Wayne Kerr B101 test bridge

[Adam183]

Hello All, and merry Christmas

I'm sure this topic has already been covered, but I'll give it a go!
I have recently acquired a Wayne Kerr B101 test bridge which currently powers up but no response from the eye tube, I'd like to do some voltage checks but cant find any schematics or even operating instructions.
If anyone could point me in the direction of a schematic I'd be extremely grateful.
Thanks for taking the time to read this post
And happy new year!

Adam

[Chris55000]

Hi!

Before I go into the theory in more detail, have you checked the two 1 megohm triode anode resistors on the back of the magic eye's holder itself haven't gone o/c?

I haven't been able to find W.K.'s original information, but the one internal picture of these I found suggests you've got the side–contact EM4 type, which is dual–sensititvity with two triode anode pins – the eye's deflection rods are internally connected to the triode anodes, so o/c anode resistors result in a fully open eye that never moves!

Chris Williams

[Chris55000]

Hi!

If it's a side contact EM4, you have four widely spaced contacts at the top of the base, which are 5, 6,7 & 8 counting clockwise, then at the bottom, four contacts much closer together at the bottom which are 1, 2, 3 and 4.

Counting clockwise, the four closely spaced contacts, 1, 2, 3 & 4 are 1:– k,g', 2:– h, 3:– h, 4, i.c., then carrying round clockwise to the four widely spaced contacts, you have 5:– a',d', 6:– t, 7:– g and 8:–a",d".

Pin 6 (t) will be directly connected to the bridge's main H.T. supply line, 150–200V, whilst the anodes (where the 1 megohm resistors are) are about 40V when the shadows are fully open.

If you have the later EM34 Octal Based eye, the pinout is:–

1:– n.c., 2:– h, 3:– a',d', 4:– g, 5:– t, 6:– a",d", 7:– h, 8:– k,g'

Both types of eye close (first shadow) at –5V and the second shadow at –16V on the grid pin for anode resistors of 1 megohm each and h t. on the target-anode of 250V.

The two anode voltages are about 40V each when the shadows are fully open.

Take some voltage measurements on the pins of the eye itself and report back, and I'll go into the theory further if need be!

Chris Williams

[Electronpusher0]

I also have a WK B101 and have been completely unable to get any service information. There is a chap on the net advertising a pdf service manual for one so I paid up but when it arrived it was for a much later model. I did manage to get my money back from Paypal, the seller would not accept it was the wrong one.
I won't break forum rules by naming him but if you are thinking of buying the manual PM me first.

I think WK still have a presence in Bognor so I may just turn up and see if they have any info lurking about.

If you manage to get the manual let me know please.

Peter

[Andrewausfa]

I've got one too, EM34 on mine. I couldn't find any information on it either. Mine's s/n 415

Photos are before a good clean.

Andrew

[Chris55000]

Hi!

All normal RC bridges of the magic–eye type I've come across use grid–leak rectification of the a.c. bridge energising supply at the control grid of the eye itself, so if the eye is fully open and the two 1M triode anode resistors shown in Andrew's rear chassis view are ok, the next step is to check the source of the eye's detection voltage.

There's two ways of doing this, the cheapest way is an auxiliary bridge supply winding on the mains transformer of about 40–60V across the Wheatstone bridge ends, this often fails due to the auxiliary winding of the mains transformer going o/c, Heathkit C3U's are notorious for this!

A more accurate and reliable method is to use the heater supply plus an amplifying stage, which is the method used in the B101 – an EF39 can be seen at the front of the chassis on the left–hand side, this may be either triode or pentode connected, so the anode (and G2 if pentode–connected) feed resistors obviously require checking!

A quick test if you have one, is to inject audio from a generator to the top cap of the amplifier valve (G1) and see if you get a movement of the eye, and also try shorting the top cap to chassis – this is quite safe as there will be limiting resistors to prevent damage to the supply source, whereupon the eye should open fully on both shadows if the amplifier and the coupling to pin 4 of the eye is OK.

EF39 pinout:–

1:– met, 2:– h, 3:– a, 4:– G2, 5:– G3, 6:– no pin, 7:– h, 8:– k, TC:– G1

If you get no change in the eye with a signal generator or by shorting the grid of the amplifier to chassis, you'll need to check the anode and G2 voltages of the amplifier and trace from pin 3 of the EF39 amplifier to the magic eye, it's usually only a simple RC coupling as found in any valved amplifier, but please obviously check wiring and switch contacts for continuity as well of course, and also the windings of the smaller upright auxiliary transformer at the rear right hand side of the chassis!

Chris Williams

[Adam183]

Thank you all for your replies,
Extremely detailed and very helpful!
I've had the unit in question on today and I think the power transformer is failing, gets very warm after a short time and smells suspiciously of hot tar and varnish, so it could be game over before I've even got going.

[woodchips]

Have a copy of the B121 manual if anyone thinks it could help?

Dated 1957, uses 12AX7 and EM34.