EKCO B74 Mains Conversion - Advice

[IJK2008]

Hello Radio experts out there!

I have an Ekco B74 that has been converted to AC mains operation. Before I could think about taking it to bits to restore it I had to check how it was wired in the first place.

I traced the circuit (see attached). There obviously is no ‘standard’ drawing but I found the relevant Trader sheets (B74, 715: AC74, 645) a good basis to make my sketch.

The set works well on strong MW stations. LW is poor but I think that is due to a coil rather then the overall circuit connections.

I am looking for ideas on how to improve and indeed correct the circuit as there are some ‘funnies’ like a resistor across the HT to chassis and 3 0.1uF caps in parallel. The HT volts are 240V dc and the screen of V1 is 145V (seems high as I expected more like 100V).

The volume control in particular seems a strange arrangement, but maybe not, I am sure someone will tell me!

The set was converted a long time ago as evidenced by the components used, but some caps have been replaced fairly recently and maybe this is where errors have crept in?

I plan to completely strip and rebuild the set so fundamental changes are not a problem

All help most welcome.

Ian

[geofy]

Well unless the person who did this wants to electrocute someone the external speaker needs to be put on the secondary for starters, The 100K resistor can't serve any useful purpose. I would put a resistor of a few K in the HT line between the output valve and the rest of the circuit, i.e., between the output anode and the top end of R9 the HT line. There doesn't appear to be any AGC and the volume control looks a bit strange.

[IJK2008]

Geof

Not guilty - but that's how it's wired. I think the speaker socket was seen as a useful connection post! The wiring is generally rough to say the least. May be I should just start again and build an AC74? That way I have a known point of reference.

Ian

[IJK2008]

Further to above - the socket is wired this way on the AC74 / B74 as far as I can see from the Trader sheets.

Ian

There seems to be no padder capacitor on the MW oscillator, C4 C20 are the padder for LW. This may not be a problem if either the IF frequency is low or the tuning cap has shaped fins.

[Coastwatch]

Is the diagram correct as V1 grid has no bias, perhaps this is where the 100k resistor should go.

Cheers n Beers,

David.....

[Cobaltblue]

Hi Ian

Although a bit dodgy lots of 1930's sets have the extension LS on the Primary side example Cossor 364 (amongst many). I am guessing that this was to reduce the loss to the extension speaker which may be in another part of the house connected with 'bell wire'. A lot of the extension speakers of the time were multi taped High impedence. I am guessing a few people got zapped.
I think as the set has already been modified it might be safer to put it on the secondary side but I don't expect many of us actually use the extension speaker socket much these days except to demonstate our extension speakers
Have fun Mike T

[geofy]

Originally Posted by Coastwatch

Is the diagram correct as V1 grid has no bias, perhaps this is where the 100k resistor should go.
Cheers n Beers,
David.

Don't think that would be right, as the grid would be positive by several volts, could be it should go to the 30K screen grid resistor junction.

Originally Posted by Cobaltblue

Hi Ian

Although a bit dodgy lots of 1930's sets have the extension LS on the Primary side example Cossor 364 (amongst many). a few people got zapped.
Have fun Mike T

Can't quite see how the oscillator gets back to V1 either, or how the signal is detected.

[Cobaltblue]

My guess is V3 is a leaky grid detector. In the battery version some bias was probably obtained by returning the grid circuit to the +ve battery supply.
V1 is a self oscillating mixer with the coupling in the Cathode circuit. I have no idea what the 100K resistor is for if its shown correctly
Have Fun Mike T

[IJK2008]

Hi All

Thanks for the help so far. The 100k is comprised of an old 50K carbon stick with a 1960s style 47k in series. It is wired as shown. I checked several times as I was unwilling to believe it.

Ian

[Cobaltblue]

Hi Ian the two 47Ks its possible it's an artefact of the conversion. This set as drawn seems to use techniques from battery and AC sets of different types. It's possible some of this was done empirically and was changed several times. This at one point may have just been a potential divider to feed a Screen grid or something similar but was was abandoned and as it does nothing it was just left in circuit. I am sure I am not the only one who leaves abandoned components when I am christmas treeing up an experiment. I have a Cossor 37 which has been brutaly changed into a mains set after the war only the cabinet, tuning capacitor and the speaker as well as a very carved up chassis are from the original set Its now an all octal post war set! I have now to decide if I should try to change it back!
Have fun Mike T

[IJK2008]

Hi Mike T

Thanks for you comments - if you had access to the AC74 and B74 trader sheets I think you would see exactly what you surmise. The set is a kind of amalgam of both sets and I was able to make my sketch up by sticking bits of both circuit diagrams together.

I think I need to drop the HT as Geof suggests and then get rid of the unneeded components. After that not sure about the volume control part of the circuit?

Ian

[Cobaltblue]

Hi Ian It looks as if the volume control operates by changing the bias on V2 which is a variable mu pentode changing the bias changes the gain of the stage, The HT does look a bit high for the FC and IF valves. I just noticed a very similar arrangement for gain is used on the front end of the Cossor 388 (trader sheet 357) I am sure its used elsewhere If I remember correctly Philips also favoured this method Have fun Mike T

[murphyv310]

Hi.
I am sure the 100k is purely to give a little load to the HT, not that elegant I must say but with a decent resistor OK, just the way things "used" to be done. The use of a speaker output at the anode/ht side although seeming unsafe does indeed have some subtle advantages! I have come across situations where an extension speaker is placed in a different room some considerable distance from the radio, using a low impedance feed and thin cabling (to be unobtrusive) the extension speaker would then be low output, using the primary feeding a separate output transformer will give excellent audio, and safe if done correctly.
I also have seen this type of "volume" control, the downside is you cannot get minimum volume, at least if you wish silence you DO turn the set off.

[Staigen]

Hi Ian
About the 100K resistor i dont know, maybe it is for discharging the electrolytics after you turn off the radio, the EXT LS can be at a high voltage otherwise.

But there is a few errors I can see(I have downloaded the PDF file you supplied and printed it out).

First, the L1 to L5 + C15 to 19 is a top capacitve(top coupled) bandpass filter, and the C15 coupling cap is connected wrong! It should go to C16/L2 conection. Then i also dont understand fully the S4 switch. Maybe it is two separate coupling caps. When is S4 open?

Then we come to the mixer. It is called an additive mixer(usally called first detector in USA, maybee so in Great Britain too, because the way it works). There MUST be a path for the first grid to ground, a resistor from the grid to ground(usally this resistor is at about 100K).

Then there is S7, that switch I dont understand at all, it will short out the IF totally, or is there a grammophone input somewhere.

It is nothing unusual about the volume control, it gives negative voltage to the first grid, with respect to the catode(a variable mu tetrode, or at least it should be that).

Do you have the original schematic to the EKCO B74? If you have, can you then send me a copy of it, so I can have a look on it, to help you further.
PM me in that case.

//Staigen

[IJK2008]

Hello Staigen

I understand it is difficult to offer advice if you don't have the original data.

The copy I have was obtained from this website so it would not be correct for me to send it on anywhere. The monies raised from the circuit diagrams help keep this website going.

I can tell you that C15 is for image suppression and is closed on MW only. According to the Trader sheet the coils are a primary and secondary arrangement with inductive coupling. This is all consistent with the original B74 circuitry.

There is a resistor in parallel across C2! Sorry I missed this when tracing the circuit and thus from my drawing - it's measured value is 2.2M. It's like a thin black tube - an early wirewound??

You are also right about switch S7 - it relates to Gram use. The other Gram switch contact is not wired. I would like to put this back into use.

Trevor - I can get rid of the parallel 'load' resistor when I sort the HT voltage?

Mike - thanks for the Cossor reference it was very useful. I see I have essentially the same set up on the volume control.

I guess that one of the 01uF decouplers should be between the 2 x 60K resistors and chassis? If so that leaves one to find a home for.

If the consensus is that the circuit is sound (but basic) then I'll rebuild as it is with only the necessary fixes. I can stick the final circuit inside the case.

Cheers

Ian

[Staigen]

Okay, its your radio, I only wanted to help.

//Staigen

[IJK2008]

Hello Staigen

You did help - and it is much appreciated.

Ian

[Staigen]

Hi Ian

Ok, do you have the schematic for this radio? If you have, what do it say about the C15 trimmer, is it connected to the antenna input?
About the 3 0.1uF decoupling caps, can it be a substitute for a 0.3uF cap?
The IF tube screengrid should of course be decoupled. Try a 0.1uF cap.

//Staigen