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Old 25th Sep 2016, 7:44 pm   #1
Voxophone
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Default RF alignment question.

A few months ago I started restoring a Marconi 911 and posted a thread here regarding a problem with the MW reception. Since then I’m pleased to report that the issue has largely been solved and that the set has plenty of volume on all bands.

The one remaining problem is that MW reception fades and eventually stops towards the LF end of the dial. The station positions are also about 300 kHz on the low site with respect to the dial. Obviously I started with the intention of performing an r.f. realignment, however the Trader sheet specifies a 510m signal to adjust the cores and this lies beyond the point on the dial where reception has faded to nothing.

Can anyone advise as to the best way to recover the alignment when one end of the band is too weak to be heard?

I suppose the problem could also lie in components although both SW and LW work for the full range, so it’s not a shorted tuning cap. The MW oscillator does appear to stop when the problem range is reached, however my ‘scope and DMM also suggested a dead oscillator on SW which works fine.

Any suggestions would be much appreciated.

Cheers

Liam
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Old 25th Sep 2016, 7:52 pm   #2
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Default Re: R.f. alignment question

First thing to do is check that the pointer is correctly set with the tuning capacitor's vanes fully meshed or at some other specified position. The service sheet will describe how to do this.

As regards the dead end of the band a strong signal from a signal generator will always get through regardless of the RF tuning, so long as the local oscillator is running at the correct frequency.

You could try setting the tuning to a dead spot, then sweeping the SG to see if you get a response somewhere.

Alternatively find a spot where you can pick up the SG, then change the SG's frequency a bit and see if you tune it on the radio. "Chase the SG's signal across the band. This will help you to see if the local oscillator is stopping, due to say shorted tuning capacitor vanes.
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Old 25th Sep 2016, 8:27 pm   #3
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Default Re: R.f. alignment question

First it could be a failing LO valve that stops oscillating as the frequency is lowered. It may well be ok on other bands.

Second, to check calibration, use another radio (portable one with digital readout if possible) to check the frequency of the local oscillator. Your stated 300 kHz low is a LONG way out!

For example, if the IF is 470 kHz and you tune to a station OF KNOWN FREQUENCY (e.g. Five Live on 909 kHz or 693 kHz - make sure you know which one!) and tune your portable to 909 + 470 = 1379 kHz or 217 m.

From this you can easily check the calibration of the MW dial and LO. Don't do it at the high frequency end of the band or the LO frequency will be out of range of the portable radio MW coverage. (If you're lucky you use an all-band SW radio in that case.)

Hope that makes sense. This technique is particularly useful when aligning SW bands so you don't tune in the image because you've set the local oscillator frequency.

Ian
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Old 26th Sep 2016, 3:31 pm   #4
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Default Re: RF alignment question.

Thanks both for the suggestions.

I'm fairly confident that the dial pointer is set correctly with respect to the gang. I took pictures of it prior to dismantling and have put everything back as it was. Besides this R4 and other LW stations line up well. It's evident that there's a problem just by looking at where the stations appear with respect to the gang position. Absolute Radio appears with the gang half open (or half shut depending on your perspective), which is clearly not correct.

I've tried 'chasing' the signal across the band, and have found that I can get a 510m signal through if the amplitude is high enough. The position is clearly not correct as it's not much past halfway on the gang.

Following Ian's suggestion I placed a portable next to the set and tried to pick up the LO. The lowest frequency station I can receive is Talksport at 1071 kHz, which is very faint. When tuned to this I can pick up LO tones at approximately 610 kHz and at just over 1500 KHz on the portable. The latter would seem to be the correct value for this station for an IF of 465 kHz.

When I've measured the LO on a 'scope in the past, the frequency read at just over 2MHz with the gang fully open, which seems correct. It progressively falls in amplitude as the frequency is reduced and disappears from the 'scope while still well over 1MHz. By contrast the LW oscillator can be viewed end-to-end and the amplitude is very large.

Next I'm going to try a substitute mixer valve.

It might also be worth mentioning that the MW aerial trimmer (Trader C4) was damaged and had to be replaced with a modern silver mica. Prior to this however I tried replacements of several different values and I seem to recall that the station positions were still incorrect.
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Old 26th Sep 2016, 4:22 pm   #5
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Default Re: RF alignment question.

Like any Mixer, take for example a food mixer, you need all of the ingredients at the right proportion to bake the perfect cake. Remove for example the Flour and the cake is useless. For a good I.F. signal you need a good input from the antenna plus a good input from the local oscillator. If either loses amplitude then the I.F. will lose amplitude. So if the scope shows a drastic reduction and final collapse in local oscillator then this should be investigated.
V1 uses one of its grids as an anode, this being fed by R5 and coupled to grid via C10, L8, C8 and L10, I would check C6, C8 and C10 for any leak particularly if any are ceramic but Mica are normally OK but still worth a check. I have found mica plate capacitors to wander way off value so check this as well as leakage. The other components around the oscillator circuit worth checking are C5 and R4 together with the cathode components R3 and C4.

If you suspect loss of the RF input signal then if you can borrow a signal generator, you can feed this into the antenna sockets and check that it appears on the grid of V1 through the whole frequency range without loss.

Certainly check the screen grid resistor and decoupler R6 and C13.

Usually a low emission V1 will give local oscillator failure at its higher frequency points i.e. I would expect it to cut off as the vanes open rather than close, but this is not 100%. I say this, as I have a short wave radio which cuts off at the lower frequency end of one band. All components have been checked, the valve replaced but to no avail. It remains a mystery but one which I hope to solve one day

I have always personally maintained that the best arrangement is to have a separate oscillator followed by a leveling amp to maintain a constant amplitude of local oscillation. Some of the most revered receivers suffer from drop off in performance as the frequency range is increased, my investigations have always found the same thing happening and that is slowly reducing amplitude of local oscillator input. But I digress.

Check the components as mentioned that exist around V1 then, if no joy, try a known good X63M.
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Old 26th Sep 2016, 5:44 pm   #6
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Default Re: RF alignment question.

1. Double check the pointer position as Graham suggests.
2. Clean the wavechange switch thoroughly.
3. Get the oscillator running at the correct frequency when in the open position where it is now working.

As Top Cap has pointed out, quite a few components are left in place when in the MW position so if you can't get the oscillator to align here we need to think again. C10 (200pF) is the LW tracker but also serves to bypass MW RF and its impedance at MW is not that low.
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Old 26th Sep 2016, 7:57 pm   #7
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Default Re: RF alignment question.

Another couple of results to share.

I tried another input valve (a 6A8GT) and have found that strength of the lowest frequency stations such as Talksport is now much better, although they are still quite noisy. Unfortunately I don't have a good X63M as a direct replacement.

I've also tried feeding the signal generator into the A-E inputs and viewing the control grid of V1 on the scope with the set switched off. Based on the results obtained, the aerial circuit alignment is not too bad. Signals from 500-200m can be fed through and tuned successfully with a relatively constant amplitude obtained and the dial positions within a few tens of kHz. This suggests that the problem lies with the oscilator being off frequency rather than the aerial circuit.

Afterwards I powered up the set and viewed the LO on the scope. With the replacement valve I found it does oscillate from end to end of the gang; however it only varies from 2.21 MHz (gang fully open) to 1.5 MHz (gang fully closed). It seems therefore that the frequencies are spread too thinly over the gang range resulting in the lowest being out of range and higher frequency stations being shifted down the band.

I'll try some of the tests suggested to see if I can figure out why this is happening. Is there any possibility it could be due to lack of metalisation on the LO valve? I only have a non-metalised X63 or the 6A8GT to choose from here.

Also, does anyone have the exact instructions for aligning the tuning pointer? Neither of the schematics I have describe how this should be done. The pointer on this set is an illuminated drum with a painted spiral rather than the usual cord and pointer.

Cheers

Liam
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Old 26th Sep 2016, 8:08 pm   #8
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Default Re: RF alignment question.

If the switch contacts etc are ok then that sounds like lack of C, maybe check the padder?

Lawrence.
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Old 26th Sep 2016, 8:30 pm   #9
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Default Re: RF alignment question.

The switches have previously been cleaned and continuity tested. They all seem to be ok.

The thing I don't understand is: how does the station get through the aerial circuit to the mixer valve if the correct oscillator frequency does not coincide with the peak in the aerial circuit? Surely the aerial signal shouldn't be strong enough away from the peak to get through to IF?
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Old 26th Sep 2016, 8:45 pm   #10
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Default Re: RF alignment question.

The fact that tuned the signal reaches the mixer has nothing to do with the oscillator frequency.

Lawrence.
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Old 26th Sep 2016, 9:14 pm   #11
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Default Re: RF alignment question.

I realise that. I'm just surprised that the aerial signal gets through at sufficient magnitude away from the circuit peak to produce strong IF and audio signals. My measurements seem to show that the aerial circuit peaks much further along the gang from the LO frequency required for an IF of 465 kHz - suggesting that it must be thoroughly de-tuned at the point where I'm seeing it on the dial - and yet the set has plenty of volume. Perhaps I'm thinking of this the wrong way.

I've attached a picture of the MW LO tracker (Trader C9) below. It's the round one to the left of the picture. Is this type usually problematic?

Liam
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Old 26th Sep 2016, 9:26 pm   #12
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Default Re: RF alignment question.

If longwave works normally and stations appear in the right places, then the tuning capacitor is working OK and the pointer is strung OK, and the IF is set up OK.

But if some component problem is upsetting the local oscillator tuning on the medium wave band only, then medium wave stations can be shifted on the dial. It is the LO which really determines the frequency being received. So when you offset the tuning to get the wanted station, the RF input tuned circuits are not faulty and so are tuned to the frequency the pointer says, while the LO is set to receive a different frequency. So for the frequency the LO will convert into the IF, the RF tuned circuits are badly off-tune and will attenuate the signal, so the signal heard will be very weak.

So it could all be explained by a single fault in the MW coil/trimmer/padder area of the LO.

David
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Old 26th Sep 2016, 9:42 pm   #13
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Default Re: RF alignment question.

Post#7 gives the max/min oscillator frequency that was 'scoped for what I assume is MW....2.21mHz-1.5mHz...less The IF of 46kHz = signal frequency coverage of 1.75mHz-1.035mHz, that implies that the oscillator is not tuning across the full range it's designed to (approx 3:1) and certainly not low enough, assuming that L9 is set somewhere near the ball park.

The only schematic I've got for reference is the one shown in post#5, in that, the oscillators MW padding capacitor is C7, and it's trimmer is T1, the value of C7 will affect the LF end oscillator tuning more than T1 will.

Whoops....Post crossed.

Lawrence.
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Old 26th Sep 2016, 9:46 pm   #14
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Default Re: RF alignment question.

Quote:
Originally Posted by Radio Wrangler View Post
So when you offset the tuning to get the wanted station, the RF input tuned circuits are not faulty and so are tuned to the frequency the pointer says, while the LO is set to receive a different frequency. So for the frequency the LO will convert into the IF, the RF tuned circuits are badly off-tune and will attenuate the signal, so the signal heard will be very weak.
This is exactly what I was thinking; however the signal received is far from weak except further down the band. For stations in the 1500-1050 kHz range, or thereabouts, the set has a huge amount of volume - despite the aerial circuit being off-peak. This is what I can't get my head around.

For example, Absolute Radio (1215 KHz) appears very strongly at about 340 m on the dial, which is 332 kHz out of position on the low side. The LO measures 1680 kHz at the same point (i.e. 1215+465 kHz). And yet the aerial circuit, taken on its own, tunes to about 850 kHz at the same point. Hence, as you describe, it is not in alignment with the LO and should be attenuating the signal.

Am I wrong in thinking that the aerial circuit can be tested with the set switched off? I assumed this would be ok to do so since there are no active components, and it helps to stop the LO interfering with the 'scope.

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Old 26th Sep 2016, 10:14 pm   #15
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Default Re: RF alignment question.

Think of the Local Oscillator as controlling just the received frequency. ie LO frequency plus IF frequency equals received frequency, which I'm sure you know.

The RF circuit on the other hand controls signal strength. A strong signal will get through it however it's tuned.

As you say the RF circuits can be tuned without the LO running, so long as you have a means of measuring the amplitude of the signal coming out of the other side of the tuned circuit.
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Old 26th Sep 2016, 10:29 pm   #16
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Default Re: RF alignment question.

Thanks. That's understood. I'll try and desolder the padder tomorrow and get an idea of the value.

Cheers

Liam
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Old 26th Sep 2016, 10:57 pm   #17
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Default Re: RF alignment question.

Yes, that's a good move.

General note: Alignment is always IF..Osc..RF in that order, with a final check/tweek of Osc and RF if needs be, scale pointer should be set correctly for Osc/RF alignment.

If there's no scale pointer reference available, then for the LF end alignment point...tuning gang fully closed then backed off a few degrees, for the HF end alignment point...tuning gang fully open then backed off a few degrees, that should get it near enough then a final tweek if needs be.

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Old 27th Sep 2016, 12:42 am   #18
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Default Re: RF alignment question.

As you tune further down the band, the Q of the RF circuit will make it progressively narrower, so the attenuation from a tuning offset will be greater.

David
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Old 27th Sep 2016, 8:29 am   #19
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Default Re: RF alignment question.

Could this problem be that the air spaced tuning gang is not balanced?

If dirty or damaged, the tracking can go out between the gangs, which would not affect LW as dramatically as MW.

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Old 27th Sep 2016, 9:04 am   #20
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Default Re: RF alignment question.

Bad tracking affects longwave more than it affects medium wave, because the RF tuned circuit will be narrower and more selective on lower frequencies. Also the tuning of the RF and of the LO are more octaves apart onLW, so tracking becomes harder to achieve as well.

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