Electronics > Repair
Fluke PM6666 counter freezing after 3 seconds from turning on.
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pizzigri:
OK...
I connected my crappy FY6900 to the 5mhz pin on the board, 4V 50% duty cycle, square wave. I checked it on the scope and sincerely the original square wave from the meter is nicer. But I digress, on how crappy the FY DDS is. Anyway, turned on and the meter never goes past the POST, all leds lit and all segments in the LCD on.
So I thought to use an external 10 MHz reference using the rear BNC, bypassing completely the internal MTCXO oscillator, and no joy. Now it works as before three secs after power on, it locks up.

Then, I imagined that the 10MHz internal ref and 5Mhz square were out of sync, yielding the POST lockup, since I was using the DDS for the external 5mhz clock. SO, I connected the 10 MHz to the rear connector and 5 mhz clock to the GPIB card, using both channels of the DDS, therefore in sync with each other - reasoning being, this way I'm bypassing everything on the board save for the 5mhz clock for the master cpu (but still in sync due to the external ref coming from the DDS). Again, no joy; locks up three seconds after post. It's getting real frustrating.
pizzigri:

--- Quote from: thinkfat on October 13, 2020, 08:33:57 am ---I found a datasheet for the MAB84X1 series. The clock input is specified as HCMOS and TTL compatible so we're good here (though I think 3.8V still a bit low), but the maximum rise and fall time of the clock is <10ns. Can you check the clock edges meet that spec?

--- End quote ---
No they dont. It looks like a sawtooth especially in the rise edge, and not a square wave. Rise time is about 60ns, fall is 20ns. Amplitude is actually 4.20V, if I measure directly the pin.
Could it be the reason? The square wave I believe is built by the custom ASIC, if this is the case, I'm done for....
coromonadalix:
you could use logic device ic's   to square up the signal ??
thinkfat:

--- Quote from: coromonadalix on October 13, 2020, 02:27:55 pm ---you could use logic device ic's   to square up the signal ??

--- End quote ---

Yes, a buffer with Schmitt trigger input for example, 74HC7541.

But, please show screenshots of the waveforms of the 5MHz signal.

I hope you disconnected the counter-generated 5MHz from the GPIB card before you fed it from the function generator? I don't see how it could be looking worse than the 5MHz from the counter itself - that one is abysmal.

Also, please show screenshot of the 5MHz waveform without the GPIB card connected.

Yes, there's one ASIC (IC104) that creates among other signals the 5MHz for the internal CPU (IC105) of the counter and the GPIB interface (BU105, pin 3). From what I can see in the schematics, the 5MHz go nowhere else.

I figure either there's something wrong on the GPIB board that loads the 5MHz signal and distorts it (that's why I want to see the signal without the GPIB card attached) or there's something wrong with the clock signal output driver in IC104, or the clock input of IC105 has a problem.
pizzigri:
Hello Thinkfat!
OK so here are the screenshots.

First one, output from FY6900 at 5MHz, using bnc end to end.
Then, output of the 5MHz coming from the pin of the GPIB option connector on the meter mainboard.
I then tried to trace the output directly out of IC104, third image. The quality is worse because I had to use the smallest grabber I had, form the logic analizer and that has a long unshelded wire. Gnd is always on the frame of the counter (except for the first image obviously). I tried to hook up the 10 MHz but if I do so, somehow the meter doesnt start giving garbage on the screen.
What I find strange how ever is that the onboard CPU, that is exactly the same part mounted on the GPIB card, can use the sawtooth signal nicely and works, and the one on the card cannot. As I mentioned before, the counter works as a charm without any glitch that I can see if the card is disconnected.

Oops I mean here are the images....
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