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Old 13th Jun 2021, 5:39 pm   #1
Gabe001
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Default KB MR10 sudden fault, help needed

Good afternoon,

I'm just after some help, I've spent a good few hours with my KBMR10 (early version - this one https://www.radiomuseum.org/r/kolsterbr_mr10mr_1.html) which has developed a sudden fault, and I'm a bit stuck

This radio was restored by myself about a year ago and has been in regular use ever since without any issues at all.

The fault is the sudden loss of AM (both bands), and this has been substituted by random loud noises. FM works, but is out of alignment. Pickup is fine and noise free.

After some signal tracing I'm 99% sure that this is due to the presence of around 100v DC on the aerial input grid of V2, pin 7 shown encircled in the picture. v2 is the mixer oscillator 6be6. The green wires come from the tuning cap and, indirectly, from the internal aerial. In fact there are 100v or so (the voltage is very unstable) on the fixed vanes of the tuning cap which I'm sure shouldn't be there. This voltage is prresent when the valve is removed, so the valve itself is blameless.

I can signal trace using the Velleman (using the minimod as a source) crystal clear music on pin 7 of V2 with the radio off, but when I turn the radio on and the voltage starts to dance on that pin everything turns to mush and this garbage is what I hear on the speaker.

I have scrutinized the wires and attachments and the waveband switch for a short but there's nothing obvious.

Does anyone have any tips on how to go about finding where the problem is?

Thanks
Gabriel
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Old 13th Jun 2021, 5:56 pm   #2
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Default Re: KB MR10 sudden fault, help needed

Have you tried removing the mixer valve and disconnecting the wires from 9 and 10 on S2, also the the HT feed to 10 on S1 front?

Lawrence.

Last edited by ms660; 13th Jun 2021 at 6:04 pm. Reason: extra info
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Old 13th Jun 2021, 6:09 pm   #3
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Default Re: KB MR10 sudden fault, help needed

Just a thought,
Valve holder tracking across? V2 Pin 6 has 70 volts on it.
Maybe even V3 causing a problem or another its holder tracking?
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Old 13th Jun 2021, 6:47 pm   #4
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Default Re: KB MR10 sudden fault, help needed

Thank you Lawrence
Removing the mixer valve made no difference.

I haven't disconnected wires yet, I was hoping to avoid this as it's a bit of a cramped space and the wires don't have much "give"- they're all at minimum viable length. Unless anyone can suggest a better way of locating a short I think this is the only way forward - divide and conquer. It's going to be a slow grind. I have a hunch the issue may be at the waveband switch. There's a part of it I can't see properly.

I don't know what you mean by 9 and 10- do you mean L8 and L9?

Gabriel
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Old 13th Jun 2021, 6:55 pm   #5
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Default Re: KB MR10 sudden fault, help needed

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Originally Posted by snowman_al View Post
Just a thought,
Valve holder tracking across? V2 Pin 6 has 70 volts on it.
Maybe even V3 causing a problem or another its holder tracking?
Can this happen? There's no dirt or corrosion. The pin sockets look suitably separated by a thickness of plastic.

I suppose when I disconnect the 2 green wires I will find out
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Old 13th Jun 2021, 7:00 pm   #6
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Default Re: KB MR10 sudden fault, help needed

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Originally Posted by Gabe001 View Post
I don't know what you mean by 9 and 10- do you mean L8 and L9?
The connections to tags 9 and 10 on the rear of S2 as per the manufacturers schematic.....which so far as all functions are concerned appears to be in error on both versions...tag 4 on S2 rear should be elongated so as to make permanent contact with the wiper.....so far as I can make out.

Lawrence.
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Old 13th Jun 2021, 7:33 pm   #7
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Default Re: KB MR10 sudden fault, help needed

I'm wondering if the cathode circuit of the mixer is going high resistance/open circuit and the 100 volts on the grid pin is some low current leakage from the HT side of things.

Lawrence.
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Old 13th Jun 2021, 8:01 pm   #8
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Default Re: KB MR10 sudden fault, help needed

Try disconnecting R7 from L8/L9. That will at least eliminate the possibility of the 100v coming via the AGC line, although I can't see why such a voltage should be there either! Jerry
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Old 13th Jun 2021, 8:31 pm   #9
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Default Re: KB MR10 sudden fault, help needed

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Originally Posted by ms660 View Post
I'm wondering if the cathode circuit of the mixer is going high resistance/open circuit and the 100 volts on the grid pin is some low current leakage from the HT side of things.

Lawrence.
There is measurable resistance between pin 6 and 7, about 30k. This is the same even with the valve removed

I don't know what you mean exactly, Lawrence, could you indicate which components are suspect? I can't see any caps or resistors that could be responsible
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Old 13th Jun 2021, 8:56 pm   #10
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Default Re: KB MR10 sudden fault, help needed

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Originally Posted by cathoderay57 View Post
Try disconnecting R7 from L8/L9. That will at least eliminate the possibility of the 100v coming via the AGC line, although I can't see why such a voltage should be there either! Jerry
Thanks Jerry,

What I will do is disconnect r7, the wire from c16 and the S2 pathway in turn and see what happens

S3 and S4 are inaccessible which could create a problem if the problem lies there. My hunch is a short on the waveband switch in one of the areas I cannot examine properly, probably involving s11 which carries HT to anode. I'm not sure if the waveband switch wafers can be removed and examined. Worst comes to the worst I'll isolate pin 7 and run it as an FM set only.
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Old 13th Jun 2021, 9:44 pm   #11
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Default Re: KB MR10 sudden fault, help needed

Quote:
There is measurable resistance between pin 6 and 7, about 30k
That's not good, and most likely where the 100v is leaking from. If the valve holder is paxolin or similar material, it can carbonize and turn resistive. If that's the case a valve holder replacement is probably necessary. Not sure why it has happened suddenly though. Disconnect the wires from either pin 6 or 7 to make sure you aren't just measuring resistance across the HT supply. Cheers, Jerry
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Old 14th Jun 2021, 7:29 am   #12
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Default Re: KB MR10 sudden fault, help needed

Quote:
Originally Posted by cathoderay57 View Post
Quote:
There is measurable resistance between pin 6 and 7, about 30k
That's not good, and most likely where the 100v is leaking from. If the valve holder is Paxolin or similar material, it can carbonize and turn resistive. If that's the case a valve holder replacement is probably necessary.
This is becoming more common with the age of the sets. Most radios are stored (or have been stored) in less than ideal conditions and dampness plays a major role. Once the valve holder starts to 'track' (leakage between two pins usually where there is the greatest voltage difference) then it gradually gets worse and the tracking causes heat in a localised spot which gradually builds up causing the Paxolin to carbonise until it reaches a point where the leakage affects the operation of the valve.....

The only answer in that case is a new valve holder. Note that it's not only Paxolin valve holders that can track......
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Old 14th Jun 2021, 8:46 am   #13
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Default Re: KB MR10 sudden fault, help needed

Thank you to all those who commented.

I realise I forgot to attach the picture

If it's the valve base I could replace it and I would consider myself lucky. Not sure if the valve base is paxolin - or whether the base I've got can also track. I hope it's not the waveband switch as I'm not sure I can fix that.

I'll pick up where I left off over the weekend. I will know whether it's the valve base once I remove the 2 green wires from pin 7

Gabriel
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Old 14th Jun 2021, 8:56 am   #14
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Default Re: KB MR10 sudden fault, help needed

That valve holder is not made from Paxolin, it's some kind of hard plastic.

Personally I've not had a valve holder like that go leaky.
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Old 14th Jun 2021, 9:24 am   #15
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Default Re: KB MR10 sudden fault, help needed

Hi Gabe, here's a link to my own K-B MR10 restoration Thread:-

https://www.vintage-radio.net/forum/...d.php?t=159373

There may be something there that might help - if you are brave enough to plough through it then you will note that at Post #120 I lost MW and LW (FM was still working) due to a dodgy chassis connection. I do remember this set as having a rat's nest of criss-crossed wires notwithstanding the complicated connection arrangements on the underside of V2.

When restored, however, this set was an excellent performer on both AM and FM.
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Old 14th Jun 2021, 9:38 am   #16
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Default Re: KB MR10 sudden fault, help needed

Quote:
Originally Posted by Gabe001 View Post
Quote:
Originally Posted by ms660 View Post
I'm wondering if the cathode circuit of the mixer is going high resistance/open circuit and the 100 volts on the grid pin is some low current leakage from the HT side of things.
I don't know what you mean exactly, Lawrence, could you indicate which components are suspect? I can't see any caps or resistors that could be responsible
I wouldn't have expected 100 volts +ve on the control grid with the valve in place due to the diode action of the grid-cathode circuit, hence the mention of the cathode circuit, there's not a lot in it to check...resistance/continuity from cathode to chassis.

Lawrence.
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Old 15th Jun 2021, 9:11 am   #17
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Default Re: KB MR10 sudden fault, help needed

Quote:
Originally Posted by Sideband View Post
Quote:
Originally Posted by cathoderay57 View Post
Quote:
There is measurable resistance between pin 6 and 7, about 30k
That's not good, and most likely where the 100v is leaking from. If the valve holder is Paxolin or similar material, it can carbonize and turn resistive. If that's the case a valve holder replacement is probably necessary.
This is becoming more common with the age of the sets. Most radios are stored (or have been stored) in less than ideal conditions and dampness plays a major role. Once the valve holder starts to 'track' (leakage between two pins usually where there is the greatest voltage difference) then it gradually gets worse and the tracking causes heat in a localised spot which gradually builds up causing the Paxolin to carbonise until it reaches a point where the leakage affects the operation of the valve.....

The only answer in that case is a new valve holder. Note that it's not only Paxolin valve holders that can track......
I had 10 minutes before work and I think I sorted it, more out of luck than anything else. On close inspection from a different angle here was a small charred area on the paxolin of the waveband switch at the edge of the metal strip linking s11 when the waveband switch is turned. This carries HT to anode (180v). It was only 2mm in diameter and would have been inconsequential if it weren't the place where the anchoring pin for the metal strip linking S2 S3 and S4 on the other side of the same switch emerges. I'll post a picture later. I have scooped out the charred area and I still need to triple check but it seems the short is gone.

There is still some work to be done. The radio isn't working so I may have either disturbed something or the abnormal voltage on V2 may have damaged it (which is what I think Lawrence is alluding to). I'll have a second look when I can but I don't think it'll take too long to sort out now.

Any advice on how I can stop this issue from recurring? Scraping out charred paxolin from a rather inaccessible area isn't an exact science, and I cannot realistically change the waveband switch. I'm tempted to disconnect the s11 wires from the waveband switch and leave them permanently connected, but I'm not sure whether this will affect the FM end of things. What do you think?

Thanks for the help

Gabriel
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Old 15th Jun 2021, 9:29 am   #18
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Default Re: KB MR10 sudden fault, help needed

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Any advice on how I can stop this issue from recurring? Scraping out charred paxolin from a rather inaccessible area isn't an exact science, and I cannot realistically change the waveband switch.
I don't think you can guarantee to stop it from happening again. Paxolin is what it is....SRBP Synthetic Resin Bonded Paper so it is subject to moisture absorption (over a very long period though) and insulation breakdown.

Having scraped away the carbon perhaps try sealing it with a small blob of Araldite or similar Epoxy.
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Old 15th Jun 2021, 9:32 am   #19
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Default Re: KB MR10 sudden fault, help needed

Possibly heat damage as a result of arcing of the HT switch contacts as they have opened and closed over the years. Not sure if it would be possible to bridge the offending contact set with a high voltage suppressor capacitor? If only switching HT this might be OK but a no-no in tuned circuits. Cheers, Jerry
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Old 15th Jun 2021, 9:51 am   #20
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Default Re: KB MR10 sudden fault, help needed

And I suppose if I just keep the switch (s11) shorted all the time and off the waveband switch , as it is in AM mode, I will probably lose FM as I'm effectively shorting the FM IFT

Last edited by Gabe001; 15th Jun 2021 at 10:02 am.
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