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Old 9th Sep 2014, 10:20 pm   #61
Phil G4SPZ
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Default Re: Roberts RMB Problems

Avo signal generators are pretty robust, so one like that should be a good bet, better still if it can be relied upon to be working and accurate.

Colin makes a good point about the valves and the LT voltage. 1.3 volts might be a bit low. You could try running the set off batteries, which I must admit I prefer to do when repairing portables, as it eliminates the PSU as a potential source of faults as well as being safer. Just lash up ten cheap pound shop PP3s in series for the HT and a single alkaline D cell for the LT.

I can test your valves for you and I probably have some spare good valves if you need any. Drop me a PM when convenient.
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Old 10th Sep 2014, 3:01 am   #62
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Default Re: Roberts RMB Problems

The Advance E2 is a good old RF generator - it's what I use. I'm sure I've seen these come up for sale in the past for £20 - £30 in the 'for sale' section.
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Old 11th Sep 2014, 12:45 am   #63
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Default Re: Roberts RMB Problems

Two posts removed, please start another thread for signal generator advice, avoiding discussion of eBay auction items.
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Old 6th Oct 2014, 8:58 pm   #64
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Hi all,
I have now acquired a sig gen and oscillascope, and familiarised myself with both of them. I have injected an AF signal into the grid of the output valve and at the wiper contact of the volume pot and gained a good tone from the speaker. I have then switched to RF signal and injected a modulated 475 kc's into the grid of valve one. This again gave an identical tone to the AF signal injected earlier at the volume pot. I then worked back to C1. On the valve side of C1 i gain a good tone from the speaker, if i move to wave change side of C1, the tone becomes ever so slightly quieter. If i inject a signal at the aerial connections it is barely audible. C1 has been replaced. Signal generator tone through the speaker can be heard loud and clear in any wave change switch position. This is when the signal is injected the valve side of C1. It is very slightly quieter on the wave change switch side of C1 but only slight. Injecting into the mw frame aerial and LW aerial it can only just be heard. I guess it does need alignment, but something should be picked up on one of the wave bands. Even if faintly or some tuning whistles etc. I feel it could be the wave change switch seeing it is effecting all wave bands, not just one. Anyway this is my latest progress. Enjoying using the new kit if nothing else. Any pointers would be great and help. Feeling like i am gaining progress now.
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Old 6th Oct 2014, 9:43 pm   #65
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Default Re: Roberts RMB Problems

Okay, Dave, glad you got the test gear up and running. It's worth checking first, with your oscilloscope, that you can see a strong sine wave at the top end of C35. If there's nothing, or just a mish-mash of signals, the local oscillator isn't running.

The problem then is that you are trying to inject the 472kHz intermediate frequency modulated RF at the wrong place. You can inject 472kHz at the grid of V3 (pin 3) and you should hear the tone. Then move back to the grid of V2 (pin 6) and the tone should be much louder. This proves that the IF amplifier (V2) is working. Reduce the output from the signal generator to get a comfortable level of sound.

The area around C1 handles modulated RF at signal frequency, so inject the signal into the grid of V1 (pin 4). Tune the radio to 300 metres MW and set the signal generator to 1,000kHz. Rock the radio's tuning either side of 300m and at some point you should hear a signal. Work back from pin 4 to the aerial side of C1, then back to the aerial socket. Keep the generator signal as low as possible.

As you work backwards in this sequence, the point where the signal vanishes shows where the fault lies. But do check that the LO is working first.

Phil
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Old 8th Oct 2014, 6:13 am   #66
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Default Re: Roberts RMB Problems

Ok, got in late from work last night and quickly checked the LO. I checked with my scope the top end of c35, well between c35 and c5. I checked on all voltage divisions and time divisions, nothing. Sorry if i do not quite understand this yet. How is it if i inject a tone at c1, a clear tone can be heard through the speaker if the L0 is not working? I guess i need to go back and check V1 voltages? If i remember from my previous tests correctly though, these were ok. I think all my heaters were running 1.3 volts.
Many thanks.
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Old 8th Oct 2014, 8:59 am   #67
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Default Re: Roberts RMB Problems

At what level are you injecting the 475kHz (Intermediate Frequency {I.F.})? It may be that the L.O. is working, but weakly, so a very strong signal will force it's way through the system.
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Old 8th Oct 2014, 9:29 am   #68
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A strong signal at 472kHz injected at V1 grid will certainly appear at its anode and pass through the IF stage, even if the LO is stalled. This whole saga suggests that the LO has been stopping and starting intermittently throughout. Some DK9x valves can be low gain and fail to oscillate, especially if the filament voltage is marginal, as yours is. Before you do any more, hook up a 1.5 volt D cell to power the filaments at full voltage - you may find the set works.

If so, you then need to resolve the low LT problem.
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Old 8th Oct 2014, 4:27 pm   #69
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Default Re: Roberts RMB Problems

Obviously I have misunderstood the use of my sig gen at the RF stages. I could not tell you what the attenuation was set to when i was injecting a RF signal. I was turning it up and down when it was too loud or quiet where as i should have been using the strength of the tone in various points to aid my diagnosis.
If anyone could run through what i should be doing with the sig gen on the RF side? What i should set my sig gen to etc and attenuation.Phil, in your post #65, you say about setting the sig gen to 1000khz and the radio to 300m. Why these figures? What is the relationship between 1000 khz and 300m? Is this a standard test setting . Just trying to get my head around all this new test gear. Many thanks for your time and help.
Dave.
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Old 8th Oct 2014, 4:41 pm   #70
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Default Re: Roberts RMB Problems

1000Khz = 1 Mhz= 300m. wavelength x frequency = 300000 , the speed of light, depending on units.
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Old 8th Oct 2014, 5:48 pm   #71
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Default Re: Roberts RMB Problems

Okay thanks for that Pete Kaye. Thats cleared that up. Also i did some reading online so i get the figures now. If anyone else can help with my questions and quieries in my poat #69? I shall do as Phil says and check my LT and install a D cell. Just want to also clear up the correct use pf my sig gen on my Roberts RMB on the RF side. Many thanks,
Dave.
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Old 8th Oct 2014, 10:37 pm   #72
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Default Re: Roberts RMB Problems

Quote:
Originally Posted by sobell1980 View Post
Obviously I have misunderstood the use of my sig gen at the RF stages. If anyone could run through what i should be doing with the sig gen on the RF side? What i should set my sig gen to etc and attenuation.
There is a wealth of helpful information on Paul Stenning's website, and you would be well advised to read the descriptive articles on basic superhet principles. Here is a good start on how the IF stages work.

The only reason I suggested 1MHz from the signal generator and 300m on the radio is that this will most likely be the way the equipment's dials are calibrated. As Pete says, 300 metres equals 1,000kHz and the numbers are easy to remember, as well as every broadcast radio will have this wavelength marked on the medium waveband. It's also a quiet spot on the band, at least hereabouts, and I set my pantry transmitters to this frequency.

The general rule is to keep the output from the signal generator as low as possible, commensurate with satisfactory reception. If you have to turn up the generator output to hear anything at all, it tells you that something's wrong. When you progress to aligning radios, a small input signal prevents the set's AGC from coming into action and reducing its gain.

Once you have got this radio working, a few minutes spent playing with the sig gen will quickly give you the idea. You have all the test gear now to perform a full alignment, and even if the set doesn't need it, you will learn a great deal by following the alignment procedure set out in the Trader service sheet.
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Old 11th Oct 2014, 8:30 am   #73
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Default Re: Roberts RMB Problems

I fitted an AA battery to LT supply. The set immediately burst into life, picking up absolute radio very strongly. I changed the selenium rectifier along time ago for a couple of diodes giving me my 1.3 volts to the heaters. Obviously this is not enough for a low emission valve. I need to change the value of L 17 to bring the voltage up. Because L17 is a choke can i substitute this for a modern day resistor? Also would anyone know what the total current draw of the heaters is so i can do yhe maths to work out a new value resistor? I have about 1.90 v after my diodes that replaced mr2. After L17 i have 1.3v. The value of L17 is 2.7 ohms. Which points would be best to connect my meter in series to measure my total current draw? I can then work out my new tesistor value. Many thanks.
Dave.
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Old 11th Oct 2014, 9:03 am   #74
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Default Re: Roberts RMB Problems

The '96' range of valves filament current is 25mA, whilst that of the earlier '91/92' series is 50mA, so, because the two filaments in a DL91/92 96 are wired in parallel, the total filament current for a set of '96' series valves is 125mA @ 1.5v, whilst that for the '91/92' series is 250mA. I was reminded of that recently, when trying to power an Ever Ready model 'C', which uses DK91, etc., from a Murphy PSU designed to power a radio using the DK96, etc. (or equivalents). The LT winding on the little transformer was being overloaded by the older valves!
To measure the filament current, insert your meter, on a low DC current range, say 500mA FSD or less, between the +ve LT supply and the connection to the valve filaments, which should be pin 7 on each valve. A reading of just under 250mA or 125mA, dependant upon which series of valves is fitted, should be obtained. Although it's important not to exceed 1.5v, you may find that no series resistor or choke is neeeded, but fitting a couple of shunt diodes ( 2 x 1n4007 or similar) across the LT supply to ensure that it cannot exceed about 1.4v may be desirable.

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Old 11th Oct 2014, 4:22 pm   #75
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Default Re: Roberts RMB Problems

The smoothing choke L17 is an essential part of the LT smoothing circuit, and probably won't be at fault. More likely C25 and C26 have lost capacitance and need replacement.
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Old 11th Oct 2014, 5:44 pm   #76
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Default Re: Roberts RMB Problems

C25 and c26 have been replaced for F and T electrolytics. All smoothing caps and all electrolytics have been replaced.
Dave.
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Old 11th Oct 2014, 5:52 pm   #77
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Default Re: Roberts RMB Problems

The original selenium rectifier is shown as a bridge rectifier containing four diodes. Can you tell us exactly how you wired the 'couple of diodes' as replacements?
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Old 11th Oct 2014, 6:23 pm   #78
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The two anodes of the 2 diodes are conneted to the LT windings of the transformer. The two cathodes of the diodes are soldered together and joined into the circuit where they eventually meet L17. A repair another forum member talked me through on this set a very long time ago.
Dave.
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Old 11th Oct 2014, 6:44 pm   #79
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Default Re: Roberts RMB Problems

What is the mains voltage coming into the set (measure carefully - insulated probes!)?

Is there a 'tapping' for different line voltage ranges, and if so, which one is selected?

If the mains voltage is low where you are, and the primary tapping is set to maximum, this might account for low LT.
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Old 11th Oct 2014, 6:59 pm   #80
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Quote:
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The two anodes of the 2 diodes are connected to the LT windings of the transformer. The two cathodes of the diodes are soldered together and joined into the circuit where they eventually meet L17.
That's your problem. Where is the negative connection? According to the Trader service sheet, the heater winding on the transformer doesn't have a centre tap. The circuit should be a full-wave rectifier, hence a bridge rectifier is necessary and you must have four diodes, not two. Wire the four diodes exactly as shown on the service sheet.

Be aware that modern silicon diodes may have a lower forward resistance than the original selenium rectifier, and the LT voltage to the valves may be higher than it should be. If you measure the LT current flowing from the battery as you have previously been advised, we can suggest a suitable value of series resistor to bring the voltage down to the correct value.
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