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HP8664A with issues

Question asked by Harke001 on Mar 25, 2019
Latest reply on Jun 4, 2019 by Harke001

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Hello Group,

I have obtained a very cheap HP8664A, but, of course, with issues. Ser no starts with 3744 but internal date codes show well into 2000. It has options 010, 004 and an oven. It has a special high shielding case. Each time upon power-on it will calibrate itself, a long operation. Then it will give the Result code: 3,504,311. MSSG gives: Hardware Failures; 5 and 1 and Calibration Errors: 2 and 1. The repair manual I have suggests to look at A10, the fractional N module. I opened it and found a beautiful pcb without any visual or smelling anomalies. The power supply is fine. On my SA (HP8562A): between about 400 MHz and 2.9 GHz everything is fine. Nice clean stable signal, frequency and amplitude spot-on. Can be modulated as well. Outside this range the signal is clearly out of lock=useless. I checked the Eeprom (28HC64) at the interconnection board and it reads fine on my Eprom programmer.
ILD test 320 gives the code; +3,504,311, quite similar to the calibration routine. The HW Failures and Calibration Errors given above can not be found in the manuals I have.
Any help in bringing this beautiful instrument back into service is highly appreciated.
Kind regards,
Harke

Harke001 Apr 3, 2019 5:30 AM

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Hello,

Thanks for your responses. After a lot of searching I nailed the problem down: the YTO is defect. It tunes correctly but only provides about 20 dB lower output than it should deliver. Relative high harmonic content (about -15 dBc). Confirmed on my sa and power meter (HP437B/HP8485A). This way the dividers can also not work.

Now looking for a replacement.......

Best regards,

Harke

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jonathan.falco Mar 27, 2019 4:26 PM
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Hello Harke,

When you say the PCB is beautiful, does it look new? If you bought it very cheap, then it's possible somebody duped you and put a false board into the instrument just to get it to "work." Check that it is a genuine board and get back to me.

Thanks,
Jonathan
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- Harke001 @ jonathan.falco on Mar 28, 2019 2:13 AM
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  Hello Jonathan,
  
  Many thanks for your reaction. The instrument is fine, seals are just broken by me.... After some initial measurements I am coming to the conclusion its the microwave section: the YTO or the first divider (or both...) have trouble. I connect a HP8562A to the auxiliary port on the first divider via a 10 dB pad and a 1m RG58 BNC cable. Looks like a coupled YTO output as it says: 3-6 GHz. SA center frequency: 4.5 GHz, span: 3.2 GHz, BW: 100 kHz. During calibration upon start-up the instruments walks through its frequency range. I see alternative a good image: just the main signal (abt +3 dBm on sa) with a very small harmonic or a bunch of signals, clearly wrong. Noise floor remains constant. When I measure at the output of the first divider I see either the correct image (F-YTO/2) or a bunch of signals with a huge irregular noise floor. It looks like when the YTO is operating near 6 GHz things go wrong.I step in 1MHz increments; 2950 MHz (on generator): all ok on sa (Pout +4 dBm on sa). From 2954 MHz till 3000 MHz the signal is unstable and noisy.
  I am really looking forward to your advice.
  Kind regards,
  Harke
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EricBaker Mar 29, 2019 10:32 AM
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Hi Jonathan,

When one of these instruments begins calibrating on power up, it means one of two things. Either the CPU battery is dead, or the previous self-calibration failed. When a unit with a timebase has this problem, it's common to see failures due to an attempt to calibrate with a cold oven. When I see one of these, I allow the unit to sit for a half hour turned on, then use Special 172 to perform a full self-calibration. Then if you get an error it's a real problem. Let us know how this turns out. I may be able to provide additional info depending on your results.
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- jonathan.falco @ EricBaker on Mar 29, 2019 10:35 AM
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  Harke001
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  - Harke001 @ jonathan.falco on Apr 3, 2019 5:30 AM
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    Hello,
    
    Thanks for your responses. After a lot of searching I nailed the problem down: the YTO is defect. It tunes correctly but only provides about 20 dB lower output than it should deliver. Relative high harmonic content (about -15 dBc). Confirmed on my sa and power meter (HP437B/HP8485A). This way the dividers can also not work.
    Now looking for a replacement.......
    Best regards,
    Harke
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    - EricBaker @ Harke001 on Apr 3, 2019 1:07 PM
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      Hi Harke,
      There was a modification we used to perform on the YTO's due to intermittant cabling. This usually caused tuning problems, but I thought I'd pass it on anyway. Originally there was an intermediate D-type connection on the case of the YTO, between the cable to the driver and the cable to the yig inside the can. The fix was to make a direct ribbon cable connection from the driver to the yig, with the cable running through the hole left in the can when the D-type connector was removed.
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      - Harke001 @ EricBaker on Apr 4, 2019 12:38 AM
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        Interesting Eric,
        
        My instrument has that mod. In fact I suspected the ribbon cable to be bad. I removed the YTO unit and tested it but the cable and semi rigid both are fine. The YTO tunes correctly but I measure 15-20 dB lower output power and a somewhat high harmonic content. I am afraid I need a new YTO.
        Now that we are here: Another generator has result code +0 when tested. But the output is at least 10 dB lower than expected, and at the lower end even lower. Confirmed directly at J2 of module A7. At J5 of same module I measure +19 dBm, much more than the denoted +11.5 dBm (at J1 of A5). However at J7 (375 - 3000 MHz) of the HF Driver module A8 I only measure something like -35 dBm. Are these latter values normal? I suspect something is wrong immediately before J2 of Output module A7. Please advise.
        Kind regards,
        Harke
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        EricBaker @ Harke001 on Apr 8, 2019 5:19 PM
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        Hi Harke,
        If there was something wrong with the output of the A8 it would show up as a self-test failure, since the levels are tested in the A7. Low-end rolloff of the type you describe usually indicates a bad center pin connection. I suspect J2 of A7 might need to be resoldered.
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        Harke001 @ EricBaker on Apr 9, 2019 8:29 AM
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        Hi Eric,
        
        Thanks for the comment on J2. I am with you here, but my concern was related to the excessive power at A7-J5: 8dB more than expected. Lots of harmonics suggest the related amplifier (the LF Driver) is saturated. I wonder if this could be caused by some ALC/AGC feedback signal being "maximized" due to the fact that the ALC detector did not get input signal. Then the fault would be just there. I will check J2 asap, but have to do some trips first. Will come back here and let you know.
        Thanks again, regards,
        Harke
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        EricBaker @ Harke001 on Apr 10, 2019 2:32 PM
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        Hi Harke,
        When you disconnected the cable at A7J5, you opened the AGC loop. Like you suspected, no RF to the detector mens the AGC will crank the signal to maximum. To measure in a closed loop mode accurately, you'd need to insert a directional coupler into the RF path.
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        Harke001 @ EricBaker on May 1, 2019 2:37 AM
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        Resume the faultfinding process after some traveling weeks..... You are completely right, Eric! Some further testing reveals that the output section of A7 is at fault. As before the unit gives +0 after test 320, but a very low output, at least 20 dB lower than expected at the upper end of the frequency range, much less even at lower frequencies. So I expect a cracked output capacitor and I measured the ESR at the output SMA connector of A7: bingo infinite! A known good unit measures about 50 ohms, that I can understand. Unfortunately the output section is a sealed nicely gold shining HP sub-module (08665-67001). Impossible to open without further damage I'm afraid. The sub-module can be replaced with some effort as it is socketed. If anyone reading this has a working A7 module or even a spare sub-module that would definitively make my day!!
        I will dig into the other (Agilent) generator asap. I now have a spare YTO that gives at least 20-23 dBm output. Looks a bit on the high side, I am not sure if it is even too much power.... To be continued.
        Kind regards,
        Harke
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Harke001 Jun 3, 2019 1:56 AM
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The YTO works fine and provides about +15 dBm output. Earlier measurements were wrong (my fault). There are adjustments indicated on the YTO: R18 and R154. R154 is easily found and accessible on Module A11. I found a R18 on the pcb of the Microwave section, with the YTO and microwave dividers. Is that the correct one and how do I adjust this pot in agreement with the YTO? On the YTO it says: R18 -78???
Please advise.
Thanks a lot,
Harke
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- EricBaker @ Harke001 on Jun 3, 2019 3:17 PM
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  Page 3-2 of the service manual shows how to do this adjustment.
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  - Harke001 @ EricBaker on Jun 4, 2019 2:15 AM
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    Thank you Eric, I have the manual but I simply missed it. Apologies!
    Now on to the other faults....
    Best regards,
    Harke
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