开云体育

Just picked up yet another 3400A. Coincidentally, I was just last week trying to fix a problem in two other recently acquired units, and this latest one acts the same. They seem to work fine with definite input signals, or with no input while a low-Z terminator is present. Without it, they tend to have some reading around 10 to 100 mV with nothing applied. In contrast, the ones that work right are clean all the way down to the 1 mV range, reading residual noise and offsets around 5% of FS. IOW, say you've got some signal reading nicely in one of the more sensitive ranges, then completely disconnect it, and it pegs the meter.

I'm assuming the problem is due to line ripple in a PS due to bad caps, causing high residual signal, or local leakage somehow getting into the front when open-input. Another possibility is maybe something is oscillating up front. The main thing is, that they all seem fine when the input is from low-Z. My test setup for these is a 50R terminator in front, and HP3325A feeding it. With 1 mV up to the most the 3325A can do (2-3 Vrms?) for input, they all show about right, in the appropriate ranges. So, I think the basic amplifier and RMS engine work fine - it's just this extra junk that's always present, but is swamped out when the input is at low Z.

Anyway, I haven't checked anything much yet, but before digging into the details, I figured on getting some background info. Does this symptom seem common or familiar to anyone? I don't recall any discussions of this behavior.

Scoping them will be the first thing, but I won't be getting around to these again for a couple months. I want to avoid total re-capping, since there are a helluva lot of caps, times a helluva lot of units (nine now) involved, if I wanted to really do a proper, thorough job on the whole stable. Over the years I've managed to patch them up as needed with minimal changes. I assume (and hope) these last three have identical problems. This last unit also means I get to do yet another IEC power connector mod - I've already done all the others (I think) that needed it over the years.

Ed

The symptoms you describe could be caused, as you point out, by several different things.? Personally I would lean towards capacitor issues as a "first check" item - I've worked on 12 of them so far (and am waiting for #13 to arrive) and capacitor issues have been the most frequent cause of problems.? In the 3400A the power supply capacitors often go bad but there is really not much required of them so, even faulty, they usually still work "good enough" - and the rest tend to be used in ways that leakage can cause numerous and fairly subtle errors. And each of the boards have multiple revisions, each of which changes what tends to go wrong.? Throw in a handful of carbon comp resistors and you get a whole other avenue for issues to crop up.? All that being said, I love working on them and have have had the best luck with starting off by replacing all the electrolytics on the power supply board then just trace the signal through the unit.? The troubleshooting steps in the manual - provided it actually addresses your particular hardware combination - are, as typical of that vintage HP, very helpful.? While I've not personally had issues with it, I have heard from others that the input JFET can get leaky and cause problems.? Don't know if that would result in your issues but it would be something to check.

On a side note, one of the very first things I do is replace the output jack on the rear with a BNC - makes later adjustments *much* easier and any other use in the future.

Hal

Thanks Hal,

I'll start on the PS caps, but measure them all in-circuit first, and replace only bad ones. I think the PS board is plugged in, so should be easy to access.

My collection has a number of different variations, including original and newer type optical choppers, and a couple with the later ICL7650 chopper-stabilized op-amp. The one thing that's the same is they all use the original 5886 cathode follower input amp. I have plenty of spare 5886s, but as I recall, I've only had to replace one over the years. I remember having to replace the ICL7650 in one unit, the neon lamps in maybe a couple, and a number of bad caps in various circuit spots. I also have a couple with the flakey (literally) meter face problem that need restoration someday.

I figured on replacing all those headphone jacks with BNCs eventually, but for expedience I think I'll just make a phone jack to BNC adapter so I can scope the output signal for clues.

One other problem they seem to all have is deterioration of the foam cushions that suspend the input amplifier board. In the past I've made various foam arrangements to try out, and painstakingly adjusted everything to keep it properly centered in the mounts. More recently I did some experiments to see just how sensitive the board is to shock and vibration in my typical lab environment - it's about zero. I could rattle the board around in the empty mounts and tap on the 5886, with hardly any effect, so I decided to keep it simple and scrape out the old foam residue, then wedge some plastic tubing in to keep it from rattling around loose. It would likely be affected in a high vibration environment, but I don't have that, so no problem - I can't picture ever using these on my tractors.

Ed

Oops - just realized I wrote 5886 tube (which was one of my favorites - electrometer tube), when I meant to say 7586 (Nuvistor).? Ed

Not only are they very nice instruments, they are (to me, anyway) enjoyable to work on - anything other than the thermocouple or meter movement can be repaired and it is easy to access the subsections.? I have several different iterations of DIY replacement boards for them and have one as a semi-permanent test bed on a bench shelf to try out different ideas.? And, of course, one is used frequently for measuring power supply noise? - my main use for it, actually.?

On 5/23/24 11:44, Harold Foster wrote:

Not only are they very nice instruments, they are (to me, anyway) enjoyable to work on - anything other than the thermocouple or meter movement can be repaired and it is easy to access the subsections.? I have several different iterations of DIY replacement boards for them and have one as a semi-permanent test bed on a bench shelf to try out different ideas.? And, of course, one is used frequently for measuring power supply noise? - my main use for it, actually.

I've never had to repair it, but I have a 3400A that I really like as well. A friend who died left me a second one; I've not checked it out yet.

-Dave

--
Dave McGuire, AK4HZ
New Kensington, PA

Hello to all members.

My 3400 developed a "stuck range switch" symptom.? At first it was a bit hard to turn and now it is completely jammed.? I cannot turn it even with pliers.? I tried IPA to dissolve the grease, a few drop of oil on front and rear sleeve.? All of this without any success.? Is it something that you experienced ?

Claude

I've had some rotary wafer switches (not in 3400A) that were totally seized up. If it's not installed somewhere, and out in the open, the quickest fix is to disassemble the unit to the point where you can pull the shaft out of the front bushing (you remove the clip ring at the front), and use brute force. This is no sweat on a raw part out in the open, but a PITA and risk of damage on one already built into something.

You can trade patience for expedience by generously applying penetrating oil to the front shaft and bushing, from both sides of it if possible, then wait for possibly quite a long time. It could take days to weeks, and maybe adding more oil occasionally and checking to see if it's loosening up. Don't use WD-40, which can get things loose, but will eventually gum up and seize it again - that's only good for something you'll be taking apart afterward to clean out and re-lube. Use real, petroleum-based penetrating oil (with additives) or just light machine oil.

I have an old variac junked from something, that had it's shaft stuck nearly solid. It's a weird kind, with a very long front bushing to recess the main part maybe 2-3" behind a panel. I could only get oil in at the front. I soaked it with good old Liquid Wrench, reapplied a few times over some weeks. It gradually loosened only a little, and I kind of forgot about it for several months. When I finally thought again to check it, it was like brand new. It took a long time to get all the way through that bushing.

Anyway, if you can let it sit for a while, slowly but surely you can fix it with low risk.

Ed

On 2024-05-23 5:07 PM, ed breya wrote:

I have an old variac junked from something, that had it's shaft stuck nearly solid.

I heard about Kroil on here or a similar group. Expensive stuff, but it really works. I've used it to free up several things that I had despaired of ever getting to move again. Apply as close as you can to the stuck pieces, wait an hour or a day and apply again, and it'll practically fall apart. Only place I've had it fail was on a 1/2" shaft threaded into a metal part, that had been submerged for a couple of decades and was rusted solid.

Steve Hendrix

I discovered my local hardware store has Kroil. Fairly expensive but you can get it in small amounts so its affordable.
There are so many penetrating oils around plus formulas for making youor own. Each has a fan club. I have no idea of which is best.

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

I have used Kroil, and a cheaper but equally good PB Blaster.

The problem is when they are left on the fasteners, really bad
corrosion happens much more quickly than I would like.

In automotive applications, where moisture exposure is what made
the problem happen in the first place, this is especially bad.

I would recommend that if you use these, or similar products,
clean them off thoroughly, and apply a real lubricant that is
appropriate to the application.

If you want something that won't corrode later, try "Ed's Red",
which is a homemade concoction that does it all:

1 part Dexron ATF
1 part K1 kerosene (low sulfur diesel is ok too.)
1 part mineral spirits (aka varsol)
1 part acetone
200g/l lanolin.

The acetone and lanolin are optional. I have never tried it with
the lanolin.

-Chuck Harris


On Thu, 23 May 2024 23:52:57 -0700 "Richard Knoppow"
<dickburk@...> wrote:

I discovered my local hardware store has Kroil. Fairly expensive
but you can get it in small amounts so its affordable.
There are so many penetrating oils around plus formulas for
making youor own. Each has a fan club. I have no idea of which is
best.

On 5/23/2024 2:12 PM, Steve Hendrix wrote:

On 2024-05-23 5:07 PM, ed breya wrote:

I have an old variac junked from something, that had it's shaft
stuck nearly solid.

I heard about Kroil on here or a similar group. Expensive stuff,
but it really works. I've used it to free up several things that I
had despaired of ever getting to move again. Apply as close as you
can to the stuck pieces, wait an hour or a day and apply again, and
it'll practically fall apart. Only place I've had it fail was on a
1/2" shaft threaded into a metal part, that had been submerged for
a couple of decades and was rusted solid.

Steve Hendrix