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Old 6th Oct 2019, 11:54 am   #81
DonaldStott
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Default Re: Kolster-Brandes MR10

I've used the voltage chart on page 11 of the KB service manual previously and you will see my valve pin readings in a couple of Posts above. My specific point related to Mike's (crackle) annotations on the schematic where there is nothing shown for V1 Pin 3 (cathode). I appreciate that page 11 of the KB service manual states that this should be 1.0V.

Rechecking all my voltages it is clear that the HT is low right across this set and this can be traced back (?) to the output from the Westinghouse rectifier which is only 181V DC from an AC input of 205V. The KB service manual states that this should be 235V DC, Mike's annotation shows 218V DC while the Trader Service Sheet 1233 states 225V DC. So I'm at least 20% down on what the rectifier DC output should be - as David G4EBT has advised me "The AM and FM frequency changer must be close to losing oscillation."

So I'm going to focus my attention on the low HT and replacing the rectifier - my options would appear to be to install four 1N4007 silicon diodes or a modern solid state bridge rectifier?

Looking more generally at the low volume on FM I remembered some advice from a previous restoration from boxdoctor (Tony) that: -

"Once the signal strength is enough to operate the limiter action in the I.F.amplifiers, the volume of the output will not materially increase further, regardless of any further increase in input signal. The proof of the effectiveness of the aerial will ultimately be revealed on the improvement to weaker stations, which will show mainly as an improvement in signal-to-noise ratio, i.e. less background 'hiss'."

- interesting?
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Old 6th Oct 2019, 12:32 pm   #82
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Default Re: Kolster-Brandes MR10

It's normal to have to replace the metal rectifier in these. You can use any suitably rated bridge, with either 4 separate diodes or a dedicated bridge. Use a 100-300 ohm resistor in the HT line to limit the switchon surge and reduce the HT voltage. You may want to experiment with the value so that the HT is absolutely spot on, but it's not really necessary.

Ideally you would use fast rectifiers such as UF4007s in the bridge to minimise noise, but bog standard 1N4007s will probably be fine.
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Old 6th Oct 2019, 1:32 pm   #83
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by paulsherwin View Post
Use a 100-300 ohm resistor in the HT line to limit the switchon surge and reduce the HT voltage. You may want to experiment with the value so that the HT is absolutely spot on, but it's not really necessary.
Thanks Paul - difficult to be spot on when we have: -

KB service manual: 235V DC,
Mike's annotation: 218V DC
Trader Service Sheet 1233: 225V DC.

Somewhere in the middle is where I'd aim for?

Quote:
Originally Posted by paulsherwin View Post
Ideally you would use fast rectifiers such as UF4007s in the bridge to minimise noise ..
Various versions of the UF4007 of course - would this one suffice: -

https://uk.rs-online.com/web/p/recti...iodes/6715470/
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Old 6th Oct 2019, 1:38 pm   #84
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Default Re: Kolster-Brandes MR10

The HT voltage isn't all that critical, so it looks as if 220-230V would be fine.

The diode in your link should be as good as any. I mentioned the UF4007 as an example of a cheap fast recovery diode that would do the job, but there are many other types that would work just as well.
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Old 6th Oct 2019, 9:09 pm   #85
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Default Re: Kolster-Brandes MR10

You may find it easier to mount a bridge rectifier, for instance one with a hole for mounting direct to the chassis, rather than 4 separate diodes. (bit of an overkill on current wise but that wont do any harm. https://uk.rs-online.com/web/p/bridg...fiers/1656141/)
It is quite possible that the selenium rectifier is not performing correctly, but I have not had to replace one of these in a KB set yet.
The questions I would be asking my self now are;
Can anything else be pulling the voltage down?
Have I replaced all of the paper capacitors including all the crumbling hunts and any of those little double ended bullet shaped ones? (they look similar to resistors and normally seem to be 1nF brown black red, I cant remember if there are any in this radio)
Is V5 cathode resistor correct, and the cathode/grid voltage correct?

Mike
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Last edited by crackle; 6th Oct 2019 at 9:19 pm.
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Old 7th Oct 2019, 7:46 am   #86
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
Have I replaced all of the paper capacitors including all the crumbling hunts and any of those little double ended bullet shaped ones? (they look similar to resistors and normally seem to be 1nF brown black red, I cant remember if there are any in this radio).
Is V5 cathode resistor correct, and the cathode/grid voltage correct?
I'm sure I've seen some of those double ended bullet shaped ones but assumed they were ceramics?

Not sure though how those would affect the voltage back at the rectifier where I am only getting 181V DC?
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Old 7th Oct 2019, 10:54 am   #87
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Default Re: Kolster-Brandes MR10

It's worth trying a replacement diode regardless as they are so cheap and cheerful. You can get 50 x 1N4007 here for 1.49 + P&P https://store.brightcomponents.co.uk...ode-50-pieces/ . I haven't found noise to be a problem in domestic receivers. Cheers, Jerry
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Old 7th Oct 2019, 12:08 pm   #88
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Default Re: Kolster-Brandes MR10

Quote:
It's worth trying a replacement diode regardless as they are so cheap and cheerful.
Agreed Jerry - building a rectifier bridge on strip board with four silicon diodes (1N4007 or UF4007) and a series limiting resistor doesn't look too challenging even for me so I can treat it as an educational project.

It may even fit inside the existing metal rectifier case?
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Old 7th Oct 2019, 12:59 pm   #89
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
Is V5 cathode resistor correct, and the cathode/grid voltage correct?
Looking at the 680k‎Ω cathode grid resistor (R21 in Trader Service Sheet 1233 and R20 in the KB service manual) it is measuring in circuit as 689kΩ.

V5 voltages are: -

Pin 3 Cathode: 3.42V
Pin 7 Anode: 164V
Pin 9 Screen: 153V
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Old 7th Oct 2019, 1:56 pm   #90
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Default Re: Kolster-Brandes MR10

I think Mike was hoping for the actual value of the cathode bias resistor as well as the voltages, that way it can be established by Ohms law what the state of things are with regard to the valve and its effect on the HT voltage etc.

Lawrence.
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Old 7th Oct 2019, 3:16 pm   #91
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Default Re: Kolster-Brandes MR10

Oops, sorry - got my grid and bias resistors confused!

Looking at Trader Service Sheet 1233 I think that the cathode bias resistor is R26 (KB service manual R25) which should be 100‎Ω - this has been replaced and is measuring 99.8Ω in circuit.

The voltage on Pin 2 of V5 is 3.2V.
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Old 7th Oct 2019, 5:01 pm   #92
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Default Re: Kolster-Brandes MR10

If measuring the grid voltage it should be measured between grid (pin2) and cathode (pin3), not chassis.
The result should always be negative with regard to the cathode.
You can also measure the voltage across the cathode resistor, (cathode to chassis), that will normally be the same as the grid voltage but it will be positive with regard to chassis.
Knowing the voltage across the cathode resistor will enable you to get a rough indication of the current through the valve..

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Old 7th Oct 2019, 5:34 pm   #93
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
If measuring the grid voltage it should be measured between grid (pin2) and cathode (pin3), not chassis.
The result should always be negative with regard to the cathode.
Getting there, eventually -grid voltage is -2.97V

Quote:
You can also measure the voltage across the cathode resistor, (cathode to chassis), that will normally be the same as the grid voltage but it will be positive with regard to chassis.
Cathode (Pin 3) to chassis is 3.18V
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Old 7th Oct 2019, 5:46 pm   #94
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Default Re: Kolster-Brandes MR10

If measuring the voltage between the cathode and the grid you need to be aware of the loading effect of the meter, especially with an analog or some of the cheapo DMM's.

Scenario....The chassis to cathode voltage across a 100 ohm resistor is actually say 4.5 volts, but you measure between cathode and grid with a DMM that has a 1meg input resistance, the grid return resistor is say 680k, the voltage indicated will be approx. 2.69 volts (bad enough) With a 20kohm per volt analog meter set to the 25 volt range and measuring between the same two points the voltage indicated will be approx. 1.9 volts With the same meter set to the 10 volt range the voltage indicated will be approx. 1 volt

In reality the actual voltage at the grid would be 4.5 volts....

All circuits are potential dividers:

https://www.electronics2000.co.uk/ca...calculator.php

Lawrence.

Last edited by ms660; 7th Oct 2019 at 5:55 pm. Reason: word swop
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Old 7th Oct 2019, 9:59 pm   #95
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Default Re: Kolster-Brandes MR10

R=100, V=3.2 and I=V/R so the cathode current is 32mA which is very much less than the trader values for combined anode and screen current of 44mA. It is therefore not the EL84 pulling the HT rail down.

It's a long thread and I am not sure if you have replaced the 3 way electrolytic but if you have then this almost certainly confirms the rectifier needs replacing.
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Old 8th Oct 2019, 12:59 pm   #96
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Default Re: Kolster-Brandes MR10

Thanks for your input PJL.

(All references below are to the KB service manual.)

The HT smoothing electrolytics have not been replaced - they are measuring: -

1st electrolytic C39 (30μF) : 181V should be 235V as we already know
2nd electrolytic C37 (20μF) : 159V should be 214V
3rd electrolytic C36 (10μF) : 142V should be 198V

One of the HT smoothing resistors i.e. R22, has been replaced: -

R22 : 1k‎Ω - Measuring 991Ω and 17.8V
R24 : 820Ω - Measuring 902Ω and 21.6V
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Old 8th Oct 2019, 1:38 pm   #97
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Default Re: Kolster-Brandes MR10

The numbers you are measuring seem broadly OK to me, apart from the low HT.
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Old 8th Oct 2019, 1:57 pm   #98
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Default Re: Kolster-Brandes MR10

According to what's been measured the rectifier is on its way out....Vout from the rectifier is less than Vin, could have gone from full to half wave due to being overstressed or an age related internal contact problem.

Lawrence.
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Old 8th Oct 2019, 2:17 pm   #99
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Default Re: Kolster-Brandes MR10

What I did next was to disconnect the HT + connection at the rectifier, and to connect my meter on the DC mA range between the rectifier + tag and the HT lead, so that the meter was in series with the HT line.

On FM : 55mA (should be 75mA)

On AM : 53mA (should be 70mA)
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Old 8th Oct 2019, 2:27 pm   #100
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Default Re: Kolster-Brandes MR10

The rectifier is obviously knackered. The current consumption will be reduced if the HT is low, so your numbers are again plausible.
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