Author Topic: Fluke 8000A Help Needed (Read 4891 times)

Goatropercu · « **Reply #25 on:** January 21, 2023, 07:30:32 pm »

Apologies in advance if any of the following is difficult to follow. All observations below were made with the range set to "2".

Point 1 (CR3 Anode)
No signal: Slightly unstable scope image. Voltage bounces between -19vdc and -20vdc
W/ 1.5vac signal: Slightly unstable scope image. Voltage bounces between -19vdc to -19.7vdc

Point 2 (Q2 Base)
No signal: Voltage bounces between -12vdc and -13vdc
W/ 1.5vac signal: Voltage bounces between -11.9vdc and -12.7vdc

Point 3 (CR22 Cathode)
No Signal: Voltage bounces between -0.9vdc and -1.3vdc. The functional 8000A measures dead nuts -1.8vdc at this location; no deviation whatsoever.
W/ 1.5vac signal: Clean sine wave that slightly shifts up and down with voltage varying between -.9v and -1.1v

Point 4 (Q1 Source)
No Signal: Voltage bounces between +1vdc and +1.3vdc. The functional 8000A measures dead nuts -0.7vdc at this location; no deviation whatsoever
W/ 1.5vac signal: Clean sine wave that shifts up and down with voltage variation between -1.9 and -2v

Point 5 (Q1 Drain)
No Signal: Voltage bounces between +19vdc and +20vdc.

Voltage across points 3 and 4 is a rock solid 2.27vdc

I am using a Rigol DS1102E (2ch) and the probes that came with it.

I know my B+ and B- are about 4v off from the prescribed 15vdc, and do seem somewhat less stable than I'd expect. However, I'm not convinced that the PS is the culprit.

What's interesting is that with a 1.5v input and the meter range set to "2", the display steadily reads between 1.3-1.7vac. This is not accurate, but close. Now when I flip the range over to 20v, still with a 1.5v input, the meter spits out all sorts of readings in the range of 0.9v to 7v. The drift is compounded when the "20" range is selected. Perhaps this is expected and the drift at lower voltages is just being magnified via the circuitry associated with the 20 and 200 range functions.

Thank you for your time and assistance.

Goatropercu · « **Reply #26 on:** January 21, 2023, 07:40:05 pm »

Edited Point 4 above.

Image of points 3 and 4. As stated the waveforms shift up and down slightly.

bdunham7 · « **Reply #27 on:** January 21, 2023, 07:49:25 pm »

There's no way that this circuit can work with unstable power supplies. Any shift in point 2 or point 5 is going to be reflected in the input to U1. So best be having a look at those PS rails as there appears to be no regulation at all. It is possible the DC and Ohms functions aren't as sensitive to this.

The reason you see more of an effect on the 20V range is that it is using much more gain in the U1 circuit. The 2V range appears to be approximately unitary gain, but the 20X range uses 100X gain with the 1000X divider if I've interpreted things correctly. This is a fairly goofy way to make a meter, but the 8000A is an early, thrifted version that really hasn't stood the test of time TBH.

Edit: this isn't a battery-equipped model, is it?

Goatropercu · « **Reply #28 on:** January 21, 2023, 08:03:34 pm »

Then back to the power supply I go.

Both of the 8000A’s here are connected to mains, no batteries.

I have been using an old 8060A for the past 5 years. My dad has had this one lying around and with the cost of 9v batteries, combined w/ my forgetful nature I finagled this one from him. I’m already on the hunt for a more modern/accurate/better bench meter, but this was my Dad’s and I want it to function as well as I can get it to.

Really appreciate the help.

Dave Wise · « **Reply #29 on:** January 21, 2023, 08:07:03 pm »

I suspect CR8 is open.

bdunham7 · « **Reply #30 on:** January 21, 2023, 08:07:40 pm »

Can you post a photo of the whole board?

Goatropercu · « **Reply #31 on:** January 21, 2023, 08:15:43 pm »

bdunham7 · « **Reply #32 on:** January 21, 2023, 08:23:43 pm »

I was afraid of that....can you post a photo of the front face as well? Your PSU issues are not going to be simple. They use a crude inverter and that is why there are the little fine ripples on your waveform. The AC circuit can probably work OK with some noise and lack of precision, but stability issues will wreck it, as you are seeing.

Are the batteries any good or are they completly toast?

bdunham7 · « **Reply #33 on:** January 21, 2023, 08:36:33 pm »

Quote from: Dave Wise on January 21, 2023, 08:07:03 pm

I suspect CR8 is open.

Oh he wishes it were that simple! He has this mess, not the simple PSU on the line-only version:

Dave Wise · « **Reply #34 on:** January 21, 2023, 08:45:43 pm »

Oops sorry. After I posted, I began to wonder if the OP's instrument is the Option 001 battery version.

The Option 001 version "regulates" only as well as the battery regulates. Yuck. If the battery pack - which also acts as main reservoir capacitor - is crapped out, we can't expect +/-15 to be anything reasonable. I would disconnect the AC power cord and the battery pack, and temporarily power the instrument from a bench supply connected in the battery's place. Add a big capacitor at the battery terminals, since the inverter draws heavy spikes of current and the external supply may not keep up.

bdunham7 · « **Reply #35 on:** January 21, 2023, 08:57:12 pm »

Quote from: Goatropercu on January 21, 2023, 08:03:34 pm

Both of the 8000A’s here are connected to mains, no batteries.

I have been using an old 8060A for the past 5 years. My dad has had this one lying around and with the cost of 9v batteries, combined w/ my forgetful nature I finagled this one from him. I’m already on the hunt for a more modern/accurate/better bench meter, but this was my Dad’s and I want it to function as well as I can get it to.

OK, so there's a big difference between a non-battery model and a battery model (which yours is) without batteries. I don't know how well they work with the batteries not installed but I would suspect not well. Does your good meter have the same battery holders on the board as yours? And if so, they are empty, no batteries installed?

IMO, it is worth the experience of fixing this even if the end product isn't something all that helpful. But I'll warn you , it can become a lifelong addiction!

Goatropercu · « **Reply #36 on:** January 21, 2023, 11:23:23 pm »

Quote

Does your good meter have the same battery holders on the board as yours? And if so, they are empty, no batteries installed?

The working meter also has the battery option w/ no batteries installed.

Quote

IMO, it is worth the experience of fixing this even if the end product isn't something all that helpful. But I'll warn you , it can become a lifelong addiction!

That is what I like to hear and I really appreciate the positive attitude. If you can't tell I have no formal electronics training. I have been repairing stereo equipment for the last few years, but that's about the extent of it. I know this meter isn't the holy grail fluke by any stretch of the imagination, but it belonged to my dad. I've already learned quite a bit up to this point, and have had a few refresher courses in things I already thought I knew as well. If you have the patience I am all ears.

Goatropercu · « **Reply #37 on:** January 21, 2023, 11:28:14 pm »

Picture of the working meter attached. The only difference in terms of battery is that this meter has the battery terminal connections still in place. It’s interesting to me to see the different vr’s, resistors, etc that have been replaced assumingely when they were sent in for cal.

Dave Wise · « **Reply #38 on:** January 21, 2023, 11:55:09 pm »

Looking at the power supply schematic, I don't expect an 8000A Option 001 to do anything reasonable without a good battery. There's no regulation or filtering ahead of the inverter except the battery itself. Disconnect mains power and hook up a bench power supply in place of the battery. Any troubleshooting done without this (or a good battery) is meaningless. "Ain't power supply happy, ain't nobody happy."

There are instruments that run on AC without the battery (e.g. Fluke 8100B), but the 8000A Option 001 is not one of them. It must have a good battery to work.

bdunham7 · « **Reply #39 on:** January 22, 2023, 12:23:13 am »

Quote from: Goatropercu on January 21, 2023, 11:28:14 pm

Picture of the working meter attached. The only difference in terms of battery is that this meter has the battery terminal connections still in place.

Well that's interesting. Apparently at least that meter works sufficiently well without batteries in place. Can you post scope pictures or screenshots of the power supply rails from both the working and the unstable unit? It would be worth it to confirm that there is some signficant difference and perhaps what might be causing it. Are you averse to 'improving' the original design a bit?

joeqsmith · « **Reply #40 on:** January 22, 2023, 02:13:50 am »

Looks like someone told you to start swapping caps and you replaced tants for alum elec.

8000A was my first DMM. Like your dads having some sentimental value, that's about the only reason I still have mine.

Those custom ICs are not well protected and those stupid trim resistors that came with the kits being plugged into the IC sockets. Not that those mechanical joints would ever get flaky. Then there was the cost for the IC repair kits.

Dave Wise · « **Reply #41 on:** January 22, 2023, 03:59:01 am »

Add a reservoir cap and a three-terminal regulator that outputs the battery voltage. Of course then you can't put a battery in it.

m k · « **Reply #42 on:** January 22, 2023, 10:59:01 am »

Quote from: Goatropercu on January 21, 2023, 07:30:32 pm

I know my B+ and B- are about 4v off from the prescribed 15vdc, and do seem somewhat less stable than I'd expect. However, I'm not convinced that the PS is the culprit.

Better believe what you see.

Your #21 is already showing that the issue starts appearing at the R32 level of the direction of input.
Then points 4 and 5 here are showing that the issue is present both sides of Q1.

Two possibilities, either something is overloading +15V or it's unstable.
The other machine seems to be stable, so my guess is the latter.

Goatropercu · « **Reply #43 on:** January 23, 2023, 01:48:15 am »

Quote

Can you post scope pictures or screenshots of the power supply rails from both the working and the unstable unit? It would be worth it to confirm that there is some signficant difference and perhaps what might be causing it. Are you averse to 'improving' the original design a bit?

Images attached with leads at C17(+) and C18(-) of both instruments. Please let me know if you need me to adjust anything and try again. The voltage isn't 100% rock solid at these points on the working meter, but clearly it is closer to spec.

I am not opposed to modifications in the effort to get it functioning properly. I have no intention of ever powering this meter via batteries.

Quote

Disconnect mains power and hook up a bench power supply in place of the battery.

I do not have a bench power supply at my disposal (yet), but I do have a tub of wall warts of various voltages.

I have a stable 8vdc at the cathodes of CR13 and CR14. I am getting between 5.3vdc and 5.5vdc at R22.

bdunham7 · « **Reply #44 on:** January 23, 2023, 01:58:15 am »

Quote from: Goatropercu on January 23, 2023, 01:48:15 am

Images attached with leads at C17(+) and C18(-) of both instruments. Please let me know if you need me to adjust anything and try again...

...I have no intention of ever powering this meter via batteries.

Could you change the timebase to 10ms/div (100ms total across the screen to correspond with the U3 measurement period) and try again on both meters?

As for batteries, one of my suggestions was going to be to put 4 alkaline D cells in it and see what it does!

However, a bit further down the road you might put a bit of circuitry in place of the batteries to regulate your 5 V supply. The +/-15V supplies can probably be fixed well enough by simply crowbarring them with 15V 5W zeners across C17 and C18. But I'm interested to see what the differences are between the good and bad meters first--you don't want to re-engineer the whole PSU and still have an issue.

Goatropercu · « **Reply #45 on:** January 23, 2023, 02:16:20 am »

Updated images in previous post. Will rustle up some D’s.

Edit: It states to use nicad batteries only. It charges them when plugged in. I don’t have any rechargeable D’s here and don’t plan to. I’ll use alkaline for testing porpoises and just not plug the thing in.

Will have to rig up some sort of battery terminal doo hickies. All for robbing parts, but i’m leaving those in the working meter intact. Might just see if I can rig up a voltage supply in place of batteries.

bdunham7 · « **Reply #46 on:** January 23, 2023, 03:59:13 am »

Quote from: Goatropercu on January 23, 2023, 02:16:20 am

Updated images in previous post. Will rustle up some D’s.

Edit: It states to use nicad batteries only. It charges them when plugged in. I don’t have any rechargeable D’s here and don’t plan to. I’ll use alkaline for testing porpoises and just not plug the thing in.

Will have to rig up some sort of battery terminal doo hickies. All for robbing parts, but i’m leaving those in the working meter intact. Might just see if I can rig up a voltage supply in place of batteries.

ms = milliseconds, 1000X slower than what you have posted which is 10µs (microseconds)! The sampling period of U3 is 100ms (1/10 of a second) so that's the timeframe we need.

Can you look at the 5V supplies as well?

Goatropercu · « **Reply #47 on:** January 23, 2023, 11:42:55 am »

Not sure what I was thinking. Long day yesterday.

Goatropercu · « **Reply #48 on:** January 23, 2023, 01:08:16 pm »

Images updated again, and attached here as well.

I have 8vdc at the cathode of CR14 and 5.3-5.5vdc at the positive lead of C25.

Thank you.

Goatropercu · « **Reply #49 on:** January 23, 2023, 02:45:04 pm »

What’s the purpose of the incandescent lamp DS5?


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Author Topic: Fluke 8000A Help Needed (Read 4891 times)

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