KB ER30 radio

[crackle]

This week I have been restoring a 1949 KB ER30 radio.
These are a 6 valve largish radio and they have a 10 inch speaker driven by a 6V6GT output valve.
The valve line up is 6K8GT 7R7 6SL7GT 6V6GT 6X5GT and a 6U5G tuning indicator.

The radio was generously donated to the KB Museum by Leslie Wertheimer from Cheshire, and was kindly brought to me, first by forum member "Boater Sam" to Miky's do, then from there to the NVCF by "Keiths TV", where it was handed over to me.
These radios are not particularly rare, a few examples of the KB ER30 (not this one) can be seen in the Radio Museum; https://www.radiomuseum.org/r/kolsterbr_er30er_3.html. This example will feature in the KB Museum website when finished.
The KB service information is available here; http://www.kbmuseum.org.uk/kb_servic...KB_ER30_kb.pdf

This example is in better external condition to my original one so was a welcome addition to the collection in the KB museum.
On examination on the workbench I was surprised by the size of the mains transformer, I didnt remember it being as big as that. It seems to be a universal replacement and as well as the necessary windings for this radio it also appears to have 4v and 5v windings. It has been squeezed in (only just) between the magic eye and the OP valve.

I checked to see which valves were fitted and found the double triode, 6SL7GT, was substituted with a 6SN7GT, and when I checked my other ER30 I found that also had a 6SN7GT fitted, maybe KB changed the spec.

I next checked the OP transformer primary and that seemed OK at something over 400 ohms. Next was to try and reform the double smoothing electrolytic, surprisingly they appeared to reform very quickly on my home made reformer, but I found they had no ability to keep a charge so it was released from its clamp and 2 temporary replacements were tacked in. A new 32 + 32 uf capacitor has been ordered from Cricklewood via Ebay. The V5 cathode bypass capacitor was reforming OK but I decided to replace it with a new one as it seemed daft to leave one original component in when all the other capacitors would be new replacements.
Some other work had previously been done in the past to replace some capacitors, there were a couple of Mullard mustards in place of the audio coupling caps C19 and C21 on the V4 anodes. But the remaining 5 waxies and a tiny brown hunts were all to be replaced with new ones.

There is a label to warn service engineers that if the plugs to back panel are removed from the chassis then the ground to the speaker is disconnected leaving the output transformer with no load.

At first switch on the HT rose to about 120v, there were sounds of static on the workshop speaker and all seemed OK, so the lamp was switched out and the HT rose to about 170v still rather low. I switched off and looked for another 6X5 rectifier. I fitted this and the HT rose to about 210v. I plugged in the aerial and the set was receiving loads of stations on all 3 bands, it was very lively, and could really have benefited from a further reduction drive on the tuning capacitor.
Long wave was very surprising, its coverage was almost into MW, covering from 142 to 410 kHz, and I was picking up a number of navigational beacons at the high frequency end. One on 363 kHz was giving out an identity signal of ". . . _ . _ . ."
RTE on 252kHz was coming in very loud and clear, as well as R4 and a host of French stations. MW was crowded with stations, Gold on 1548 was unusually loud and clear and Caroline on 648 at the other end of the band was also very clear with no noticable background noise.

The magic eye had only a faint red glow from the heater. So I borrowed the very good one from my other ER30.
Altogether an excellent performing radio, a credit to the KB design engineers, and it will sound even better when it is back in the cabinet and connected to its 10 inch speaker instead of the 4 inch workshop speaker.

Hope the new smoothing capacitor arrives soon so I can hear the radio as it should sound, back in its cabinet.

Mike

[HamishBoxer]

I see a replacement Elstone mains tx has been fitted.

[Heatercathodeshort]

My Gran in Thornaby had a KB ER30. It was in the back parlour sitting on a large Victorian chest of drawers.

As a very young lad on holiday I remember it began to shriek it's head off, similar to when the EF39 looses it's screening earth.
How Grandad heard the football results above this racket nobody will know but he did!
I presume a decoupling cap had gone O/C.

It was collected for repair and returned the next day. A very good radio.

I can see it now sitting on that Victorian chest.

Alas Gran and Grandad have gone, So are the gas lights, the milk horse, cobbled streets, outside toilet, Gran's house, Number 5 and even the part of Walker Street where Gran lived. [A66 widening etc] It was another age. John.

[boxdoctor]

Lovely set, Mike. KB certainly built things properly in those days! really substantially made.
Had the replacement "Elstone" (Well spotted, H.B.) transformer mounting flange been "engineered" somewhat to accommodate the 6V6 and 6X5 valveholders? A very tight fit, anyway.
Should sound really good with the 10" speaker, and in it's own cabinet. Cheers, Tony

[crackle]

Yes some of the extra wide mounting flanges on the replacement mains transformer had been cut away to accommodate its fitting.
I cant admit to this being one of the prettiest of the KB sets, (which ones were?) but it is well designed from a functional, and performance point of view. Everything is easy to get at and component wires were not wrapped 3 times through the solder tags.

Mike

[Nanozeugma]

A very satisfying endeavour by the look and sound of it, congrats.
That replacement mains tx - no other word - HUGE.

[boxdoctor]

Originally Posted by crackle

I cant admit to this being one of the prettiest of the KB sets, (which ones were?) but it is well designed from a functional, and performance point of view.

"Functional" and performance is what ticks the boxes for me with KB models.
My uncle had a KB when I was a small boy, (https://www.radiomuseum.org/r/kolsterbr_640.html ), and exploring what that would do (mainly on short-wave) was one of the starting influences that got me into electronics.
I have a nice green FB10 in the kitchen. Often on, and never had any trouble with it. Tony.

[crackle]

The nice thing with "Tygan" speaker cloth is it cleans up easily. I have been cleaning the front of the radio. With the speaker removed I brush on foam cleaner, brush it in well then use a vacuum cleaner to suck off the remaining foam. I do this bit by bit so as to not allow what's under the cloth to get too wet.
Here you can see the difference between the top two strips which I have cleaned and the rest.

It helps get rid of that dirty circle which always shows where the speaker aperture is and where the dirt and dust has collected.

Dont try this with conventional cloths though, only with the woven nylon "Tygan" type cloth and certainly not with silk cloth, foam cleaner dissolves the silk.

Mike

[crackle]

The new 32+32 uF capacitor has arrived from cricklewoodelectronics, wow that was fast, I only ordered it yesterday about 10am.

Mike

[Sideband]

It's amazing how some sets perform so well with a conventional valve line-up while others struggle. Just looked at the circuit (I was curious as to why the double triode stage with only one output valve)....the Trader sheet says 'electronic tone control' via the second triode and....very unusual for a domestic radio of the time....negative feedback from a tapped secondary winding of the output transformer. It should sound very good when connected to it's 10" speaker.

[crackle]

You know its funny, this is the only set so far which responds VERY well to plugging the new RF earth I rigged up last year. Even though the ground is so dry, and there is a mains earth connected via the mains cable to the chassis. When I disconnect the RF earth Gold on 1548 returns to the usual weakish signal with some background noise. When the earth is plugged back in it returns to much stronger reception. the same can be said for MW and particularly LW.

So why does this radio work so much better with my new 11m long wire aerial and separate earth than many of my other sets, when they were all designed for a long wire aerial and separate earth.
I might just experiment and see how it performs with the mains earth disconnected.

Mike

[Heatercathodeshort]

I have often wondered why this is so. My guess is loss of 'Q' in the I.F. transformers and coils due to damp and changes in the wax they were often dipped in.

Were the older KB models more popular 'Up North' than down south? Maybe a link to the 'Queen Mary'.

My uncle had a KB radio, television and radiogram in the 1950's. He worked in the laboratories at Head Wrightson steel works Middlesbrough and was very well paid.

Exciting holidays at the steel works. John.

[crackle]

Reception was not quite so good with the mains earth removed and just the RF earth connected. However with the mains earth connected, connecting the RF earth does "clean" up the signal a little, less background noise etc.
I have not noticed this so much on other radios designed for long wire and earth.
There is a RF choke across the aerial and earth terminals, is this an IF trap or something else.

Mike

[ms660]

Originally Posted by crackle

There is a RF choke across the aerial and earth terminals, is this an IF trap or something else.

Mike

I'm not absolutely sure but it might be performing a similar function as a high impedance primary in a conventional transformer coupled antenna/RF circuit, where by the inductor and its self capacitance in conjunction with any of the receivers input capacitance and the antenna capacitance broadly resonates below the low end of the MW band in order to help give a more even response across the whole tuning range.

Lawrence.

[crackle]

Hi Lawrence
The choke in question can be seen in this photo at the top.

L15 in the circuit diagram.

Other than that it looks like a fairly conventional circuit.
Mike

[ms660]

Yes, I can imagine that resonating sub MW.

SW is transformer coupled, MW/LW bottom coupled so far as I can make out.

Lawrence.

[crackle]

One other interesting thing about this radio is the use of a 7R7 valve with a B8B base for the IF amp and detector, as opposed to an octal base.
It seems KB were more or less, the only manufacturer to ever use this valve in a domestic radio, and that was only in 8 models. This ER30 and the other ER30 I have are the only examples I have in the KB Museum.

Mike

[Heatercathodeshort]

The DV40 television receiver [1948] employs two 7R7 valves as sound I.F. amplifiers. J.

[crackle]

Hi John
What make is the DV40, is that KB as well?
Mike

[Heatercathodeshort]

Yes Mike. As you probably know post war KB model numbers were prefixed with a letter from 1946. A=1946, B=1947 C/D= 1948 E=1949 etc.
The DV40 is a very rare model. J.