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Re: HP 478A Thermistor Sensor. Input return loss and matching
Just to clarify, the latest service manual for the standard 478A sensor is available on the Keysight website and this lists the different internal circuits for the early and later variants of the
By jmr · #151909 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
OK thanks. I think the standard 478A sensor uses a tiny (and very thin) SMD disc cap of about 1350pF for the AC ground end of the thermistor. The later 478A sensor uses 1200pF here.? I don't know
By jmr · #151908 ·
Re: Recap 6205B
Hi Joe, If you just change some parts without knowing why you only make it worse. You give exact 0 information what was your error-picture, what you did or what voltages did you measure. My advice:
By RolandK · #151907 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
On Sat, 17 May 2025, jmr via groups.io wrote: There are 0.1uF C0G capacitors in 1206, nothing special. I have a reel of those and use them often. Dunno what capacitors are inside H72 probe although I
By Sergey Kubushyn · #151906 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
Here's an image showing the completed matching adaptor that contains the series 11nH inductor. This is constructed using two precision N bulkhead connectors, one male and one female. I salvaged them
By jmr · #151905 ·
Re: Recap 6205B
For C14 & C20 the can has several tabs that secure them to the board - these are common with the "-" of the capacitor and the can itself is often used to complete the circuit.? Make sure that all of
By Harold Foster · #151904 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
I'd expect the 0.1uF caps in the H72 sensor to be class 2 ceramic types as it's hard to imagine COG/NP0 being used here. This would presumably make the reactance at 10kHz susceptible to temperature
By jmr · #151903 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
I can't be certain, but I think the reason the H72 can't be used with the 431 meter type is because the H72 sensor uses a 0.1uF decoupling cap on the output of the 100R + 100R thermistors inside the
By jmr · #151901 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
The HP 478A Thermistor Sensor Option H72 and others state only to be used with HP 432 So it sounds like the new HP 478A Thermistor Sensor. Without the option might also not be useable / compatible
By Paul Bicknell · #151900 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
I was under the impression (although possibly incorrect) that resistance and capacitance elements found in early 478A sensors were needed corrections for 431 power meters that utilized a 10 kHz bridge
By ebrucehunter · #151899 ·
5 files uploaded #file-notice
The following items have been added to the Files area of the [email protected] group. * /All HP, Agilent and Keysight instruments in folders by part numbers/3000 to 3999/E3610A,
By Group Notification <noreply@...> · #151898 ·
Re: HP8757A Display Problem (Scalar Network Analyzer)
Hmmmm?? messages passing in the night (or day) It looks like you were typing message 151895 while I was typing 151896. Hopefully my info will be helpful. Philip
By Philip Freidin · #151897 ·
Re: HP8757A Display Problem (Scalar Network Analyzer)
I think the 8757A uses an HP 1345A display module, which is used in multiple different instruments. You can isolate the display subsystem by unplugging the 26 signal flat flex cable, at either end.
By Philip Freidin · #151896 ·
Re: HP8757A Display Problem (Scalar Network Analyzer)
This afternoon I read more of the manual. I then checked the "X-Y-Z Analog" board that drives the video - gently "banging" (tapping with a plastic rod!) the rear corner of that board suddenly but
By Trevor Gale · #151895 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
When I fitted a ferrite bead to my early 478A I found that the VSWR curve became smoother but it degraded from about 1.068:1 at 50 MHz to about 1.075:1 with the ferrite added. This is unavoidable as
By jmr · #151894 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
Thanks. I tried lots of ferrite types on a VNA and quite a few were OK to use but I chose one that gave at least 70R ESR across 50 MHz to several GHz that was fairly stable on a VNA when heated.
By jmr · #151893 ·
Re: HP8757A Display Problem (Scalar Network Analyzer)
It sounds like it could be in the analog display area, the video and sweep drive to the CRT. Check the horizontal and vertical drive to the CRT during the analog portion of the sweep. With no signal
By Don Bitters · #151892 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
Here's a comparison showing the VSWR of an early sensor (VNA measurement of my HP 478A) against a model based on the schematic of the later sensor. You can see that the VSWR at 50 MHz should improve
By jmr · #151891 ·
Re: HP 478A Thermistor Sensor. Input return loss and matching
It would be reasonable to try 61 mix NiZn ferrite with a 300C curie temperature, and mu of 125. ________________________________ Sent: Saturday, May 17, 2025 8:34:17 AM To:
By Ed Marciniak · #151890 ·
Re: HP 8566B repair
It almost seems like the display is in a test mode. This first pic shows the 0-10V ramp of the first band sweep, the second pic shows the ramp voltage for each of the separate bands B, C, D, E, but
By Don Bitters · #151889 ·
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