General Microwave 460B / n420c head - UK Vintage Radio Repair and Restoration Discussion Forum

loulou31 · 15th Dec 2019, 1:17 am

I have a General Microwave thermoelectric power meter that is functionnal, but is showing a very bad frequency reponse in the low end. The power is reduced of 2 dB from 1GHz to 500 MHz. and about 10 dB at 100MHz. It seems that there is a very low capacitance coupling at the input of the probe of about 10 pF only.
Is there any schematics / mechanical drawing of the n420c probe; I am also imterested on the service manual of the 460b unit.
thank you and best regards.
Jean-Louis

Radio Wrangler · 15th Dec 2019, 8:00 am

I think that power meter was also sold branded as Sanders, and later as Marconi Sanders.

It is possible that mechanical stress has cracked the input capacitor due to movement of the centre pin of the type-N connector.

I expect there will also be an RF decoupling capacitor inside to decouple the centre point of the resistor bridge. This will also affect your low frequency response.

Are you sure it is your power meter which is rolling-off, and not your signal source? Have you another power meter to check it against?

David

loulou31 · 15th Dec 2019, 10:00 am

Thank you for your reply. I have found the manual of the power head.
https://usermanual.wiki/Datasheet/N4...ual.155845927g
May be the problem is on the two decoupling capacitors of the resistor bridge. The input coupling capacitor is not a component is done by mechanical construction. I have measured 10 pF input coupling capacitor that explain the frequency response I have.
I expect that the problem is not on the thin film thermoelectric.

Jean-Louis
PS : I am still interested on the 420c manuel

WME_bill · 2nd Jan 2020, 7:14 pm

General Microwave 460B head for power meter.
I have the Marconi-Sanders version of the power meter, 6460. It looks a direct copy of the General microwave version, and the 6421 series thermocouple heads.
The MoD version of the power meter, 6460/1 / CT596, had a built in RF calibrator operating at audio frequency. Nothing sphisticated.
Manuals for both the meter and the various heads, both GMC and Marconi publcations. I can scan once I know which versions you want.
The different heads are with waveguide (X420c/U420C/K420C/a420C) or with type N for different power settings. Model N420C for 3W peak, N421C for 30w peak. N422C for 300mW.
wme_bill.

15th Dec 2019, 1:17 am	#1
loulou31 Tetrode Join Date: Jan 2019 Location: Toulouse, France Posts: 65	General Microwave 460B / n420c head I have a General Microwave thermoelectric power meter that is functionnal, but is showing a very bad frequency reponse in the low end. The power is reduced of 2 dB from 1GHz to 500 MHz. and about 10 dB at 100MHz. It seems that there is a very low capacitance coupling at the input of the probe of about 10 pF only. Is there any schematics / mechanical drawing of the n420c probe; I am also imterested on the service manual of the 460b unit. thank you and best regards. Jean-Louis

15th Dec 2019, 8:00 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,894	Re: General microwave 460b / n420c head I think that power meter was also sold branded as Sanders, and later as Marconi Sanders. It is possible that mechanical stress has cracked the input capacitor due to movement of the centre pin of the type-N connector. I expect there will also be an RF decoupling capacitor inside to decouple the centre point of the resistor bridge. This will also affect your low frequency response. Are you sure it is your power meter which is rolling-off, and not your signal source? Have you another power meter to check it against? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

15th Dec 2019, 10:00 am	#3
loulou31 Tetrode Join Date: Jan 2019 Location: Toulouse, France Posts: 65	Re: General microwave 460b / n420c head Thank you for your reply. I have found the manual of the power head. https://usermanual.wiki/Datasheet/N4...ual.155845927g May be the problem is on the two decoupling capacitors of the resistor bridge. The input coupling capacitor is not a component is done by mechanical construction. I have measured 10 pF input coupling capacitor that explain the frequency response I have. I expect that the problem is not on the thin film thermoelectric. Jean-Louis PS : I am still interested on the 420c manuel

2nd Jan 2020, 7:14 pm	#4
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: General Microwave 460B / n420c head General Microwave 460B head for power meter. I have the Marconi-Sanders version of the power meter, 6460. It looks a direct copy of the General microwave version, and the 6421 series thermocouple heads. The MoD version of the power meter, 6460/1 / CT596, had a built in RF calibrator operating at audio frequency. Nothing sphisticated. Manuals for both the meter and the various heads, both GMC and Marconi publcations. I can scan once I know which versions you want. The different heads are with waveguide (X420c/U420C/K420C/a420C) or with type N for different power settings. Model N420C for 3W peak, N421C for 30w peak. N422C for 300mW. wme_bill.