Re: Variable capacitors and IF cans

by **Ivan** Sun Dec 16, 2012 1:24 pm

Hi Harry,
you are 100% right. Nevertheless, both methods give similar results with common IF cans on AM - 455 kHz. Old (before surface wawe filters appeared) video IFs were more difficult to set: a flat response cca 6 MHz wide was required. Each IF can had to be tuned to a different frequency and with the proper Q factor. Do you still remember ?

VBR from Ivan

by **admin** Sun Dec 16, 2012 11:56 am

One other interesting technique for aligning IF amplifiers with basic equipment is to modulate the signal generator with 4Hz AM and tune for maximum recovered 4kHz at the detector diode using an oscilloscope.

What you are doing is optimising the bandwidth by detecting the sidebands and not the carrier. Old AM had a miximum bandwidth of 4.5kHz with the trebble emphasised to give the impression of higher frequency response.

If you tune for max DC from the AGC detector then you are only peaking the IF stages at the centre of the IF band which will not give you a flat response.

Just thought I wopuld add my two-penneth.

BR Harry - SM0VPO

by **Ivan** Sat Dec 15, 2012 5:12 pm

Hi,
you are right. The postage is the same unless the total weight exceeds some limit. It is increased in steps.
Re your IF strip: when tuning it, AGC should be disabled, tuning should be performed with maximum gain. The injected AM signal should be attenuated so that no overload of any stage takes place.

BR from Ivan

by DragonForce Fri Dec 14, 2012 9:57 am

Ok, I realise that postage would cost more than the items themselves, but that could be just a matter of how many are purchased - assuming he is willing to do a deal on postage. It would cost the same to post two of those cans as it would to post one wouldn't it? Assuming the destination address is the same of course.

I have plenty of the white cans, it's yellow I have none of, although Harry says that the difference is simply in the input/output impedance and therefore pretty much interchangeable with a tiny loss in performance.

The cans I have came from Jaycar is Australia, two white cans, a black and a red LO can. I've had a play with them, and whilst I got the IF strip working well, I couldn't quite get the LO to mix with the antenna signal - probably just a simple error on my part.

When I tested the IF strip, I had a quite low level tone at the detector diode when injecting a modulated 455KHz signal to IF3, a much larger one on injection at IF2, and a large drop if injected at IF1. I am assuming that the AGC action is the cause of this drop.

The LO frequency is good and stable, and on frequency - as tested on my frequency counter and the LO signal is received on my Grundig MW radio. I just couldn't get the mixing action to work - ho hum, I've obviously made a mistake somewhere. That project has been shelved for now as I've moved onto other things for the time being - still radio related.

FWIW the circuits I used are both on the home pages, the LO circuit is described in the section on rewinding IF transformers and the IF strip came from the building blocks/IF amps section.

The signal injector I've used to test the IF strip is simply an 800Hz oscillator modulating a 455KHz oscillator based on one of those small yellow 455KHz resonators. It works quite well, although I have no real idea about it's power output. I can easily pick up the harmonics at 900 KHz (and beyond) on the Grundig.

Audio amp is a simple LM386 design.

by **Ivan** Fri Dec 14, 2012 7:54 am

Hi,
yes, I know him, I bought enough interesting and useful parts from him. He lives in Prague, too, although in a bit far quarter. I picked those parts personally, as postage would cost more than the parts.
The parts are new, stored, from terminated repair-shops. You are right about the prices. Notice he has no white IF cans, he sold them out.

BR from Ivan

by DragonForce Thu Dec 13, 2012 6:56 pm

Wow Ivan, those look great!

From my point of view, at 31 CZK to 1 GBP, those components are very inexpensive, do you know the seller Purdy?

by **Ivan** Thu Dec 13, 2012 8:03 am

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