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Old 9th Oct 2019, 6:44 am   #101
crackle
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Default Re: Kolster-Brandes MR10

Yes agreed, it looks like a new bridge rectifier is now needed.
I would suggest the addition of a 100 or 150 ohm 3 watt resistor in series before the new rectifier.
Mike
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Old 26th Oct 2019, 5:25 pm   #102
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
I would suggest the addition of a 100 or 150 ohm 3 watt resistor in series before the new rectifier.
Hi Mike - any circuits I've seen for a rectifier bridge with four silicon diodes all show the series resistor after the new rectifier?
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Old 26th Oct 2019, 6:35 pm   #103
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Default Re: Kolster-Brandes MR10

In this case I dont think it will make much difference, you could put it before or after.

Mike
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Old 26th Oct 2019, 9:35 pm   #104
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Default Re: Kolster-Brandes MR10

Thanks Mike - just checking in case there was a difference in the KB-MR10.
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Old 27th Oct 2019, 3:16 am   #105
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Default Re: Kolster-Brandes MR10

The current through a rectifier is the same on the input as the output.
(Because it is current.)
This caused a heated discussion on a newsgroup several years ago with people arguing that it was AC on the input, but DC on the output...
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Old 29th Oct 2019, 10:46 am   #106
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Default Re: Kolster-Brandes MR10

Yes, but the resistor doesn't care which way up the current pulses are and will have the same effect regardless (and dissipate the same power).
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Old 29th Oct 2019, 11:05 am   #107
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Default Re: Kolster-Brandes MR10

Thanks to Mike, MildMartin and Chris - understood!

Getting this Thread back on track after a sojourn in Spain ...

With the help of David G4EBT I've used a short length of turret board to construct a replacement rectifier bridge with four UF4007 silicon diodes and a series resistor.

I'll be removing the existing rectifier today and fitting the replacement - results to follow later.
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Old 29th Oct 2019, 9:55 pm   #108
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Default Re: Kolster-Brandes MR10

First results with the replacement rectifier bridge - four UF4007 silicon diodes and a 150‎Ω series resistor:-

Input: 202V AC

Output: 235V DC


Success?? I'm already picking up a lot more stations on FM.

On FM the voltage drop across the series resistor is 10.8V and thanks to George Ohm we can calculate that the current is 0.072A (72mA). Multiplying the current times the voltage we get 0.072 x 10.8 = 0.77W.

On AM the voltage drop is 10.6V so the current is 0.070A (70mA). Multiplying the current times the voltage we get 0.070 x 10.6 = 0.74W.

These current values are almost spot on for this set and the wattages are well within the capabilities of the 3W 150‎Ω series resistor?

As a consequence the HT is up right across this set which seems to be performing much better - still a bit quiet on FM but this KB-MR10 is now out of Intensive Care.

Any critical valves/pins I need to check out - most values are up by 10-15% on what was measured previously.
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Old 30th Oct 2019, 8:17 am   #109
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Default Re: Kolster-Brandes MR10

excellent
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Old 30th Oct 2019, 10:20 am   #110
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Default Re: Kolster-Brandes MR10

Be wary about that power calculation. The current through and hence voltage across the resistor will have a very peaky waveform which may have an rms value rather higher than a typical meter will indicate.

That's why it's recommended to fit a resistor of around twice the wattage rating than a simple sum suggests.

If you have a floating 'scope (battery operated for example) you can look (carefully!) at the waveform and see.
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Old 30th Oct 2019, 10:24 am   #111
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Default Re: Kolster-Brandes MR10

I also checked the replacement V1, a 12AT7WA kindly supplied by Forum member David G4EBT as this set was completely silent on FM previously.

Pin 1 - Anode (a) 168V
Pin 6 - Anode (b) 160V
Pin 3 - Cathode (a) 1.14V

Slightly down on what they should be so I'll go back and do a final check on all components in the FM stage shown in the picture below but I have been wary of replacing anything here for fear of upsetting the alignment.

There is a Hunts cap in the middle left which may need replaced - anything else that's obvious?

Click image for larger version

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Also with V1 we have: -

Grid Voltage: -1.13V
Cathode Voltage 1.13V

Resistors across the set will need a final sweep as I seem to remember that some of them were marginal.

In Post #76 above I mentioned issues with the Tone Control which is very bright and harsh with a lot of sibilance! You can only turn the control a very small way round before the audio becomes too harsh to listen to! Don't really want to think about replacing this switch/pot so what are my options to alleviate this issue? From the Trader Service Sheet 1233 I note that between the Anode of V5 and the Output Transformer we have R23 (4.7kΩ) and C44 (0.01μF) in series. Would increasing the value of any of these help - if so, what values would be recommended??

On the home straight now, hopefully ...
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Last edited by DonaldStott; 30th Oct 2019 at 10:46 am. Reason: Typo
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Old 30th Oct 2019, 10:36 am   #112
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by Herald1360 View Post
Be wary about that power calculation. The current through and hence voltage across the resistor will have a very peaky waveform which may have an rms value rather higher than a typical meter will indicate.

That's why it's recommended to fit a resistor of around twice the wattage rating than a simple sum suggests.

If you have a floating 'scope (battery operated for example) you can look (carefully!) at the waveform and see.
Thanks Chris - noted.

I'm conscious of the simplistic power calculations in my Post which gave 0.77W on FM and 0.74W on AM but I have fitted a 3W resistor which should suffice?

I'm not sure what a "floating 'scope" is but it gives me something else to go off and investigate!
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Old 30th Oct 2019, 12:44 pm   #113
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Default Re: Kolster-Brandes MR10

Quote:
I'm conscious of the simplistic power calculations in my Post which gave 0.77W on FM and 0.74W on AM but I have fitted a 3W resistor which should suffice?
It's all to do with the conduction period of the rectifier and the current during that conduction period that's demanded by the load as well as that demanded by the partially discharged reservoir capacitor which supplied the load when the rectifier wasn't conducting, remember that the conduction period of a rectifier into a capacitor filter (the reservoir capacitor) is much less than the +ve half cycle period.

From that it can be deduced that the current flowing through the rectifier (and its series resistor) will be greater than that of the DC load current and therefore the power dissipated in the series resistor will be much greater than the figure calculated using the value of the DC load current alone.

Rough and ready guide for calculation purposes is to assume that the current flowing through the rectifier is approx. 2.2 times the DC load current.

Eg: DC load current = 0.07 amps, 0.07*2.2 = 0.154 amps, R = 150 ohms therefore Pdiss = 3.5 watts, if it were me I would fit a 5 watt job.

Lawrence.
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Old 30th Oct 2019, 3:39 pm   #114
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Default Re: Kolster-Brandes MR10

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Originally Posted by DonaldStott View Post
I'm not sure what a "floating 'scope" is but it gives me something else to go off and investigate!
Simply, an oscilloscope whose input is not connected to mains Earth. So, fitted at the factory with a two-core power lead; or battery powered.

Note, though, that any exposed metalwork on the 'scope will be connected to the mains .....
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Old 30th Oct 2019, 4:44 pm   #115
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Default Re: Kolster-Brandes MR10

Hi. This is what I am going to do with my MR10 receiver, the thread has motivated me to get going with my project (i.e replace the metal bridge rectifier with four silicon 1n4007) plus a resistor. I wasn't too sure on the wattage, although I have plenty of the 3 watt power resistors that I bought from Maplin earlier.
The original electrolytic capacitors had been replaced by 300 volt types.
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Old 30th Oct 2019, 5:21 pm   #116
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Default Re: Kolster-Brandes MR10

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... the thread has motivated me to get going with my project (i.e replace the metal bridge rectifier with four silicon 1n4007) plus a resistor.
Good to hear my Thread has been of some value - keep us posted on progress.
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Old 30th Oct 2019, 5:32 pm   #117
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Default Re: Kolster-Brandes MR10

Hi. Many thanks for that. I must admit it is a great looking set. It's a shame that it receives more negative than positive reports with regards to performance from collectors. Can not remember where I bought this from.
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Old 30th Oct 2019, 7:55 pm   #118
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by telstar View Post
It's a shame that it receives more negative than positive reports with regards to performance from collectors. Can not remember where I bought this from.
I have said it before, my sets performs nicely, plenty of volume on FM and AM, and they still have their metal rectifiers.
I guess it really depends on where you live. But I dont think it is fair to blame KB for making inferior sets. The circuit design is not that far removed from the Bush VHF70 and the performance is comparable.

Of course I'm not biased.

Mike
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Old 31st Oct 2019, 2:52 pm   #119
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Default Re: Kolster-Brandes MR10

I am not blaming KB for making inferior radios, it is just from what I have heard from other people about various models made by many companies.
When I worked at Comet (After sales) in Leeds, I took my K.B RB10 to show various engineers, told them about my childhood memories of the original. Most were really impressed, but you had to have some people that "turned their noses up" at it.
I really enjoy working on all radios from manufacturers worldwide, and take them all on their merit.
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Old 5th Nov 2019, 9:37 pm   #120
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Default Re: Kolster-Brandes MR10

I was on the home straight with this KB-MR10 but have suffered a setback!

Replaced the Hunts capacitor and the waxy paper capacitor (in the "FM section") along with R5, the V1 H.T. feed, and this has had the effect of boosting the audio volume - now quite acceptable!

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?

Replaced some of the other over-tolerance resistors i.e. +20% or more - good practice for my floating spiral connections! And of course the 150Ω series resistor in the new rectifier bridge has been upgraded to a new 5W resistor (thanks Lawrence).

This is where it has all gone awry - I replaced R17 (10MΩ) as it was reading 13.2MΩ and then Long Wave and Medium Wave stopped working - aargh! I seem to have disturbed a number of connections to and around V4 but (as usual for me) I am having problems relating what I see in the schematic to what is physically present in this set.

Can someone confirm where the two ends of R17 should be connected - I can see from the schematic that one end is connected to C32 (0.05μF) and then to Pin 8 of V4 while the other end is connected to earth, but where are these connections physically??

Similarly there is a co-axial cable that runs from the volume control to the vicinity of V4 - again not sure where these two connections (inner conductor and shield) should go physically??

I'm assuming that this is where the problem lies as FM is still working - remarkably!

Almost there ...
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