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Old 17th Oct 2017, 10:00 pm   #21
Nuvistor
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Default Re: 'I.F Alignment Aid'

Quote:
Originally Posted by Philips210 View Post
I seem to recall some sets have two slugs in their IF transformers and are stagger tuned to give a broader response so tuning is not as sharp. The primary is tuned to about 2kHz below the nominal IF frequency and the secondary 2kHz above the nominal IF frequency. So peaking the transformer won't give the best performance.
I think the correct term is bandpass not stagger tuning, stagger tuning would be successive stages being tuned slightly different.
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Old 17th Oct 2017, 10:25 pm   #22
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Default 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!
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Old 17th Oct 2017, 10:27 pm   #23
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Default 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.

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Old 17th Oct 2017, 10:53 pm   #24
Argus25
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Default Re: 'I.F Alignment Aid'

Quote:
Originally Posted by Philips210 View Post
I seem to recall some sets have two slugs in their IF transformers and are stagger tuned to give a broader response so tuning is not as sharp. The primary is tuned to about 2kHz below the nominal IF frequency and the secondary 2kHz above the nominal IF frequency. So peaking the transformer won't give the best performance.
It would be interesting to see what forum members think of this.

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 10:57 pm. Reason: spelling
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Old 17th Oct 2017, 11:44 pm   #25
Terry_VK5TM
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Default Re: 'I.F Alignment Aid'

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Originally Posted by David G4EBT View Post
Just curious to know what the cost might be - as I said earlier, electronic modules sourced from the orient (DDSs etc), are ludicrously cheap.
Finally got to see the article.

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.

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Old 18th Oct 2017, 8:32 am   #26
David G4EBT
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Default Re: 'I.F Alignment Aid'

Quote:
Originally Posted by Bazz4CQJ View Post
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.
I made that in 2010, and the follow up RF probe, which reinforces the fact that for me, an IF alignment aid is completely redundant, but it won't stop me making one, just for fun, and because I can!

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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