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Old 15th Aug 2005, 10:10 pm   #1
johnlees
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Default Kolster-Brandes (KB) Model MR10 - dead and smoking!

This set was rescued from a skip. The audio section (gram. socket onwards) works fine but the radio section before that is dead. Valves light up and there is HT. Anode voltages read considerably lower than the data sheet says, e.g. 150V instead of 199V to V3. (I have the valve data and circuit diagram.)
LW and MW are just dead but FM is dead with fireworks, i.e. arcing and smoking from the upper part of the rear wafer of the wavechange switch, though the cramped design makes it difficult to see exactly where it's coming from.
V2 anode voltage is 150V on gram/MW/LW (should be 193V) but drops to 25V on FM and immediately the smoke starts, so obviously I don't leave it on FM for more than a couple of seconds to take readings.
V1 is designed to get no HT supply except on FM, but then it only measures 70V and should be 172V on both anodes.
I'm bamboozled by all this. Any ideas, please? Clearly current is going somewhere it shouldn't on FM, but where? (C12 and C30 seem to be non-leaky.) Thanks, all. John.
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Old 15th Aug 2005, 10:43 pm   #2
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

The obvious cause is wavechange switching problems. This switches the ECC81 HT and something may have gone badly wrong. You could try disconnecting the HT supply from the switch and seeing what happens.

Have you tried running it with the RF valves removed? A valve failure could possibly cause this.

Some of the wiring in the MR10 is pretty messy and you should check carefully for shorts, especially around the smoothing electrolytic base.

Finally, the output of the metal rectifier is likely to be down if it's original. This won't explain the fireworks of course, but may be a secondary fault. If the HT is too low the mixer won't oscillate.

Good luck, Paul
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Old 15th Aug 2005, 10:54 pm   #3
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Brilliant, Paulsherwin. I'll look into all of that. Actually the low rectifier output did cross my mind. Yes, the wiring on this set is really messy and I've already spotted several near-short-circuits, though none of them actually were. Thanks again.
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Old 16th Aug 2005, 10:07 pm   #4
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

I'm still baffled by this. The smoke still starts on FM with ALL the valves removed ! It comes from the rear wafer of the wavechange switch, so, looking at the circuit diagram, it's from the switch marked "S2 FRONT" on the diagram. Does that give anyone a clue? It could well be the switch itself, of course, but I just can't get at it. Any further help, please ? Cheers, all !
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Old 16th Aug 2005, 11:46 pm   #5
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

John,

you have two options........

1. (more likely) a leaky or short circuit de-coupling capacitor in the FM tuner head.

2. Leaky switch wafer or valve holder - check with a Megger if you have one, these kind of faults often need a high voltage before they show.

(There is a third option - the phantom bodger has been at your set, and something has been put back wrong).

Jim.
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Old 17th Aug 2005, 12:29 am   #6
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Hi, I can't remember what kind of set it was, but I've had the burning switch one before. Turned out the switch had arced due to dirt build up, then once it had arced, the wafer was charred, causing more arcing etc and total switch failure. I ended up writing the set off as the switch looked like a nightmare to replace, and was a fire hazard. It wasn't due to any other component failure in the set, though.

Biggles.
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Old 17th Aug 2005, 11:14 am   #7
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

As the arcing is obviously coming from the FM HT switching arrangements, one approach would be to disconnect the HT wiring from the switch and wire it permanently on. This will shorten the ECC81 valve life somewhat if the radio is mainly used on MW/LW, but they're easy enough to get. You should obviously investigate the possibility of shorts in the FM tuner head while doing this.

If you really can't get at the switch, you could disconnect the HT wiring at the 'other end'. I imagine you've already given it a good squirt of contact cleaner?

Best regards, Paul
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Old 17th Aug 2005, 9:46 pm   #8
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Thanks for all those new suggestions. I will reply in more detail when I have looked into them. Thanks again, John.
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Old 17th Aug 2005, 10:08 pm   #9
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Hi, John, Prompted by your postings on the MR10, I've just been working on my own example, and after changing the O/C smoothing capacitors and all the Hunts Mouldseals and leaky waxies, It actually works! . A new ECC81 has brought FM back to life and it is currently belting out Classic FM. All the voltages are close to spec, and none of its smoke has escaped . A lot of work still needed, like cleaning dirty switches and valveholders and replacing the tuning cord, but so far its looking good. If you need any help with readings around the switch, etc for your set let me know as mine will be in bits for a while.There is hope for these things after all! .

Regards, Mick.
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Old 18th Aug 2005, 10:29 pm   #10
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Thanks for that, Mickjjo.I shall probably act on that. There is good progress today. The primary fault is DEFINITELY and indisputably arcing from "S2 REAR" to "S2 FRONT", proved by snipping the earthing connection on "S2 FRONT" (contact number 6 on the circuit diagram). (Housepoints each to Paulsherwin and to Biggles !!) HT was leaking badly on FM between the front and back of the rear wafer of the wavechange switch.
Whether this is repairable has yet to be seen. One solution might be to get rid of the two connections from L10 (which are at HT potential) to that switch and fit another switch somewhere, though only time will tell if there are other problems with this switch.
If this part of the problem is solved, the set still won't work, probably because of low HT. I will keep you all informed.
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Old 19th Aug 2005, 9:43 am   #11
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

As I suggested in an earlier post, you should simply rewire the HT so that it doesn't go via the switch and is always on. This is a standard workaround for this sort of problem.

Once this is fixed, the next thing to do is to measure the HT current. You can do this by measuring the voltage drop across the resistor between the 2 smoothing electrolytics, and calculating the current using Ohm's Law. If it's too high you need to work out where it's going. If it's OK the metal rectifier is on the way out. I replaced my rectifier with a silicon bridge and 200 ohm resistor.

Good luck, Paul
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Old 19th Aug 2005, 9:55 pm   #12
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

It's fixed !! It works !! Switch "S2 REAR" was arcing to switch "S2 FRONT". A couple of people (thanks Dave T) have pointed out that switch "S2 REAR" is a bit superfluous so it's just been disconnected, leaving L10 in series with L12 on both AM and FM (thus clearing any HT off the leaky wafer). Disconnecting it has made no audible difference to AM reception and the set now works on AM and FM. (Paulsherwin - the switch ("S1 FRONT") which switches HT on and off to the FM section was not the problem. You were right about the rectifier, though. I replaced it with a full-wave bridge of suitable diodes. Up came the HT to proper values and the set came to life.)
I changed the anode to grid capacitor in the audio section and that made a tremendous difference to the volume and sound quality.
The set is loud on stronger stations. FM reception seems quite good with plenty of stations, but MW and LW seem much poorer with only the stronger stations being received even with a long aerial in the garden.
Thanks, everyone, as this set was within a gnat's whisker of going in the dustbin. Cheers, John.
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Old 19th Aug 2005, 10:19 pm   #13
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Hi John, Well done and full marks for perseverance , You may find it worthwhile to change most of the other capacitors, particularly the Hunts and wax jobs as they were all bad on my set. Mine is working quite well although tends to go into oscillation at the lower end of the FM band, effectively wiping out FM reception on other sets in the next room, . My LW is good but the set lacks sensitivity at the low frequency end on MW. I may have to try swapping valves or checking the alignment. . Oh well, two MR10s brought back to life in one week can't be bad..... .

Regards, Mick.
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Old 19th Aug 2005, 10:40 pm   #14
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Yes, thanks, Mick. All the caps. look pretty ropey. The chocolate coloured Hunts ones tend to shed their skins when disturbed and the waxy caps. have all seen better days. Having replaced just one cap. today (which made a fantastic improvement) I might well have a go at others.
I'm glad to know that there is someone else who has got one of these beasts. The people who wired it up should be ashamed of themselves as I think it is a real mess. Let me know how you get on. Any advice on the not-too-sensitive reception on MW and LW ?? Thanks again, John.
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Old 20th Aug 2005, 12:52 am   #15
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Glad to hear you've been making progress. You should certainly change all the mouldseals. The poor AM performance may be due to RF alignment - try sliding the MW and LW coils along the ferrite rod and see if things improve.

You should be aware that the performance of these sets isn't wonderful even after a full restoration. Mine still has some FM distortion and AM sensitivity is only average. It should work acceptably on the internal ferrite rod though.

Good luck, Paul
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Old 20th Aug 2005, 10:57 am   #16
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Quote:
Originally Posted by johnlees
Any advice on the not-too-sensitive reception on MW and LW ??
Replacing all the capacitors as mentioned tends to fix all manner of problems. Leaky caps on the AGC line will mess thaat up; leaky caps on the screen grid decoupling of the IF stages could mess up the biasing affectin the gain; you get the idea. Change the capacitors first then investigate the lack of sensitivity if it is stll a problem.
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Old 20th Aug 2005, 10:33 pm   #17
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Default Re: Kolster-Brandes (KB) Model MR10 - dead and smoking!

Thanks to Paulsherwin and to Paul Stenning. On internal ferrite rod there is barely any reception except on the very local (just up the road, I believe, half a mile away) Gloucestershire Classic Gold station, and that's only faint. With the long external aerial connected there are some of the main BBC stations, but no foreigners, and Gloucestershire Classic Gold is blasting ! (FM seems much better.)
I'll follow your advice, Paul S. and replace caps. before I do anything else. I will come back here with results.
John.
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