HP 8673e - Summing and Output Buffer Amp Fault - Page 1
Forum section: electronics / repair

author: tec5c
author_rank: Frequent Contributor
timestamp: April 12, 2015, 11:36:16 am
content: I have a HP 8673e synthesised sig. gen that is giving a frequency offset, i.e. try to output a 3GHz signal, the display reads 3.0GHz but the output is always 200MHz less than what the display reads, so 2.8GHz output for a 3GHz selected output.Looking at the service manual/schematic, I have narrowed down the (well, an abnormal reading which I suspect could be the cause of the fault) fault to the summing and output buffer amplifier.I have attached the schematic of the said amp. circuit.The service notes say that pin 6 of Q5 should be at or near ground, yet it reads ~18v. The notes then say due to the feedback configuration of the circuit it is hard to fault find any further.Does anyone have any suggestions on what might be causing this reading or where to go from here?

author: MarkL
author_rank: Unknown
timestamp: April 12, 2015, 07:57:50 pm
content: Q5A, Q5B, Q8, and Q11 form a differential amplifier. What are the voltage levels on the other pins of Q5? And when you say "~18V", do you mean +18V or -18V?I have some more general questions too that might help:Is the offset always 200MHz at any frequency? Does the offset change with the band? Is it exactly 200MHz or approximately 200MHz?Are any of the sweep settings suspiciously set to 200MHz?Did this just start happening, or was it like this when you got it?Is it reporting any errors/unlock conditions?Did you work through the service BD1 checks 33 through 41, page 8-37?Also, it would help if next time you could provide references where the schematic snippet came from. A little more context was needed. For others, it's from the service manual, page 8-125 (PDF sheet 398):http://literature.cdn.keysight.com/litweb/pdf/08673-90053.pdf

author: jlmoon
author_rank: Unknown
timestamp: April 13, 2015, 02:25:49 pm
content: What is the voltage at pin 4 of Q5 when you see the 18 Volts on Pin 6?

author: cha_ly
author_rank: Newbie
timestamp: August 13, 2015, 09:54:34 am
content: Hello,just joined after I found this blog during a HP/Agilent 8673 search.I am not a professional but a ham-radio enthusiast and like to build and measure my home brew radio equiment.Recently I purchased an HP 8673m (should be electrical identical to the 8673E except for a 400 Hz fan)The unit is working fine from about 3,5 Ghz to its upper limit > 18 GHz with full output and accurate frequency.Between 2 GHz to about 3,5 GHz the Generator output is low and shows spurious and increased noise level.At very low signal level (-30dBm or less) the output signal appears clean after a large frequency change (> 1GHz) but when the peak tuning finishes the first spurious show up on the spectrum analyser, at higher output lever much faster.Reading older HP User/Service manuals, I noted that it is mentioned that spurious can appear around 2 GHz at output levels above +7dBm, but no hints what causes these effects.I do not have 8673 extenderboards, neither the test software mentioned in the service manual, so I have limited my preliminary tests measuring the supply voltages (which are all within 10mv) except one 5V supply test point which has an offset of approx. 65 to 70mv. No AC rippel imposed on either supply voltage.At the moment I wait for a printed 8673e service manual to arrive rather than using the pdf version to go further.I checked for loose interconnecting cables or connectors - but athose accesible are all ok.My feeling (based on my limited knowledge of this particular generator) leads to verify the biasing of the output amplifier and the functioning of the yig filter(& peaking circuit)May be there is a way to veryfy the yig filter & peaking circuit without the dedicted test software. and without disturbing the yig multiplier function.Thanks for reading