Wayne Kerr B421/CT492 LCR Bridge - UK Vintage Radio Repair and Restoration Discussion Forum

Andrew Sinclair · 28th May 2021, 9:00 pm

Hello,
I have been struggling to repair one of these bridges, but I am now reasonably happy that it is working. Like many WK instruments it uses transformer ratio arms and has a synchronous detector and uses an auto-balance technique. The best manual I could find on the web is a Royal Navy document, BR1771(63) dating from 1967. My instrument dates from 1975, judging by the component date codes, and is the B421 civilian version.
Mine has more modern transistors (BCY70 in place of OC202) and many value changes from the manual I have. The inductance calibration procedure cannot be used with my bridge. I have attached some rough notes for anyone else trying to repair one of these instruments.
Like a lot of WK kit, the design is much cleverer than it seems, and it is a very useful instrument when working properly.
I spoke to a helpful gentleman at WK, and he sent me a manual dated 1973, but it does not have the modifications fitted to mine.
Does anyone have a copy of a more recent manual?
Andrew

Jez1234 · 28th May 2021, 9:30 pm

I have a CT492 and was kindly afforded a manual by a member here. Mine is serial number 58 so a very early one. It's an impressive instrument in many ways and quite clever design.

I've since obtained a B521 from the late 50's and although it's not auto nulling nor as accurate it can do tricks like measuring up to 5 Farads plus simultaneously display capacitance and series resistance so a very good guide to ESR.

MotorBikeLes · 29th May 2021, 10:47 pm

I just checked mine. Type B421-Mk. 2
Ser. No. 712
I bought the manual from admiralty getting on for 40 years ago. The manual is dated 5/67, but it includes amendment sheets dated 11/73.
It sounds as if you have the fully amended manual. I replaced my sheets with the amendments, but still have to original 5/67 sheets in the front.
Mine needs a service, it has not behaved properly for decades. One day maybe.
Les.

Andrew Sinclair · 30th May 2021, 10:43 am

Thanks for the replies.
I downloaded Admiralty manual some years ago, but I can't remember where from!
The copy I have is the original 1967 version.
Les, if you could check your manual to see if the value of R55 is 200k or 500k that would be most helpful.
Les if you need help fixing yours please ask, though I would not claim to be an expert!
Andrew

MotorBikeLes · 30th May 2021, 5:58 pm

Interesting. I purchased my manual April 1980 from admiralty. I had to jump through some minor hoops. It included two amendments. The first, dated 11/73 simply replaced 5 complete drawings, the second from 7/78 was a partial spares listing, one double sided sheet starting at R103 and finishing at VT20, so no changes to any resistors below circuit ref R103. R55 thus 200K. Since yours appears to be of 1975 manufacture, the 200K should still apply.
Cheers, Les.

Andrew Sinclair · 31st May 2021, 7:33 am

Thanks for checking Les.
It could be that the Admiralty only bought CT492s in the early years of production and the mods were applied to equipment in service.
Mine has serial number 875 and may be a special for some purpose, although they usually added a letter, or a / number, for versions of their products. The resistors look original and the preset minimum capacitance mod is on a nice looking PCB that fits on two of the fixing holes for the oscillator PCB.
I guess we will never know...
Hopefully my notes may help somebody who has one like mine!

28th May 2021, 9:30 pm	#2
Jez1234 Banned Join Date: Mar 2021 Location: Morpeth, Northumberland, UK. Posts: 936	Re: Wayne Kerr B421/CT492 LCR Bridge I have a CT492 and was kindly afforded a manual by a member here. Mine is serial number 58 so a very early one. It's an impressive instrument in many ways and quite clever design. I've since obtained a B521 from the late 50's and although it's not auto nulling nor as accurate it can do tricks like measuring up to 5 Farads plus simultaneously display capacitance and series resistance so a very good guide to ESR.

29th May 2021, 10:47 pm	#3
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Wayne Kerr B421/CT492 LCR Bridge I just checked mine. Type B421-Mk. 2 Ser. No. 712 I bought the manual from admiralty getting on for 40 years ago. The manual is dated 5/67, but it includes amendment sheets dated 11/73. It sounds as if you have the fully amended manual. I replaced my sheets with the amendments, but still have to original 5/67 sheets in the front. Mine needs a service, it has not behaved properly for decades. One day maybe. Les.

30th May 2021, 10:43 am	#4
Andrew Sinclair Pentode Join Date: Sep 2007 Location: Fareham, Hampshire, UK. Posts: 104	Re: Wayne Kerr B421/CT492 LCR Bridge Thanks for the replies. I downloaded Admiralty manual some years ago, but I can't remember where from! The copy I have is the original 1967 version. Les, if you could check your manual to see if the value of R55 is 200k or 500k that would be most helpful. Les if you need help fixing yours please ask, though I would not claim to be an expert! Andrew

30th May 2021, 5:58 pm	#5
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Wayne Kerr B421/CT492 LCR Bridge Interesting. I purchased my manual April 1980 from admiralty. I had to jump through some minor hoops. It included two amendments. The first, dated 11/73 simply replaced 5 complete drawings, the second from 7/78 was a partial spares listing, one double sided sheet starting at R103 and finishing at VT20, so no changes to any resistors below circuit ref R103. R55 thus 200K. Since yours appears to be of 1975 manufacture, the 200K should still apply. Cheers, Les.

31st May 2021, 7:33 am	#6
Andrew Sinclair Pentode Join Date: Sep 2007 Location: Fareham, Hampshire, UK. Posts: 104	Re: Wayne Kerr B421/CT492 LCR Bridge Thanks for checking Les. It could be that the Admiralty only bought CT492s in the early years of production and the mods were applied to equipment in service. Mine has serial number 875 and may be a special for some purpose, although they usually added a letter, or a / number, for versions of their products. The resistors look original and the preset minimum capacitance mod is on a nice looking PCB that fits on two of the fixing holes for the oscillator PCB. I guess we will never know... Hopefully my notes may help somebody who has one like mine!