17-04-2012, 06:44 PM
eddited post 8 to show link to wobbulator not the oridgnal
rob t
rob t
if its made by hand it can be repaired by hand
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Hacker RP33 Autocrat
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17-04-2012, 06:48 PM
That's ok Rob, that looks like the Jasonkit one, I think Alan Beckett has one of those.
Lawrence.
17-04-2012, 07:16 PM
Yes, I have one, which I'll get to one of these days.
I've also got the Manuals. Alan
17-04-2012, 08:06 PM
Hell Joe, just had another look at the test specifications pic in the original post, it says to connect the 'scope across the volume control, I am assuming this is to monitor the IF passband when adjusting IF's with sweep generator, not the ideal place to monitor or am I way off hear. You might be able to use that Avo Test Bridge you have as an ac milivolt meter if it is sensitive enough in the V position.
Lawrence.
17-04-2012, 08:23 PM
Perhaps the scope is just to check for overloading.
The last sentence is interesting. Apparently an Output of 25 Megavolts can be achieved with an input of 25 Microvolts  Alan
17-04-2012, 08:36 PM
Gosh..national Grid stuff....
Re: the 'scope, the wording suggests that IF's are adjusted for a response of +- 3Kcs at the half power points ( -3db) which seems to imply that the envelope is being monitored, there won't be an envelope at the volume control. Lawrence.
17-04-2012, 08:49 PM
Putting the 'scope across the volume control is a way of connecting the 'scope to the output of the A.M. detector. That way, as the generator sweeps through the I.F. passband, the resultant R.F. output from the I.F. amp. is rectified by the detector and a d.c. voltage is thus produced. The resultant display on the 'scope is thus a curved line - this being the graph of the response of the I.F. amp.
If the 'scope was connected to the O/P of the I.F. amp., prior to the detector, (which is another way of using such a test set-up) what you would see on the 'scope would be two 'humpy' curves (a mirror image, one above the other) with the region between those curves being a 'blur of R.F.' What the detector is doing is chopping off one of those curves and its filter is removing that blur of R.F. - thus leaving just the curved line - the d.c. O/P from the detector. Al.
17-04-2012, 08:54 PM
(17-04-2012, 08:23 PM)AlanBeckett Wrote: The last sentence is interesting. Apparently an Output of 25 Megavolts can be achieved with an input of 25 Microvolts Wow! Now that is some I.F. amplifier!  That's a whopping 140 dB !  Al. (17-04-2012, 08:49 PM)Skywave Wrote: Putting the 'scope across the volume control is a way of connecting the 'scope to the output of the A.M. detector. That way, as the generator sweeps through the I.F. passband, the resultant R.F. output from the I.F. amp. is rectified by the detector and a d.c. voltage is thus produced. The resultant display on the 'scope is thus a curved line - this being the graph of the response of the I.F. amp. Yes, I understand that Al but the wording seems to suggest ( by virtue of the +- 3 kcs quote) that the envelope is being monitored ie: pre detection, so in my head I am having a hard job imagining a trace on the 'scope showing the -3db points at +- 3kcs on a volume control ie: post detection, mind you I've been having a hard day so maybe I am reading things wrong and thus trying to make something out of nothing. Lawrence.
17-04-2012, 09:57 PM
Having done this, it's really quite simple...
Picking up the signal post-detector seems to be pretty standard practice in these sorts of sets, and works well. I've got a diode probe that does the same job, though I can't remember the last time I used it... I mentioned earlier that I don't use a wobulator; instead I use two oscillators. One is a Marconi TF2008, which generates the 470kHz. This is swept by a Wavetek 162, which is a lovely old function generator. This also drives the X input of the 'scope, of course. The Wavetek can be stopped at any point in the cycle, and that's really convenient. It means that I can set the output to 0V, then centre the spot on the centre of the screen using the X-shift, then adjust the Marconi to give exactly 470kHz. Then, I can move the Wavetek output to the peak of its output, then adjust the amplitude so that the Marconi is giving a sensible frequency - say 460kHz. Then I adjust the X-gain on the 'scope, so that the spot has moved 4 divisions left. That means 1 horizontal division equals 2.5kHz. Once all that is done, the Wavetek is set to oscillate... Estimating the -3dB point is quite easy, as you can see the peak at the centre, and the baseline. If the difference between the peak and the baseline is 8 divisions, then the -3dB is 5.6 divisions above the baseline. It's harder to explain than to actually do  I've never played with a wobulator, but do they offer the option to stop the sweep at any frequency so you can check calibration? |
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