26, 2023, 05:17:28 pm »
Thinking about it in terms of "bogdability" - You could probably also get away with a TO-220 package like:

https://www.digikey.com/en/products/detail/microchip-technology/MIC29500-5-0WT/771612

Might make it easier to get a test component. Will need to ensure that it has an appropriate heatsink etc.

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #109 on: February 26, 2023, 07:00:27 pm »
Thanks Tony_G! All good thoughts.

I tested U2 just like Buddy did on his video. As I recall, his U2 held things to just over 5V. I'll test it again and loosen up the current limit a little bit to see if that makes a difference, but at this point, I'm thinking he's dead Jim. I won't jump to that conclusion just yet though, I'm going to test the 15V regulators too. All the thermal compound has dried up on these guys, so I'm going to re-seat them anyway, might as well test them while they're out.

Some surplus houses seem to have NOS 1905's for various prices. One even had some 1905B's in stock for about $5 each. From what I can tell it is virtually identical to the 1905, just one small bit of data difference that looks to be better, but not important. Several sources have NOS 1905 units for around $10 each. I would like to stick something in there briefly though, before I order anything to see if the 5V rail perks up.

More to come...


JRH
 

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #110 on: February 27, 2023, 03:28:58 am »
Quick 8656A update here, work is on going.

Testing U2 with a power supply, current limited to .25A seems to say that she aint regulatin'. Once I climbed past 5 V on the input, the output tracked the input all the way up to 8 or 9 volts, when I shut it down, nothing else to see there. Seems to me it should hold the 5V pretty close.

I checked my stash of regulators for a test bodge, but don't have any 5V units, and definitely no 5 Amp beasts, but I do have a pile of adjustable LM317T's that I suppose I could work up on a piece of perf board to output 5 V. They're 1 Amp components, but maybe good for a quickie, not sure. The original LAS1905 TO-3 packages seem to be available, albeit from varied sources, nothing at the big parts houses that I could find.

I'll ponder it for a bit.........

JRH

The 5V regulator is an adjustable, so wouldn't regulate if it was disconnected from its sense divider (R78, R79 on the A10 board). As you ran it up to 9V I'm guessing that you tested it out of circuit.
 

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Re: Fool for the 8656A Sig Gen
« Reply #111 on: February 27, 2023, 05:00:26 am »
Hey Swainster - I don't think that is correct for the 8656A - And I need to defer to the fact that you both have devices and I'm just looking at the schematics & datasheet - The LAS1905 is a fixed regulator device according to the DS and the Schematic shows it being used as a fixed device - The schematic seems to imply that it has a virtual GND instead of an adjustable voltage.

That said, I'm not sure you can test it by simply attaching a voltage source on one side and measuring the voltage on the other but, as I said above I defer to your experience with the actual implementation.

TonyG

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #112 on: February 27, 2023, 07:14:41 am »
Hi Tony,

I was just going by the schematic in the HP 8658A manual on manualslib.com. That said, even my HP 8656B has at least two different A10 boards, so it is quite possible that there is a fixed regulator version of the 'A'. Although... I thought the 'A' version also has the 5.4V adjustment pot? (funny thing, the 'B' has the "5.4V" pot, but despite the markings on the silkscreen, the manual says to adjust it to 5.25V)

Off topic, I recently acquired a HP 461A from the 'Bay and when I got it open it turned out that the output transistor and... the actuating lever for the attenuator had gone missing. Funny coincidence! I enjoyed watching your video for pointers on what the internals should actually look like (wouldn't mind a few more of those pixels though ;)). I've not got to the stage of performance tests yet - need to put my metalwork skills to work.
 

Online Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #113 on: February 27, 2023, 03:41:41 pm »
Yeah, I don't really understand what they were getting at with the 5.4V as it's labeled the same on the 8656A schematic:



To me what it looks like is a virtual GND for U2 as all rails are measured relative to the chassis GND. Anyway, the LAS1905 is a fixed regulator so JRH should use the same type of component if possible.

On the 461A - I went back and looked and I didn't take any photos on the S1/S3 Cam - Let me know if you want me to grab one for you - The unit is sitting right next to the bench and I'm happy to do that.

TonyG

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #114 on: February 28, 2023, 01:45:15 am »
This is such great discussion guys! I'm compelled to comment, even as I'm in mid exploration on this segment of the project. But here's a couple developments, and a couple notes:

  • Yes, the instrument rails, and pretty much everything else, are referenced to CHASSIS ground in this beast.
    The regulators can be tested in the simple manner described previously, unless I can't believe my "Lyin' Eyes" on the Radio Shop video.
    All these T0-3 packages are being tested out of circuit.

As for developments:

  • The +15V regulator, U1, tested rock solid. I ran the voltage up to 18V and it didn't even flinch, holding at 15.00V on the DMM. It's good to go methinks'.
    I'm working on the -15V, U3 now. Right now, things are inconclusive, I'm not currently believing those same "Lyin Eyes"........ stand by as I test it some more.
    Interesting discovery regarding the heat transfer pads under the TO-3's. They're like nothing I've ever seen before, but others are probably experienced with them. Their form is like a traditional mica pad for TO-3's, but they're made differently for sure. They feel and look like they're a silicone coated, double thickness mica, definitely more durable than the classic mica pad.

Anyway, too much going on to download at the moment, but more details later. Awesome conversation guys, I really appreciate it!

JRH
 

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Re: Fool for the 8656A Sig Gen
« Reply #115 on: February 28, 2023, 02:25:18 am »
Good to hear.

I'm sure that someone who does this all the time could chime in and comment but I've had regulators that will regulate correctly under no/light load but then sag under the expected load. The "proper way" to test them (in my view) is to use an Electronic DC Load (I have one that does up to 300W I think, would have to dig it out as it is in the "don't use often" pile of equipment) - You provide them the correct/designed input voltage and then vary the load up to the designed amount of current and ensure that the voltage doesn't sag.

What I mean in this case is that the unregulated voltage coming in is +9.5V to +13.5V and U2 should kick out +5.19V to +5.61V but that might be the case when U2 is providing 0.1A but when asked to provide 3.5A the voltage might drop to +2V. This would mean that it would "pass" when simply checked with a DMM but "fail" in operation.

Buddy's test was just the DMM version and not a load test - He did test whether or not it would stop regulating with higher voltage and he also pointed out that his U2 was drawing 21mA (from memory) which he, correctly, thought was strange - Where he sort of missed it was that the quiescent current of a LAS1905 expected is 10mA, so his was showing something different from expected but according to the datasheet the max was 25mA - So his U2 could have still be working correctly even though it showed something was beyond what would be expected.

This is why I think you need to actually put the regulator under the expected load to really test whether it is working or not.

All that said, if I didn't have either a DC Load or the desire to bodge up something to test that with the correct power dissipation, then I would just go ahead and replace it.

Looking forward to your next update.

TonyG

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #116 on: February 28, 2023, 02:43:27 am »
Yes, you are correct, of course - its a 5V regulator. I guess that what I am getting at is that if you disconnect it from its load i.e. board A10, you need to make some other arrangement for the virtual ground (which is basically the same as an adjustable regulator with a 5V reference voltage). In short, to measure the output of A12U2, assuming you have completely disconnected it from A10, then you need to at least ground pin 8 of A12J1, or jumper over a ground to the case of U2. As you say, a suitable dummy load would also be needed for a reliable test. If no electronic load is available then some HP power supplies have a "down programmer" which can sink some current.
 
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Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #117 on: February 28, 2023, 10:13:10 pm »
All good points, well taken guys, appreciate the feedback.  :-+
No doubt loading the components is the ultimate test. I've always had an electronic load on the "maybe someday" list, but I don't have one on my bench and acquiring one is unlikely in the near future. It's interesting to browse the Bay for them, they tend to be pricier than I would have expected.
Anyway, to the point - the 5V regulator seems to support exactly the scenario you've described Tony_G. Presumably when pin J2 was connected, it loaded the regulator down. Since it tested poorly with the DMM test also, I'm going to replace that sucker. That's enough evidence for me. Both 15V rails tested good at the test points on the schematic and with the DMM test, so I think I'll just re-seat those guys. (Although I'm still poking around with the -15V regulator, its pin-out is different, so I'm still looking at that for curiosities sake.)

As I've removed the TO-3's from their mounts, it's become clear that things were a little rough under there. It's turned out to be a good thing that I'm re-seating them with fresh thermal compound and cleaning the surfaces. I'll send a pic of the final install when I get a new LAS1905. My overall mindset on this unit is one of "reasonable restoration", so I'm inclined to repair and replace as I go, with the hope that I can bring it back into service, knowing full well that I could ultimately hit a dead end and have to scrap the effort. My fun comes from the journey and the learning. Thanks for helping with that!

More to come I'm sure,

JRH


 
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Online Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #118 on: February 28, 2023, 11:58:33 pm »
Just dug out my DC load (at least enough to see it) - http://www.maynuo.com/english/pro.asp?tid=97 - These guys I think are the OEM for BK Precision (or at least there was some back and forth about who did what/stole what) - Still not an inexpensive thing to have but useful if you're building your own supplies rather than using off the shelf converters.

To add to the list of projects you have, you might want to look at what GreatScott did to build one:



He does seem to have a newer one in his latest videos but my SearchFu failed me in trying to find anything on that.

TonyG

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Re: Fool for the 8656A Sig Gen
« Reply #119 on: March 01, 2023, 12:03:40 am »
In short, to measure the output of A12U2, assuming you have completely disconnected it from A10, then you need to at least ground pin 8 of A12J1, or jumper over a ground to the case of U2.

Absolutely, my apologies if I wasn't being clear enough in my post - You're completely correct to point out that the "in place" test needs to do those things.

Thanks again Swainster for the comments - Love hearing from you - Again, let me know if you need a more hi-res photo of that cam for your 461.

TonyG

Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #120 on: March 01, 2023, 10:02:24 am »
Hi Tony,

No problem - I am just worried that JRH's latest observations dont seem consistent with the rail drooping under heavy load but being fine at light load. Having the case floating while under test might explain the discrepancy.

Regarding the 461A I am thinking about starting a separate thread, but first waiting for the postman to deliver some RF transistors as possible candidates for the missing output driver.

Hi JRH,

as far as electronics load go, I find that I rarely use them, but on that rare occasion then they are indispensible. I actually have 2 (or maybe 2 and 1/2) electronic loads. The one I'm calling "1/2" is actually a 10k pot cobbled together with an op-amp and a random power mosfet with heatsink from my parts collection - this was improvised to give me a controllable few 10s of mA load for a project, once I had got fed up with playing with combining resistors is various combinations (I also didn't want to risk my decent resistor decade boxes on prototype circuits). The first "proper" -ish load was an aliexpress 80W load like the one discussed on a popular thread here on eevblog. Finally my most recent load is a huge 19" Chroma mainframe with a single 300W module installed (and 3 empty slots for future expansion). I bought this one when I needed a bit more oomph and it was available locally and cheap - I think it was less than USD150. The type that Tony has actually looks really convenient, but like you say, that form factor at least always seems quite expensive on the 'Bay.

I just recalled that I have another home made load which is just a bunch of power resistors mounted on perf board with a fan strapped to the front. Its got a pretty limited input voltage as the fan is driven directly from the load voltage  - it was a another project specific 'special' to stress test a 12V 2A supply for a couple of weeks. Anyway, the point is that you can work around not having a fancy electronic load (but they are nice to have available).

For a another example of a self built load, Kerry Wong has a nice video. I have a feeling that he touched on MOSFET SOA and the difference between linear and switching power MOSFETs
 

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Re: Fool for the 8656A Sig Gen
« Reply #121 on: March 01, 2023, 07:29:51 pm »
Oh yeah, I forgot that Kerry did the video on those - He has some great videos on loads & power supplies - Well worth the sub and watching.

TonyG

Offline jrharleyTopic starter

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Re: Fool for the 8656A Sig Gen
« Reply #122 on: March 01, 2023, 09:21:27 pm »
Thanks swainster, you guys know my weak spot for any new project!

I've got a beefy old case from an Astron power supply that I parted out a few years back, that would be a great home for a DIY electronic load. Although they are only the basic guts of a working unit, what do you guys think of those $20 bag of parts kits you can see on the bay and other places, bangood, Aliexpress, Amazon? (Not the USB loads). Just curious since I have enough salvaged parts packed away to probably create a power supply, display, etc. to put something like that together. Plenty of old fans and heat sinks too.

At any rate, new old stock LAS1905 regulators should arrive this weekend and I'll stick one in to the 8656A for the next phase of the journey. Should be interesting no matter what happens. Seem to come from a reputable source in the States, "warranted", and came in at about $12 each, shipped.

JRH
 

Online Tony_G

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Re: Fool for the 8656A Sig Gen
« Reply #123 on: March 01, 2023, 09:50:50 pm »
JRH - The journey is as important as the destination - So if grabbing an electronic load kit for $20 from a website gets you enough bits to start building your own load then go for it - I just wouldn't put any stock in the numbers they rate and in the actual working of the unit - But, and this is the journey part, getting something that doesn't work, learning why and then correcting it is worth the time as well.

Just make sure that you characterize the device before you depend on it.

TonyG


Offline Swainster

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Re: Fool for the 8656A Sig Gen
« Reply #124 on: March 02, 2023, 03:10:15 am »
Hi JRH,

The other day I tried to list all the projects and repair candidates on my to-do list - I realised that I probably need a spreadsheet to keep them all organised :-DD

I'm not sure which kit you mean, but if it's the simple constant current source based on LM324 (i.e. no microcontrollers involved) then that looks like a fun project - for that price (seems to be a low as USD10) then I'd be tempted to add it to my next aliepress buy just for fun (actually you could build it on prototype board but the kit PCB would save a lot of donkey work).

Anyway, if it is that one, then be aware that the kit mosfets (110N8F6 in the listing that I was looking at) are not specified for DC loads i.e. no DC Safe area of operation. You can't expect reliable operation when operating outside the datasheet specs so treat it as a bit of fun and maybe see how far you can push it before the supplied FETs pop. At that point you can investigate SOAs and linear power mosfets etc, and maybe identify some more suitable parts (though proper linear power mosfets are not cheap).

Basically I'm just paraphrasing what Tony says.
 


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