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Old 8th Nov 2017, 11:01 am   #1
newlite4
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Default BEME Mini Loop - silent

Anyone got any ideas on the Mini Loop receiver. This one has the bar antenna on top. Injecting a signal from the signal injector into the grid of the IF amp (DF96) produces a signal but everything around the mixer/oscillator (DK91) is silent, nothing in, nothing out.
When I first got the receiver, I made up the 1.5V and 90V battery packs and initially I had the 90V pack the wrong way round, this just resulted in the ground resistor from negative supply to chassis smoking. The resistor remained intact and upon correct connection of the 90V the receiver still did not work ( it may well have not have worked for for many years, I can't imagine any damage that could have been done by reversing the HT polarity). If I can't get this receiver to work, I will probably cut my losses and scrap it, but before I do, I just thought I would put out some pointers, it is quite a complex set with a "busy" front end and now that I am getting into the RF section, it is becoming more complex.
Neil
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Old 9th Nov 2017, 6:18 pm   #2
Jon_G4MDC
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Default Re: BEME Mini Loop - silent

I think that "just smoking" might appear harmless but of course it meant it was seriously overloaded.

If "remained intact" means "it still reads the right value" then you have to question where did all the rest of the "smoke current" go?

If it was down some inductor wound with very small gauge enamelled wire it might have "smoked" too and now you have a lump not an inductor.

Only thinking out loud since I have no idea of what this is in reality. Good luck with it.

<meant to say - like your Cat!>
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Old 9th Nov 2017, 7:17 pm   #3
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Default Re: BEME Mini Loop - silent

I would sub the DK91 with a known good one for a start. Then check the valve sockets for poor connections. I am not familiar with this set but suspect it may be a DF radio? Check all voltages around the DK91 also, but sometimes these valves just get tired and won't oscillate. If you have a means of checking if it is oscillating that would be good. The IF stages onwards seem to be working ok from what you have said, so maybe a poor connection around the frequency changer is the culprit. It's possible that the supply decoupler electrolytic (if it has one) has given up the ghost, and taken too much current which would cook the bias resistor as you describe. It may have recovered but maybe it got too much grief and failed. Try removing it and subbing it with another.
Alan.

Last edited by Biggles; 9th Nov 2017 at 7:21 pm. Reason: More info.
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Old 9th Nov 2017, 8:26 pm   #4
ex seismic
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Default Re: BEME Mini Loop - silent

And please don't bin it before offering it here!

Gordon
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Old 10th Nov 2017, 9:08 am   #5
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Default Re: BEME Mini Loop - silent

Thanks Alan, I'll have another poke around the DK91 to see if I can get it oscillating, I'll check the main electrolytic again for value.
Neil
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Old 10th Nov 2017, 9:13 am   #6
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Default Re: BEME Mini Loop - silent

No Gordon, I never said that I would bin it, but it does contain a considerable inventory of precious parts possibly worth more than the set cost me originally which would be better used in several other sets than sitting in a steel box on the shelf.
Neil
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Old 13th Nov 2017, 1:37 pm   #7
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Default Re: BEME Mini Loop - silent

Have you measured the volts on G2 of the DK91? I think that they normally use G1 as the oscillator grid, with a 47k or similar to deck and G2 as the oscillator anode. The HT rail is fed via a resistor and tickler coil to G2 and a tuned coupling coil feeds back to G1 so the whole thing then oscillates.The feed resistor is usually decoupled to ground so check the potential on the DK side of that resistor. Be wary of the dreaded McMurdo B7G valve holder. The metal bits inside the socket break and although you see a voltage on the soldered part of the socket, it doesn't get to the valve pin. I expect you will have checked these things anyway. A signal at IF fed to the DK91 signal grid should appear at the LS so that would check the IF and back end stages. I expect that you will have fed a signal generator as a substitute LO at LO+IF into G1 of the DK91? I suspect that the radio you have is a bit more sophisticated than a simple battery portable and you will have done all the logical tests anyway.

Ian
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Old 13th Nov 2017, 2:37 pm   #8
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Default Re: BEME Mini Loop - silent

Hello Ian, yes, Sean was over yesterday and we carried out measurements around the DK91 (heptode wired as a pentagrid converter), it is a very crafty but established layout. I have full HT of 82V on pin 2 (anode) I have around 50V on pin 3 (grid 2), I will scope pin 6 (grid 3) to look for amplified RF and pin 4 (grid 1) for oscillator feedback later.
I am beginning to suspect the valve base, there is no other reason why this stage should not work apart from a very stubborn oscillator section.
Thanks for getting in touch,
Neil
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Old 14th Nov 2017, 10:23 am   #9
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Default Re: BEME Mini Loop - silent

Just about got this going last night, faults centered around the DK91 as expected. It turned out to be a bad valve socket although there is lots more clearing up to do. I had a good 0.75Vpp signal on grid 3 (RF in) at around 5MHz, the local oscillator grids 2&4 sprang to life to give 13.5Vpp at around 4MHz and after a little movement in the socket, the 82VDC on the anode became modulated with the IF signal. I would think that all valve bases need a check over (this set would have been at sea after all) and a re-cap may not go amiss as it uses the brown mouldseal caps.
Getting there,
Neil
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Old 14th Nov 2017, 9:49 pm   #10
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Default Re: BEME Mini Loop - silent

Well, just finished for now. I have treated the rest of the valve sockets to a clean and tweak and it is working very well with good volume and intelligibility. It only receives Long Wave at the moment but I am quite happy for now. I need to get things ready for RWB now so it will have to go on the "to do" pile until I can get back to it.
Thanks for all your help,
Neil
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Old 15th Nov 2017, 9:40 am   #11
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Default Re: BEME Mini Loop - silent

Well done Neil. At least on those valve holders it's possible to change out the faulty pins without having to remove the valveholder. I seem to remember that not all B7G holders have the same socket orientation with respect to the mounting holes. That can be a bit tricky .

Ian
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