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Old 5th Nov 2019, 9:49 pm   #121
ms660
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Default Re: Kolster-Brandes MR10

R17 physical connections....third photo from Post#9:

https://www.vintage-radio.net/forum/...6&d=1567428993

Ie: between pins 8 & 7 of the EABC80 valve holder.

Pin 6 is the AM detector.

Lawrence.

Last edited by ms660; 5th Nov 2019 at 9:55 pm.
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Old 6th Nov 2019, 11:20 am   #122
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Default Re: Kolster-Brandes MR10

Thanks Lawrence - confirmation that I have R17 wired up properly i.e. physically connected between pins 8 & 7 of the EABC80 valve holder.

The inner conductor of the co-axial from the volume control is connected to the tag strip below and between V4 and V5 in the third photo from Post#9 but the shield has become disconnected - where should that go physically?? Looking at the schematic I think that this should also be connected to pin 7 of V4?

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Old 6th Nov 2019, 12:11 pm   #123
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by DonaldStott View Post
Thanks Lawrence - confirmation that I have R17 wired up properly i.e. physically connected between pins 8 & 7 of the EABC80 valve holder.

The inner conductor of the co-axial from the volume control is connected to the tag strip below and between V4 and V5 in the third photo from Post#9 but the shield has become disconnected - where should that go physically?? Looking at the schematic I think that this should also be connected to pin 7 of V4?

Attachment 193173
Hard to tell from the photo's, could be pin 7 or chassis....First lesson of forensics...every contact leaves a trace.

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Old 6th Nov 2019, 1:38 pm   #124
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Default Re: Kolster-Brandes MR10

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Originally Posted by ms660 View Post
Hard to tell from the photo's, could be pin 7 or chassis....First lesson of forensics...every contact leaves a trace.
Agreed - there appears to be a connection from pin 7 to the chassis - really confused now?

Would be helpful if someone with a working KB-MR10 could comment?
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Old 6th Nov 2019, 2:53 pm   #125
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Default Re: Kolster-Brandes MR10

In Post #36 above Mike (crackle) helpfully posted this link to a photo of the whole under chassis showing the components; -

http://www.kbmuseum.org.uk/kb_images...er_chassis.jpg

Much clearer than my own picture but that co-axial screen connection is still elusive?
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Old 6th Nov 2019, 6:00 pm   #126
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Default Re: Kolster-Brandes MR10

I have not specifically checked but screens do not always need to be connected at both ends, where does the other end connect?

Mike
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Old 6th Nov 2019, 7:34 pm   #127
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Default Re: Kolster-Brandes MR10

The co-axial cable connects with the volume pot - it's the cable with a bit of blue tape for identification purposes.

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The inner conductor and screen of both co-axial cables are connected at the volume pot.
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Old 7th Nov 2019, 7:45 am   #128
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Default Re: Kolster-Brandes MR10

Is the radio working without any excessive hum now that you have replaced R17. If so dont worry about the screen.
Is the other coax cable screen connected to chassis? Only one of these screens need be connected to the chassis, if both are connected to different parts of the chassis then sometimes it can introduce an earth loop which can cause problems with hum.

Mike

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Old 7th Nov 2019, 8:03 pm   #129
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Default Re: Kolster-Brandes MR10

No excessive hum from the set.

I note that both the co-axial screens are connected at the volume pot with one of these going to chassis.

All voltages for V4 are in keeping with those on the KB Museum Service Data sheet.

When I first started this restoration I had fairly decent AM and no FM, now I have no AM but very good FM - especially now that I've rigged up my ribbon cable FM aerial!
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Old 8th Nov 2019, 1:35 pm   #130
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Default Re: Kolster-Brandes MR10

Here's one for Lawrence, many moons ago in a Thread Far, Far Away you Posted: -

"The best servicing tool you have is knowledge, that's the skill, you've either got it or you ain't, if you ain't then go get some, that's my tip of the day."

As you hopefully appreciate I have been gathering knowledge as my skills have slowly improved over the years but sometimes people like myself just get stuck!

I thought I had been careful to try to follow the rule to replace and test components one at a time but may have got a bit lax towards the end as I had both FM and AM working very well. "Just a few over tolerance resistors to replace and that's us" or so I thought - oops!

I will of course dig out my Velleman K7000 Signal Injector/Tracer and attempt to pin point where the problem lies, but as usual any advice or guidance would be welcomed.
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Old 8th Nov 2019, 2:56 pm   #131
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by DonaldStott View Post
Here's one for Lawrence, many moons ago in a Thread Far, Far Away you Posted: -

"The best servicing tool you have is knowledge, that's the skill, you've either got it or you ain't, if you ain't then go get some, that's my tip of the day."
You're correct, I did...it still stands.

Post up the results of your checks and tests first.

Lawrence.
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Old 8th Nov 2019, 4:14 pm   #132
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Default Re: Kolster-Brandes MR10

if the screens on the earthy end of the volume pot are connected to ground then I dont think the hum is caused by a screen problem, and besides it would effect FM and AM equally.
Switch to gram and see if the hum goes, use your signal injector to put an audio tone into the gram socket socket and then into the "hot" side of the tone control track on the grid to the EL84.

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Old 8th Nov 2019, 5:27 pm   #133
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by crackle View Post
if the screens on the earthy end of the volume pot are connected to ground then I dont think the hum is caused by a screen problem, and besides it would effect FM and AM equally.
Switch to gram and see if the hum goes, use your signal injector to put an audio tone into the gram socket socket and then into the "hot" side of the tone control track on the grid to the EL84.
There are no hum issues with this set - I had fairly decent AM and no FM, now I have no AM but very good FM?

Trying to locate my Velleman K7000 Signal Injector/Tracer ...
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Old 9th Nov 2019, 6:38 pm   #134
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Default Re: Kolster-Brandes MR10

Managed to find my Velleman K7000 Signal Injector/Tracer along with the excellent Automatic AF/RF probe constructed for me by Forum member David G4EBT.

As I still have FM but no AM then it seems like the audio stage is working. So I'm working back from there to find where/why the AM signal is not getting through?

Did some basic checks with the Signal Injector and got a loud tone at the middle tags of both the Treble and Volume pots. Also a loud tone on Pin 2 (Control Grid) of V5 (EL84) and on the gram socket when switched to Gram.

I was also advised to check R17 which is 10MΩ to ensure I had connected this new resistor correctly and I have 10.2MΩ between Pin 8 of V4 (EABC80) and the chassis.
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Old 9th Nov 2019, 8:08 pm   #135
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Default Re: Kolster-Brandes MR10

Those injector tests confirms that the audio section is working, as you already know.

You need to use the tracer function of the Velleman K7000 to check for AM signals.

With your tracer probe on the centre tag of the volume control, do you get tunable AM stations? You should do. If you don't hear AM signals at the centre tag, could you have disturbed the wiring around the rear wafer (S2 on the KB Service Sheet)? Check for a signal from the 2nd IF where R14 (100k) on the KB Service Sheet joins point 2&3 on the rear switch wafer. If there's a signal there, but not at the volume control, that suggests problems around the 'switchery'.

I must say that I find the unconventional style of the KB circuit hard to follow as compared to the Trader Sheet.

On the Trader Sheet rather than the KB circuit, you can see more clearly that V4a - the AM detector diode, rectifies the IF signal, picking off the AM signal which should appear at the junction of R16/R17, before R17 goes to the switch wafer, S12. If you don't hear AM signals there, that suggests that the AM detector isn't working. If you hear AM signals at the junction of R16/17 (Trader Sheet!), but not at the volume control, that could point to a problem with S12.

I think signal tracing give you the best chance of finding where the AM signal is being lost. The best tutorial I've come across is at the link below. It's a circuit of a conventional 'AA5' ('All American five tube') AM superhet tube/valve radio. The instructions tell you to click on the circuit, then hover your cursor at different points - A, B, C etc. Click on any of those points and text will appear to tell you what you should hear (or not hear in some cases) from your signal tracer at each point, and if you don't, it suggests where the fault may lie with that stage of the radio:

http://www.nostalgiaair.org/Referenc...ost/post01.htm

Given that the set was working fine till you change the 10Meg resistor, you need to discover what you've disturbed to cause the AM section to stop working. Could you for example have inadvertently caused any wires to short out in the scary rats' nest of wiring around the valve-holder? (Which I only view from behind the sofa!

These are just some 'off the top' thoughts, but I'm not at the top of the food chain so others may have some better ideas.

None of this makes any sense of course without both the KB circuit and Trader Sheet 1233 in front of you.

Hope it might help a bit.

Good luck with it!
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Old 10th Nov 2019, 7:42 am   #136
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Default Re: Kolster-Brandes MR10

Let's not forget that this could easily be just a poor connection in a valve base, for instance a broken fork in the valve base. Gently wiggle the valves V2 V3 and V4 whilst the radio is on and tuned to MW with the volume up.
It is normal under these circumstances to establish if the AM oscillator V2 is working.
This can be done using another transistor radio close to the MR10, and see if any interference can be heard in the transistor when you tune through the MW band on the MR10.

Mike
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Old 10th Nov 2019, 6:23 pm   #137
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Default Re: Kolster-Brandes MR10

Just a quick update to let you all know that I have resolved the missing reception on AM - both LW and MW are back!

As I had all three wavebands working previously, checking there was nothing I had done subsequently that appeared to be connected incorrectly in any way and that the AF stage was working I concluded that the problem must lie elsewhere.

I wasn't getting anywhere with David's tests using my Velleman K7000 to check for AM signals - nothing at any of the points indicated in the helpful link to Nostalgia Air: Signal Tracing.

Then I tried Mike's suggestion to try another transistor radio close to the MR10 - there was no interference in the transistor when I tuned through the MW band on the MR10.

Conclusion - there was something amiss with the AM oscillator at V2!

Those with patience and a good memory may remember that I had some difficulties understanding and replacing components on the underside of V2 and how busy things were at that location. I had replaced three Hunts Mouldseals and two resistors here:

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

R32 (220Ω) Pin 7 to Ground
R39 (22kΩ) Pin 9 to Ground

Only problem was that R39's connection to Ground had worked its way loose and was floating a fraction of an inch above the chassis joint - probably a poor solder joint at this location () as it shares this chassis connection with C15 and R32. Fortunately there is a rivet that comes through the chassis at this location and this was used as a new Ground connection for R39 - what a nice, bright solder joint we now have! V2 now oscillating away nicely on AM while FM reception is still good.

So there we have it - got there in the end with a bit of methodical detective work which was greatly aided and abetted with support from the Forum.

Thanks again to everyone -we're almost there!
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Old 10th Nov 2019, 9:36 pm   #138
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Default Re: Kolster-Brandes MR10

Phew, that will be a relief - a high note on which to end the week - onwards and upwards!

Just a point about placing another (working) radio close to the MR10 to check if the oscillator is running or not by listening for the non-working set's heterodyne on the working radio, doesn't the second radio need to be on the same IF frequency? The IF frequency of the MR10 is a bit odd, in that it's 422 kHz, whereas most UK post-war radios tend to be 465kHz (EG Bush DAC90A) or 470kHz (EG Bush TR82).

Now that the MR10 is working again on AM as well as FM, if only out of curiosity and a learning experience (for me, at any rate!), it might be worth tuning the MR10 across the AM bands to see if - despite the second radio placed next to the MR10 having a 465/470 kHz IF - nevertheless, a heterodyne is heard on the second radio.

As to 'not getting anywhere with the Nostalgair tests and your signal tracer', in fact you were, because the tests showed that the oscillator wasn't working, just as if - for example - you put an Ohmmeter across a fuse and 'get no reading', it proves that the fuse has blown.
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Old 10th Nov 2019, 10:32 pm   #139
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by David G4EBT View Post
Phew, that will be a relief - a high note on which to end the week - onwards and upwards!

Just a point about placing another (working) radio close to the MR10 to check if the oscillator is running or not by listening for the non-working set's heterodyne on the working radio, doesn't the second radio need to be on the same IF frequency? The IF frequency of the MR10 is a bit odd, in that it's 422 kHz, whereas most UK post-war radios tend to be 465kHz (EG Bush DAC90A) or 470kHz (EG Bush TR82).

Now that the MR10 is working again on AM as well as FM, if only out of curiosity and a learning experience (for me, at any rate!), it might be worth tuning the MR10 across the AM bands to see if - despite the second radio placed next to the MR10 having a 465/470 kHz IF - nevertheless, a heterodyne is heard on the second radio.

As to 'not getting anywhere with the Nostalgair tests and your signal tracer', in fact you were, because the tests showed that the oscillator wasn't working, just as if - for example - you put an Ohmmeter across a fuse and 'get no reading', it proves that the fuse has blown.
The IF of the 2nd receiver is irrelevant, what matters is that the 2nd receiver can be tuned to the oscillator frequency of the receiver being tested, the oscillator frequency of the receiver being tested should, if it's working normally, be the sum of the receivers IF and the frequency that the receiver is tuned to, if there are no interfering frequencies the oscillator frequency of the receiver being tested should be received by the 2nd receiver as a blank carrier just like an unmodulated carrier from a signal generator would be received, after all that's what the receivers oscillator is in effect if you think about it.

Lawrence.

Last edited by ms660; 10th Nov 2019 at 10:38 pm. Reason: word shuffle
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Old 10th Nov 2019, 10:39 pm   #140
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Default Re: Kolster-Brandes MR10

Quote:
Originally Posted by David G4EBT View Post
doesn't the second radio need to be on the same IF frequency?
I am not sure that is the case, I believe what you are listening for on the second radio is the frequency generated by the first radios oscillator, not the resultant IF of the first radio. But I can never remember if this is higher or lower than the IF.
Perhaps someone will explain it to me again.

Good work for locating and fixing the problem.

thanks
Mike.
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