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Old 22nd Apr 2015, 12:08 pm   #21
Andrewausfa
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Default Re: Confusing Sobell 519P.

Hi Ric,

The service sheet, to start RF alignment, says 'Check that the pointer lies along the horizontal scale at the top of tuning dial with ganged capacitors fully meshed'. When we looked at it, the pointer was down the bottom with the gang fully meshed.

What are the wavelength markings at the top of the scale?

For those unfamiliar with this set the dial and stringing arrangement is vertical, rather like a post-war Pye woodie or Ekco A28.

Thanks - Andrew
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Old 22nd Apr 2015, 12:16 pm   #22
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Default Re: Confusing Sobell 519P.

Just fully mesh the tuning capacitor vanes then set the pointer at the long wave length end of the scale ie near 550 metres medium wave.

Of course the pointer will have to be threaded so that as the vanes unmesh the pointer moves towards the 200 metre end of the scale.
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Old 22nd Apr 2015, 12:28 pm   #23
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Default Re: Confusing Sobell 519P.

Disabling the LO and AGC.
I have never needed to disable the LO on the simple consumer MW/LW radios but you would only do that for the IF alignment.
The AGC can mask the peak when aligning a radio, so you can either disable the AGC or keep the signal generator output low enough not affect the AGC.
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Old 22nd Apr 2015, 10:16 pm   #24
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Default Re: Confusing Sobell 519P.

Hi,
Have you checked the screws you are adjusting on the various rf/if coils are actually attached still to the ferrite cores ?There have been occasions when these have become unattached !
Also check the tuning mica caps for the various coils for changed/open circuit values
I don't know if they are fitted inside the cans on your radio ?
Regards,Pete
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Old 23rd Apr 2015, 9:44 am   #25
Riccampbell
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Default Re: Confusing Sobell 519P.

The service sheet I have is definitely wrong then. It says the cursor must be at the top of the scale (low wavelength) with the capacitor fully meshed. (the top of the scale is marked 200 m )

Good point concerning the cores at the end of the RF screws. I did check one (LW coil) but it is a bit of a fiddle, the trimmers are fixed over the open end of the coils with metal clips and then the wiring is soldered on. After reading all the posts I feel I should have the rest of them to bits. I can check the trimmers properly at the same time.

Thanks for the advice,

Ric.

Last edited by Riccampbell; 23rd Apr 2015 at 10:08 am. Reason: I didn't read all the posts first.
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Old 28th Apr 2015, 7:08 pm   #26
AlanC
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Default Re: Confusing Sobell 519P.

Ric, the service sheet is indeed wrong if it says that. As the vanes of the capacitor unmesh, the capacitance reduces so the frequency of the tuned circuits will increase. On MW, you'd expect the radio would tune to 200m (1500 kHz) with the vanes almost fully unmeshed, and 550m (545 kHz) with the vanes nearly fully meshed.

As I understand it, you're getting no response from the sig gen at all on MW... except at the IF frequency. So it sounds like the oscillator maybe isn't running. Do you have an oscilloscope?

Cheers,
Alan
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Old 30th Apr 2015, 1:13 pm   #27
Riccampbell
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Default Re: Confusing Sobell 519P.

The puzzle continues, this is the state of play so far; Having re-jigged the scale cursor drive cord I have the tuning cap. fully meshed and the cursor at the 550m end of the scale.

I have followed the albeit sparse information on my service sheet and after much going over again and again I have the alignment as good as I can get it, my gen is a Nombrex 27. All the coil screws are responsive as are the trimmer caps. I have checked all the contacts on the wave change switch and checked the wiring as per the sheet, nothing seems amiss.

This is the result; SW all OK.

MW; With the aerial connected I get a few stations but feint. However, if I clip the aerial wire direct to the top cap (grid) of V1 (6K8G) I get no end of stations loud but not too clear.

LW. This was difficult to get right, and in the end I had to connect another trimmer cap in parallel with the one on the coil (C19 on the sheet) to get the alignment right. I now get loud humming/buzzing noises where the stations should be with an aerial wire connected, however at 1500m behind the buzzing I get Radio 4 and strangely with the aerial disconnected I get the station with much less interference but not very loud. Clipping the aerial onto the grid of V1 only causes the interference to return.

I do have an oscilloscope, so I can check the local oscillator. Obviously my lack of experience is letting me down, but I am finding things out on the way, any suggestions gratefully received. Ric.
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Old 30th Apr 2015, 1:23 pm   #28
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Default Re: Confusing Sobell 519P.

If the IF is set correctly and stations or injected signals appear at the correct place on the scale, then the local oscillator is working correctly.

The problem is likely to be with the aerial tuning coils and/or their associated trimming capacitors. You'll have to work out which are the MW coils.

Buzzing can be down to fluorescent lights etc. in the vicinity.
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Old 3rd May 2015, 7:39 pm   #29
Riccampbell
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Default Re: Confusing Sobell 519P.

Thanks for the reply, which lays to rest any doubts concerning the local oscillator. However the other faults listed are still with me. The interference is not from any local source, I have my workbench in a back bedroom with no fluorescent lighting or other source of interference. Other sets work fine.

Does my problem stem from mis-alignment? although I have been over the process many times. Also I have been over the set time after time, checking components in desperation, I must be missing something but I can't see what it is. Ric.
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Old 3rd May 2015, 10:18 pm   #30
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Default Re: Confusing Sobell 519P.

Modulation hum?
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Old 26th May 2015, 12:19 pm   #31
Riccampbell
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Default Re: Confusing Sobell 519P.

At last I have the set working properly. I was advised by forum members about the need to check LW coil cores, this I initially did, but was not thorough enough. On removing the trimmer from the end of the coil I screwed in the adjuster and saw that the core moved down, what I missed was that had I screwed the adjuster the other way the core would have remained stationary.

On a subsequent and more thorough checking I found the core in two bits. My mistake, I have learned to conduct more complete investigations in future. Another thing I missed was the braiding of a screened cable supplying the screen of V (6Q7) had become detached, again a more rigorous approach was needed. After correcting these faults I was able to align the set easily, and it works well now.

Many thanks to all the forum members who posted advice. Ric.
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