Edit: See the complete report here:
http://wunderkis.de/pm6654/Got an nice instrument from eevblog member Ice-TEA, supposedly in a defective state, just to have some fun with it:
![](http://wunderkis.de/pm6654/teardown/P1160927.medium.jpg)
It's alive and at least does something but showing the E4 message:
So the normal eevblog member would follow the "don't turn it on, take it apart" workflow. For some particular reasons (basically because I'm curious), I decided to turn it on before I take it apart. Nothing happened, the display was blank, some LEDs were lit at the front panel. Randomly pushing and turning buttons and knobs revealed this:
The A and B inputs apparently worked, since I could turn on and off the trigger LEDs by changing the trigger level. With an input signal applied, these LEDs start blinking, showing the input is working to some extent.
So one might think now, at least some analog supply is alive, operating the inputs, but maybe the digital supply is dead. The next step shall prove this assumption wrong:
Pushing the reset button brought some more life into the unit, the display showed a row of zeros and more buttons operated something else but the button was intended to. E.g. one of the numeric keypad buttons changed the mode. Normally one is intended to change this setting by the arrow up/down buttons.
OK, We've at least power to the digital components, and the processor does something though not what it's supposed to. Maybe there's a faulty EPROM?
Now I've tried to hold the reset button while turning on the unit. This apparently works, the display shows "E4" now. Pressing the reset button again now brings the unit to a somewhat better state - still no counting or measurement working, but all the buttons work as expected, and one can read out e.g. the trigger level settings.
So the power on reset circuitry most probably is toast in some way.
I had to look up "E4" in the manual (didn't read that until now). It's an internal self test, indicating "Measurement Logik Fail". First step in the fault finding flow is to check the int/ext ref source switch (so I did, was OK), next is to swap the "COAC" ASIC. Can't do that, since this chip most probably is unobtanium now. So my next thought was: That darn COAC (counter on a chip) ASIC might be dead, could one replace that by some CPLD or FPGA stuff if neccessary? For sure this would be an interesting task!
Reading the manual revealed some more self testing procedures, this showed "E4" as the only failure. So RAM and EPROM and other stuff is considered OK now.
More playing around shows the counter can display the "Hold Off" setting (time), though the knob doesn't work.
The knob is stuck, one cannot turn or pull, obviously another point to repair. According to the manual, a special internal path to the mighty "COAC" is used to display the "Hold Off" setting. So now, I came to the conclusion, the COAC is OK as it can measure this setting. Phew.
Now, with Hold Off turned on, the counter actually shows plausible frequency measurements up to around 200kHz. Above its garbage.
This is another hint the COAC most probably is fine, but the CEX board might be faulty, as this acts as a prescaler and high resolution extension to the COAC.
See some preliminary teardown pictures here:
http://wunderkis.de/pm6654/teardown/So up to now, this looks like an interesting fault finding and repair process to happen. I'll keep you updated here.