Kolster-Brandes MR10

[ms660]

Originally Posted by DonaldStott

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??

It's quite straightforward once you know the system that the Trader sheets use.

Almost all Trader sheet representations of the switches in the circuit and the actual switch contact layout diagrams use the same system, this makes it easy to understand for any manufactures model because they are all drawn using the same system.

1) All the switches are numbered on the schematic with a circuit reference, ie: S1, S2, S3 etc.

2) They are also shown in the switch table, the switch table shows which of the switch contacts are closed and which are open for the particular function that's been selected, a dash indicates open and a c indicates closed, for example when the control is set to gram in your receiver switch contacts S5 and S14 are closed, all the rest remain open.

3) Below the switch table are shown the switch contacts diagrams, each one has a number within a diamond for identification purposes in the chassis layout drawing, in this case there are only two switch wafers so they are numbered 1 and 2 respectively, if you look at the chassis layout pertaining to the inverted chassis you will see the two diamonds with the numbers in them close to the switch wafers, there's also an arrow attached to each one to show the direction viewed.

So far so good.

4) Next it's back to the switch diagrams again, each tag that is part of the switching in the schematic has a number corresponding to the switch number in the schematic, ie: S2, S3, S4 etc, you will notice that in the diagram that the curve that they relate to also has another contact that isn't labeled, that contact is the wiper for those contacts on that curve, in other words it's common to those switches as depicted in the schematic.

As an example for switch S8 look at switch drawing number 2, switch 8's contacts as shown in the schematic are the one at 12 o'clock and the one at 10 minutes before 12 o'clock.

Lawrence.

[crackle]

Originally Posted by DonaldStott

On the assumption that turned fully anti-clockwise is the Gram position then two clicks clockwise would be LW.

Yes to question
The Wave Band knob should have Gold, Red & Green dots for the MW, LW, and FM bands. The Blue dot is the gram position.
Looking at the knob from the right of the radio then turning fully clockwise should be MW (yellow dot), then Next click is LW (red Dot) then next click is FM (green dot)
The odd thing is the colours on the dial do not go in the same order as the coloured dots on the waveband knob.
Do you have the little gold pip which indicated which colour you are on or is it missing.

One point to watch is dont strain the knob if it is very stiff on these sets the plastic can go very brittle and crack. If the shaft of the knob does crack it can be repaired by building up a good layer of good quality epoxy around the shaft. I dont use quick setting araldite I find it useless and does not set fully hard.
Mike

[crackle]

If you have not yet changed all the Hunts capacitors then voltages will be down all over the radio due to excessive current being drawn on the HT.
I generally find that most resistors with the exception of the cathode resistors are nor especially critical if they have wandered in value a little. But a leaky capacitor can draw excess DC current and drag down the screen and some other voltages quite considerably.

Mike

[DonaldStott]

Originally Posted by crackle

The Wave Band knob should have Gold, Red & Green dots for the MW, LW, and FM bands. The Blue dot is the gram position.
Looking at the knob from the right of the radio then turning fully clockwise should be MW (yellow dot), then Next click is LW (red Dot) then next click is FM (green dot)
The odd thing is the colours on the dial do not go in the same order as the coloured dots on the waveband knob.
Do you have the little gold pip which indicated which colour you are on or is it missing.

Interesting but my Wave Band knob has lettering that says VOLUME while the on/off switch tone control says OFF-TONE: -



The knobs at the front of the set for tuning and volume are smaller and have the gold inserts: -



I do have the little gold pip which, in addition to the function you mention, appears to have been used to connect the internal plate aerial on the inside of the set.

[ms660]

Originally Posted by DonaldStott

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

The Trader sheet is also the same as the manufactures manual so far as the switch positions go, in the switch tables in the Trader sheet the left most column is the fully anti-clockwise position of the selector switch and the right most column is the fully clockwise position so going from fully anti-clockwise to fully clockwise = Gram, FM, LW, MW.

The Trader sheet switch system is easy to understand once you've done a few and are familiar with what's what, it makes following the switching paths much easier than what's shown on a lot of manufactures schematics.

Lawrence.

[paulsherwin]

The original knobs are all the same and either cream or brown depending on the case. They are very fragile and frequently break, so it's quite common to find MR10s with replacement knobs. I suspect *all* the knobs pictured in #24 are replacements.

[crackle]

The volume and tuning knobs in tour photo look original. But you dont often see these radios with all their original knobs. For the reasons stated earlier they do get weak and crack if the wave change switch gets stiff or is is forced on the end stops.
Here are a couple of photos of the wave change knob.


I always smear a little capacitor wax onto the spindle and into the hole of the knob to help making removing them and replacing easier.

Mike

[DonaldStott]

Thanks Mike - good to know that my volume and tuning knobs are originals.

As for the wave band and on/off switch tone control knobs I'll stick a Post in the Wanted section in the hope that I get lucky.

I note that the wave band knob has coloured dots so just wondering if there were any markings or lettering on any of the other knobs?

[DonaldStott]

Some initial readings for the Valve pin voltages: -



All values seem to be on the low side even though the Rectifier is chucking out a healthy 184V D.C. As Mike (crackle) advised in Post #23 above "If you have not yet changed all the hunts capacitors then voltages will be down all over the radio due to excessive current being drawn on the HT."

Clearly something not quite right with the 12AT7 (V1) at the FM stage - but we'll tackle that in due course.

So several Hunts Mouldseals to replace and some cathode resistors to check.

I'll probably replace that big electrolytic as well C37, 2μF

[DonaldStott]

Making steady progress with replacing out of tolerance resistors along with the waxy paper capacitors and Hunts Mouldseals. Replacing one at a time and testing each time just in case something goes wrong but so far so good!

The tone control has had several doses of DeOxit D5 Contact Cleaner but you can only turn it about one third of the way round before the audio breaks up and you get a hum! I'm thinking that there may be mechanical wear issues rather than just needing to clean the internal track and contacts - advice please?

I'm also still struggling with the fact that there is no schematic that matches the components I have in this set. I've already mentioned the replacement of the 7-pin 6BE6 with a 9-pin Mullard ECF82 but there are components in this set not showing in any schematic and similarly there are components missing that are shown in the schematic.

One example is the 47kΩ resistor shown in the middle of the picture below - anyone shed some light on what this is: -



In Post #9 above I mentioned the underside of V2 which is the 9-pin Mullard ECF82 and how busy things were at that location. This next picture shows this in more detail where we have three Hunts Mouldseals and two resistors (22kΩ and 220Ω) but I cant seem to relate these to the schematic?

[crackle]

Have you got the KB service manual from my web site, have a look at page 8, this should cover the version you have.
The anode resistor for the V2 6U8 (ECF82 in your case) pentode section is shown as 47K, By the way all the voltages on that circuit diagram are what I actually measured on my radio, I added them to the schematic just as a reference point.
I tend to not worry too much about resistors at this stage, until after I have replaced all the horrible hunts. Then change what's required after checking the voltages.

Mike

[ms660]

Originally Posted by DonaldStott

In Post #9 above I mentioned the underside of V2 which is the 9-pin Mullard ECF82 and how busy things were at that location. This next picture shows this in more detail where we have three Hunts Mouldseals and two resistors (22kΩ and 220Ω) but I cant seem to relate these to the schematic?

220 ohm = mixer cathode bias....R32

22kohm = oscillator grid leak....R35

The three capacitors...clockwise..

1) In the circuit the position of C45 ?

2) In the circuit position of C44 ?

3) In the circuit position of C15 ?

Circuit refs are per the manufactures manual.

It should be easy for you to see what they are connected to....pin numbers etc...let the forum know.

Lawrence.

[DonaldStott]

Thanks Mike and Lawrence for the helpful feedback.

I had trouble relating the schematic on page 8 with the View Under Chassis on page 9 of the KB Museum service manual - realised that not all components are shown on page 9.

Had a long hard look at the circuitry around V2 and what I have in my set. To assist myself I've highlighted in RED in the schematic below the capacitors and resistors in the vicinity of V2.

[DonaldStott]

Trying to unpick the connections on the underside of V2 was quite challenging but this annotated picture (labels and values correspond to the KB Museum service manual) is what I have surmised so far :-



This is where I think the connections are: -

C45 (0.01μF) Pin 1 to Pin 7
C44 (0.01μF) Pin 1 to Pin 3
C15 (1000pF) Pin 3 to Ground

C46 (10pF) Pin 8 to Pin 3 of V3
C12 (410pF) Pin 9 to S2

R39 (22kΩ) Pin 9 to Ground
R33 (220Ω) Pin 7 to Ground

I'm struggling a bit with R31 (15kΩ) which I think is connected to Pin 3 and S1 - assuming that it is the one identified in the above picture just visible on the other side of the metal dividing plate in the FM stage?

Now on the assumption that the above is correct I need to replace the Hunts Mouldseals along with R33 and R39, both of which are at least 25% above their stated values, in circuit of course.

As it's a real guddle of connections in there what's the best practice advice on how to un-solder the old components, solder the new ones and in what order - not really enough space for the usual spirals I've used elsewhere?

[crackle]

The component layout on page 9 is a part of the original service manual and so only refers to components on the original schematic.
I included the updated schematic issued the following year, but unfortunately there was no component layout drawing to go with it.
Maybe it would have been better if I had included the updated schematic and put it at the end of the document. But that concerned me that it may not have been noticed.

Mike

[crackle]

I have taken a photo of the underside of the chassis of my radio.

If the resistor you have arrowed as R39 is the brown green brown then I believe it is R29. The 10k resistor behind this area is odd, I cant find it on the schematic at all.

Here is a photo of the whole under chassis showing the components.

By the way I found it much easier fitting the chassis back in the cabinet if the cabinet is laid down on its front. I replaced the hex head bolts with philips type screws, I believe they are 2BA, a blob of wax or bluetac on the screw head helps hold it on the screw driver.

Mike

[ms660]

The component that's being pointed to in Post#34 looks like one end of R31, the oscillators anode feed/load.

One end to tag 1 of the valve holder the other end to the rear of S1 (contact 8)

Lawrence.

[DonaldStott]

Originally Posted by crackle

If the resistor you have arrowed as R39 is the brown green brown then I believe it is R29. The 10k resistor behind this area is odd, I cant find it on the schematic at all.

Thanks Mike - the resistor I have arrowed is R31 and according to the schematic should be 15kΩ - brown-green-orange.

As Lawrence has correctly pointed out this connects Pin 1 on V2 to to the rear of S1 (contact 8) - thanks for that emendation.

R31 is shown more clearly in this picture - only seems to have two bands, green and orange??



The other brown-green-brown resistor shown in both our pictures is indeed R29 (150Ω) and this is connects L5 in the 1st FM.IF to the rear of S1 (contact 12).

Mike - I have a good stock of metallised polyester film capacitors (630V) that I would normally use to replace old waxy and paper types, including the Hunts. I noticed that you have used different types of capacitors to replace C15, C44 and C45.

Any reason for this and what replacement types have you used - thanks again?

[ms660]

Originally Posted by DonaldStott

R31 is shown more clearly in this picture - only seems to have two bands, green and orange??

Read as in the body, tip dot system:

Body....Brown....1

End band (tip)....Green....5

Middle band (dot)....Orange....000

Other end band not there so tolerance = +-20%

Lawrence.

[DonaldStott]

Thanks Lawrence - every day is indeed a school day and today I've learnt about the body, tip dot system!

"The 'body-end-dot' or 'body-tip-spot' system was used for radial-lead composition resistors sometimes found in vacuum-tube equipment; the first band was given by the body color, the second band by the color of the end of the resistor, and the multiplier by a dot or band around the middle of the resistor. The other end of the resistor was colored gold or silver to give the tolerance, otherwise it was 20%"

Main outcome is that the resistor R31 should be 15kΩ