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Old 21st Jul 2018, 10:26 pm   #1
Doris The Diode
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Default Staggered I.F. tuning help

Hi,

I have just restored a Radio For The Blind aka RFTB set and especially on MW the tuning is best descibed as twitchy i.e. fidelity is bad and the slightest touch of the tuning dial off tunes the set. ItÂ’s ok on LW with radio 4 booming and sounding alright, acceptable.

I ran into the though of staggered IF tuning but donÂ’t really know where to start.
I donÂ’t have a wobulator but do have a sig gen and scope.
Perhaps there is a already a thread on this subject (I did do a search) or a set which uses staggered tuning and there already exists a procedure.

Perhaps someone could point me in the right direction or thread how to go about staggering the IF e.g. how many KHz away from the centre frequency should a stagger be?

The set has the standard 2 IF cans with 2 preset caps in both and is a typical 1950Â’s set with UL41 etc valves.

Cheers,
DD
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Old 22nd Jul 2018, 8:44 am   #2
boxdoctor
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Default Re: Staggered I.F. tuning help

I would think it unwise to interfere with any I.F. transformer tuning unless it was found to be in real need of adjustment.
Its far better to find the cause of the problem, and put that right. Staggering the I.F.s will increase the amount of SMPS and other rubbish picked up, and at night make interference from stations on nearby frequencies much more evident.
Have you checked the condition of the earthing arrangements on the tuning capacitor? If it uses metal earthing brushes from the shaft of the rotor to the frame, are they clean, and in good contact with the shaft? Is the tuning capacitor soundly earthed to the chassis? Might the problem be a poor contact on the wavechange switch? Many things to check, including dry joints on the local oscillator and aerial coils. Broadning the bandwidth to mask this problem is simply doing that - masking the real problem. What is known in the trade as a "Bodge". Tony.
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Old 22nd Jul 2018, 12:27 pm   #3
Skywave
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Question Re: Staggered I.F. tuning help

What make and model is this radio? I ask that since if it uses staggered tuning IF for MW reception, that sounds very unusual. Moreover, the fact that you say it O.K. for LW - which will almost certainly mean using the same IF - seems to suggest that the problems are not related to the tuning of that IF. However, if it also has FM, then staggered tuning for that IF sounds more credible - but by reading your post, seems that you are only referring to reception on MW.

Al.
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Old 22nd Jul 2018, 1:55 pm   #4
Doris The Diode
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Default Re: Staggered I.F. tuning help

Hi,

Unfotunately I don't know what the model is as there is nothing printed anywhere.
I'll check these joints although there is no crackling etc but worth cleanng either way.

I have already peaked the IF chain as when I restored the radio there was just a hint of radio 4 coming through and nothing on MW.
Once peaked it burst into life and then I tuned the antenna coil/osc sections.
I treated it like a DAC90 which I have as the circut is almost the same.

I have a thread how I went about restoring it, I did not have a diagram and had to slowly draw one out as I went along.

I'll leave the IF as is and clean up othe parts first.
Thank you for reminding me that staggered IFs are more FM based radio techniques.

DD
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Old 22nd Jul 2018, 2:48 pm   #5
Skywave
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Arrow Re: Staggered I.F. tuning help

In the light of what we now know, this set will not use staggered IF tuning. All the IFTs should be centred on the nominal IF - probably 465 KHz. It may well be worthwhile checking that that is indeed so with your sig. gen. & 'scope.

Quote:
Originally Posted by Doris The Diode View Post
Unfortunately, I don't know what the model is as there is nothing printed anywhere.
A picture or photo of this set will help us determine what exactly you have there.

Al.

Last edited by Skywave; 22nd Jul 2018 at 2:49 pm. Reason: Add quote - and its response
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Old 22nd Jul 2018, 7:07 pm   #6
Doris The Diode
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Default Re: Staggered I.F. tuning help

I thought I had sent the link, here it is with photos and my hand drawn circuit diagram:
https://www.vintage-radio.net/forum/...d.php?t=147510

The IF freq is 470kHz.

Judging by the replies perhaps what I wnated to do is to slightly off tune the IF instead of peak it (which in theory is stagering) but ever so slightly.
Either way, I will try all methods sugested first before experimenting.

DD

Last edited by Doris The Diode; 22nd Jul 2018 at 7:18 pm.
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Old 22nd Jul 2018, 9:34 pm   #7
Skywave
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Arrow Re: Staggered I.F. tuning help

Setting up staggered tuning in an IF amp., just using a sig. gen. and a 'scope, even with small shifts from 470 kHz centre freq. and aiming to get a symmetrical skirt response curve with a reasonably flat top can be tricky and certainly time-consuming. To do that, you really need to use the 'sweep method' with a wobbulator.
And just for what it's worth, with an IF of 470 kHz, to me, that set sounds like it has a Philips chassis, yet the under-chassis looks very much like a Bush.

Al.

Last edited by Skywave; 22nd Jul 2018 at 9:38 pm. Reason: Amend last para.
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Old 22nd Jul 2018, 10:38 pm   #8
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Default Re: Staggered I.F. tuning help

To broaden the peak is most easily done by adding a suitable resistor in parallel with the secondary of the first IF transformer. Try 2.2k initially and by trial and error either do the same for the second or adjust the value. The objective is to keep the gain/selectivity satisfactory.

LW tuning will, by nature of the frequecy, be easier to tune even with peaked IF's.

Generally because of high audio compression and Optimod tailoring of the sound, losing sidebands makes for listening fatigue.

Good luck

Chris
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Old 24th Jul 2018, 8:03 pm   #9
Doris The Diode
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Default Re: Staggered I.F. tuning help

Problem resolved.

I first of all noticed that a 24K resistor on the ht had changed value again and it was now 27K as well as the 5K in the smoothing section had yet again changed in value and is now 4K2, so those were replaced to modern versions and the supply rails are now stable.

I had a listen but no real improvement, perhaps long term warm up there shoild be.

I cleaned all contacts and put the 2K2 in place as Chris suggested and retuned all IF cans and this made a noticable difference and the set sounds much nicer!

Thank you to everyone for their help.

DD
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