ggabe  Your HP3400A is an old-school analog meter. I personally have a FLUKE 113 which does "true RMS" I got mine in a pawn shop for about 50 bucks. but if you like new Lowes (Box Store) is about 150 bucks. Now it's time to ponder Fix or Replace. If fix Did you check out the parts available from HP ops? Agilent or now Keysite??

My motivation is “restoration”. This is the only analog instrument I have. I like the retro vibe of the mirrored gauge. Three different rms converters in my parts box already, time to make them work all together. Spicing it up to meet or exceed the 10MHz 5% rms measurement accuracy. Which can easily met by any of today's oscilloscopes, just that I like analog and want to push analog to its limits.

The meter movements on the 4xx series of HP instruments are really nice (so long as the paint hasn't flaked.)

I'm not sure that I'm up to the challenge of attempting to replicate the external power sensor for my 435s but it would be nicer than altering the insides.

MK

 ggabe Keysight just got back to me.. Im copying the pdf into this: 

 KeysightCare Case # 01362418, 3400A Questions [ref:_00D1aawb5._5005a2ZlXvL:ref]

Good Afternoon Cris and thanks for Contacting Keysight!

I left you a vm in the morning and now following up with an email as well.

Not sure if we got the model number wrong but I had to dig deep to find this really really old 3400A. :)

Unfortunately we do not have a direct replacement but for RMS and thermocouple measurements, please check if our 34465A or 34470A (data sheet attached) meet your needs. (( its just their new test equipment

Below are also the compatible t/c offerings to the 34465A/70A:

34307A Thermocouple Kit (10 J-type, bare wire termination)
U1180A Thermocouple (J and K-Type) and Temperature Probe Adapter
U1181A Immersion Temperature Probe (K-Type Thermocouple)
U1182A Industrial Surface Temperature Probe (K-Type Thermocouple)
U1183A Air Temperature Probe (K-Type Thermocouple)
U1184A Temperature Probe Adapter (Thermocouple NOT included)
U1185A Thermocouple (J-Type) and Temperature Probe Adapter 
U1186A Thermocouple (K-Type) and Temperature Probe Adapter
U1188A Thermocouple (K-Type) with built-in Adapter

 ggabe Keysight just got back to me.. Im copying the pdf into this: 

 KeysightCare Case # 01362418, 3400A Questions [ref:_00D1aawb5._5005a2ZlXvL:ref]

Good Afternoon Cris and thanks for Contacting Keysight!

I left you a vm in the morning and now following up with an email as well.

Not sure if we got the model number wrong but I had to dig deep to find this really really old 3400A. :)

Unfortunately we do not have a direct replacement but for RMS and thermocouple measurements, please check if our 34465A or 34470A (data sheet attached) meet your needs. (( its just their new test equipment

Below are also the compatible t/c offerings to the 34465A/70A:

34307A Thermocouple Kit (10 J-type, bare wire termination)
U1180A Thermocouple (J and K-Type) and Temperature Probe Adapter
U1181A Immersion Temperature Probe (K-Type Thermocouple)
U1182A Industrial Surface Temperature Probe (K-Type Thermocouple)
U1183A Air Temperature Probe (K-Type Thermocouple)
U1184A Temperature Probe Adapter (Thermocouple NOT included)
U1185A Thermocouple (J-Type) and Temperature Probe Adapter 
U1186A Thermocouple (K-Type) and Temperature Probe Adapter
U1188A Thermocouple (K-Type) with built-in Adapter

Very nice, both of you phoenixcomm and Keysight to care asking and replying.