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EIP 545B Frenquency counter
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douardda:

--- Quote from: HighPrecision on January 17, 2018, 10:38:41 am ---Another weak point maybe the YIG current sense resistor (the big one on A108) is out of tolerance.

--- End quote ---

Just did a quick check and it's bang on. It would have been too easy :-)
douardda:

--- Quote from: HighPrecision on January 14, 2018, 10:41:03 am ---Special WB68 is a BAND 3 Low end frequency extension to 0.6GHz (600MHz) with reduced specs for sensitivity and power meter dynamic range.

Frequency offset and resolution setting have not to do with this special option, in the EEPROM there are saved the instrument power-on defaults.

--- End quote ---

After digging more in the disassembly of the firmware, and comparing the RAM content juste after booting the and after having changed these settings, it appears that these default values **are** hardcoded in the firmware. So, special really means special: it's most probably a very specific crafted firmware for a specific customer...

I "dumped" the RAM using TEST10 via GPIB (with a simple python script).

Now, I need to try to 'fix' this firmware, fix my sensitivity problem, and calibrate the device...

 
HighPrecision:
Informations about WB68 are very scarce, all I have is this:

--- Code: ---WB-68  B3  .6GHZ to 20 GHZ
            600 MHZ to 1 GHZ   0 to -20 dBm
            1 GHZ to 20 GHZ   +5 to -20 dBm
            BCD remote programming (this is a special on a 545B)
            GP-IB  (option 08)
          instrument configurated with BCD interface. GP-IB board
          and rearpanel cable physically located in unit.
          BCD to GP-IB and vice versa can be done within 15 min.

--- End code ---
That's all, this option is possible ONLY on 545B and not on a 548B, the extra space for YIG DAC calibration (to 26.5GHz) is used for this low end extension, probably data shift as I said.

This probably mean also that the Microwave Convertor is a special version for this WB68 option, my informations on A203 testing are pratically useless.
douardda:

--- Quote from: HighPrecision on January 21, 2018, 10:25:30 am ---
--- Code: ---          instrument configurated with BCD interface. GP-IB board
          and rearpanel cable physically located in unit.
          BCD to GP-IB and vice versa can be done within 15 min.

--- End code ---

--- End quote ---

I was wondering why I have this BCD board in a blank slot with the device, now I know!
douardda:
I've been investigation a bit more this evening the sensibility problem of my 545B. I haven't been able to improve anything injecting a small current on the negative input of the log amplifier driving the YIG.

I've also tried a quick calibration of the YIG, without noticeable effect either.

Then I've passed (again for some of them) the troubleshooting tree from the service manual, and I've found something wrong:

when locked on a (strong enough) signal on band 3, the IF sits at roughly 127.9MHz instead of 125MHz as described in the manual, for which the troubleshooting tree concludes that "VCO or LOCK CIRCUITRY on A108 BAD".

So I've probed most of the passive components on A108 involved in the lock circuitry of the VCO, and every looks OK so far. I've also removed all the chips to clean (with a bit of Deoxit) the DIP sockets. It's probably useless but cannot hurt.

Tomorrow I'll try to probe a bit the several test points of this section of the circuit to see if I can find something strange, but I'm afraid the problem might be in A201 in which I would have preferred not to put my nose...

David
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