|
Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
|
Thread Tools |
13th Jun 2021, 6:39 pm | #1 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
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 |
13th Jun 2021, 6:56 pm | #2 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
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 7:04 pm. Reason: extra info |
13th Jun 2021, 7:09 pm | #3 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
|
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?
__________________
Never Leave Well Enough Alone... |
13th Jun 2021, 7:47 pm | #4 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
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 |
13th Jun 2021, 7:55 pm | #5 | |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
Re: KB MR10 sudden fault, help needed
Quote:
I suppose when I disconnect the 2 green wires I will find out |
|
13th Jun 2021, 8:00 pm | #6 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: KB MR10 sudden fault, help needed
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. |
13th Jun 2021, 8:33 pm | #7 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
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. |
13th Jun 2021, 9:01 pm | #8 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
|
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
|
13th Jun 2021, 9:31 pm | #9 | |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
Re: KB MR10 sudden fault, help needed
Quote:
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 |
|
13th Jun 2021, 9:56 pm | #10 | |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
Re: KB MR10 sudden fault, help needed
Quote:
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. |
|
13th Jun 2021, 10:44 pm | #11 | |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
|
Re: KB MR10 sudden fault, help needed
Quote:
|
|
14th Jun 2021, 8:29 am | #12 | ||
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
|
Re: KB MR10 sudden fault, help needed
Quote:
The only answer in that case is a new valve holder. Note that it's not only Paxolin valve holders that can track......
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
||
14th Jun 2021, 9:46 am | #13 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
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 |
14th Jun 2021, 9:56 am | #14 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
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.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
14th Jun 2021, 10:24 am | #15 |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
Posts: 1,840
|
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.
__________________
BVWS Member |
14th Jun 2021, 10:38 am | #16 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: KB MR10 sudden fault, help needed
Quote:
Lawrence. |
|
15th Jun 2021, 10:11 am | #17 | |||
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
Re: KB MR10 sudden fault, help needed
Quote:
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 Last edited by Gabe001; 15th Jun 2021 at 10:17 am. |
|||
15th Jun 2021, 10:29 am | #18 | |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
|
Re: KB MR10 sudden fault, help needed
Quote:
Having scraped away the carbon perhaps try sealing it with a small blob of Araldite or similar Epoxy.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
|
15th Jun 2021, 10:32 am | #19 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
|
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
|
15th Jun 2021, 10:51 am | #20 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
|
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 11:02 am. |