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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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17th Oct 2017, 11:00 pm | #21 | |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,431
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Re: 'I.F Alignment Aid'
Quote:
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Frank |
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17th Oct 2017, 11:25 pm | #22 |
Triode
Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 37
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Re: 'I.F Alignment Aid'
When I was trained, it was called "offset tuning" and was a method to slightly broaden the selectivity of an AM receiver in an effort to make the tuning less critical and to improve the audio frequency response somewhat - particularly as some of the European stations didn't bother much about bandwidth limiting their modulation! It was made clear to us that it was a compromise, and that peaking the IFTs at the centre IF frequency would enhance sensitivity and selectivity at the expense of easy tuning and narrowed audio....
In the case of FM receiver strips, some models required offset tuning of the IFTs to get the requisite bandwidth to give the discriminator enough to demodulate! |
17th Oct 2017, 11:27 pm | #23 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,934
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Re: 'I.F Alignment Aid'
Anyone thinking about building a wobbulator might also like to take a look at the design published in Radio Bygones #82 (April/May 2003) by Raymond Haigh. This has four switched ranges for frequencies from 400kHz to 14MHz (it can be built with just the 450-460 range) and uses a pair of 2N3819. The 8 page article gives good constructional details (inc pcb layout) and describes the use of a wobbulator quite nicely.
B
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Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch. |
17th Oct 2017, 11:53 pm | #24 | |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: 'I.F Alignment Aid'
Quote:
There are a number of transistor radios that have double tuned IF transformers, a good one could be the Eddystone EC-10. And those typical vintage flat Philips one too in other radios. My understanding was that even though these are double tuned, they are critically coupled or close and the transformer has been designed by the manufacturer so that when the signal amplitude is peaked on both adjustment cores (just as they are done in a valve radio IF where these are often tuning capacitors) the correct bandwidth occurs due to the design of the transformer. I have not actually seen any alignment instruction for radios where its suggested the IF's are stagger tuned to widen the bandwidth, but there may be some that do suggest this, has anybody seen this for any AM radio model where a sweep is suggested and a characteristic broadened response curve is required ? I think gain and selectivity are the prime concerns in the radio, not bandwidth. Last edited by Argus25; 17th Oct 2017 at 11:57 pm. Reason: spelling |
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18th Oct 2017, 12:44 am | #25 | |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,339
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Re: 'I.F Alignment Aid'
Quote:
Cost of the Micromite module is around £32 + postage (from UK) and the DDS module about a fiver from the usual source. Add in other ancillary bits and pieces and a box to put it in, say £50 total. Terry |
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18th Oct 2017, 9:32 am | #26 | |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,760
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Re: 'I.F Alignment Aid'
Quote:
The R.B. Raymond Haigh project been covered in several forum threads, such as these: http://www.vintage-radio.net/forum/s....B.+Wobbulator http://www.vintage-radio.net/forum/s....B.+Wobbulator There's no point in anyone who might be interested in building this project to even consider it unless they can make or source the PCB, and obtain the set of four Toko inductors, which have been out of production for several years. (All the other bits are standard things - caps, rs, pots, switches etc, but I'm not sure about the KV1236/KV1235 varicap). The Toko inductors needed for this project are: RWO6A7752EK (green core) 154FN8A6438EK (violet/deep red core) 154FN8A6439EK (yellow core) KXNK3767EK (pink core) I incorporated a sawtooth generator on a small PCB designed by forum member Colin Armstrong, and Colin kindly supplied the artwork for a modified front panel, which is far better than a scan of the original layout in the RB article. (The sawtooth generator was based around a 555 IC, and initially didn't work, but when substituted for the CMOS version - the '7555' it worked fine. A note of caution if anyone has built or intends to build that nice little project: On the PCB overlay in the article, the range 3 & 4 coils need to be swapped over, as a glance at the circuit in comparison with the layout will show. I didn't discover that until I checked the coverage of the ranges on my frequency counter. Some pics attached. A bit off topic, but hopefully acceptable as a response to Bazz4CQJ's post.
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David. BVWS Member. G-QRP Club member 1339. |
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24th Oct 2017, 3:02 pm | #27 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,573
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Re: 'I.F Alignment Aid'
Hi.
I also built the Raymond Haigh wobbulator featured in RB magazine and it is working well. I also seem to remember there's a mix up with the location of two of the four Toko coils. It's probably the board layout rather than the circuit diagram but will have to verify this. When I eventually finalise the layout of my workshop then I hope to use this unit more frequently. I used the front panel legend as per the original article, it's just a paper copy with a piece of perspex protecting it. A couple of pics of the completed unit attached. Regards Symon. EDIT: Just reread David's post #26 re the transposed Toko coils Last edited by Philips210; 24th Oct 2017 at 3:07 pm. Reason: Update on transposed Toko coils |
24th Oct 2017, 4:05 pm | #28 | |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,934
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Re: 'I.F Alignment Aid'
Quote:
B
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Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch. |
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24th Oct 2017, 6:15 pm | #29 |
Octode
Join Date: Mar 2011
Location: Sheffield, South Yorkshire, UK.
Posts: 1,453
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Re: 'I.F Alignment Aid'
Haigh PCB artwork attached.
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