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Fool for the 8656A Sig Gen

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jrharley:
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

jrharley:
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

Tony_G:
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

Tony_G:
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.

jrharley:
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

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