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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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16th Apr 2021, 8:06 pm | #1 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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TESTGEAR MC/12 and FM Alignment Help
Good Evening,
Recently I bought an 'MC/12 Alignment Analyser' by TESTGEAR (Acton) LTD from a local member. It is a sweep generator that covers 400KHz - 80MHz and can be used to measure small capacitors. I bought it intending to learn sweep alignment of FM receivers. Not a great deal seems to be known about the MC/12. It came up a few times on this forum. It looked to often feature in Practical Wireless/Television during the '50s and '60s. My first question would be: does anybody have any practical, hands on experience with the MC/12? My MC/12 responded well to a good clean and service. The front bezel was easily removed. T-Cut and wax got it looking like it was new. The rear case was treated the same way after being washed. This removed the light smattering of rust (?) that was breaking through the paint in some places. I checked the RF output and was satisfied that the unit was outputting the signals it should be. With my 'scope cranked all the way up, I could see that the sweep was functioning when the 'sweep amp' control was manipulated. Satisfied that things were working OK, I changed out the Belling Lee connectors for BNC and the Wander sockets for 4mm Banana sockets. After building a diode probe, I was ready to try aligning a set. I picked out my Roberts R700 as it had sweep alignment instructions in its Trader Sheet. I have followed the instructions up to the end of step two. I'm not sure what I should be expecting to be displayed on my 'scope. I photographed what I saw. I expected a typical curve. My 'scope is in XY mode. Everything is set up as per the instructions and diagram. Does anybody know what I am doing wrong? |
17th Apr 2021, 11:06 am | #2 |
Heptode
Join Date: Jan 2011
Location: Croydon, London, UK.
Posts: 767
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Re: TESTGEAR MC/12 and FM Alignment Help
This appears to have only one valve, so it is likely to be a double triode, one half as a simple RF oscillator the other acting as a reactance valve. From information on this forum, the X output is a 50Hz sine wave.
To avoid any phase shifts which will confuse matters, both channels of your oscilloscope will need to be DC coupled. The diode probe won't work properly if it is feeding directly into a capacitor with no resistive path to the signal return. The photo shows that you are using AC coupling. When the diode probe is connected to the Roberts, without the wobbulator connected, just check that the IF strip isn't oscillating on its own, using your oscilloscope in normal timebase mode. Is the waveform in the photos showing the output of the wobbulator, or the signal at the test point in the Roberts? I would have expected to see a resonance type curve being displayed, what the photo is showing is nothing like that. You should hear a 50Hz hum from the radio when the wobbulator is tuned to 10.7MHz, the level and distortion of the hum should be adjustable by the frequency sweep amp. At the correct sweep setting for alignment, the hum should be rather distorted sounding. Paula |
17th Apr 2021, 3:19 pm | #3 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
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Re: TESTGEAR MC/12 and FM Alignment Help
Your right about the valve, it's marked 12AX7.
I'll keep the 'scope DC coupled. When the wobbulator (with diode probe still in place) is disconnected the radio receives OK. No sequels or oscillating. The waveform in photograph 3 shows the output on the collector of TR5 via the top end of R20. The scope was connected, via the diode probe, to R20. The wobbulator was connected to the aerial. I set the wobbulator bang on to 10.7MHz with my frequency counter. When I adjust the frequency sweep amp I start to hear 50Hz though the radio's speaker. It does change as the knob is rotated. With the amp turned all of the way up two curve looking things appear. I hope this helps. |
17th Apr 2021, 5:23 pm | #4 |
Octode
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
Very interested to see how you get on with this, I have one and tried all sorts but could never get a sensible waveform on the scope.
Andrew
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17th Apr 2021, 6:18 pm | #5 |
Heptode
Join Date: Jan 2011
Location: Croydon, London, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
That looks fairly promising. What you could do is connect the output of the Advance E2 to the Ext Ref in socket. Set the Advance to 10.7MHz and that should give you a reference pip on the display. You will need to play around with the output level for best results. It's possible that the centre modulation sweeping isn't exactly centred on 10.7MHz once the sweep width is increased. You can use the Advance to check that the upper and lower frequencies on the display to confirm that the frequency sweep is linear. If it isn't, don't adjust anything on the Roberts!
You can afford to reduce the frequency sweep width, as it will be easier to adjust if it appears wider on the screen. That may help to reduce the double image too. I suspect the Z output socket is to allow blanking of the oscilloscope trace during the flyback period and probably would remove the double image entirely. Your Hameg doesn't appear to have Z modulation capability, so you can't use that. What you have to do instead is to pick one of the images and ignore the second one. Is your diode probe connected directly to the Roberts, or are you using some sort of connecting lead, I can't tell from the photo. The diode probe must be connected directly, to avoid problems with stray capacitance which will skew the shape of the pattern that you see on the oscilloscope. Paula |
17th Apr 2021, 7:27 pm | #6 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
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Re: TESTGEAR MC/12 and FM Alignment Help
Some wobulators are sync to the mains waveform, some have there own frquency sweep arrangements, which should preferably around 20 to 30 Hz I think. Your timebase is too fast, but surely it should be in XY mode (I have forgotten, 40+ years since I aligned a radio) such that as the trace moves over the screen, it displays the diode probe output on the A channel. When it reaches the end of sweep, you may or may not (depends on scope and setup) see the retrace. I think you just need to get your scope settings correct. There was a series of articles about alignment in Television magazine in the '70s, (by Harold Peters) get hold of that and read it, then you should get a handle on it all.
Les. Last edited by MotorBikeLes; 17th Apr 2021 at 7:29 pm. Reason: Added citation. |
17th Apr 2021, 9:18 pm | #7 |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,958
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Re: TESTGEAR MC/12 and FM Alignment Help
Some wobbulators e.g. Metrix 210, have a retrace phase adjustment to line up the forward and reverse frequency sweeps, or as another option, the retrace sweep can be blanked.
You might find that a small adjustment to the carrier centre frequency will line up both traces better if you have neither of the above options. Injecting the 10.7MHz carrier into the external aerial circuit, as you are doing, (I agree Roberts suggest this), will cause the swept IF signal to be attenuated by the tuner IF trap. In some circumstances it may be better to inject the signal via a blocking capacitor a little further down the IF chain, as a smaller signal amplitude can then be used. Ron |
18th Apr 2021, 12:13 pm | #8 | ||||
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
Quote:
1. No circuit diagram. I've never traced out a circuit before but we could try and cobble something together? 2. We have no idea of how well this generator performs. How good is it at its intended function? Quote:
Quote:
Quote:
I have attached an instruction sheet for the MC/12 which members may find useful. It mentions the Z output. Also find 'The Art of Alignment'. Thanks for all of the replies! |
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18th Apr 2021, 12:22 pm | #9 |
Moderator
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Re: TESTGEAR MC/12 and FM Alignment Help
Have you tried manually sweeping the injected IF frequency (sic) to see if a peak is displayed on the scope?
ie Turning off the sweep function of the signal generator and SLOWLY turning the tuning knob from say 10.5 to 10.9 MHz whilst observing the trace on the scope. This will show whether the SG, diode probe and scope are working correctly.
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18th Apr 2021, 12:50 pm | #10 |
Heptode
Join Date: Jan 2011
Location: Croydon, London, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
The scope probe is going to be introducing a lot of capacitance which will upset the tuning of L13. That explains the very lop sided response. The diode probe must connect by the shortest possible leads to the test point - think mm, not metres here!
The X out signal is derived from the mains transformer and will be 50Hz. There aren't enough valves for it to be anything else! Paula |
19th Apr 2021, 2:44 pm | #11 | ||
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
Quote:
When I manually sweep the signal generator around 10.7MHz, the trace starts as a line, then takes a rectangular form then goes back to being a line. Quote:
I'll keep the lead length in mind and try and reduce them. When I feed the MC/12 a marker signal I can't see anything resembling a marker on my 'scope, despite adjusting my E2. |
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20th Apr 2021, 2:21 pm | #12 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
I followed step three in the R700's alignment instructions to check the discriminator . An S curve starts to appear as the sweep amp is turned up. Again, feeding in a marker did not do much.
At least we know that the wobbulator is working somewhat. I'm not too sure what this says about the diode probe. I wonder if I could find a way to feed the Z out from the MC/12 into my 'scope? Assuming that this would eliminate the double curve. The instruction sheet suggests connecting the Z out is important. Andrewausfa, I'd be curious to know what you tried to get this wobbulator going. I can get an OK looking sine wave from mine when I adjust my 'scope. |
20th Apr 2021, 8:01 pm | #13 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
Hi OTF, I think the key to the marker pips may well be the Z mod as from memory these pips appear quite bright on the screen
Ed |
20th Apr 2021, 8:55 pm | #14 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
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Re: TESTGEAR MC/12 and FM Alignment Help
OTF96, I think that last pic IS your "SD" curve, NOT a sine wave. you are nearly there I think!
Les. |
21st Apr 2021, 11:56 am | #15 | |
Octode
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
Quote:
Andrew
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21st Apr 2021, 1:12 pm | #16 |
Octode
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Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
I should have been a bit clearer about my last comment. The sine wave I mentioned would be the one you get when you connect the RF out of the MC/12 to a 'scope and turn the Y amp all the way up.
Having nowhere to plug the Z out puts a stop to things at the moment. I'll have a look at some other radios and their alignment data. It would be interesting see how far you get, Andrew. |
21st Apr 2021, 8:41 pm | #17 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
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Re: TESTGEAR MC/12 and FM Alignment Help
In my last posting I typed "SD" curve, but of course meant "S" curve, but too late to edit it.
Looking again, I will say it is NOT a sine wave. If you adjust your timebase/sync./trigger, I think you will see it is a single wave. Furthermore, if you look at the rising flank on the negative half, surely that kink is your marker. I would revisit those settings, and not affect of firstly shifting your marker a little each way. If I am correct, it will move up or down the curve. It will distort the curve less if you can reduce its amplitude such that it is only just visible. Then carefully vary your timebase settings and see if you can improve the displayed curve. An S curve, correctly aligned, has a good straight line section of the central rising flank. Les. |
28th Apr 2021, 9:29 pm | #18 |
Octode
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
I've had a look at the service manual for my Hameg HM203-4 and it looks like a Z in can be easily added with a few components. I'm not sure what C3107 is for though.
Does anybody know what the Z out does on the MC12? It's fed from the negative side of the rectifier diode via a resistor and a capacitor. It stays at 13VAC no matter how the controls are manipulated. The Art of Alignment does mention that some markers will only be visable with the Z out connected. Hopefully, connecting the Z out sorts out the double trace. |
29th Apr 2021, 10:38 am | #19 |
Octode
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
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Re: TESTGEAR MC/12 and FM Alignment Help
Z out should go to Z input on the scope which I'm not sure your Hameg 203 has (I have a 203-6 and can't remember!). Z out will provide the signal to the scope to modulate the intensity of the trace on the scope thereby giving a little pip brighter than the rest of the trace as per the piccie attached.
Andrew
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