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 Refurbishing the RME receiver part 2, some side trips.
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 Return to top of page · Post #: 1 · Written at 7:00:28 AM on 7 February 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1049

Hi All, here is a part 2 where I diverge a bit and look at the Lamb filter and ask a question about the valves used.
I'll send the usual PDF to Brad for attachment.
Cheers, Fred.

RME 69 Receiver Part 2 - Some Side Trips

 Return to top of page · Post #: 2 · Written at 6:39:29 PM on 7 February 2018.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1073

The Radiotron Designers' Handbook has a brief mention of various noise limiters (the simplest only appear to require a double diode + a few other components). No circuit is given for the Lamb filter, but a reference is given for it which I can post if needed.

 Return to top of page · Post #: 3 · Written at 10:29:56 PM on 7 February 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2109

The thing about the Lamb noise silencer is it gates out the large noise pulses while they are still very narrow, before they get to the IFs and get widened by the tuned circuits all ringing like crazy. By all accounts it can be highly effective.

The two diode circuit is called a Rate of Rise noise limiter. It's inserted in the audio path, after the detector.
Fast rising pulses simply reverse-bias the diodes to disconnect the audio path for the duration of the pulse.
Similar idea to the Lanb system but not as effective because it has to work with wider pulses.

A well designed rate of rise limiter can work well though. I've used VHF AM radios that had this system and I can vouch for its effectiveness. Receivers for the old British 405 line TV system, which used AM sound, were usually fitted with rate of rise noise limiters.

 Return to top of page · Post #: 4 · Written at 11:06:31 PM on 7 February 2018.
Brad's avatar
 Location: Naremburn, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6851

Document uploaded.

A valve a day keeps the transistor away...

 Return to top of page · Post #: 5 · Written at 7:04:48 AM on 9 February 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1049

Hi STC yep I have the 4th ed of RDH and the limiter section is on page 1130 where they detail the double diode limiter but skip the Lamb.
I read the paper by Wald and of course the big one was the original patent paper by Lamb. A master piece of patent attorney writing to cover the commercial application.
As Ian points out the Lamb is a pulse type of gater distinct from the limiter or mute type of function done by the others.

 Return to top of page · Post #: 6 · Written at 12:49:46 PM on 11 February 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2109

Hi Fred, love your noise generator!

Over 20 years back I had to get our line of microcontroller-based gear ready for EMC testing and export to Europe.

One of the pieces of gear we rented was similar to this:


It puts rapidly repeating, fast rise time, 2.5kV noise pulses into the mains supply to the device under test. It can telescope years of testing into a few minutes.

We fired it up, it instantly crashed every computer in the building! It generated noise that could be picked up on an AM car radio several suburbs away! (I wonder since what the mains earth impedance was like....)

But by testing out of hours we were able to get our product to withstand the onslaught. It went on to pass CE compliance testing with flying colours.....

 Return to top of page · Post #: 7 · Written at 5:56:30 AM on 12 February 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1049

I did have a look at the crystal filter in this set and found the crystal is inactive.
I wrote a report on this investigation and will forward a PDF to Brad as a part 3.
I wont bother to get another crystal don't need it for MW reception and all I could find were units for silly prices $100+.
Possibly someone may have some advice on waking the crystal up?
Its the first one I have ever seen being a "Bliley CF1" at 465Kc.
The crystal block does not have any obvious cracks or chips, its just inert to signal as I explain in part 3.

 Return to top of page · Post #: 8 · Written at 1:07:58 PM on 12 February 2018.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 513

If possible it would be worth pulling the crystal block apart.
Might be fixable. Slivers of crystal fallen out of place?, Contact cleanup?.
Or just broken due to unit being dropped.

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