HP8753B "Caution: No IF Found Check R Input Level"

[BalticLab]

Hey everyone,

hope somebody can give me some helpful pointers to repairing a HP8753B network analyser.

On start-up, the frequency range from ~1.5 GHz to 3 GHz works just fine. In the lower range up to 1.5 GHz, the device displays a "Caution: No IF Found Check R Input Level" error message. If I set the start frequency to around 1.6 GHz and the stop frequency to 3 GHz, the message disappears and the measurements appear to be correct. After a short warm-up period, the same error message is displayed in the 1.5 to 3 GHz range. However, there's a small range around 1.7 GHz +- around 100 MHz where the VNA works fine.



I have checked all supply rails on the main power regulator card. All self-checks yield a "PASS" result. If I connect the RF OUT connector to an external spectrum analyzer, and set the HP8753B to a CW frequency around 1.6 GHz, a 1.6 GHz signal is clearly visible. However, the displayed level is around -30 dBm with a power level on the 8753B set to 0 dBm. In the range where the "Caution: No IF Found Check R Input Level" message occurs, no output signal can be detected.

Before I pull the source assembly out of the device and check the outputs of the two oscillators with an external spectrum analyzer individually, I was curious to hear if anyone has experienced the described behaviour before and could give me some pointers. Also, while I do have the entire service manual, there seems to be no documentation on the source assembly in the service manual. There are quite a few test pins. For instance, there are two pins, 10- and 10+ (could also be ID+ and ID-), which both carry a constant 14.7 Volts. Given the + and minus, this seems a bit suspect.



On a side note: I saved the data from all 4 EPROMS on the main CPU card. So if anyone needs the hex files, let me know.

[Maurizio1957]

Hi, I repaired my old 8753C in this way:

https://www.simonsdialogs.com/2020/04/hp-8753c-network-analyzer-a-new-old-yto-and-a-new-old-firmware/

with the help of mr. Simons!
73, Maurizio IU5BEC

[khach]

Try to reduce output power settings   to  -5 or -10dbm. At my partially damaged 8753 that settings INCREASE output power to desired level and device working properly except 2-th harmonic generator at 85047 test set.

[Bud]

Dr Joel Dansmore, the designer of the 8753, frequents the "RF and Microwave" board of the forum. You can move your topic there to help grab his attention.