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Old 10th Nov 2019, 10:05 pm   #141
David G4EBT
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Default Re: Kolster-Brandes MR10

Thanks for the clarification Lawrence.
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Old 11th Nov 2019, 6:25 am   #142
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Default Re: Kolster-Brandes MR10

Thanks Lawrence you answered my question whilst I was still struggling to ask it.

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Old 11th Nov 2019, 10:13 am   #143
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Default Re: Kolster-Brandes MR10

No problem.

If no second receiver is available, the time honoured method of measuring what's what at the oscillators anode and grid is the way to go as a first port of call.

Lawrence.
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Old 11th Nov 2019, 1:39 pm   #144
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Default Re: Kolster-Brandes MR10

All very technical stuff guys and well above my pay grade but I think I'm just about hanging on in there?

The YouTube link below shows the Kolster-Brandes MR10 (at rear) with my Roberts R303 - both on the MW band but MR10 speaker disconnected.

Some interference heard on the Roberts at one particular spot when the MR10 is tuned through the MW band.

Is this what you mean: -

https://youtu.be/a37mgMNXgT8

While I had YouTube open I thought I'd upload another video which hopefully lets you hear the issues I am having with the Tone Control. This is very bright for most of it's rotation then dull and flat - "dead spot" with excessive hum and crackling. Treated with De-Oxit several times but problem getting progressively worse!

The hum in the "dead spot" is much more pronounced in real life!

https://youtu.be/bzU6QzLeL6A
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Old 11th Nov 2019, 2:37 pm   #145
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Default Re: Kolster-Brandes MR10

I am assuming you changed C39 (trader).
It looks like you may need to remove the tone control and dismantle to check for and clean away excessive dirt or corrosion on the metal wipers and tracks.
Be careful about not scraping the carbon track with anything hard, the layer of carbon is very delicate and thin. I normally reassemble with a little smear of grease on the metal and carbon tracks.
If that does not fix it then a new 220k with double pole switch may be needed.

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Old 11th Nov 2019, 4:38 pm   #146
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by David G4EBT View Post
As to 'not getting anywhere with the Nostalgair tests and your signal tracer', in fact you were, because the tests showed that the oscillator wasn't working, just as if - for example - you put an Ohmmeter across a fuse and 'get no reading', it proves that the fuse has blown.
Apologies David, poor wording on my part as the tests did indeed show that the oscillator wasn't working - a crucial step in finding where the actual problem lay i.e. a poor solder joint on the underside of V2. Nostalgia Air: Signal Tracing is a very good resource so thanks again for the link.

Quote:
Originally Posted by crackle View Post
I am assuming you changed C39 (trader).
I know it's been a long Thread but way back in Post #2 I mentioned: -

"Next Step is to replace both C38 and C39 A.F. coupling capacitors - before our first live test, with the lamp limiter of course.

The Service Sheet shows C39 as 0.02μF although what has been fitted in my set is 0.001μF. On the assumption that Kolster-Brandes made this change for good reasons (?) I'm going to replace like for like ..."


Both C38, C39 (Trader) and a number of Hunts Mouldseals and waxy paper capacitors have all been replaced - like for like.

I note what you write about removing, dismantling and cleaning the Tone control - something I have been avoiding but now know I have no real choice.

I have looked around for an appropriate 250k with double pole switch but can't seem to find an exact match, specifically in relation to the d-shaped metal spindle.

The KB Museum Service Data sheet lists this as: -

Component: Potentiometer 1/4 M.ohm and D.P.S.
Part No. : 80670 / 5
Price : 8/-

I make that 40 New Pence as we used to say back in 1971!
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Old 11th Nov 2019, 6:04 pm   #147
David G4EBT
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Default Re: Kolster-Brandes MR10

You won't fine a current production 250K potentiometer as for a long time, the standard value has been 220K, which would be fine, (in the same way that 470K has superceded 500K).The next value above 250K would be 270K, but you're unlikely to find one of those either. Most suppliers tend go from 100K to 220K, 470K, then 1 Meg, 2 Meg.

A current production standard pot would have a 50mm long x 6mm diam Delrin (plastic) shaft with no flat, whereas I guess your MR10 one has a metal shaft and will be 1/4" diam (6.4mm) and with a flat. A 1/4" knob will fit a 6mm shaft but will be very slightly eccentric, though on your radio, as the tone control and combined on/off switch are mounted on the side, I doubt that slight eccentricity would be noticeable. That said, I think your knobs fit with a spring clip rather than a grub screw, so that could be problematical on a 6mm shaft.

You can however obtain a new switched pot made to order from Blore Edwards with a 1/4" metal shaft on request and if you wished, could ask if they could do a 250K linear track, but I doubt it, and as I said, 220K would be fine. Being a tone control, it will almost certainly have a linear track, but you can make sure by setting it to the middle of its travel and checking with an Ohm-Meter from the centre tag to each end, to check that it reads approximately 125K either side of the centre tag.

If you got to this link at Blore Edwards, you'll see that they state that their standard Type 45 potentiometer is supplied with a 6x50mm (two inch) plain Delrin spindle, but that metal spindles are available on request and are also available in both metric and imperial diameters. You'd have to ask them to add a flat to the shaft if you want to avoid having to file a flat on it yourself.

The person to ask for is Brett, who is very helpful, as others will testify:

https://www.blore-ed.com/series45potentiometers

Phone No: 01685 882 043

The simplest and cheapest option would be a 220K switched pot with a 6mm diam x 50mm Delrin shaft from Bowood, but as I said, the push-on knob might be an issue, and the pot shaft won't have a flat on it.

https://www.bowood-electronics.co.uk...switched-p-651

Hope that may help.
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Old 11th Nov 2019, 6:56 pm   #148
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by DonaldStott View Post
All very technical stuff guys and well above my pay grade but I think I'm just about hanging on in there?

The YouTube link below shows the Kolster-Brandes MR10 (at rear) with my Roberts R303 - both on the MW band but MR10 speaker disconnected.

Some interference heard on the Roberts at one particular spot when the MR10 is tuned through the MW band.

Is this what you mean: -

https://youtu.be/a37mgMNXgT8
That's basically it when using another receiver to check if the oscillator in the receiver under test is working, what you are getting there is the oscillator in the KB mixing with the signal frequency that the Roberts is tuned to, that produces an audible "beat" frequency as you tune through, it's frequency varying according to the difference frequency between those two signals, basically that's how the IF is obtained in a superhet receiver, the difference being that the difference frequency is much higher, in the case of the KB it's given as 422kHz.

The oscillator frequency should always be higher than the frequency of the signal being received and the difference between the signal frequency and the oscillator frequency should always be the IF frequency, so to that end here's the maths behind it, it's not complicated.

Suppose you tune the receiver to a station you want to listen to and let's say the station is transmitting on a frequency of 1,000kHz, the oscillator frequency needs to be higher than that by an amount that equals the IF, we know that the IF is 422kHz therefore the oscillator frequency should be at 1,000+422 which equals 1,422kHz, that's it in essence and to prove the point, when those two frequencies are fed into the mixer the difference frequency produced at the anode will be 1,422kHz (the oscillator frequency) minus 1,000kHz (the signal frequency) That results in a frequency of 422kHz....Voila..the IF.

From that you can see that the signal tuning and the oscillator tuning should be tuned to maintain that difference frequency, the IF.

In practice they never track together exactly across the tuning range but near enough not to cause any great concern if the receiver is up to spec.

That's it, not as technical or complicated as might be first assumed.

Keep at it, learn the basic theory and apply to practice, first principles won't let you down.

Lawrence
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Old 11th Nov 2019, 7:04 pm   #149
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Default Re: Kolster-Brandes MR10

The plastic shaft is very easy to cut to length and if the flat is filed carefully, with just a fine flat, the knob should push onto it and grip the small flat sufficiently. If not then make the flat bigger and pack out the flat with a thin piece of hard plastic and try again.
Alternatively, am I right in thinking the knobs are not the original ones, maybe keep the new shaft long and fit a new knob with a grub screw to the new pot shaft.

But you may be able to clean up and get the existing pot working nicely.

One point I have noticed about a radio with a non working oscillator, you can sometimes use another radio to get the faulty radio to work.
By tuning through the dial on the good working radio you may hear stations being received on the faulty radio.
It is rather spooky when you adjust one radio and the other starts to receive a station.

This can be even more noticeable if you are using an RF frequency generator. As you sweep the frequency generator across the MW frequencies + the IF frequency the radio should start to work.

Mike
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Last edited by crackle; 11th Nov 2019 at 7:13 pm.
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Old 11th Nov 2019, 9:41 pm   #150
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
But you may be able to clean up and get the existing pot working nicely.
Can't help but feel that any attempt to remove, dismantle and clean the Tone control is a one way street to a new potentiometer? I'll contact Blore Edwards regarding a new pot before attempting anything too ambitious!

I'm still hopeful that I can source some original knobs or suitable alternatives - surely there are many Forum members with boxes of old knobs kicking about their workshops?

Lots of chatter about making such items using a 3D printer but it's probably early days for such technology in this context.
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Old 12th Nov 2019, 12:35 pm   #151
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Default Re: Kolster-Brandes MR10

Quote:
One point I have noticed about a radio with a non working oscillator, you can sometimes use another radio to get the faulty radio to work.
By tuning through the dial on the good working radio you may hear stations being received on the faulty radio.
It is rather spooky when you adjust one radio and the other starts to receive a station.
Since most oscillator coils are not shielded in valve radios, I guess the leakage RF oscillator signal of the working radio must be coupled to the antenna of the defective radio and then to the RF input grid of the mixer.
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Old 12th Nov 2019, 2:11 pm   #152
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Default Re: Kolster-Brandes MR10

Nothing to loose by trying, only experience and skill to be learned.
Mike
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Old 22nd Nov 2019, 7:10 pm   #153
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Default Re: Kolster-Brandes MR10

As the issues I was having with the Tone Control continued to deteriorate I decided to contact Brett Cryer at Blore Edwards as recommended by David G4EBT.

I explained that I needed a replacement for the 250K linear potentiometer with a Double Pole, Single Throw switch (DPST) and he directed me to this link: -

https://www.blore-ed.com/product-pag...er-metal-shaft

There are a number of other measurements required, some of which I was uncertain about, such as: -

Click image for larger version

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Brett and I agreed that the best course of action would be to disconnect and send them the existing pot/switch and they will take it from there - "It would be great to have the original to work from."

So my Tone control/switch is currently in transit to Mid Glamorgan - Brett at Blore Edwards could not have been more helpful so I am anticipating a favorable outcome with this.

Meantime I'm in the process of replacing the existing mains lead and polishing the cabinet - hoping for a final reassembly in the near future.
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Old 23rd Nov 2019, 7:02 am   #154
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Default Re: Kolster-Brandes MR10

It looks like you should get a nice drop in replacement.

Mike
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Old 17th Mar 2020, 7:03 pm   #155
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Default Re: Kolster-Brandes MR10

Finally got my replacement 250k linear pot with dual gang mains switch and fitted that today - this radio has never sounded better although I'd still like a bit more gain

Almost ready to put it all back together (if I can remember!) but just one last hurdle that I need advice on!

The new pot is very good BUT it is the Tone control and is only really effective at the top 10% of rotation - everything else is much too bright! Now I know that I can ameliorate this by changing the value of one of the capacitors associated with the Tone control - but which one and which way, up or down in value??

Here is a snip from the circuit diagram: -

Click image for larger version

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So it's either C43: Part tone control (0.003μF) or C44: Tone correction (0.01μF) - advice would be welcomed.

And when I do come to final reassembly, how do I secure the glass tuning scale to the case - it was just held in by some tape I seem to remember - see Post #2 above!
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Old 17th Mar 2020, 8:53 pm   #156
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Default Re: Kolster-Brandes MR10

C44 and R43 across the OP TX primary will start to roll of the HF response at a frequency determined by C44 and the transformed speaker resistance. At a frequency determined by C44 and R43 the roll off will flatten out again giving an overall "shelf" type of response.



R22 and C43 (the tone control) will give an increasingly lower frequency HF rolloff as the wiper of R22 approaches C43. I would expect the effect to seem quite non linear since the rolloff point will have to come down below about 4kHz before anything will be noticed on an AM broadcast signal.


Add in the fact that the NFB circuit is also frequency shaping too and the whole shebang gets quite "interesting".


If the "tone" at max treble setting is fine for the FM signal sound to your ears I'd be inclined just to leave things as they are.
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Old 17th Mar 2020, 9:00 pm   #157
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Default Re: Kolster-Brandes MR10

I think you may have some sort of fault condition. These sets shouldn't sound excessively bright. They liked a lot of top cut in the 50s, and most radios of that era tend to sound murky to modern ears.
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Old 18th Mar 2020, 9:47 am   #158
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Default Re: Kolster-Brandes MR10

@Chris - thanks for the explanation most of which I understood. I'm inclined to leave things just as they are unless there is a quick fix that anyone can recommend?

@Paul - hopefully we haven't got a new "fault condition" at Post #157. Can you be more specific?
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Old 18th Mar 2020, 10:05 am   #159
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Default Re: Kolster-Brandes MR10

Can you clarify - does the radio sound excessively bright unless the tone control is in the 'maximum top cut' position, or does it sound excessively dull unless the control is in the 'minimum top cut' position? You seemed to be saying the former in #155, which would be a fault, while Chris has assumed you mean the latter, which is normal for radios of this era.
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Old 18th Mar 2020, 10:16 am   #160
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Default Re: Kolster-Brandes MR10

Youtube clip with tone control action, on AM and on FM..?

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