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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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15th Feb 2009, 1:23 pm | #1 |
Heptode
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 648
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Residual FM on a Marconi TF2015
Does anybody have any experience of sorting out residual FM on CW in a Marconi TF2015 please? A recent check on mine has shown that (as measured on a deviation meter) it's up to 20 times what the manual claims it should be.
I suspect it's been like this for a while but it came to light after I repaired the associated synchronizer and realised that the frequency wasn't as stable as it should be. Watching a deviation meter connected to the output (without the synchronizer!), it seems obvious that the problem is noise rather than hum and this is confirmed by listening to the audio output from the deviation meter. On each range the problem is worst with the Tune control at its LF position and decreases progressively as you rotate the control. So far I've Confirmed that the problem lies in the genny rather than the deviation meter (by feeding the same meter, cable set etc. from another generator).and eliminated the power supplies as a source of the problem (by temporarily replacing them with batteries).I suppose I could embark on a progressive replacement of components associated with the tuning control but the thought of removing the switch and its 20 plus resistors isn't very appealing. Is there any better way of finding out where the noise is coming from (or is it a lost cause anyway)? All thoughts gratefully received! Hugh |
16th Feb 2009, 6:57 pm | #2 |
Pentode
Join Date: Jan 2006
Location: Almeria, Spain
Posts: 221
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Re: Residual FM on a Marconi TF2015
Haven't worked on these for many years, Is this the MF or VHF (up to 560 MHz) version? I Can't remember how the tuning was achieved; presumably
range switching by coil and a variable capacitor to set the frequency. If I remember correctly there is a varicap diode across the variable capacitor for FM generation and possibly the control voltage from the syncroniser is also applied to this circuit, this is where I would look first (possibly disconnect the varicap). That's all that springs to mind at present, I don't have a service manual so this is from memory. As an afterthought there also a fine tune control (dark red & concentric with the tuning knob I think this was a pot also connected to the above circuit, 2nd afterthought; the generator isn't sitting on something with a fan inside causing vibration? Good luck, nice compact generators these. Regards Mike A |
17th Feb 2009, 7:01 pm | #3 |
Heptode
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 648
|
Re: Residual FM on a Marconi TF2015
Hello Mike, thanks for responding.
Yes, it's the VHF version. Tuning is entirely by varicap, with the range control switching in a new coil/varicap combination for each band. The main tuning control is a switched potential divider and the fine & extra-fine tune controls are pots on the output of that divider. Deviation is fed into the divider at 4 points(!). I'm pretty certain that the cause is inside the generator. It's not sitting on anything obviously vibrating so, unless the whole workshop is trembling, I don't think that vibration can the cause. I've only got a PDF copy of the manual and an A4 printer, which means that, until today, the circuit diagram needs better eyes than mine to read it! Fortunately I've managed to get to an enlarging photocopier so I now have an A3 copy, which is a lot easier on my eyes. I suspect that the next step will be to disconnect the modulation inputs from the tuning switch and see if the problem goes. It would be a lot easier of course if I could see the noise on a scope but it seems to be to small to be detectable (other than by its effect on the signal). Regards, Hugh |