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Old 5th Sep 2021, 10:08 pm   #1
ManxDave
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Default Mediator La Chaux M233AB maybe?

I got this battery valve radio on eBay. From the case, dial and valve line up the nearest ID I can find is a Mediator La Chaux M233AB, which it may well be??
Valve lineup is DK40, DF91, 2*DAF91 2*DL41
The only service data I can find ( radiomuseum ) is in german. Can anybody point me in the direction of english service sheets for this radio or a similar alternative?
I won't be powering it up until I have made up the batteries but any other tips greatly appreciated. I have some limited knowledge of valve radios but am much more comfortable with early transistors.
Thanks Dave
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Last edited by Cobaltblue; 5th Sep 2021 at 10:16 pm. Reason: euphemism for eBay
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Old 6th Sep 2021, 2:12 am   #2
Maarten
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Default Re: Mediator La Chaux M233AB maybe?

From the part number on the valve layout label, it was made or at least assembled by Philips Sweden. According to the language on the controls, for the local and/or scandinavian market. Could have been sold as Philips or Dux, or even another brand within or outside the Philips universe. Designs were sometimes shared with other Philips factories, so it might well share part or all of the design with the Mediator model.

Often, a model reference can be found on the dial scale, underneath a knob or underneath the bezel/case.

Last edited by Maarten; 6th Sep 2021 at 2:18 am.
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Old 6th Sep 2021, 7:29 am   #3
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Default Re: Mediator La Chaux M233AB maybe?

Thanks Maarten. The Phillips Sweden reference is helpful. When I remove the chassis I shall look under all knobs, bezel etc.
If it is a Mediator La Chaux, it is the M233B not M233AB.
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Old 6th Sep 2021, 11:02 am   #4
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Default Re: Mediator La Chaux M233AB maybe?

It certainly appears to have a Swedish tuning scale. The mixture of valve types is distinctly odd - I've never come across B8A 1.5V battery valves before.
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Old 6th Sep 2021, 11:50 am   #5
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Default Re: Mediator La Chaux M233AB maybe?

Quote:
Originally Posted by ManxDave View Post
Thanks Maarten. The Phillips Sweden reference is helpful. When I remove the chassis I shall look under all knobs, bezel etc.
If it is a Mediator La Chaux, it is the M233B not M233AB.
It's actually the M that refers to a Mediator model, the A refers to mains supply while the B refers to battery supply. A number on a dial could be without or partly without letters.
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Old 13th Sep 2021, 10:14 pm   #6
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Default Re: Mediator La Chaux M233AB maybe?

I haven't had much time for this yet but here are a few more photos. Someone has been in the IF can by the looks.
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Old 14th Sep 2021, 11:10 am   #7
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Default Re: Mediator La Chaux M233AB maybe?

Philips made a range of B8A base 1.4volt valves for high quality battery receivers. DK40, DAF40, DAF41 and DL41.
Only British made radio I know of to use one of these valves was the Bush BE15. It employed a DK40 as the frequency changer.
Philips suggested using two DL41s in a push-pull output stage. Two watts of audio power would be possible.
http://www.tubebbs.com/tubedata/sheets/046/d/DL41.pdf

DFWB.
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Old 14th Sep 2021, 3:19 pm   #8
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Default Re: Mediator La Chaux M233AB maybe?

https://www.radiomuseum.org/r/philips_lch382b.html

This manual is in German as well but would it be any use ?

Lawrence.

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Old 14th Sep 2021, 7:08 pm   #9
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Default Re: Mediator La Chaux M233AB maybe?

The dial was designed in Sweden, from the small code number in the lower right corner. Unfortunately no model designation there.

I'd say the set as a whole was designed or at least heavily modified by Philips Sweden. There's likely some LSxxxAB set that would come close.
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Old 14th Sep 2021, 8:43 pm   #10
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Default Re: Mediator La Chaux M233AB maybe?

Thank you for the links and extra info. Progress will be slow on this one but I'll update when I can. Dave
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Old 16th Sep 2021, 11:40 am   #11
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Default Re: Mediator La Chaux M233AB maybe?

It's possible the push-pull output stage circuit is similar to the circuit employed in the Ever-Ready Sky Monarch.
This is most likely a circuit design developed by Philips.

DFWB.
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Old 19th Sep 2021, 11:33 am   #12
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Default Re: Mediator La Chaux M233AB maybe?

Thanks, the push pull does look similar, to a novice on valve circuits.

A capacitor replacement question. What type of capacitor should I replace C4 with, bottom right of the schematic snip and the big brown one in the photo. I think it should be 50uF 12.5V but what type? If electrolytic which way round?
As it just goes across the two 0V rails in parallel with the wirewound 1K resistor I don't understand its purpose.

Also the large black capacitors test up to 3 times their value on my cheap tester, and when connected to my HV supply appear to pass about 25uA with 200V across them, so I assume they will need to be replaced. They are marked 400V and listed as 500/1500V do they need to be this high? E.g. C62 in the snip.
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Old 19th Sep 2021, 12:20 pm   #13
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Default Re: Mediator La Chaux M233AB maybe?

C4 is the de-coupler for the bias supply, the bias voltage being developed across the 1k resistor, a 47uF at 25V will do, +ve to ground.

Lawrence.
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Old 19th Sep 2021, 1:09 pm   #14
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Default Re: Mediator La Chaux M233AB maybe?

"Also the large black capacitors test up to 3 times their value on my cheap tester, and when connected to my HV supply appear to pass about 25uA with 200V across them, so I assume they will need to be replaced. They are marked 400V and listed as 500/1500V do they need to be this high? E.g. C62 in the snip."
Leaky capacitors show a much higher value on digital tester instruments.
You'll find most of the black tar capacitors will be leaky.
It's essential that the coupling capacitors between the anodes of the DAF91 amplifier and phase inverter stages to the control grids of the DL41s must be replaced.
Information for the DL41: https://www.radiomuseum.org/tubes/tube_dl41.html

DFWB.
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Old 19th Sep 2021, 8:34 pm   #15
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Default Re: Mediator La Chaux M233AB maybe?

Thanks Lawrence, because C4 is connected between 90V -ve and 1.5V -ve I wasn't sure which end was 'ground' but it is obvious looking at the set. However if I had fitted an electrolytic I would have installed it -ve to ground, so glad I asked.

That is what I assumed thanks DFWB, I just thought 400 or 500V rating looked at bit high but better on the high side. Yes the couplers were the first I tested and will be first changed.
Thanks Dave
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Old 23rd Sep 2021, 11:54 am   #16
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Default Re: Mediator La Chaux M233AB maybe?

After cleaning the on/off switch. Replacing all the 'tar' capacitors and the electrolytic across the HT, which wouldn't reform, this set is working on all 3 bands, surprisingly well on SW with a long wire attached.
With only German instructions I am glad I haven't had to do any tweaking ( so far, fingers crossed ).
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Old 23rd Sep 2021, 12:18 pm   #17
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Default Re: Mediator La Chaux M233AB maybe?

Nice!

The 400V voltage rating is the nominal voltage. The 1500V is reference to a peak voltage they can withstand for a certain time.

400V was a standard value at the time, 125V being the next lower which is a bit on the low side. As a rule of thumb in an unknown circuit, capacitors need to have a working voltage of at least twice the highest supply rail but in most positions they will see much less than that.

Capacitors across or to ground from transformers might need to be rated higher and for pulse use (polypropylene instead of polyester). Capacitors across the mains or from mains to ground or from mains to touchable parts, need to comply to X or Y ratings respectively.
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Old 23rd Sep 2021, 8:48 pm   #18
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Default Re: Mediator La Chaux M233AB maybe?

Thanks Maarten.
The step from 400V to 125V answers my question. Thanks to everyone for the help and links.
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