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Fool for the 8656A Sig Gen

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Swainster:

--- Quote from: jrharley 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

--- End quote ---

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.

Tony_G:
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

Swainster:
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.

Tony_G:
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

jrharley:
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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