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Old 2nd Dec 2014, 8:46 am   #141
peter_sol
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Default Re: Roberts RMB Problems

Try the new DK92
As those valves age they will not oscillate at higher frequencies IE MW and SW.
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Old 2nd Dec 2014, 10:49 am   #142
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Default Re: Roberts RMB Problems

I will try that tonight Peter and let you know. Do you know if that valve you kindly sent me is tested?kind regards.
Dave.
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Old 2nd Dec 2014, 11:13 am   #143
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Default Re: Roberts RMB Problems

Yes on MW and LW not SW though
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Old 2nd Dec 2014, 6:39 pm   #144
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Default Re: Roberts RMB Problems

As Peter has said, valves sometimes deteriorate and the gain tends to fall off with increasing frequency. Valve testers often only test the valve under DC or low frequency conditions, so even a tested valve is not guaranteed to work in a circuit. This is even more apparent at VHF.

One function of the valve in a local oscillator is to provide sufficient gain at the desired frequency to create oscillation in conjunction with the positive feedback network of passive components. The DK92 is a heptode in which two of the grids form a triode oscillator which modulates the electron stream. The incoming signal from the aerial then mixes with the LO within the same valve to produce a mish-mash of frequencies; the one you want is the difference between LO and the desired signal, which is selected by the first tuned winding of the IFT.
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Old 2nd Dec 2014, 8:19 pm   #145
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Default Re: Roberts RMB Problems

Ok, i tried the valve Peter kindly sent and I'm still getting the same result. Good strong trace but only on LW. I've tried adjusting my time base and voltage settings to see if I can pick anything else up but nothing. So I have identical readings on both DK92 valves I have. I guess looking at the diagram I Could check continuity across the wave change switch again to make sure the ocsillator tuning coils are being switched on? Or they may be wildly out of tune or not working. I've learned to not overlook anything with this set . I suppose I could align and set up LW. At least that would prove the majority of the set is up and running, then focus on the MW and Sw problems. Let me know your thoughts. Many thanks as always.
Dave.
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Old 2nd Dec 2014, 8:24 pm   #146
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Default Re: Roberts RMB Problems

Yes, do check the wavechange switching. On mine, it seemed fine one minute, then hopeless the next, for no apparent reason.
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Old 2nd Dec 2014, 9:12 pm   #147
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Default Re: Roberts RMB Problems

Is yours now fully functional Nick? Did you fix the wave change switch? It's as if the ocsillator tuning coils are not being switched on for MW or SW. Is it R4 that feeds the tuning coils as I can also see a feed from valve 1 to the tuning coils also? I could have dislodged a wire perhaps to loose both MW and Sw or a bad connection. I feel the fault lies amongst these areas.
Dave.
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Old 2nd Dec 2014, 9:18 pm   #148
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Default Re: Roberts RMB Problems

Sort of functional, Dave! The waveband switch is intermittent and I haven't had the time or inclination to repair it yet.
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Old 2nd Dec 2014, 9:55 pm   #149
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This set has although been nothing short of a nightmare, it certainly has been a huge learning curve and a frustrating one at that. I have at times walked away and done some other easier projects in between to keep my enthusiasm going, I do love the hobby though but it's time to put this set to bed. I have invested quite a bit of money on new kit to further my knowledge and skills in this hobby. I am now efficient with my new kit and confident in using it after all your help on this forum. Especially using the scope. It has also aided me in my job. Using the scope I can now see you can diagnose faults easily with visible results with out educated guess work. Hopefully I can get sat afternoon on the set.
Dave.
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Old 3rd Dec 2014, 9:27 am   #150
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Default Re: Roberts RMB Problems

Hello Dave
As you can see the HT voltage goes through R4 and then to the bottom the secondary of all 3 tuning coils. The top of the coils goes to a switch which connects to pin 3
(I think)
You could check for Voltage there to see if those contacts are OK.
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Old 3rd Dec 2014, 9:48 pm   #151
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Default Re: Roberts RMB Problems

Also check the values of R4 and R3, plus continuity through wavechange switch contacts S7, S8 and S9, although the latter would appear to be working.
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Old 6th Dec 2014, 8:04 am   #152
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Default Re: Roberts RMB Problems

Good morning all. Progress again. I found why I was not getting a scope pattern for MW at least. I traced it to a bad joint at the ocsillator tuning Coil L7. The fault was at the soldered joint of where c37 meets L7 and S8. Prodding the joint made the wave form appear and dissappear. I now have the DK92 ocsillating on LW and MW. SW is still inoperative though, but I think I know where the fault lies with this. The only capactor on the set now that I haven't changed is C8. The reason for this is that I was not fussed about using shortwave and also modding the set for a shortwave aerial. But, seeing how much agro I have had and the time spent I thought I may as well continue and get the set fully working. I think replacing C8 should sort it out. The cap itself looks in a bad way. I also know that I also got it very hot, the wax melted off it when I was soldering in the vicinity of it. I shall get this replaced then have another go at the alignment. I still have one stubborn ocsillator tuning core but I'm leaving well alone. The set may work with getting the others adjusted, I don't want to create more work or problems. Not sure if I will get time this weekend with Christmas so near. I achieved all this in the early hours before work yesterday. Watch this space.
Dave.
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Old 6th Dec 2014, 8:26 am   #153
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Default Re: Roberts RMB Problems

Not quite Home and dry, but there's Light at the end of the tunnel.
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Old 18th Dec 2014, 2:51 pm   #154
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Default Re: Roberts RMB Problems

Guess what chaps?

All aligned. MW and LW all working as should and checked with oscilloscope. On test now out of the case. Been on test for several hours belting out good strong stations on the right wavelength and position on the dial.
last question, I have ordered some 3 core braided cable to compliment this repair from online. Is it safe to earth this ac set? I've read it can also improve reception . It is just live chassis sets you can't earth? Many thanks.
Dave.
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Old 18th Dec 2014, 2:56 pm   #155
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Default Re: Roberts RMB Problems

Wow, Dave, I never thought I'd hear that, so WELL DONE!!!

Yes, you can indeed earth it. It's got an isolating transformer so you're safe to do so. A proper BS 1A fuse in the plug is also a good idea.

Nick.
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Old 18th Dec 2014, 5:03 pm   #156
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Default Re: Roberts RMB Problems

Check, 1amp fuse has been fitted all along.
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Old 18th Dec 2014, 10:18 pm   #157
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Default Re: Roberts RMB Problems

This calls for a celebration! I bet you're pleased.

Looking back, do you think the cause of the fault was the bad soldered joint you found on the coil that you mentioned in post #152 all along?
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Last edited by Phil G4SPZ; 18th Dec 2014 at 10:18 pm. Reason: Typo
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Old 19th Dec 2014, 12:46 pm   #158
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Default Re: Roberts RMB Problems

Well done Dave. I must applaud your stamina and I should imagine you feel pretty confident now as a radio repairer.

Jim
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Old 19th Dec 2014, 4:14 pm   #159
sobell1980
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Default Re: Roberts RMB Problems

Yes I think you are right Phil. I think this intermittent poor connection was the main issue, with the valve heater voltage also being low. I can now see where I was going wrong. Investing in the scope and sig gen earlier would have revealed these problems much earlier. I also thought I understood the circuit well, but I'd learned it incorrectly. My understanding of how the tuning coils were part of the circuit etc was my biggest bug bear. To be honest I really enjoyed the alignment which in the end revealed the fault on the coil. I am no expert and have a long way to go in this fantastic hobby. But the purpose of this set was for me to gain this knowledge. I shall contine with the scope on working sets. This will aid my future diagnosis of what I should see where. 2011 I started this set! I have since acquired many other valve sets to continue this hobby. The RMB is being given to my very good friend for Christmas as my first completed valve set. I would like to take this opportunity to thank everyone on here who has had input on this repair and taken the time to mentor me.
Jim, thankyou for the valve sockets and the afternoon in your workshop and lesson on alignment. Also for the boxes of useful valves etc. All of you have been great. When I get time i shall post my success story with some photos. Kind regards.
DAVE.
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