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Old 14th Jan 2019, 11:27 pm   #1
fastdave
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Default Help with alignment of a Sobell 515P

Two edged tale here from someone involved in the digital world too long - I now have 3 valve amplifiers, a Murphy 122m, a PYE 48A, a Phillips in the loft (can't remember it's No.) and they all work.
Some time ago, I indulged myself in a 'FeelTech' signal generator, which seems capable of delivering all sorts of signals.
I recently Acquired a Sobell 515P - absolutely no sensible signal. I fear the alignment has been seriously tampered with, for on the allowed 1 turn back or forward, I can create whistles and burps, but no good signals.
My problem? I have no clue how to inject signals - I even have a 'scope, with no idea where to look. Can anyone direct me on how to inject IF and RF, and where to look whilst 'peaking the coils? - I think I have to use a capacitor to input, but where to look with the scope? Do I create a coil for the Genny to give the 475 to the Arial? Can the genny input direct to a grid?
I think you'll see that I am untrained in good ole' am fault tracing - any quick lessons?
Dave - ex audio engineer, turned industrial electronics, electro mechanical, then AP now retired.
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Old 15th Jan 2019, 9:44 am   #2
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Default Re: Help with alignment of a Sobell 515P

Unless you know the alignment has been got at, do some standard fault finding first. Alignment can drift but it’s unusual to drift that far out to be stop the set working.

If after failing to find any faults then you can then check the alignment.

Look at this web page for information and fault finding on old radios.
https://www.vintage-radio.com/repair...ion/index.html
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Old 15th Jan 2019, 11:10 am   #3
TrevorG3VLF
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Default Re: Help with alignment of a Sobell 515P

I can create whistles and burps,

These are likely to be due to poor capacitors.
The critical one drives the output valve grid and must be in good condition or severe damage can be done. Measure the cathode voltage and switch off quick if too high.

Look at the DC voltages and if a screen is low, change the bypass capacitor. There will also be feedback on the AGC line if the capacitor is poor.

You can input signals through a short wire as an aerial. Direct connection is only necessary if sensitivity is low.
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Old 15th Jan 2019, 3:29 pm   #4
fastdave
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Default Re: Help with alignment of a Sobell 515P

Thank you both. The voltages are fine and the components do not look suspect, and I know that the alignment has been tampered with - scores on the top of the coils, etc, and the fact that I'm getting anything - I know who had it, and he starts to fit a light then gets a tradesman to finish it - I have rescued this Sobell (valiant trumpet blasts).
I was looking to find out how to apply the signals - e.g. if I connect the genny directly to the aerial for the 475 IF, or go to the grid of the mixer/osc.
I don't know what to do with the output of the signal genny - do make a coil for it to transmit? or do I connect it via the aerial socket?
I shall have a look at the pages you suggest, although most text I have perused seems to avoid these very basic questions, perhaps expecting a Radio engineer to do it second nature.
Thank you,
Dave.
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Old 15th Jan 2019, 5:02 pm   #5
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Default Re: Help with alignment of a Sobell 515P

Full alignment instructions in the Trader service sheet, they appear very straightforward, injecting a signal via a capacitor.
If you don’t have the sheet it well worth having, just follow the text.
https://www.service-data.com/product...894/688/t15894
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Old 15th Jan 2019, 5:06 pm   #6
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Default Re: Help with alignment of a Sobell 515P

Does the sig gen have AM function?

Lawrence.
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Old 15th Jan 2019, 7:20 pm   #7
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Default Re: Help with alignment of a Sobell 515P

Snippet from the Trader sheet.
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Old 16th Jan 2019, 3:07 pm   #8
fastdave
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Default Re: Help with alignment of a Sobell 515P

Thank you Nuvistor (wow! that was difficult to remember!), I do have that service sheet - what I have is an Aerial input and earth wanderplug sockets, and having stuck an earth onto a live chassis as a young teenager, did not want to put my nice Chinese signal generator to death so early in its career! Not terribly sure if I need to put the -ve of the bnc connection of the geny to earth, and worse still if I was injecting into a grid.
Next question relates to Lawrence as an answer- yes, the sig geny certainly has AM, and its output is digitally rendered into a little 'scope screen which displays it's shape and amplitude - question is, what level would I inject?
Thanks again, folks.
Dave
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Old 16th Jan 2019, 9:51 pm   #9
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Default Re: Help with alignment of a Sobell 515P

Quote:
Next question relates to Lawrence as an answer- yes, the sig geny certainly has AM, and its output is digitally rendered into a little 'scope screen which displays it's shape and amplitude - question is, what level would I inject?
Typical old style AM sig gen would have been 30% modulation at 400Hz, eg: an Advance E2 sig gen.

Set the output level of the sig gen so that the receivers AGC is just below it's operating threshold, that way your aligning at maximum gain, the Trader sheet for your receiver indicates where to inject and where to apply damping.

If your doing the Osc/RF alignment connect the sig gen to the antenna socket via a standard dummy antenna, this should ensure that the tracking aligns as designed if all's well, you can also try to align to the antenna by wrapping the sig gen output a couple of turns around the antenna lead but sometimes (depending on the antenna) the tracking might not line up as intended.

You can monitor with your 'scope no problem, anywhere in the signal path between the wiper of the volume control and the loudspeaker should do, if you feel the need to disconnect the loudspeaker make sure a suitable dummy load is connected in place of it.

Lawrence.
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Old 17th Jan 2019, 5:46 pm   #10
fastdave
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Default Re: Help with alignment of a Sobell 515P

Thank you for that detail, Lawrence, that helps a lot - the grid I was really quite happy about, but the dummy aerial foxed me - I shall construct one, but not sure about the 20H choke, and feel much more comfortable than jabbing in a hefty sine wave!
I know you guys probably think I'm a transistor wimp - yes, there is something easy about setting the quiescent current on the output of a B & O Tuner/Amplifier, and it took me weeks to get round to re - bias the new matched quads on my Marshall JCM 900, but having shocked myself silly as a young person (I was fooling around with an old valved radio, putting a signal onto the radio amp valve grid from a gramophone with a dead valve - in order to hear my records) gaining respect for higher voltages - (I am a retired AP, coming away from High voltages Grid style, yes, 11kv and above, so I don't like sparks, and I like isolated circuits), so I am a wimp!
I shall try to source a choke, and then I can approach all RF with more confidence - in the meantime, a capacitor to grid and onward.
Thanks guys,
Dave
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Old 18th Jan 2019, 10:12 am   #11
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Default Re: Help with alignment of a Sobell 515P

Not sure where the 20H choke comes in to this. A 20 Henry choke would be a large A.F. choke as used in some power supplies' smoothing circuits. In R.F. & I.F. stages (and in signal generators/dummy loads, etc., a 20mH ( millihenry or 20uH Microhenry choke might be found. These would be small dust-cored or air-cored components.
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Old 18th Jan 2019, 2:14 pm   #12
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Default Re: Help with alignment of a Sobell 515P

A six inch wire in to the aerial socket and a six inch wire from the generator will probably couple sufficiently to tune. Direct coupling and a generator with a calibrated output level will be needed to measure sensitivity.

Have you got rid of the whistles and burps? This must be done first.
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Old 18th Jan 2019, 6:13 pm   #13
fastdave
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Default Re: Help with alignment of a Sobell 515P

Thanks livewire - now I am totally confused - The circuit I picked up had a 200pf cap in series with a 20H choke which was in parallel with a 400ohm and a 400pf cap - this was the radio end - now Trevor is talking about a 6 inch wire? all I want to do is inject an RF signal after I get the IF right - as to the whistles, they only occur at certain stages in the IF adjustment - I always return these to the original position within half a turn or so.
At one point, I got a faint radio station, but lost it without any good means of tuning the IF.
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Old 18th Jan 2019, 7:00 pm   #14
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Default Re: Help with alignment of a Sobell 515P

Dummy aerial diagrams from “Radio servicing pocket book” by J P Hawker.

Check the size of the inductor.
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Old 18th Jan 2019, 7:22 pm   #15
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Default Re: Help with alignment of a Sobell 515P

That inductor is 20uH ([B]20 Microhenries and, as I noted earlier will be either air-cored or possibly have an iron dust or ferrite core.
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Old 18th Jan 2019, 7:43 pm   #16
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Default Re: Help with alignment of a Sobell 515P

I was really meaning for Dave to check the inductor size.
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Old 19th Jan 2019, 9:51 am   #17
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Default Re: Help with alignment of a Sobell 515P

Frank, my comment with the highlighted terms & value, was intended for Dave as I'm not certain that he is familiar with the different sizes/values of inductors, etc.
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Old 19th Jan 2019, 12:25 pm   #18
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Default Re: Help with alignment of a Sobell 515P

Quote:
Originally Posted by 'LIVEWIRE?' View Post
Frank, my comment with the highlighted terms & value, was intended for Dave as I'm not certain that he is familiar with the different sizes/values of inductors, etc.
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Old 21st Jan 2019, 6:49 pm   #19
fastdave
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Default Re: Help with alignment of a Sobell 515P

I cannot thank you guys enough for your friendly advice. I am by no means a novice, but new to refurbishing radios, and thoroughly enjoying it. The audio is a breeze, and technically I should be well up on inductors and Amplitude modulation, but my career careered away from what I enjoyed most, and now I am retired and have time to learn and re learn, I appreciate even more, professional hints and tips from such as yourselves - this gives me keys which I can use to jump steps - a bit like the keys in maths which only a few teachers can hand to you, and once you have them, it's plain sailing. I did baulk a little at the mention of 'big' henrys, but what you say makes sense, and as yet, I have no inductor - I am about to order a few 20uH chokes, and look forward, Sheldon fashion, in true Geek spirit, to have 'fun with chokes' and brush up my electronics maths again.
Thank you,
Dave.
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Old 21st Jan 2019, 7:10 pm   #20
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Default Re: Help with alignment of a Sobell 515P

If you want to make a choke.
http://info.ee.surrey.ac.uk/Workshop/advice/coils/
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