Prema 6048 teardown

[Rax]

After performing the DCV cal on the Prema on the .2V, 2V and 20V ranges - using my 5440A, which seems to be vetted by a KS-calibrated 3458A - I am seeing what to me like pretty reassuring agreement. For instance, between the 2V "non-DIV" input and the 2V "DIV" input, there's sub-unitary (ppm) divergence between the two hardware paths (preliminary conclusion). Of course, not very easy to say if what I'm seeing confirms the Prema, the Fluke, of both of them in "joint venture." I'd need to do more work to clear my data and experimental conditions.

But to me this provides some level of comfort with the linearity of the Prema ADC away from zero crossing, as it compares with the pretty good linearity the 5440A in known to have.

[Rax]

The other provisional conclusion I feel I can draw is that I'm seeing much more consistency from the Prema than before the upgrade.

I feel that the zeroing is now pretty reliable and and the drifts I used to see (and not know whether they are caused by the Prema or the 5440A) are no longer occurring. If anything, the zeroing of the Prema seems just about as reliable and resulting in a level of relative accuracy (between the two) as the AUTOCAL the 3458A seemed to provide (and direly need all the time). In fact, for my particular samples, the Prema needs zeroing less than the 3458A needed the AUTOCAL.

[Kleinstein]

The upgrade via better OP-amps can be really woth it.  The change OP77 to OPA140 seems to largely fix the weak point in the INL.  The OPA140 and LT1007 for the reference are major contributors to the zero point drift. Here there can still be variations in the actual parts used. Even in the original configuration the stability of the zero can vary between units. If the zero is stable there is no need to change things. If it is drifting it can make sense to try an alternative chip (could be just another LT1007 or similar or an AZ OP).

How often the zero for the 6048 or ACAL for the 3458 is needed  can also vary. Some units can be more stable than others.

For the prema meter the gain of the ADC and input is set by 1 resistor ratio. The 2 V range may be slightly more stable in this respect, using 2 x 5 K and thus equal value resistors. The other ranges still also still have only 1 pair of resistors, just exchanging one of the resistors to a different value. So there is no clear prime range with like with the 3458. The 6048 does not look that it could easily measure it's own reference directly. So a kind of ACAL would not work so easy for the Prema meter. The test in 2 ranges is more like a self test, but no way to correct drift.

The 3458 has additional resistors in the ADC gain and amplifier gain. So less stable to start with, but able to correct. On the other side it has a stable zero.

[Rax]

A bit of a follow up post (medium term observations, at least in my book) to the issue of the op amps upgrades and zeroing. It partly reconnects with some offline conversations with dietert1.

I'm very happy for how it behaves from that perspective. I had it off for a few days and just turned it on. From a cold turn on it started around maybe .00000250V with a short on the input, and after a good one hour warmup it's at about .00000035-45. I don't think that's a bad zero by saved values from the prior run by any stretch of the imagination.

I think this is further proof the op amps upgrades are conducive to bettering the performance of this great meter.

[branadic]

We learned something new at MM2024 about Prema 6048. Some participant, let's call him Manfred L., gave us some interesting info. In case you were wondering about the bodge wires in your unit (soldering side), in particular at the 5 V logic rail, this is not someones repair attempt of a broken trace but a factory bodge made by Wilfried Stricker himself to decrease trace resistance. Don't attemp to remove them, but leave them as they are and where they are!
Hopefully we can soon report what removing them will lead to

-branadic-

[Rax]

We learned something new at MM2024 about Prema 6048.
-branadic-

OK. Bring on more novel finds on the 6048.

[meandeev]

We learned something new at MM2024 about Prema 6048. Some participant, let's call him Manfred L., gave us some interesting info. In case you were wondering about the bodge wires in your unit (soldering side), in particular at the 5 V logic rail, this is not someones repair attempt of a broken trace but a factory bodge made by Wilfried Stricker himself to decrease trace resistance. Don't attemp to remove them, but leave them as they are and where they are!
Hopefully we can soon report what removing them will lead to

-branadic-

Hi Branadic,
got the broad hint  ;-)
Background for everyone else: I bougth a prema 6048 3 years ago with a strange error, that wasn´t solved by us until now.

In the picture that I made when the unit arrived one can see the bodge wires at the digital board.



I removed them, because I couldn´t imagine that they are an "official repair". I didn´t made any measurements before that, but later the meter shows a strange behavior, shown in the second picture:



strange jumps of the measured values.  The red line are the values from the Prema 6048, blue are the values from a HP34401 that measures PIN6 of  U10 (~ the voltage of the Prema LTZ1000). Please ignore the jump at 1:05:00, at that time I switched off/on the prema for a second.

If I have some spare time I will start to investigate that again, also with installed bodge wires.

[dietert1]

When i started working on that board - after receiving our Prema 6048 from former contributor essele - i found those bodge wires, too. As far as i remember it is an improvement of the +5 V supply network.
I also remember adding several buffer caps on that board. The original design is poor in comparison to a typical HP device of that time. Those have almost one buffer cap per logic chip.
I also bought a new 2 MHz crystal, as the oscillator wasn't in good shape. Don't remember whether that solved the issue or other mods were necessary, like grounding the crystal housing.

Regards, Dieter