Roberts RMB Problems - Page 8 - UK Vintage Radio Repair and Restoration Discussion Forum

peter_sol · 2nd Dec 2014, 8:46 am

Try the new DK92
As those valves age they will not oscillate at higher frequencies IE MW and SW.

sobell1980 · 2nd Dec 2014, 10:49 am

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.

peter_sol · 2nd Dec 2014, 11:13 am

Yes on MW and LW not SW though

Phil G4SPZ · 2nd Dec 2014, 6:39 pm

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.

sobell1980 · 2nd Dec 2014, 8:19 pm

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.

Nickthedentist · 2nd Dec 2014, 8:24 pm

Yes, do check the wavechange switching. On mine, it seemed fine one minute, then hopeless the next, for no apparent reason.

sobell1980 · 2nd Dec 2014, 9:12 pm

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.

Nickthedentist · 2nd Dec 2014, 9:18 pm

Sort of functional, Dave! The waveband switch is intermittent and I haven't had the time or inclination to repair it yet.

sobell1980 · 2nd Dec 2014, 9:55 pm

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.

peter_sol · 3rd Dec 2014, 9:27 am

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.

Phil G4SPZ · 3rd Dec 2014, 9:48 pm

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.

sobell1980 · 6th Dec 2014, 8:04 am

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.

Phil G4SPZ · 6th Dec 2014, 8:26 am

Not quite Home and dry, but there's Light at the end of the tunnel.

sobell1980 · 18th Dec 2014, 2:51 pm

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.

Nickthedentist · 18th Dec 2014, 2:56 pm

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.

sobell1980 · 18th Dec 2014, 5:03 pm

Check, 1amp fuse has been fitted all along.

Phil G4SPZ · 18th Dec 2014, 10:18 pm

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?

G4XWDJim · 19th Dec 2014, 12:46 pm

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

Jim

sobell1980 · 19th Dec 2014, 4:14 pm

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.

2nd Dec 2014, 8:46 am	#141
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Roberts RMB Problems Try the new DK92 As those valves age they will not oscillate at higher frequencies IE MW and SW.

2nd Dec 2014, 10:49 am	#142
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	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.

2nd Dec 2014, 11:13 am	#143
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Roberts RMB Problems Yes on MW and LW not SW though

2nd Dec 2014, 6:39 pm	#144
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	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. __________________ Phil Optimist [n]: One who is not in possession of the full facts

2nd Dec 2014, 8:19 pm	#145
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	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.

2nd Dec 2014, 8:24 pm	#146
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	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.

2nd Dec 2014, 9:12 pm	#147
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	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.

2nd Dec 2014, 9:18 pm	#148
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	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.

2nd Dec 2014, 9:55 pm	#149
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: Roberts RMB Problems 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.

3rd Dec 2014, 9:27 am	#150
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	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.

3rd Dec 2014, 9:48 pm	#151
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	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. __________________ Phil Optimist [n]: One who is not in possession of the full facts

6th Dec 2014, 8:04 am	#152
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	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.

6th Dec 2014, 8:26 am	#153
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: Roberts RMB Problems Not quite Home and dry, but there's Light at the end of the tunnel. __________________ Phil Optimist [n]: One who is not in possession of the full facts

18th Dec 2014, 2:51 pm	#154
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	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.

18th Dec 2014, 2:56 pm	#155
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	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.

18th Dec 2014, 5:03 pm	#156
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: Roberts RMB Problems Check, 1amp fuse has been fitted all along.

18th Dec 2014, 10:18 pm	#157
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	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? __________________ Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 18th Dec 2014 at 10:18 pm. Reason: Typo

19th Dec 2014, 12:46 pm	#158
G4XWDJim Octode Join Date: Oct 2008 Location: Littlehampton, West Sussex, UK. Posts: 1,465	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

19th Dec 2014, 4:14 pm	#159
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	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.