Re: HP 141T Power Supply Help Needed
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Mike,? I now get a rather high impedance?at the output of the regulator, think it was ~50k.? ? ?The previous issue was in fact a physical short on A5 board (lead clipping...) Currently, things look as follows:? Q9 C .65? B? 1.36 E .65 Q10? C 2.4 B 0 E 0 Q11 C 1.3 B -.65 E 0? Output Voltage -0.002 Output Impedance? ~400k? I get the sneaking suspicion that my driver is bad as the CE voltages are roughly equivalent (maybe .002 different).? ? ?The voltage I see at the base of Q9 seems close, so I'm guessing the sensor amp is working better..? ?Q10 base at 0v is a little weird.? ?
Like I say, I'll go back through things this afternoon and see if any remaining parts saw issues in the last round of testing.? ? ?This feels like progress though.? ? ?
I don't know how to fix Q4 socket long term, but I'm mulling that over.? I kind of want to insert / solder a pin in place of the spring contacts that have been causing me so much frustration.? ? ?
On Thu, Sep 5, 2024 at 10:42?AM Michael Bafaro via <m.bafaro= [email protected]> wrote: Bill, ? The big question is what resistance do you measure at the pins of the A2 board that have the white / Violet wires going to them with the wires removed.? At this point you should read >1K Ohm at the output of the regulator.? If the resistance at the output of the regulator is < 4 Ohms don¡¯t put in a new pass device.? As I remember you had one of the white / violet wires that measured < 1 Ohm.? I seem to remember that you were going to trace out that wire to see where it went and why it read so low.? ?I remember you had one of the White / violet wires that went to the A5 assembly which runs the storage functions and you were going to find the cause of that later.? Let me know what you find. ? Mike ? ? Ugh, I put it down for a few days to work on an old Zenith my wife got for my birthday.? Today, I put a fresh pass trans in, and fired it up.? I got -2.5v for a few seconds and then it looks like it took out the transistor.? Replaced and same story.? ? ? Mike - with F4 removed, I measure ~50k ohm at the terminal you mention, so that rules out C9?? ? R49 does look ok..? my DMM only measures tenths, so I get a floating .2 to .3 ohm.? I guess I typo¡¯d the decimal or mentally transcribed wrong.. ?
Dave - I ran through each part you mention.? R44 was a little suspect in that the schematic says 20k but it measured 18k, still in range but I¡¯d have expected it to go UP. ? In a fit of rage, I replaced C8, C9 , R44, R46 and still blows pass trans.? ? ?Raw voltage in??is around 22v last I checked.
At this point, I¡¯m wondering if there is something under the board fighting me. ? It didn¡¯t look like I was the first person in here, and I¡¯ve been working from the top side only.? ? Is it worth pulling the board to look at? ? ? I could go eBay a bunch of old exotic parts, I could try to shotgun some moremodern parts in.. ? ? I do feel like I¡¯m loosing the battle on this rail though. ? Anyone in North Carolina?? :-)? ? Hello All
I have a hp 141t when it is powered up there is no trace but when I turn off the power the trace briefly appears in top right corner.
Is it the blanking circuit or do I need to do an trace? alignment or something else. I need some guidance
Has anyone encountered this problem before and let me know what you did.
73's Leroy de VK5HL
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Chuck Harris
Sent: Tuesday, 3 September 2024 1:41 PM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP 141T Power Supply Help Needed
To amplify your comment, many times older linear supplies, when they are brute force recapped, with the best, long life, low esr caps you can find, will become beacons of chaotic oscillation.
I put a low ESR ceramic capacitor in the feedback loop of a unity gain OPAMP circuit the other day, and was amazed that it went totally unstable.? It was an older LM301 variety, where the response characteristics of the opamp are a blank slate until the designer adds in the stability network.? The designer's efforts were wiped out by my ceramic capacitor replacement for his 1uf back-to-back emulation of a non-polar electrolytic.
Sometimes the modern best replacement is not suitable for use in an older instrument.
-Chuck Harris
On Mon, 2 Sep 2024 22:40:20 +0100 "Dave_G0WBX via "
<g8kbvdave=[email protected]> wrote:
> Hi.
>
> It's just a generic term (damping is another) to slow down the
> response of a loop, to prevent it going unstable.
>
> Not an uncommon issue in some linear PSU's like those -12.6V or -100V
> regulator topologies that are not "conventionally" laid out.
>
> Unless you 'scope the thing, you often don't know it's gone unstable,
> other than some passive parts sometimes cook for no obvious reason.
> (That R49 for example!? See Mike's comment in another mail.)
>
> It is also not uncommon, when modern transistors are used to replace
> older failed parts (that are not available any more) and the new
> device has "a lot" more gain at HF than the originals, that regulator
> or other control loops suddenly become "unruly" under some operating
> conditions.
>
> Sometimes a low value "Base Stopper" resistor (a few Ohms, or a
> ferrite bead) in series with the new device's Base connection can
> restore sanity!
>
> Take care.
>
> Dave B.
>
>
>
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