Author Topic: ZHAOXIN HC-F2700L (Atten F2700C) Frequency Counter un-stable count  (Read 18229 times)

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Offline orin

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Re: ZHAOXIN HC-F2700L (Atten F2700C) Frequency Counter un-stable count
« Reply #25 on: February 07, 2015, 08:45:07 pm »
I investigated the buffer board further.  The 13MHz from the main board goes through two 74HC04 inverters in series, then through the remaining four in parallel.  Three of the outputs are connected in parallel to a 51 ohm resistor.  The other end of the resistor goes to the BNC on the rear.  The final inverter output is unconnected.  So, at least there are no floating inputs.  FWIW, there is an unpopulated SOIC14 on the main board (surprising, the rest of the board is through hole) which almost does the same thing, apart from one input being unconnected.

The buffer board is clearly designed for something else.  The 13MHz goes all over the place.

I put the scope on the buffer and as expected, the ringing is bad, partly due to the scope probe of course.  To start with, here is the trace of the 13MHz clock on the main board with the buffer board disconnected, then the clock on the buffer board, then after the first inverter on the buffer board (yuck!) and finally, the output of the buffer board.

I do not know what that drop is before the falling edge of the clock on the main board.


« Last Edit: February 07, 2015, 09:22:36 pm by orin »
 

Offline orin

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Re: ZHAOXIN HC-F2700L (Atten F2700C) Frequency Counter un-stable count
« Reply #26 on: February 10, 2015, 05:39:18 am »
Well, that ringing was so horrible that I got out the HP 1120A 500MHz active probe.  Result for the signal on the main board is attached.  The buffer board is in purgatory and I couldn't be bothered to go get it.

Yes, that drop before the falling edge is real.  But I was pretty sure of that anyway as previously I'd seen a similar glitch on the rising edge at the BNC output (there are three inversions in line on the buffer board, so the glitch appears on the rising edge).

The rising edge on the main board is good, but after the glitch, it looks like an RC decay.  Do I really want to follow this through further or should I just but the top back on and forget about it?

Edit: I used a 100X tip on the probe so as not to overload it.
« Last Edit: February 10, 2015, 05:43:13 am by orin »
 

Offline amyk

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Re: ZHAOXIN HC-F2700L (Atten F2700C) Frequency Counter un-stable count
« Reply #27 on: February 10, 2015, 12:45:40 pm »
I would suspect the last active component that signal came from; it looks like a defective pulldown transistor is causing the signal to go down at first when it turns on, then somehow it turns off (while the pullup is still half-on) making it shoot back up, and then decay slowly as the pull-up transistor turns off and the remaining charge leaks out.
 

Offline orin

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Re: ZHAOXIN HC-F2700L (Atten F2700C) Frequency Counter un-stable count
« Reply #28 on: February 11, 2015, 08:41:06 am »
I would suspect the last active component that signal came from; it looks like a defective pulldown transistor is causing the signal to go down at first when it turns on, then somehow it turns off (while the pullup is still half-on) making it shoot back up, and then decay slowly as the pull-up transistor turns off and the remaining charge leaks out.


Yes, sure looks like the failure of a pull down transistor.

What we know is the TCXO is known to fail so this might be one of its failure modes.  There is a mixture of LS TTL and HC CMOS chips on the main board.  I'd guess the TCXO output is CMOS and it wouldn't surprise me if they are driving too many LS TTL loads resulting in the death of the TCXO output.
 

Offline kt6ln

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Re: ZHAOXIN HC-F2700L (Atten F2700C) Frequency Counter un-stable count
« Reply #29 on: April 05, 2020, 01:22:12 am »
I was looking for some information on HC-F2700L, I came across this page. Although I have been reading through the eevblog forums for several years, this will be the first time I write.

I recently got one of these freq counters. When I plugged in, I noticed exactly the same symptoms as mentioned in this thread. I know this thread is a bit dated but F2700L seems to be the same. I was going to open the case by taking out four screws on the back frame, I noticed that AC input is for 220V +/-10%. I am in US, here the power is 120V. Luckily, I have a 120V/240V transformer. When I plugged it to that, viola everything worked perfectly.

Not knowing if the units that folks in this thread had also required 220V, I wanted to put my comments that it looks this is a good unit. I went ahead and opened the case to look what I could do. Out of pure luck, I found a similar sized 120V transformer in my scrap box. It has about the same output voltage also. Replacing the transformer was not difficult at all. Now, the counter is operating with 120Vac in. It's very stable but like all crystal oscillators, it will drift I am sure. So, I would think installing a better 13MHz reference is a necessity.

Just to let everyone know.
 


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