[SOLVED] HP 8713B network analyzer no output - Page 1
Forum section: electronics / repair

author: hjf83
author_rank: Contributor
timestamp: August 28, 2025, 08:54:14 pm
content: I recently bought an 8713B. it was in rough shape but it seems to work.At first it wasn't able to make it work, it would just display basically just noise. I followed the troubleshooting guide which has been very painful because you really need an extender board to be able to work with this thing so i've been soldering wires to test points, slide the module back in, measure, repeat.At one point I got a stable reading (was measuring an HF antenna and the reading matched my nanoVNA)... but after a few minutes the "noise" problem started again. I thought it must be a broken solder joint so i reflowed most large components and big connectors prone to breaking. And after installing it back in.. now it doesn't work at all. The FRAC-N board seems to be outputting a constant 48MHz but no signal in the RF output port. (in CW mode the manual states several frequencies that should be present at the FRAC-N output port, but it's always 48MHz)I took the FRAC-N board out, and fed it 5, +15, -15 and started measuring and at least the VCO part seems to behave. I haven't found any other strange voltages or anything around the board. No components are getting warm except for U19, which seems to be an HP ASIC, but it's not really hot.so anyways I'm not really sure if this could be a problem with either the U19 ASIC IC that runs the show, or with some interface IC. There seem to be a bunch of logic ICs near the backplane which I assume are used to talk to the CPU.So.. any ideas or any experience with this device?

author: hjf83
author_rank: Contributor
timestamp: August 29, 2025, 01:36:58 am
content: I've tested the rest of the device by injecting the expected frequency into the source board. The manual specifies that for a 500MHz output, the FRAC-N output should be at 38.825057 MHz, so I set up the device to 500MHz CW, and connected a spectrum analyzer to the RF output. When I injected 38.825057 MHz I got the expected 500MHz at the output.So while I'm happy that the rest of the device works, I'm still confused at what could be the issue with the current board. The complete replacement module is $180 on ebay so I'd really prefer to fix this.I re checked the entire board with a loupe looking for any bridges I may have added during reflowing but I couldn't find anything.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 29, 2025, 01:37:57 pm
content: I am interested in following this thread, I am currently working on an 8714C with no RF output. I to am having to solder wires to test points, the slide-in module design is nice for changing parts but terrible for actual component level repair.Best of luck!

author: hjf83
author_rank: Contributor
timestamp: August 29, 2025, 05:13:01 pm
content: So I managed to fix this. The problem was a silly RCA jumper wire that was "almost" shorted.The repair guide for these devices mentions that you can look at the 3325A schematics for component-level repair. This device, from 1996, was still using a FRAC-N Frequency Synthesizer design from 1980! A giant 45x15cm (or so) board that perform the same functions as an (already outdated) AD9850. Amazing. (makes me wonder if you could build a replacement frac-n board by reverse engineering the backplane protocol and adapting it to an AD9850). But i digress.Anyways, I started from the back. I disconnected the VCO cable and grounded it. The manual mentioned it should stay at 53MHz in these conditions. It was close but I also adjusted it for good measure. I connected the cable again and checked U6, the opamp that acts as an ouput buffer for the sample-and-hold circuitry. I measured the opamp's input pin at -2V. The integrator connects to the sample-and-hold module via a wire jumper that was measuring -12V. Which seemed rather strange since S&H should just sample and not mess with the signal. So i followed the voltage until I found a 10K resistor to pin 3 of the opamp. There was a 10V drop across the resistor! it had -12V at the input, and -2 at the output. Whoa! So I thought, the opamp must be bad!I replaced the tin can LM310 with a UA741 (it only seemed appropriate for 1980. it's also just a DC buffer). But nope, the voltage was still dropping. I had the bright idea of disconnecting the RCA jumper and the opamp went back to -12V. So I thought it must be a problem with the VCO input. But the whole VCO circuit "hides" besides a 500 ohm resistor. OK, so the other end must be shorted to GND? Nope. it had a voltage.So I decided to remove the RCA wire and as I touched it (didn't remove it) I saw the voltage go between -12 and -2..... wtf?It turns out the coax inside the RCA was shorting to GND. But it wasn't a dead short. It had enough resistance to keep the output at -2V instead of 0.I ended up replacing this wire with a soldered RG174 and voila! My 8713B is back to life and it now identifies as an 8714BRepair hints for anyone trying to repair a 03325-66540 or 03325-66521 board(they seem to be the same board, 540 being the complete module and 521 being the DDS alone).You can test this out of circuit easily but you do need several instruments:3 power supplies to provide 5V @ 1A, 15V @ 300mA and -15V @200mAOscilloscopeMultimeterFrequency counterFrequency generatorHow to set everything up:Connect your voltage leadsConnect your frequency counter, set it up for 50 ohm termination or provide one (this is important, frequency stability depends on Z=50)Frequency generator: you need to provide 100KHz pulses at an amplitude of 2V, with the pulses being 0.5uS wide at +2V, then 0V. The module from the 871x VNA only enables this when the CPU tells it to do so. So you need to disconnect the RCA from the 100KHz output from the main "carrier board" and inject your signal.use "HP 3325A Synthesizer Function Generator Operating and Service Manual.pdf" from here:https://archive.org/details/hp-3325-a-synthesizer-function-generator-operating-and-service-manualbasic hints: disconnect VCO from J18 and ground it (the side of the RCA that goes to the metal can on the right. Your frequency counter should read 53MHz. if not, VCO may be bad. Open the metal can on the daughter board and check. The guide tells you several VCO voltages vs frequencies.If VCO control is OK, measure TP11 (next to J18). It should be about -12V from my experience. Connect J18 again and check that the voltage you measured previously stays the same. If not, trace your way back from J18 through U6 all the way up to W1. This is the wire bridge connecting the integrator to the S&H circuit. W1 should have the same-ish voltage as TP11. if they differ, your S&H may be bad.I haven't gone any further back but the guide has full schematics and theory of operation.Tip: There is a big ASIC in the middle of the board, this is obviously some unobtanium part from HP. It seems to be some sort of ASIC or maybe a GAL, I can't be sure. The schematics show that it has a CLK pin (check that CLK works), and also a S&H control pin. I "confirmed" that U19 was working by measuring the U19 pin 19 output. It looked like a series of short pulses, and a long pause.Good luck!

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 29, 2025, 11:37:33 pm
content: Nice work!Unfortunately I don't think module A3 is the faulty module in my 8714C. Also I discovered that the service manual available from agilent is missing a bunch of figures in the troubleshooting section, they just wont make it easy.

author: hjf83
author_rank: Contributor
timestamp: August 29, 2025, 11:41:15 pm
content: Quote from: Alex_Baker on August 29, 2025, 11:37:33 pmthere is a scanned version available at some .ru domain that contains all figures.try the procedure described in the guide. set it to CW mode, 300khz frequency, scan time of 1 minute, and measure the output of the A3 module. you should have 31.892351 MHz at the output. if you do, then your A3 module is fine and your problem is downstream.