Electronics > Repair
Fluke 289 not quite sorted.
MihaiV:
Hello Towlerar,
Thank you for the advice.
I checked first that the contacts of the rotary switch are clean. Next, I checked that it was not installed in a wrong way. Someone was inside the meter before me (the heads of the input screws are slightly damaged). Since all parts are indexed, there is no way to assemble it wrong. I removed it to gain access to the contacts.
Next, i checked all vias around the switch, by buzzing to some visible traces through the PCB (all ok).
A visual inspection and a buzz of each branch of the resistor divider revealed the contacts at the switch. I traced a crude schematic (in the attachement). Pin 95 of the MSP appears to be connected to a position in the resistor ladder. It is P6.0/A0, obviously used as ADC input.
First, I must say that the 5V regulator is immediately disabled by the MSP, so I had to remove R88 (a 0R resistor) and to apply 5V externally. I noticed that if I turn the 5V too soon after pushing the power button the boot logo does not appear anymore and the power led remains on. If i apply 5v later (or never) it shows the boot logo, and the led turns off, simultaneously bringing the gray screen.
The 5V rail takes around 1mA (indicated by the power supply). I will measure it with a proper meter at the next debugging session.
Next, I measured all voltages of the resistor ladder with the rotary switch removed (see photos, positions on the ladder marked in red). They appear fairly well spaced apart in voltage, and are stable.
I buzzed the MSP pin, to see where it connects on different functions (with power removed, of course). The results are in the top right corner of the photo. Interestingly, some functions connect the pin at 2 positions in the ladder (VAC, mVAC, VDC). The others connect to a single position.
After that, I applied power an measured the voltages on each function. And they do not match the ones with the rotary switch removed! Obviously, the voltages in the ladder are also deranged. The values are indicated in the top right of the attached photo. I started buzzing all over the board, but couldn't find another connection anywhere else.
Is guess that is not normal. Is it?
It is a bit late here and my eyes are burning, so i will continue tomorrow, before i damage something else. I will try to hardwire one of the functions to the pin (like the VAC), without the rotary switch, and see what happens.
Towlerar:
--- Quote from: MihaiV on October 25, 2021, 09:05:35 pm ---Hello Towlerar,
Thank you for the advice.
I checked first that the contacts of the rotary switch are clean. Next, I checked that it was not installed in a wrong way. Someone was inside the meter before me (the heads of the input screws are slightly damaged). Since all parts are indexed, there is no way to assemble it wrong. I removed it to gain access to the contacts.
Next, i checked all vias around the switch, by buzzing to some visible traces through the PCB (all ok).
A visual inspection and a buzz of each branch of the resistor divider revealed the contacts at the switch. I traced a crude schematic (in the attachement). Pin 95 of the MSP appears to be connected to a position in the resistor ladder. It is P6.0/A0, obviously used as ADC input.
First, I must say that the 5V regulator is immediately disabled by the MSP, so I had to remove R88 (a 0R resistor) and to apply 5V externally. I noticed that if I turn the 5V too soon after pushing the power button the boot logo does not appear anymore and the power led remains on. If i apply 5v later (or never) it shows the boot logo, and the led turns off, simultaneously bringing the gray screen.
The 5V rail takes around 1mA (indicated by the power supply). I will measure it with a proper meter at the next debugging session.
Next, I measured all voltages of the resistor ladder with the rotary switch removed (see photos, positions on the ladder marked in red). They appear fairly well spaced apart in voltage, and are stable.
I buzzed the MSP pin, to see where it connects on different functions (with power removed, of course). The results are in the top right corner of the photo. Interestingly, some functions connect the pin at 2 positions in the ladder (VAC, mVAC, VDC). The others connect to a single position.
After that, I applied power an measured the voltages on each function. And they do not match the ones with the rotary switch removed! Obviously, the voltages in the ladder are also deranged. The values are indicated in the top right of the attached photo. I started buzzing all over the board, but couldn't find another connection anywhere else.
Is guess that is not normal. Is it?
It is a bit late here and my eyes are burning, so i will continue tomorrow, before i damage something else. I will try to hardwire one of the functions to the pin (like the VAC), without the rotary switch, and see what happens.
--- End quote ---
Good work getting the rotary switch reverse engineered. It seems like you might possibly be onto something.
I just had another thought. Is it possible the MSP is disabling the 20V bias boost supply after bootup? The boost converter IC is U21 and the signal to enable it appears to be on the gate of Q43 PFET via a bootstrap transistor Q42.
MihaiV:
Hello,
The 20V power supply is enabled at power-up and remains on as long as the meter is on. I can't turn it off without removing power. Holding the power button for more than 4 seconds reboots the unit. By the way, I have 21V, not 20. All other voltages generated by U19 are a bit higher too.
I tried to hard-wire one function to the MSP pin 95, but no results (the Ohms measurement). Tested with and without the rotary switch contacts, with and without external 5v supply.
But the fact that the voltages seen by the MSP are so different from the ones at the resistor ladder seems weird. I hope the selector was not changed with one of a different model, as someone was inside this unit before.
For a moment I suspected that the MSP pin 95 might be faulty and drag the voltages down, as they were connected to it by the selector, but hard-wiring proved it is ok, I measured 1.47V connected at position 5 (Ohms measurement).
Next, I will focus on the communication between the measurement processor and the MSP.
MihaiV:
Hello everybody,
I tried some SPI bus probing between the MSP and the other components.
I started with the MSP to MXS communication, and I saw that the MSP sends commands and the MXS responds. I did not decode the actual commands, I wanted to see that the traces/vias are intact and that there is no short-circuit between them.
Waveforms in photo 1 appear when the power button is pressed first time after applying 9v. The timebase is large, so the clock appears simply as a few pixels (channel 1 is CS, channel 2 is the CLK, channel 3 is MISO and 4 is MOSI).
Photo 2 is zoomed in a little; it seems that there is some communication with the CS line high too and the messages are different (photos 3 and 4). No activity appears on the bus when other buttons are pushed or the rotary switch is switched (not even with 5v applied externaly). But the traces up to the MXS are ok.
Next, I tried the MSX to ADG714. Accodrind to post 153 from KJK24, MSP pin 48 is clk, 50 SDI and 49 SDO. There is no activity here at all, with or without 5V from outside. A quick buzz indicated that pins 48 and 50 buzz to pins 1 and 3 (SCLK and DIN) respectively on the ADG714 switches. Pin 48 doesn't ring to pin 22 (DOUT) of the ADG! It goes to R291 (unpopulated) and nowhere else on the board. Pin 22 of ADG does not buzz anywhere, too.
I checked the photo from post 153 because I remembered it had a photo of the vias under the MSP but noticed that my board has a slightly different layout. My DIN trace goes away from the MSP, to the south-west part of the board (photo 5), but the photo in 153 goes right under the IC. Seeing post 139 I noticed that I have a ton of parts unpopulated in the south of MSP (my unit is 287/NUC). My board says FLUKE-28x-3001 REV 015.
Can anyone check that pin 49 of MSP is in direct connection to pin 22 on the ADG714 on the other side of the board (U48) ?
Thank you in advance!
MihaiV:
Hi,
Ignore the second part of my previous message... Just being dumb :palm:
I rechecked the datasheet and DOUT of ADG714 does not need to connect to anything (unless you wnat do daisy-chain multiple devices). No need for it to go anywhere.
Took a photo of what's under the ADG, while at it.
Back to square 1. And to the SPI between MXS and the measurement processor.
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