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Old 26th Oct 2014, 10:34 pm   #121
ronbryan
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Default Re: Roberts RMB Problems

Sorry Nick. The way I read it is 1.6 divisions at 5us/div or 0.8 divisions at 10us/div. Does that not equate to 8us period, equivalent to 125kHz?

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Old 26th Oct 2014, 11:05 pm   #122
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Default Re: Roberts RMB Problems

Yes, I'm sure you're right, Ron.

N.
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Old 27th Oct 2014, 1:13 am   #123
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Default Re: Roberts RMB Problems

From Dave's first photo, five full cycles occupy exactly 7 divisions or 35 microseconds. The period is therefore 35/5 or 7 microseconds, which makes the frequency 142kHz.

Correct me if I'm wrong, but I think the local oscillator should normally be running on the high side of the wanted signal, not the low side. What I think is happening here is that the L.O. is way off frequency. To create a signal at the set's 472kHz intermediate frequency, with the L.O. running at just 142kHz, must mean that the second harmonic of the L.O. is mixing with the 198kHz signal frequency and the IF is responding to the sum of the two to give what appears to be reception of Radio 4 at roughly the right place on the dial. It would also explain why MW isn't apparently working at all.

Dave, can you try repeating the test but with the set switched to medium wave and tuned to 300 metres on the dial? If it's working at all, the local oscillator should be running at 1,472kHz under these conditions. The period will be 0.68us, or roughly two-thirds of one division with the timebase set to 1us/div.
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Old 27th Oct 2014, 7:30 am   #124
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Default Re: Roberts RMB Problems

It was set to 10 microseconds in both wave forms. It was the voltage that was changed but the same time base was kept. I don't have an option on my scope of "cal " to set the fine timebase to as Phil mentioned.

I have freed off the IF tuning core L12. I removed the core completely. Can anyone explain why it asks you to adjust the same tuning core from the top and then from underneath the chassis ? I removed the core of L12 and I can see straight through the if transformer? Are you adjusting the same core from either side of the chassis or should there be two cores in there? If it is a single core, why is it peaked up from both sides? Sorry if my questions seem simple or strange. This is my first alignment. Many thanks.
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Old 27th Oct 2014, 7:36 am   #125
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Sorry Phil, the last 3 or 4 posts have only just appeared on my phone from your replys last night. Sorry if the the thread now appears jumbled. I shall digest what you have written and try that later. If you could let me know about what I asked regarding L12, that would be great.
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Old 27th Oct 2014, 8:51 am   #126
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Quote:
Originally Posted by sobell1980 View Post
It was set to 10 microseconds in both wave forms.
I beg to differ!
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Old 27th Oct 2014, 10:35 am   #127
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Originally Posted by sobell1980 View Post
I have freed off the IF tuning core L12. I removed the core completely. Can anyone explain why it asks you to adjust the same tuning core from the top and then from underneath the chassis ? I removed the core of L12 and I can see straight through the if transformer? Are you adjusting the same core from either side of the chassis or should there be two cores in there? If it is a single core, why is it peaked up from both sides? Sorry if my questions seem simple or strange. This is my first alignment. Many thanks. Dave.
The Trader sheet shows that the IF transformers are double tuned. They should have two cores fitted. Have you looked at the second IFT to see how many cores it has?

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Old 27th Oct 2014, 11:35 am   #128
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Default Re: Roberts RMB Problems

Ron is quite correct, L12 is a separate coil from L13 and they should have one tuning slug each. It certainly won't help matters if the coil of L13 is missing, but it doesn't explain why the L.O. is running at such a low frequency.

Have any modifications inadvertently been made to the set? I know you have replaced virtually every capacitor, Dave, but are the values all correct? Too low a frequency suggests too large a capacitance or inductance. I presume the inductors haven't changed, so are you absolutely sure that the replacement capacitors have the right values? C5, for example, is part of the frequency-determining circuit on all bands and should be 100pF; if 100nF has been fitted instead, it could give rise to the symptoms you are seeing. Modern capacitors are marked in strange ways, such as 103K, so it's easy to get confused.
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Old 28th Oct 2014, 3:36 pm   #129
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Default Re: Roberts RMB Problems

Dave,

I've been following this thread with great interest and not become involved because I couldn't improve on the first rate advice that Phil has been giving you.

There is just one suggestion I'd like to make however to help you familiarise yourself with your new equipment and that is to connect your sig gen to your scope with the sig gen set to the various frequencies you expect to be encountering. You can then set up your scope and sig gen to display the traces with various time base speeds and input sensitivity settings so that you have a clearer idea of what to expect when you are testing the radio.

All the best. By the way I still have a box of stuff for you to collect one day.

Jim
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Old 30th Oct 2014, 9:38 pm   #130
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Thanks for all the support and help. It is very hard for me to get time on the set in the working week. I didn't want you to think your advice and kind help was being ignored. I have read and digested all your replys. Hopefully at the weekend I can take another look and go over my work and act on your replies to my help. I will be back on through the weekend. Many thanks.
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Old 30th Oct 2014, 11:07 pm   #131
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Dave,

I've been reading back through this thread a bit. In post #73, you used a 1.5 volt battery for the LT, and you said the set sprang into life and you were getting Absolute Radio very strongly. It's not clear whether this was with the set switched to medium wave. Since then, as far as I can tell, all you've worked on is the power supply. In the process it appears that something else has gone wrong, presumably with the local oscillator frequency, although even that is debatable.

As a general point, it is often tempting to try to reconcile fault symptoms with a single cause, but it is very often the case that two or more simultaneous faults exist. This happens, in my experience, more frequently than you can imagine.

Here you do indeed seem to have been trying to solve two separate and unconnected faults simultaneously. The radio worked fine one minute and not the next - this could have been due to the low LT voltage. Now you have fixed the LT voltage problem, it seems like the reception is once again intermittent, meaning that it could have been due to another cause entirely. You went to a great deal of trouble to replace all four valveholders, for example, yet it isn't entirely clear that these were at fault.

When fault-finding, you should always start with the basics and eliminate the simple things first, like confirming the correct LT and HT voltages, that the valve electrode voltages are roughly correct, and that the speaker is working properly - I've missed that one many times myself! Then, and only then, can you confidently move towards the less obvious faults and eliminate each stage in turn, working backwards from the output, as you have been doing. So commence by consolidating what you know so far; (a) that the slug from coil L13 is missing, (b) that the local oscillator is running but apparently on the wrong frequency, and (c) that the power supply and audio stages are working properly. Until you've fitted a new slug into L13, there's no point going any further with realignment. Once you've fitted a new slug, you can realign the IF stage. Then you will need to repeat the tests recommended earlier to establish the LO's frequency and, if necessary, confirm that the capacitors that you have fitted are all the correct values.
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Old 15th Nov 2014, 4:19 pm   #132
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Hi Phil and all who are following this thread.
I have finally had some time today to sit and take a look at the set and get back to your replys. Okay, going back to post #113 I have taken a look at the last few remaining caps I replaced for correct value and fitment. C9 as asked in post #113 should be to the value of 547pF. It is replaced with a cap to the value of 560pF.
And the last remaining ones i replaced around the same area are C7,C1 and C5. these were all replaced with caps to the value of 100pF.

Quote.
"I've been reading back through this thread a bit. In post #73, you used a 1.5 volt battery for the LT, and you said the set sprang into life and you were getting Absolute Radio very strongly. It's not clear whether this was with the set switched to medium wave. Since then, as far as I can tell, all you've worked on is the power supply. In the process it appears that something else has gone wrong, presumably with the local oscillator frequency, although even that is debatable."

I can confirm Phil the set was set to LW when this occured and not MW.

In a rush to reply to your messages, i quickly looked at the set early before work a few weeks back to urgently reply to your post. I did this without the service data sheet and made a school boy error. Neither of the double tuned IF transformers have a core or slug missing and are rounded off / seized. I was infact that morning looking at one of the single tuned oscillator tuning coils in a rush before work. This is why I mistakenly thought there was a slug missing etc. So L12 and L13 and L14 and L15 have all their tuning slugs, if they are all but seized and rounded off. I think this set has previously been severly got at, as i have till only now having coming to set the alignment never touched these adjustments. I now need to remove both IF transformers and remove the tuning slugs and replace them. I will not be able to do this with them in situe. They need drilling out in someway carefully. I have done a search on the forum to see how this is done. The question is, where do i get replacement cores or even new transformers if that is easier? If any one can help with this repair that would be great.
In response to one of your previous questions Phil, i think also in post #113, c3,c4,c12,c13, none of these have been replaced as they are i assume in the IF transformers themselves. I look forwards to your replys and getting these seized IF transformers sorted once and for all.
Dave.
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Old 16th Nov 2014, 12:47 am   #133
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[The cores] need drilling out in some way carefully. I have done a search on the forum to see how this is done. The question is, where do I get replacement cores or even new transformers if that is easier? If any one can help with this repair that would be great.
Dave,

As you recognise, drilling out old dust iron cores isn't easy. Often they were sealed in position with wax, shellac, nail varnish or even screwed in with a rubber band down the core, which over the years goes sticky or brittle and jams the whole core. Heavy-handed application of a metal screwdriver then cracks the core and causes the problems you describe.

I've only ever had to do one or two, and the drilling is difficult because the core material is very hard as well as brittle. It's wise to remove the IFT from the chassis first. Try dribbling WD40 or a penetrating oil down the thread, which may loosen the slug. You can start off drilling down the centre of the slug, only to find that the slug cracks up and the drill moves off centre, with the very real danger that you will drill into the soft wall of the former and damage the threads. If I remember correctly, I did it by drilling a couple of millimetres using a very small (1.5mm) drill, then shaking out the cracked debris, then drilling a few more millimetres, and repeating the process until all the core had been broken up. The challenge then is to remove the tiny bits of core material that will remain inside the threads of the former. I used a pipe cleaner folded in half, with a drop of oil on it, screwed into and out of the former several times. It's a painstaking process, but can be done successfully with patience.

Finding replacement cores may be less of a problem. If you can extract enough of one intact core to identify the thread diameter and the number of threads per inch, you can post a wanted request. The easier way all round would be to obtain some new IFTs, but unless you can find a complete scrap set, that's unlikely. I have a few IFTs and some dust iron slugs, if you can identify the size.
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Old 17th Nov 2014, 12:17 am   #134
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Dave,

I just came across a very helpful description of ways of removing broken tuning slugs, in a 2008 article by Ian Batty in an Australian vintage radio journal. I scanned it and hope you will find it useful.

Phil
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Old 30th Nov 2014, 8:02 pm   #135
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Good evening all. Managed to grab an afternoon on this dreaded set . To be honest it's taken me a couple of weeks to regain some enthusiasm back for it.
So, this afternoon I thought rather than go to the hassle of removing two IF transformers I would try and drill the cores out in site. I used a very very slow drill speed and my top of the range Snap on tools drill bit set. I managed to slowly drill through both cores in both transformers. I then went up various drill sizes and got all the broken ferrite from the formers or threads. I then matched the thread of the new cores I have received from a very kind forum member Peter. I matched these to a very long bolt I had and used this as a "tap " to perfectly clean the threads out through the entirety of both IF transformers. The new cores fit perfectly and snuggly. I had to stop at this point as my services were required with other household matters! But, and a big but, progress. Ready for alignment now. I stand a fighting chance. Perhaps one evening neXT week I can sit and concentrate on my alignment. Many thanks as always for your continued support and interest.
Dave.
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Old 30th Nov 2014, 8:09 pm   #136
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Very impressive, well done!
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Old 30th Nov 2014, 10:36 pm   #137
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Yes, well done, Dave, you've succeeded with one of the more difficult radio repair tasks. You can at least console yourself with the thought that your next repair - if you ever decide to tackle another - will be a doddle compared to this one!
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Old 1st Dec 2014, 9:48 pm   #138
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Good evening. I had a quick look at the set this evening. Not to align, but to check I still had a waveform at the top end of c35 after drilling out the cores. I have a waveform on my ocsillascope that lengthens and shortens as c35 is adjusted. This was on LW. However keeping my probe on c35 I have no waveform on MW or Sw. Am I missing something obvious or carrying out an incorrect check for the other wavebands? Many thanks
Dave.
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Old 2nd Dec 2014, 12:03 am   #139
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Dave,

The oscillations you are seeing on C35 should be present on all wavebands, but obviously at different frequencies. If the oscillator is working on LW but not on MW or SW, it's possible that the valve is a bit tired. Remind me which valve it is and I'll see if I have a good one.

Phil
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Old 2nd Dec 2014, 8:29 am   #140
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Hi Phil,
What could be happening in the valve exactly for this to occur? Could it be that the ocsillator tuning coils are so far out of tune that this could be the cause? They all have except one been moved to check they are free. So everything is out of alignment. One is tuning Coil is being stubborn, but I'm leaving this for now as it may not need adjusting.
DAVE.
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