HP4280A

[Keith956]

Anyone got one of these, a 1MHz C-V (and C-t) meter?

Yesterday mine was working intermittently, usually passing power on and self tests, today it comes on with Err 11 (occasionally Err 12).

Alas all the manual says is that this is a failure of the timing circuit... not very helpful.

[wn1fju]

I have a 4280A and fortunately mine is still working.  The schematics in the manual I have are barely readable and the whole service manual, while describing the circuitry, is not very helpful in terms of diagnosing/debugging.

Good luck with it.  Maybe it is something simple like a (nearly) shorted tantalum on one of the boards that is dragging a power supply rail down.  That's where I would look first.

[Keith956]

I have a 4280A and fortunately mine is still working.  The schematics in the manual I have are barely readable and the whole service manual, while describing the circuitry, is not very helpful in terms of diagnosing/debugging.

Good luck with it.  Maybe it is something simple like a (nearly) shorted tantalum on one of the boards that is dragging a power supply rail down.  That's where I would look first.

Luckily it came with an original manual in the 3 ring binder with fold out schematics. But as you say, the description is not very helpful.

When it arrived, it would power up ok but none of the buttons on the front panel did anything. It was, however, able to read a capacitor plugged into the hi/lo inputs correctly. As the seller had pictures of it working, I assumed something had got loosened in transit, and indeed a bit of random prodding brought the thing into life; it passed the self tests so I assumed it was OK. It would do a C-V sweep on an 1N4148 correctly; it was when trying to use the high resolution mode on a 48pF cap that it seemed to get stuck, and when I powered it off/on it started coming up with the Err 11 messages.

I wasn't aware the unit had tantalums in it - thanks for that info, I will go check them.

[wn1fju]

I'm not necessarily predicting tantalum failure.  It was only my not so subtle suggestion to get you to check the power supplies.  That is usually a good place to start in any repair.  Check all the power rails (and there are multiple supplies in the 4280A) for DC voltage with a voltmeter and AC ripple (usually 120 Hz if a capacitor is open) with an oscilloscope. 

You also said that it started to malfunction when you switched to high resolution mode.  Is there any chance the front panel button is stuck down?  HP pieces generally don't like starting up with buttons depressed.  Just a thought...

[Keith956]

Thanks. I have checked the power supplies (floating and grounded) and they are within spec e.g +5V reads +4.89V.

The buttons have been toggled quite a bit and 'feel' ok.

I have hooked it up to a scope and probed some of the control signals, the DISP_ signal (to select the display) goes momentarily low as the device comes out of reset (BRST_ going high, t ~= 200mS). This I think is setting all the display LEDs on. Then there is some activity on the TIMER_ and LOGIC_ signals at about t = 1.79s  followed by some on DISP_ (presumably setting the Err 11 msg on the LEDs) then it all goes quiet.

So it does look like it's testing something in the timer circuit,  but it's not clear what. Next job is to try and probe stuff on the timer card A9.

[Keith956]

So while trying to test the timer board, I happened to flick the front panel Bias switch to the +/- 100V position. And power cycled the unit.

It passed power on test and was working again! It ran through the 8 self tests OK too. That tends to suggest there is no component failure, just something going open/short?

Maybe I was a bit hasty as I put the covers back on again, and guess what it once again comes up with Err11.

So, I'm beginning to wonder if you are right, wn1fju, and there is an intermittent issue with one of the buttons. I tried De-oxit on the bias switch to no avail, I guess I need to test every front panel button.

[SilverSolder]

Yes, de-oxiting all buttons is step 1 with older equipment.  Do the same with any internal connectors...

[Keith956]

Well the issue is not the buttons, I've tested all and they are fine. The switch was a red herring.

The unit intermittently fails; typically it will work first thing in the morning for a while. Then after a period that can be seconds to hours, it will read nonsense values for C and G. Running a self test at this point gives the Err11 message. Switching it off and on will usually give the Err11 on powerup, and it will stay that way for the rest of the day - I suspect temperature may be influencing it?

Anyway probing the logic on the timer board A9 seems to indicate that the SAMPLE line is not going low and hence starting an integration ramp. I've narrowed it down to (I think) the area around U17 / U7; I'm suspicious that the timer U17 a MC6840 is intermittently faulty, as its successful count down is enabling the SAMPLE signal.

Next step is to try some freezer aerosol on the chip to test the cold hypothesis...

[Keith956]

So I had another look at this one recently. I'd narrowed it down to the A9 timing board, and had an extender card made up at JLCPCB with 2x22 way edge connectors, so I could probe on ICs directly on the board.

Even with the logic analyser it was hard to see exactly what was wrong. The fault appeared some time from turning the instrument on - up to a couple of hours - and manifested itself with (a) capacitance readings wildly out or overflowing and (b) failure of the ramped C/V part to work. At first I thought it might be temperature related but a good squirt of freezer aerosol on the ICs on the board failed to alter it from working, or not working. When a fail occurred, self test on power on would always give ERR11.

So I started replacing ICs one by one. Fortunately they are not difficult to remove aided by a Hakko FR-301 and a Quick hot air station. I concentrated on the VFD section - U17, U7, U6, U19 then U30/U29/U28/U18. Nothing, the error still appeared after a while.

This morning I replaced U27 a LS245 data buffer and U32 a LS367 control bus buffer. The unit has been powered on and on a ramp up/down CV cycle for the last 7 hours and has worked faultlessly. 

I tested the 245 and 367 on my retro chip tester (https://8bit-museum.de/sonstiges/hardware-projekte/hardware-projekte-chip-tester-english/) which said both ICs are working. But this only does logic tests. I am wondering if I can test this chip any more as I'd love to know what the failure mode was.