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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 22nd Nov 2020, 8:17 pm   #1
Gabe001
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Default The mouth of hell - a KB MR10 story

A few weeks ago I decided that I needed a challenge, and what better way to improve your skills than by taking on a set dubbed "The Mouth of Hell" by the author of "It came from Outer Sidcup" which was published in BVWS volume 24/3 Aug '99. So when a lovely cream KB MR10 was put up for sale on FB marketplace, I took a gamble and went for it.

But first, a bit about me. I am in no way affiliated with the electronics profession. I have been restoring radios for about 6 months only, and have no specialist equipment. All my radio tools still fit into one box. So when I changed the grid coupling capacitor and applied power, and the radio was completely dead, I wondered whether I bit off more than I could chew.....but I'm rushing ahead.

Aesthetics
The set looks great, and I have no difficulty believing that many were sold despite their hefty price at the time. It reminds me of a smiley face emoji , something to cheer you up every morning. This particular set was very good cosmetically, just a small hairline crack on one of the feet. It only required a polish with brasso, superglue applied to the crack and I reinforced the area with fibreglass + epoxy, although this was probably overkill.

Dismantling the set was straightforward enough. I had the necessary tool to remove the 2 oddly placed bolts attaching the chassis to the front of the radio. The dial which is held in place with tape fell out when the chassis was removed, but I was prepared for that. Other than that all the expected nuts and bolts.

The dial cord was torn and required restringing, this was achieved without major problems.

Electrics
So here is where the challenges began. The previous owner told me that the radio had been in the family since new, so I expected it to be original inside. He had also applied power to it and told me there was no sound.

So when I changed the grid coupling capacitor, applied power via the lamp limiter and built up the voltage gradually, I was disappointed but not completely surprised when the set remained completely dead, on all bands....... great ......

Having established that HT was fine, and having no signal generator, I used a small Bluetooth adapter to inject signal at the gram input. By signal I mean "radio" by Johnny Flynn....still nothing ....

Let's try volume control.... Voila...music from the speaker. This localised the problem to s14, part of the waveband switch. Let's try bridging it.... that worked... and music from the gram socket was playing on the speaker with the volume cranked up to maximum, and the sound quality was quite good.

So after a good clean of the waveband switch with servisol and a few contacts that needed to be twisted back into position, I had the set working again on the MW, LW and gram settings. Actually though, only the gram socket worked well, although the volume pot was behaving oddly in that it was really low unless I cranked the volume up to maximum. Some stations were audible on MW and LW, but the set had barely any gain and I had to put my ear to the speaker to hear anything. Moreover FM was completely dead. Mike (crackle) kindly suggested I continue replacing the capacitors, and all the hunts and waxies were replaced, one at a time, checking the set in between. I expected the gain to improve after replacing the AGC decoupling capacitor (c24) but sadly it made no difference.

So time for some more fault finding..... With the only instrument at hand, my trusty DMM, and being reasonably confident that I hadn't introduced new faults when recapping, I proceeded to try and localise the fault by measuring voltages as the valve bases.

V1 - all good
V2 - all good
V3 - all good
V4 - d (anode pin 9) measured 4.5v instead of 65v. Let's check the drop along the resistor r20.....almost 200v

So either the resistor was high or the valve was drawing too much current. A replacement resistor made no difference, so by exclusion, v4 was faulty. Removal of the valve and some tests on the pins with my trusty DMM revealed what I think is a short between pin 8 and 9 (a resistance of 250kohms) which would account for the increased current draw (I think).

Long story short, a new eabc80 arrived in the post and the set burst into life on all bands, including FM.

Enhancements
Having got this set to work, and bursting with confidence at this stage, I decided to go the full monty and add Bluetooth sympathetically. I built my usual half wave rectifier power supply on a prototyping board and attached a krc-86b module, wired to the gram input. Stealing 6.3v AC across one of the bulbs worked well for power. The problem was that there was nowhere to attach the module to on the chassis. After some head scratching and some prototyping on Blender I decided on a custom adapter that would latch onto the "holder" being used to mount the ferrite rod. So I 3d printed a mounting adapter for the BT module and mounted it vertically as you can see in the picture.

Issues and snags
The set works well on all bands, including FM, however...

The proximity of the bt module to the ferrite rod causes some minor interference on LW (clicking sound) although this is drowned out when tuned into a strong station. The interference is hardly audible on MW and not present on FM. This is completely my fault and a compromise I was ready to accept considering there are no other mounting points.

FM also takes a little while to get going, about a minute or two before I can start receiving stations. It works well once it's warmed up, so to speak. I've ordered a new 6bj6 to see if it makes a difference, although if it doesn't it's not a deal breaker.

The stations tune correctly on LW and MW and appear where they ought to, buy FM seems to be 'off' by about 0.5mhz or so ie radio X which should be 97.7 is audible with the needle pointer at 97.2mhz. Again not a biggie and I don't really want to do my first RF alignment on this set.

The internal FM aerial is rubbish. I used a piece of wire instead which works much better and I've got a dipole on order (3 from eBay) so I'll do some tests once it arrived.

Conclusion
I wanted a challenge and I got one. Unusually, this set had two major faults, either of which was serious enough to retire the set. Luckily they could be fixed easily.

Its definitely a looker, especially lit up, and the other half loves it. I was going to sell it, but I've changed my mind.

It has a rather bad reputation of being a poor performer, but it's actually not bad at all. The speaker is large and sounds good too.

Finally some pictures, of the chassis with the BT module mounted and the set, and a video of me tuning part of the FM band

https://youtu.be/nXH8m_K-ikU

Hope you enjoyed this write up. Any words of wisdom appreciated

Gabriel
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Last edited by Gabe001; 22nd Nov 2020 at 8:45 pm.
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Old 22nd Nov 2020, 9:26 pm   #2
paulsherwin
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Default Re: The mouth of hell - a KB MR10 story

Nice writeup. These are attractive sets, but KB obviously had problems with the electronic design, and circuit changes were made throughout the production run, not all of which were documented. I have one which doesn't perform particularly well, especially on FM.
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Old 23rd Nov 2020, 12:13 am   #3
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Default Re: The mouth of hell - a KB MR10 story

"It came from outer Sidcup" - what a very clever - and accurate title! As a young teenager I used to travel by bus to school in Sidcup - right past the Footscray KB factory, which was indeed in outer Sidcup.

Mike
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Old 23rd Nov 2020, 12:16 am   #4
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Default Re: The mouth of hell - a KB MR10 story

A lot of perseverance and a bit of luck got you there. The end result looks good.
It was an American golfer who came out with "The more I practice, the luckier I get!"
There was enough practice in that set to increment the luck factor.

David
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Old 23rd Nov 2020, 7:54 am   #5
David G4EBT
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Default Re: The mouth of hell - a KB MR10 story

A stylish set - sounds good too!

This one was rather more of a challenge, but ended happily:

https://www.vintage-radio.net/forum/...d.php?t=159373
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Old 23rd Nov 2020, 9:41 am   #6
Gabe001
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Default Re: The mouth of hell - a KB MR10 story

Thanks for the comments

David I've seen the thread you posted a link to, it's why I ordered the external ribbon aerial, although it does work reasonably well with a length of wire.

I think I need to invest in a velleman k7000. I was lucky in that the faults were diagnosable with a simple multimeter and some common sense this time round.

Gabriel
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Old 23rd Nov 2020, 4:05 pm   #7
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Default Re: The mouth of hell - a KB MR10 story

Nice work, Gabriel. It's a good looking set. If the slow warm-up occurs only on FM it's more likely to be the 12AT7 (ECC81) than the 6BJ6, since the former valve is unique to FM. Cheers, Jerry
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Old 23rd Nov 2020, 4:33 pm   #8
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Default Re: The mouth of hell - a KB MR10 story

I see two tabs on a choke or transformer which could be use to mount your Bluetooth module.
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Old 23rd Nov 2020, 6:27 pm   #9
Gabe001
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Default Re: The mouth of hell - a KB MR10 story

Quote:
Originally Posted by FStephenMasek View Post
I see two tabs on a choke or transformer which could be use to mount your Bluetooth module.
Yes I considered those. Not enough space.

Last edited by Gabe001; 23rd Nov 2020 at 6:55 pm.
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Old 23rd Nov 2020, 6:40 pm   #10
Gabe001
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Default Re: The mouth of hell - a KB MR10 story

Quote:
Originally Posted by cathoderay57 View Post
Nice work, Gabriel. It's a good looking set. If the slow warm-up occurs only on FM it's more likely to be the 12AT7 (ECC81) than the 6BJ6, since the former valve is unique to FM. Cheers, Jerry
It a bit of a gamble, that valve seems to be the weak link in the FM chain according to the bvws article, so I thought I'd see if replacing it makes a difference. Maybe langrex will allow me to return it if it makes no difference. In cases like this I wish I had a set of spare valves to stay swapping around.

Gabriel

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Old 23rd Nov 2020, 9:48 pm   #11
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Default Re: The mouth of hell - a KB MR10 story

Nice work Gabriel, that all looks very professionally done.
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Old 24th Nov 2020, 1:39 pm   #12
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Default Re: The mouth of hell - a KB MR10 story

Nice work. I have just repaired a very same cream MR10! To cut a long story short that is very similar to yours, lack of FM was caused by the decoupling capacitors around the ECF82 AM mixer/osc, FM IF amp. They were those brown mini tubular type that look like resistors and were O/C.

The FM now works OK with a decent aerial. I think KB failed to realize they were only a stones throw from Wrotham..

The knobs are utter rubbish. They were all broken but I rebuilt them by removing the old spindles with a mini grindstone, cutting the spindles from a set of suitable sacrificial knobs. They were secured into the cut down KB knobs with the aid of a G clamp and fibreglass resin. The rear of the knobs forms a perfect 'well' for the resin.

Reception on AM is very good even with the ferrite rod aerial operating in very unfavourable conditions.

It is a well constructed radio. Pity about the low gain FM but if you think the MR10 is bad try the FM 'Toaster' and FM 'Minuette.' Enough said. Regards, John.
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Old 24th Nov 2020, 2:03 pm   #13
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Default Re: The mouth of hell - a KB MR10 story

Thanks John, that's very interesting. How did you diagnose the fault?

the ribbon fm aerial arrived yesterday so at the first available opportunity I'll remove the coax plug from it and hook it up to the radio for some more testing. Maybe I can attach it permanently to the back panel.

Unfortunately I haven't done enough radios for a meaningful and fair comparison. The bush vhf71 was stellar on all bands. On this, FM works well enough with a length of wire. AM, I completely agree, is very good, better than my dac90a which in turn is better than the cossor 464 from an AM reception perspective.

Ive got a really nice Murphy a372 up next, so it will be interesting to see how that stacks up.

Gabriel
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Old 24th Nov 2020, 3:20 pm   #14
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Default Re: The mouth of hell - a KB MR10 story

No diagnosis really. Just the sight of those capacitors is enough to switch on the soldering iron.
Its a bit like a patient arriving at the doctor's surgery carrying his head under his arm. No diagnosis necessary.

It's worth looking at those caps. If they are the Hunts brown crunchy ones, they are as bad as the ones previously mentioned. They were of course designed for a 10 year service life. Maybe we expect too much. Regards, John.
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Old 24th Nov 2020, 6:50 pm   #15
Gabe001
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Default Re: The mouth of hell - a KB MR10 story

New 6bj6 sorted the FM slow start.
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Old 2nd Dec 2020, 9:18 pm   #16
Gabe001
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Default Re: The mouth of hell - a KB MR10 story

Just a little update on this set.

I had to go back in fix what turned out to be a loose contact on the waveband switch, which is now sorted. The dismantling process cost me the spindle on the waveband switch which was re-glued and encased in milliput and has held up well so far.

The ribbon FM aerial from eBay wasn't as good as I expected, so in the end I removed it and reverted back to using a wire, about 5 feet long, as a VHF aerial instead. This wire is hidden behind the set and behind the kitchen appliances and is not visible.

I am pretty pleased with the overall performance. I almost exclusively use it on FM. It has deposed the bush dac90a as the kitchen radio (the latter was used mainly to listen to BBC R4 in the mornings) and it gets regular use.

From the reviews and previous comments I certainly didn't expect much in terms of performance to start with, but I was pleasantly surprised. With the current arrangement it has got a pretty full vhf waveband and it's more than fit for purpose as a small kitchen set. It's not a hacker mayflower but it doesn't aspire to be one either.

Hope this helps someone. I think this project is finished now.

Gabriel
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Old 3rd Dec 2020, 11:20 am   #17
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Default Re: The mouth of hell - a KB MR10 story

That certainly looks in place in your kitchen and a lot safer than a DAC90.

All the knob spindles were broken on mine. I removed what was left of the spindles with side cutters and a mini grind stone.

A hunt through the knob boxes turned up three with stouter shafts. The holes in the MR10 case were more than big enough to accept these. The knobs themselves were unsuitable for the MR10 but they could be sacrificed for their shafts!

The tops of these were cut off just leaving a thin disc of the control knob itself. With the aid of a G clamp to hold it central, resin was poured into the rear of the original knob front forming a very strong fixing..

Even after lubrication the waveband switch was rather stiff. An Aluminium spindle extension coupler was employed in this case with one end filed into a 'D' shape to prevent it rotating in the hardened resin. [impossible!] Care has to be taken to line up the coloured dots with the index mark on the cabinet side. Secured with it's twin grub screws the feeling is very positive.

This method could of course be used with any control knob with suitable modification. Poundland's twin tube adhesive also makes an alternative fix in place of the resin and it is very cheap.

Regards, John.
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Old 3rd Dec 2020, 1:15 pm   #18
Gabe001
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Default Re: The mouth of hell - a KB MR10 story

Thanks John. That's really informative.
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Old 3rd Dec 2020, 1:16 pm   #19
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Default Re: The mouth of hell - a KB MR10 story

Quote:
Originally Posted by David G4EBT View Post
A stylish set - sounds good too!

This one was rather more of a challenge, but ended happily:

https://www.vintage-radio.net/forum/...d.php?t=159373
Apologies for coming late to this Thread and thanks to David for the link to my own Thread that you should only tackle if you have the time and patience!

It really was a bit of a struggle and I felt that my own jack of experience was only saved by David's ongoing support and encouragement - WE got there in the end.

When I eventually got the set up and working on all wavebands it was a great performer on FM even with a cheap internal ribbon aerial from Amazon!
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Old 3rd Dec 2020, 2:17 pm   #20
Gabe001
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Default Re: The mouth of hell - a KB MR10 story

Hi Donald, yes I saw your thread and used it for reference. It's quite a dissertation

If you still need the knobs there is a brown set for sale on FB marketplace for 10 in York with the knobs you need. No affiliation with seller.

Gabriel
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