UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Reply
 
Thread Tools
Old 8th Dec 2018, 1:01 pm   #1
NottsIan
Triode
 
Join Date: Mar 2018
Location: Nottingham, Nottinghamshire, UK.
Posts: 18
Default Wayne Kerr low inductance bridge data sought

Good morning,

Many years ago I was given a Wayne Kerr low inductance bridge model B321 which I have never managed to get to work.

There seems to be no information on this model on the internet but I have managed to find a manual for the Universal Bridge B221. Mine appears to be a simplified version of this operating at a higher frequency.

The oscillator is producing a strong output at 9.92 KHz and the detector seems to work as well leading me to suspect that there is a fault either in the "standards" leg of the bridge or the current transformer but without a circuit diagram I'm at a loss.

The current transformer (if that's what it is) has leaked a small amount of black oily gunge.

Does anyone have a manual or circuit diagram for this? or any advice on how to pin down the problem?

Ian.
NottsIan is offline   Reply With Quote
Old 8th Dec 2018, 8:31 pm   #2
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 5,137
Default Re: Wayne Kerr low inductance bridge data sought

Hi Ian, the current transformer may well be a "ratio arm transformer". This was a device whereby the number of standard R,L,C parts could be reduced by using different ratios in the transformer.
Such bridges were normally pretty accurate.
Leakage of oil may simply be due to poor seals rather than the unit having been "cooked"

Ed
Ed_Dinning is offline   Reply With Quote
Old 9th Dec 2018, 4:05 pm   #3
NottsIan
Triode
 
Join Date: Mar 2018
Location: Nottingham, Nottinghamshire, UK.
Posts: 18
Default Re: Wayne Kerr low inductance bridge data sought

Hi Ed. Thanks for the reply.
I did forget to mention that the transformer has quite a few wires coming out of it so I guess several taps - probably for the ranges. Before I dive into the switches / standard components / transformer I think I should check out the detector a bit more. There are two EF91 stages each with an EM34 giving coarse and fine indications. This looks very similar to the detector in the B221 so I have the manual as a guide even if it's not identical.
Investigation of the range/decade switching etc will have to wait until I've got a clear bench.
NottsIan is offline   Reply With Quote
Old 13th Dec 2018, 1:23 pm   #4
NottsIan
Triode
 
Join Date: Mar 2018
Location: Nottingham, Nottinghamshire, UK.
Posts: 18
Default Re: Wayne Kerr low inductance bridge data sought

Progress of sorts.

I've been working with the circuit diagram for the B221 which is a similar instrument by the same maker. The one I have is simpler in that it doesn't have the "Voltage Transformer" and is only designed to measure inductors. The second attachment shows what I think is the basic idea. Please correct me if I've missed something.

The alternating voltage from the oscillator is applied across the unknown Zu producing a current in the transformer. Similarly the alternating voltage is applied across Rs and Cs producing currents in the transformer. The magnitude of the sum of the currents is displayed by the indicator. When the values of Cs and Rs are adjusted correctly the currents all cancel out and the indicator shows a null. The direction of windings on the transformer allows us to use a capacitor to null an inductor.

So, on the basic circuit, we should have an oscillating voltage on point A and also on point B. Although I have drawn them connected, they are separate wires coming out of the oscillator. There is a strong signal at point A (about 60v at 9.92kHz) but nothing at all at point B. I think this explains why I cannot get any sort of null and I need to find out why there is no signal.

Next step will be to disconnect all the wires from point B (about 8 of them!) to determine whether the signal is being shorted to earth or is not coming out in the first place. The oscillator is a screened module and it looks like a tricky job to get into it so I have no idea yet what differences there are to the one in the B221.

I'll report back when I have more details.
Attached Thumbnails
Click image for larger version

Name:	Basic_Bridge.jpg
Views:	21
Size:	75.7 KB
ID:	174516  
Attached Files
File Type: pdf B221_Circuit.pdf (171.1 KB, 13 views)
NottsIan is offline   Reply With Quote
Old 14th Dec 2018, 10:47 pm   #5
NottsIan
Triode
 
Join Date: Mar 2018
Location: Nottingham, Nottinghamshire, UK.
Posts: 18
Default Re: Wayne Kerr low inductance bridge data sought

Well, two steps forward and three back today!

I disconnected the oscillator output from point B (see my previous post) and determined that there was no signal from the oscillator. Temporarily connecting the other oscillator output to both A and B I was able to balance the bridge and see the null on the magic eyes. So the standards and associated switchery appear to be working - also the current transformer (T2) and indicators.

Removal of the oscillator unit proved much easier than I expected and once the screen was off I could get a look at what's inside. How about the size of that capacitor (C4)?

I traced out the circuit (I'll scan and post that tomorrow). I unsoldered transformer T1 to determine the windings and when I put it back, I've now lost the other signal

Time to leave it for a few hours and sleep on it.
Attached Thumbnails
Click image for larger version

Name:	B321_Osc_Top1.jpg
Views:	21
Size:	86.0 KB
ID:	174610   Click image for larger version

Name:	B321_Osc_Bottom.jpg
Views:	20
Size:	73.4 KB
ID:	174611   Click image for larger version

Name:	B321_Osc_T1_L2.jpg
Views:	18
Size:	87.8 KB
ID:	174612  
NottsIan is offline   Reply With Quote
Old Yesterday, 4:03 pm   #6
John_BS
Heptode
 
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 942
Default Re: Wayne Kerr low inductance bridge data sought

Ian: Telford Electronics appear to have the manual: if you're keeping this instrument it might be worth buying: they don't charge the earth IIRC.



John
John_BS is online now   Reply With Quote
Old Yesterday, 10:49 pm   #7
NottsIan
Triode
 
Join Date: Mar 2018
Location: Nottingham, Nottinghamshire, UK.
Posts: 18
Default Re: Wayne Kerr low inductance bridge data sought

Thanks for the tip John. I'll drop them a line tomorrow.
NottsIan is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 2:33 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.