Prema 5001 Repair (Measurements are only near Zero)

[MarcX]

I‘ve got a defective Prema 5001 Multimeter with Scanner Option for free, doesn‘t turned on, found a bad electrolytic capacitor which shorted the -15V rail of the power supply and a bad ASIC (Prema CD), after replacement of these components, the multimeter is turning on and the digital circuit seems to be working. Self Test is passing completely.

There seems to be another problem with this multimeter, the readings of the multimeter are constantly near Zero on every range and function (including Ohms with open leads).

I don’t know what i should check now, maybe someone can give me a tip. The schematics are available at the pdf below starting at page 88:

https://www.prema.com/index.php/en/service-en/maintanance-of-measuring-instruments/item/download/176_49254bc9daa2b2087a972ba47f90261d

[Chris56000]

Hi!

Thanks for posting the schematic – it helps me give you accurate tips!

There are a number of things to check in the preamplifier chain before you get to the ASIC you replaced, and there's another Prema OEM special part that needs to be ruled out first!

To begin with, check the +5V. +15V and -15V lines are OK on the preamp board, then make sure the +7V feeding the amplifier block "SH10" is OK –  this is a simple ovened zener regulator diode U4 LM399 that's designed to deliver a very stable and accurately trimmed reference voltage over a long period of time, and normally runs very hot to the touch as it contains it's own heater – it's mounted in a circular off–white ceramic package.

Feel this device very carefully with just the briefest touch of your finger – if it's very hot, check the +7V from it at the junction of S22/C15/SH10 – if low or missing either U4 or the SH10 module could be defective, and desoldering the pin of SH10 connected to the +7V rail will isolate the module from U4. If the +7V is being affected by SH10 you'll need to talk to Prema again about a replacement SH10 module!

If U4 is cold and/or there's no +7V at all, bin it and get a replacement LM399H!

If the +7V is OK and U4 is very hot, connect a 1K potentiometer across a 9V PP3/1604 battery and adjust the potentiometer to give about 200mV DC between wiper and negative, connect positive  from pot wiper to Pin 3 (+ input of U3, LF411 buffer amplifier) and negative to meter 0V rail – the Meter's display should respond to this without causing an overflow!

If no response or change at all, again U3 or the SH10 module might be defective!

If ok at this point, try the same 200mV at pin 3 of U2  (+ input of '7650 chopper–stabilised amp) – no change here but OK at U3 pin 3 points to U2 or it's associated power supply etc., being defective, or possibly either (or both!) of the protection diodes CR4 & CR7 (PAD50, 5pA ultra–low leakage!) short circuit!

If CR4/CR7 are duffy you MUST get proper ultra–low leakage (5pA or less!) reverse current diodes as replacements – '4148s, etc., will NOT do – you'll never get the meter to zero properly unless you fit the correct types!

If you get a display change at U2 pin 3 with 200mV applied, then you're looking at the switching and voltage–divider components, which are self–explanatory from the circuit–diagram, which is drawn in the "DC Volts" position!

If you get the DC voltage ranges to respond but no "Ohms", measurements, check Q1, Q2, U1 and their associated components, and switch contacts S1, S2, S4, S5 and S22 are making when Ohms is selected, and any associated PCB tracking, joints, etc!

If you find the switch contacts S1 to S24 are not opening/closing at all when the front panel operation keys are pressed, you'll need a scope' to check the outputs from U10 pins 4–10 and 12, unfortunately Prema don't give a truth–table for the operation of U10, but it shouldn't be too difficult to compile your own!

Remember the relays K1 to K8 are self–latching pulse–operated types driven from the pins of U10 – the meter is designed like this to improve the noise immunity from random d.c. or a.c. voltages finding their way in from stray pick–up, etc!

A carefully placed ear should pick up faint clicks from the relays when you prod all the various front–panel keys, also, are there any sounds of clicking–noises during switch–on/self test? If there is then at least U10 is driving the relay coils during the self–test phase!

Faults in U10 or the relays themselves, again, need a chat to Prema about spares!

This is a reasonably straightforward DMM to fault–find but does rely entirely on OEM support because they've used custom ASIC devices in many places!

See how you go with all the above and come back if still stuck!

Chris Williams

[MarcX]

Hello Chris, thanks for your response, I’ve already checked the function of LM399, which is working. I will do the further tests when I’m back from journey on friday.

Relay clicking on Prema Multimeter is very loud and can not be ignored, if my working Prema 5000 runs, with open test leads, I always disable auto
ranging because the clicking relays are very loud in my working room (similiar to a contactor )

Relays are clicking on the broken 5001 aswell, zero offset compensation is working on most ranges except mA/DC if I am remembering correctly, (Error „Offset too large“)

[Chris56000]

Hi!

AH!

That's handy having loud–operating relays – means you don't have to clutter up your bench to test the pulse O/Ps from U10 with an oscilloscope!

You can bench–test the relays by discharging an electrolytic of about 2.2 or 4.7uF pre–charged up to 5V thro' the coils, (remove each from the PCB first so you're not risking U10!), which should cause them to latch over, then you can test the contacts with a good ohm–meter!

Otherwise, the rest of the fault–finding runs along the lines of my previous note!

Chris Williams

[1audio]

i have a Prema 6001 that worked fine for years but recently failed. It took out the fuse (100mA but should have been 200 mA). Replaced the fuse and a few elements of the display light up but nothing showing useful info. The usual relay chatter is not there.

I checked the supplies and they are all correct (drat, a dead cap would be easy). Power consumption when on is 20W. Checked and all the digital chips show some action and no obvious dead data or address lines. Not sure why self test and dead display. Disconnecting the analog board cables and the display stays dark. Not really sure where to go with this. I can contact Prema and get some of the ASIC's but that feels like shotgunning with no clue.  Any advise or direction would be really helpful.

[1audio]

More from further exploration- It seems something is holding it from starting. Several times I got it to start by doing "something". First was bumping the cables between the micro board and the preamp board, Then by bumping the preamp board with the scanner board loose. Once it starts all is fine and working normally. However I could not find any marginal connection or cable issues so far. Wiggling and flexing the boards and tapping with a plastic handle and nothing.

Letting it sit and warming up its trying to start. Maybe some signal is marginal. Maybe old optocouplers?

This unit does not match the schematic. Instead of an "SH10" there are several ICL7650  where it would be and there are a few other small differences.

I checked all the signals between the two modules and nothing obvious missing. There is something about the turn on sequence that is blocking it. The manual is pretty light on troubleshooting so I'm still not sure where to look. Since it works fine once started I think its well work the effort to get it running.

[1audio]

It seems if I leave the unit on for a while it will eventually start and work. it does report a ram error after it starts. I have ordered new ram. cpu and optoisolators for it. I may get a new eprom since that has a 25 year old battery in it. If these fix it I will post here and a short tear-down in metrology for those who would like to look inside.