Réf. : Re: Réf. : Re: 50 ohm thermocouples was RE: [HP-Agilent-Keysight-equipment] 478A power sensor recal for HP meters (was diode mounts)

#92816

开云体育

where .. where !

as usual

in the box with some transistors

under another box containning some screws

behind the big parcel on the floor !

same problem here !

hi ...

-------Message original-------

De : Harke Smits via Groups.Io

Date : 23/11/2018 10:25:49

A : hp_agilent_equipment

Sujet : Re: Réf. : Re: 50 ohm thermocouples was RE: [HP-Agilent-Keysight-equipment] 478A power sensor recal for HP meters (was diode mounts)

Merci Chef! I must have some original HP spare thermistors...... But where......

Best 73 de

Harke

On Friday, November 23, 2018, 12:58:27 AM GMT+1, f1chf <F1CHF@...> wrote:

may be this news can help

F1CHF

-------Message original-------

De : Harke Smits via Groups.Io

Date : 22/11/2018 17:34:46

A : [email protected]

Sujet : Re: 50 ohm thermocouples was RE: [HP-Agilent-Keysight-equipment] 478A power sensor recal for HP meters (was diode mounts)

Thanks Jeff! Nice read.

Best 73 de

Harke

On Thursday, November 22, 2018, 5:30:52 PM GMT+1, Jeff Kruth via Groups.Io <kmec@...> wrote:

The screws are weird!? Back in the late '70's when a 478A head was worth $250 working (real money then!), I used to repair these. I ran a lab at Westinghouse Defense in Baltimore that did MIC work, microwave micro-assembly and wire & ribbon bonding. I used to pay the technicians on their lunch hours to mount salvaged thermistor beads for me. If you blow out a 478A with too much RF, the reference pair is still good. I used to salvage these and match them to the Ref pair in another head, replacing the blown RF pair.? Made a bit of cash that way!

The screws do NOT contact anything, instead they "approach" the thermistor beads, being aligned directly above them. My take on it is that they change the ambient temperature environment by adding a small bit of "hotter" metal closer to the bead, sort of like the DC offset adjustment on an op-amp. However, I could be wrong: instead of adding heat, they may change the reactance of the bead to the chopper frequency, but this seems less likely, as the size is so small (not much capacitance and the frequency of the chopper is low (IIRC, about 10 KHz).

The general failure in adjustment in an attempt to re-calibrate the head is to insert the screw a little too far so that it hits the bead and snaps a wire lead off it.

Hi Paul!? I found that the "driftiness" of the head can sometimes be improved by working the re-balance adjusting the screw pair alternatively, back and forth. 1/8 turn out on one, 1/8 turn in on the other, etc.? YMMV.

I used to leave the head shell off and hold the mount in my hand to heat it, release and watch the behavior of the re-compensated mount as it settles back to ambient. I could get them pretty good after a while.?

Oh well, all that was a really long time ago now, it seems, in a period when I had more time than money, now the other is true. Found a box the other day of a pile of 478 & 8478 guts, left over bits from these days, some beads missing. Off to gold scrap, I guess!

J. Kruth

Hi Jeff??

good to talk to you again regarding the 478 and rebalancing the head using the screws

I have found that the temperature? compensation isn’t as good after you have to rebalance using the screws

By the way never opened the 478? to find out how the screws work

Paul B

In a message dated 11/22/2018 11:01:39 AM Eastern Standard Time, yrrah53@... writes:

Interesting point. I have not worked with thermistor heads since ages but I always wondered how the screws work on the HP478 and HP8478?

73 de

Harke

On Thursday, November 22, 2018, 4:58:38 PM GMT+1, Paul Bicknell <paul@...> wrote:

Hi Jeff??

good to talk to you again regarding the 478 and rebalancing the head using the screws

I have found that the temperature? compensation isn’t as good after you have to rebalance using the screws

By the way never opened the 478? to find out how the screws work

Paul B

Show quoted text

From: [email protected] [mailto: [email protected] ] On Behalf Of Jeff Kruth via Groups.Io
Sent: 22 November 2018 15:38
To: [email protected]
Subject: Re: 50 ohm thermocouples was RE: [HP-Agilent-Keysight-equipment] Issue with homemade diode power sensor for HP meters

Really? Because HP used the same idea in the 478A & 8478A heads, they were 200 ohms series under AC bias, 50 ohms to RF, but when measured "cold" with an ohm-meter they were approx 3-4K ohms. I would have thought G-M would have used a similar method.

The big thing with the HP heads was the balance between the ref thermistor pair and the measurement thermistor pair. If the measurement pair was damaged by excess RF power, the "rest" resistance was too different from the temperature reference pair (say 7K instead of 4 K). SO you could no longer balance them with the screws.

J.Kruth

In a message dated 11/22/2018 9:29:03 AM Eastern Standard Time, paul@... writes:

Regarding the TFT ?Marconi? power meter ?they had 2? thermisters? at 100 ohm each

?And connected so that DC they added up to 200 ohm but presented a 50? to an AC signal

The power sensor was easily tested as the 2 large pins on the convector should read 200 ohm

From: [email protected] [mailto: [email protected] ] On Behalf Of RFI-EMI-GUY
Sent: 20 November 2018 05:12
To: [email protected]
Subject: Re: 50 ohm thermocouples was RE: [HP-Agilent-Keysight-equipment] Issue with homemade diode power sensor for HP meters

I have such an animal here a GM 460B with that TFT hermocouple. It is an ancient beast. They got bought up by Marconi down the line.

https://patents.google.com/patent/US3384819

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#92816

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