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Old 31st Aug 2019, 3:52 pm   #1
DonaldStott
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Default Kolster-Brandes MR10

Picked up this K-B MR10 very cheaply at an auction over a year ago but only now finding the time to look at restoration, or should that be refurbishment?

Had a bit of a peek inside the set and it seems almost untouched although it appears that someone has wired up an external aerial of sorts - I'm hoping that this is an add-on and not something K-B rigged up themselves.

I am using "Trader" Service Sheet 1233 for the MR10 and will undertake all the usual checks before trying it with my lamp limiter.

Some initial pictures note that I have all the original knobs: -

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Thanks to Mr Google I came across the article "It Came From Outer Sidcup" by John Ounsted in the BVWS Bulletin volume 24 number 3 Autumn 1999: -

https://www.bvws.org.uk/publications...letin_24_3.pdf

This makes slightly depressing if not alarming reading and I initially thought that the K-B MR10 might be beyond my capabilities - but wasn't sure until I got the chassis out the cabinet and did some basic checks!

Of course I had to order up some imperial hex sockets as I didn't have the required size to extract the two 4BA bolts securing the chassis mounting brackets to the cabinet! Removing the left hand bolt was a bit of a fiddle as you need to rotate the drive drum until a suitable hole aligns itself with the bolt head

I've also benefited from some early advice and guidance form Forum members David G4EBT (David Taylor) and crackle (Mike Edwards) I had some extremely embarrassing beginners gaffs which we'll just pass over quickly but now I think I'm back on track!

The next picture shows a general view of the upper chassis and valve layout and the one after that gives some indication of the usual muck that had to be cleared out. The main variation from what is shown in the Service Sheet is the replacement of the 7-pin 6BE6 with a 9-pin Mullard ECF82 but I'll put that to one side for the moment. This set also has the MR1 Westinghouse contact-cooled metal rectifier bolted to the underside of the chassis.

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Old 31st Aug 2019, 4:01 pm   #2
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Default Re: Kolster-Brandes MR10

The article by John Ounsted referred to in the Post above did mention the fact that the glass tuning scale is not really fixed at all it was simply gaffer taped into the cabinet during assembly being then supported more securely by the scale backing plate when the chassis was in place. In this set the glue on the gaffer tape had turned to dust which meant that .. it's very likely that the glass may fall down and smash if precautions aren't taken when withdrawing the chassis for the first time after forty years. Suitable precautions were taken see picture below and the glass tuning scale was safely extracted and bubble wrapped for later.

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In the previous Post I also mentioned that someone had fitted an external aerial wire of sorts as I'm assuming this is not factory fitted. The internal plate aerial had a short length of wire attached to a Wander Plug? Both these wires were attached by insulating tape to the inside of the cabinet? The picture below shows this clearly and of course removing the insulating tape ripped the plate aerial: -

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So chassis removed from cabinet, speaker disconnected and glass tuning scale safely stored we can move on and do some basic electrical checks.

With a 1 amp fuse in a three pin plug I'm reading about 35Ω across the LIVE and NEUTRAL pins with the set switched-on but not plugged in of course - Service Sheet states that this should be 32Ω. No reading when the set is switched-off and still not plugged in of course. This means that the two pole mains switch is working and the Mains Transformer Primary winding is good.

The Mains Transformer Secondary winding was checked by first disconnecting one of the AC wires from the transformer to the metal rectifier and then measuring the resistance across that wire and the other on the rectifier. This gave a reading of 95.5Ω Service Sheet says 90Ω.

Moving on to the Output Transformer I'm getting a healthy reading of 650 ohms on the Primary and 0.4 ohms on the Secondary. I note that the Service Sheet lists 5.0 ohms at point a and I'm getting 4.7Ω. All these readings for the OT seem to be in the ball park?

Turning our attention to the Rectifier I took all the valves out as advised by David G4EBT with a view to measuring the AC input and DC output. After sending David on a wild goose chase the main lesson I have learned here is next time I need to look more carefully for the ~ symbols! So after that faux pas and measuring from the correct pins on the rectifier I have: -

219V A.C.

267V D.C.


So far, so good remember all valves removed up until this point.

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 just waiting on parts arriving. After that and assuming everything is ok we can move on to testing the valve voltages.
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Old 31st Aug 2019, 5:55 pm   #3
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Default Re: Kolster-Brandes MR10

There were lots of revisions during the production run. Although pretty, these sets aren't great performers and I suspect KB kept tweaking the design to try to improve things.
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Old 31st Aug 2019, 6:47 pm   #4
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by DonaldStott View Post
With a 1 amp fuse in a three pin plug I'm reading about 35Ω across the LIVE and NEUTRAL pins with the set switched-on but not plugged in of course - Service Sheet states that this should be 32Ω. No reading when the set is switched-off and still not plugged in of course. This means that the two pole mains switch is working and the Mains Transformer Primary winding is good.
I think that means that the mains transformer primary is _probably_ good (you would not detect a few shorted turns with an ohmmeter reading but such shorts will cause the transformer to draw excessive current and overheat if not worse) and that the switch is closing on both poles when 'on' and opening _at least one_ pole when 'off'

What you do not know is that both poles of the switch are opening. It would be possible for the pole in the live wire to be stuck closed and the one in the neutral wire to be opening. In which case the transformer primary will be live if the set is swtiched off. Not a major hazard it's true, but if it were my set I'd check the switch a bit more carefully.
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Old 1st Sep 2019, 5:32 pm   #5
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Default Re: Kolster-Brandes MR10

Reading through the article "It Came From Outer Sidcup" by John Ounsted in the BVWS Bulletin volume 24 number 3 Autumn 1999 I have come to the conclusion that he may have been unlucky with his experience of the MR10.
I also have both versions of this model and found the electrical restoration straightforward and easy, reception is adequate with the aluminium foil antenna but agree it is better with a decent external FM aerial, much like any other make of FM radio.
The advantage of this type of set and many others with a mediocre foil plate aerial for FM and ferrite rod for AM is, that in a good signal area the radio is self contained and can be moved from room to room with no external aerial connections.
As you may know I have a few KB sets, and my collection includes 7 different KB valve radios with FM, I also have 2 radiograms with FM. They have all been straightforward to get working.
I have not found any appreciable difference in the performance of KB FM sets to other early valve FM sets that that I have restored.

I wish Alistair success with his restoration.

Mike
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Old 1st Sep 2019, 7:03 pm   #6
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by TonyDuell View Post
I think that means that the mains transformer primary is _probably_ good (you would not detect a few shorted turns with an ohmmeter reading but such shorts will cause the transformer to draw excessive current and overheat if not worse) and that the switch is closing on both poles when 'on' and opening _at least one_ pole when 'off'

What you do not know is that both poles of the switch are opening. It would be possible for the pole in the live wire to be stuck closed and the one in the neutral wire to be opening. In which case the transformer primary will be live if the set is swtiched off. Not a major hazard it's true, but if it were my set I'd check the switch a bit more carefully.
Thanks Tony - a couple of very interesting points for me to look into further.

I suppose that I'm just relieved after my recent experiences with a Bush AC91 and a Bush DAC90A that neither the Mains Transformer nor Output Transformer are open circuit !
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Old 1st Sep 2019, 7:15 pm   #7
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
Reading through the article "It Came From Outer Sidcup" by John Ounsted in the BVWS Bulletin volume 24 number 3 Autumn 1999 I have come to the conclusion that he may have been unlucky with his experience of the MR10.
I also have both versions of this model and found the electrical restoration straightforward and easy, reception is adequate with the aluminium foil antenna but agree it is better with a decent external FM aerial, much like any other make of FM radio.
The advantage of this type of set and many others with a mediocre foil plate aerial for FM and ferrite rod for AM is, that in a good signal area the radio is self contained and can be moved from room to room with no external aerial connections.
As you may know I have a few KB sets, and my collection includes 7 different KB valve radios with FM, I also have 2 radiograms with FM. They have all been straightforward to get working.
I have not found any appreciable difference in the performance of KB FM sets to other early valve FM sets that that I have restored.
Thanks Mike for your early advice and guidance prior to this Thread appearing and for your ongoing support.

So far so good and things will become clearer this week when we finally get to grips with some of the electrical components on the underside of the chassis.

One of my earliest restorations was a Philips B3G63A which has a mediocre foil plate aerial for FM and ferrite rod for AM - this sounds good, especially on FM, and the set has indeed been moved round various rooms in the house.

Hopefully this bodes well for the K-B MR10?
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Old 2nd Sep 2019, 9:03 am   #8
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Default Re: Kolster-Brandes MR10

They work well enough, but the performance of mine is nothing special, even using an external aerial for VHF. I found VHF reception to suffer some distortion on deviation peaks and tried various dodges to cure this without success. Admittedly I didn't do a complete realignment but there was no evidence of previous twiddling.

It's quite likely that different models of this set will have different characteristics given the production changes.
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Old 2nd Sep 2019, 2:16 pm   #9
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Default Re: Kolster-Brandes MR10

Here are some pictures of the underside of the chassis hopefully in the right order (left to right) - as we would say in Scotland, a right old guddle: -

A few waxy paper capacitors including C38 and C39, the A.F. coupling capacitors, and several Hunts capacitors.

One obvious feature (even to me) was pointed out in the article by John Ounsted referred to in Post #1 above is that there is no screening for the FM tuner section?

At the top/middle of the second picture is the underside of V2 which is the 9-pin Mullard ECF82 which replaced the 7-pin 6BE6. Very busy in there although I'd like to discern what's connected where and how this differs from what is shown in the Service Sheet 1233 for the MR10. Just for my own knowledge and ongoing understanding.

Comments and feedback welcomed.
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Old 2nd Sep 2019, 4:18 pm   #10
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by DonaldStott View Post
At the top/middle of the second picture is the underside of V2 which is the 9-pin Mullard ECF82 which replaced the 7-pin 6BE6. Very busy in there although I'd like to discern what's connected where and how this differs from what is shown in the Service Sheet 1233 for the MR10. Just for my own knowledge and ongoing understanding.
In the 6BE6 version the 6BE6 operates as a heptode (pentagrid) oscillator/mixer, in the ECF82 version the ECF82 operates as a triode pentode oscillator/mixer, both use the oscillators cathode circuit for regeneration to maintain oscillation.

The circuit differences are shown in the manufactures MR10 schematics which are available from Mike's (Crackle) KB museum web site.

Lawrence.
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Old 2nd Sep 2019, 5:39 pm   #11
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Default Re: Kolster-Brandes MR10

If you find there is a need to change the Hunts moldseal capacitors (and there probably will be a need to) around the VHF RF sections then be careful to not disturb the other components. Just cut the leads of the hunts capacitor close to the cap and tack in the replacement as close to the original position as you can.
I just solder the new cap to the stubs of the old cap, I dont worry about wrapping the wires in this sort of cramped position.

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Old 3rd Sep 2019, 1:57 pm   #12
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Default Re: Kolster-Brandes MR10

So having replaced C38 and C39, the A.F. coupling capacitors, it was time to put all the valves back in place and try a live switch on, via the lamp limiter of course.

The K-B MR10 behaved very well when the lamp limiter was switched to Lamp - initial lamp brightness which gradually dimmed as the valves heated up and then some of the brightness gradually returned as the valves reached operating temperature. And of course the main thing is that the set "sounds alive".

I had the usual strong signal for BBC Radio 4 but not much else - really nothing at all on any of the other bands!

Switched the lamp limiter to Direct which increased the signal strength and made the set bulbs glow brighter - but still nothing on any of the other bands!

A quick visual inspection showed that there was no sign of life from V1 (12AT7) a dual triode in the FM section. Carefully touching the valve confirmed that there was no heat whatsoever? Don't know to what extent the lack of FM will be down to the fact that the internal plate aerial is not attached?

Another couple of initial observations: -

1. Even with the volume turned right up this set is (currently) very quiet so some more gain somewhere would be welcomed.

2. The treble pot is very scratchy with dead areas even after a good squirt or two of De-Oxit - why is the tone control and not the volume control coupled with the on/off switch?

Now we can move on to measuring the valve voltages along with the values for any suspect resistors or capacitors. A few of the Hunts look suspicious e.g. one is cracked, and these may need to be replaced.

In two of the pictures in my Post #9 above there is a large blue electrolytic capacitor which looks fairly new - just wondering if this is a replacement and why?
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Old 3rd Sep 2019, 2:13 pm   #13
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Default Re: Kolster-Brandes MR10

That blue electrolytic may be 'relatively new' in that it could be 60 years old! Possibly replaced in the 60s, that doesn't mean you should assume it's OK.
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Old 3rd Sep 2019, 2:30 pm   #14
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by DonaldStott View Post
why is the tone control and not the volume control coupled with the on/off switch?

In two of the pictures in my Post #9 above there is a large blue electrolytic capacitor which looks fairly new - just wondering if this is a replacement and why?
I would say that the mains on/off switch isn't fitted to the volume control because any stray 50Hz pickup from the mains lead at that point will be much more amplified compared to what it will be with the switch fitted to the tone control.

The blue capacitor looks all original to me (the soldered connections look original) the capacitor is C40 in the KB manual, it's the ratio detectors reservoir capacitor, usually most folks would replace it as a matter of course.....+ve to chassis.

Lawrence.
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Old 3rd Sep 2019, 3:40 pm   #15
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Default Re: Kolster-Brandes MR10

I think Alistair is working to the Trader sheets so the capacitor is probably C37 2uF.
What band did you pickup BBC R4 on, LW?
All the old brown Hunts on the screen grids will be leaky and will be draining away the voltage.
As you said, voltage checks on the valves are the next thing to do.
I normally work forward from the Audio stages to the RF and sort out each stage as I go.

Mike
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Old 3rd Sep 2019, 3:41 pm   #16
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Default Re: Kolster-Brandes MR10

The Mouldseals will almost certainly be bad.

It goes without saying that there won't be any VHF reception if the 12AT7 is duff.
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Old 3rd Sep 2019, 4:31 pm   #17
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Default Re: Kolster-Brandes MR10

The other thing to check if the 12AT7 heaters check out OK are the valve socket pins.
I think they may be the little brass fork types, and they sometimes go very brittle and snap off.

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Old 3rd Sep 2019, 5:27 pm   #18
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Default Re: Kolster-Brandes MR10

Before going any further I need to answer one key question - the K-B MR10 has a waveband/gram switch and page 3 of the Service Sheet 1233 has details of the Switch Table and Diagrams.

I can't really make much sense of this or properly view the waveband switch units from the end of the inverted chassis??

From the MR10 schematics on Mike's (crackle) helpful KB museum web site it says "Turn clockwise for FM-LW-MW" so I am assuming that turned fully anti-clockwise is the Gram position??

Can someone confirm which switch positions to find Gram, FM, LW and MW.

Hope someone can help?

Meantime with all the valves in place and the loudspeaker reconnected I re-measured the Rectifier voltages but unsure as to which waveband I was on: -

207V A.C. (Service Sheet lists 225V A.C.)

184V D.C. (Service Sheet lists 185V D.C.)
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Old 3rd Sep 2019, 5:34 pm   #19
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Default Re: Kolster-Brandes MR10

Very nice receiver and as with all KB products well built by the ladys at Footscray.
I have a couple of these but have found as with certain other KB models, the VHF/FM performance is below even well below it's contemporaries.

I have put this down to the location of the KB factory, literally within touching distance of Wrotham!

Some of the best fringe area television receivers were built by PYE at Cambridge then very much a fringe area for AP. The model V4 solved 90% of the problems associated with fringe reception.

Maybe if KB had been a little further distant from the VHF/FM transmitter they may have designed more sensitive receivers. Other than that, a first class company. Regards, John.
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Old 3rd Sep 2019, 6:38 pm   #20
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
I think Alistair is working to the Trader sheets so the capacitor is probably C37 2uF.
You are correct I am using Trader Service Sheet 1233 - I note that the Valve pin voltages are different from those in the MR10 schematics on your KB museum web site

Quote:
What band did you pickup BBC R4 on, LW?
On the assumption that turned fully anti-clockwise is the Gram position then two clicks clockwise would be LW.

Quote:
I normally work forward from the Audio stages to the RF and sort out each stage as I go.
After a few restoration jobs I now appreciate that this is the best way to progress - this means that with the MR10 it leaves the FM stage until last, no bad thing!
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