HP 436A RF Power Meter - Page 1
Forum section: electronics / repair

author: xrunner
author_rank: Super Contributor
timestamp: August 24, 2021, 01:09:36 am
content: I have a couple of posts about this project elsewhere, but I think I'll consolidate them and the rest of the project here so the progress can be seen more clearly. I have a hp 437B power meter and hp 8481A sensor, but I wanted the previous version of the power meter. I found this 436A for a good price so I'm going to restore and calibrate it. It's in pretty good shape but there are a few things I'll be doing to restore it, so I'll post what I've done already and the rest of the steps here.First we had an unboxing and an initial checkout. I measured the REF output of the 436A on my 437B power meter and it checked OK (0.002 dBm). The 436A drive for the sensor checked out OK (negative going square wave w/respect to GND) @ 220 Hz) so I connected it to the 436A and it worked as it should. I measured it's own REF and it was very close to correct so I just tweaked the front panel CAL ADJ pot and it's now as it should be (0.00 dBm). I also verified the CAL FACTOR % settings worked by rotating the dial and observing the reading going up incrementally with each dial setting.Another member wolfy007 noted that there were two RIFA caps in the back (which are not shown on the schematic) so I will need to order them (X2 class safety caps).

author: xrunner
author_rank: Super Contributor
timestamp: August 24, 2021, 10:17:26 pm
content: Pics of the boards Set #1 -A2 AC GainA3 A-D ConverterA4 Counter RelativeA5 Controller

author: xrunner
author_rank: Super Contributor
timestamp: August 24, 2021, 10:18:31 pm
content: Pics of the boards Set #2 -

author: xrunner
author_rank: Super Contributor
timestamp: August 25, 2021, 01:11:42 am
content: Here's the first repair to the unit -

author: Hamelec
author_rank: Regular Contributor
timestamp: August 25, 2021, 08:05:55 am
content: Quote from: xrunner on August 24, 2021, 01:09:36 amchecked it yesterday and found (and replaced) only one RIFA cap across the power switch.

author: xrunner
author_rank: Super Contributor
timestamp: August 25, 2021, 11:02:26 am
content: Quote from: Hamelec on August 25, 2021, 08:05:55 amHi Hamelec,Mine has two (see pic). One is at the back on the fuse/AC input assembly. They are not shown on the schematic.

author: xrunner
author_rank: Super Contributor
timestamp: August 26, 2021, 09:57:43 pm
content: RIFA caps replaced.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 27, 2021, 11:15:44 am
content: well done

author: xrunner
author_rank: Super Contributor
timestamp: August 27, 2021, 11:19:57 am
content: Quote from: Hamelec on August 27, 2021, 11:15:44 amThanks. I just want to mention that the RIFA in the rear was very difficult to replace. I was wondering, since you said you found only one (by the switch), if someone didn't just cut off the rear one (to prevent it from failing). They may have just cut it off and didn't bother to replace it. Just an idea ...

author: xrunner
author_rank: Super Contributor
timestamp: August 28, 2021, 10:28:17 pm
content: All boards have been cleaned/re-installed. Unit comes up and works OK. I am also going to replace two 500uF 50V caps on the PS board (have to be ordered). After that I will do as many checks as I can per the manual.

author: xrunner
author_rank: Super Contributor
timestamp: August 31, 2021, 01:07:23 am
content: I found an Agilent 8482A sensor + cable on Ebay for this unit and I should get it tomorrow - 100 kHz to 4.2 GHz, +20 to -30 dBm.Also received the 500 uF caps and will install them tomorrow.

author: rfclown
author_rank: Frequent Contributor
timestamp: August 31, 2021, 03:30:28 am
content: It's a crime how little money 436A units go for. I bought a couple in about 2009. I think it was around 2010 when Agilent opened up a calibration lab in my area and had an open house. I was surprised to see that most of the equipment they used were older models; included in their setup was a 436A. Made me feel good about the older HP gear that I had at home. The only way I was able to get any money selling them was to offer a "complete solution" (meter, sensor and cable). Otherwise it's hardly worth the trouble listing, boxing and shipping them to sell. I sold my last one with a 8484A sensor and reference attenuator. Picture of the last one I sold.

author: xrunner
author_rank: Super Contributor
timestamp: August 31, 2021, 04:55:42 pm
content: They really do seem to be priced lower than you would think, the 437B is pretty low priced too. I get that the power meter is pretty much useless without the sensor (it does have a nice stable 0.0 dBm power reference though). However, you can say the same about the sensor - it's not much use without the power meter. But it's the sensor that gets the high price.Keysight sez newer power sensors (N series) aren't compatible with the older meters like the 436A and 437B. They have an integrated EEprom with the cal factors that communicates with the meter. So I wonder how many older power sensors are left out there?I replaced two 500 uF caps on the PS board. Waiting on the sensor to be delivered today by FedEx.

author: xrunner
author_rank: Super Contributor
timestamp: September 04, 2021, 01:26:14 am
content: Some final testing, and she looks good to go.

author: niconiconi
author_rank: Frequent Contributor
timestamp: April 12, 2022, 10:12:36 am
content: Sorry for bumping this old post. But do you remember how is the N connector on the HP 436A front panel connected to the circuit board? I just got this meter in working conditions, but the N connector for the reference oscillator from the 1970s has worn off at this point. It succeeds or fails to make a connection depending on the initial angle when the connector is screwed in.I inspected it as carefully as I could and didn't notice any mechanical damage, so I assume it's just wear and tear.I'd like to get the connector replaced but I still need to do some measurements right now so I don't want to risk disabling the meter. (Yes, I do realize a precision connector costs more than the meter itself! And I do realize a potentially costly recalibration is probably required... But that's life... And the worst thing is wearing the power sensor off as well!)Thanks in advance.

author: niconiconi
author_rank: Frequent Contributor
timestamp: April 12, 2022, 11:01:38 am
content: Never mind. I studied the service manual and found it's connected to circuit assembly A8 via a N-to-SMC coax cable. So all I need to fix that is ordering a coax cable with a panel-mount N connector on one side and a SMC on the other side.

author: xrunner
author_rank: Super Contributor
timestamp: April 12, 2022, 11:14:18 am
content: Quote from: niconiconi on April 12, 2022, 11:01:38 amHi niconiconi,OK on the connector, if you want me to check anything else on nmine in the future post here.