Kolster-Brandes MR10

[DonaldStott]

Just to clarify that the tone is excessively bright and only sounds what I would regard as "normal" at the 'maximum top cut' position.

I appreciate that radios of this period can sound murky and I have had earlier experience of this - not the case with this set?

Hope that makes sense now?

[paulsherwin]

That suggests something is amiss in the tone control circuitry. Did you change the tone control capacitors, and if so are you certain you used the correct replacement values?

[Station X]

Was it ever established whether the original tone control had a log or linear track?

[DonaldStott]

Conscious of the fact that this restoration may attract the attention of the "Thread-Length Police" I'm very eager to push on to a successful conclusion. To save the sanity of Forum members who have contributed here I went back and re-read all the Posts as I had forgotten which capacitors associated with the Tone control had been changed i.e. C39, C43 and C44 (Trader Sheet).

1. This was way back in Post #120 "I tried different values of capacitor across both C43 (0.003μF) Part tone control and C44 (0.01μF) Tone correction but this made no discernible difference, at least to my ears?"

2. And in Post #146 "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 ..."

In relation to Point 2 above I decided to replace the 0.001μF with a 0.022μF as per the Trader Service Sheet. It's also shown as C34 0.02μF on the K-B Museum circuit diagram. Why I didn't try that earlier I'll never know - probably because it appeared to be a K-B factory fit!

What a difference that has made to the tone which has now been, what I would describe as, normalised!

Could this be that successful conclusion - final reassembly over the next few days ... !

[paulsherwin]

Good news. Service data often contains errors, and MR10s are notorious for design modifications during the production run.

[crackle]

I can confirm the Trader sheet and the KB service manuals (both examples) all agree on the values of the tone correction and tone control capacitors.
The 0.001 capacitor you had in the radio would appear to have been a mistake at the factory or changed at a later date.
Capacitors pass higher frequencies easier than low frequencies. So C39, on the trader schematic, being in series with the signal, will have a profound effect on the bass, the .001uf value will filter out (resist) far more bass than the designed .02uf .
The tone control cap C43, in effect in parallel with the signal, will have the opposite effect to C39 in as much as it will filter out the treble as the control is turned clockwise.
A more scientific explanation could be given but the above is my simplistic view of the function of those components.
C44 performs a function to help prevent any unwanted high frequencies or oscillation from occurring in the output valve getting to the speaker.
Mike

[DonaldStott]

Thanks Mike for the explanation which makes perfect sense. I now have a much better understanding of how Tone Control circuits work.

Any advice on how secure the glass tuning scale to the case?

[paulsherwin]

Isn't it held in place by the chassis?

[DonaldStott]

BVWS Bulletin 24/3 Autumn 1999:-



See also my Post #2 above!

Gaffer tape it is then!