Welcome to Australia's only Vintage Radio and Television discussion forums. You are not logged in. Please log in below, apply for an account or retrieve your password.
Australian Vintage Radio Forums
  Home  ·  About Us  ·  Discussion Forums  ·  Glossary  ·  Outside Links  ·  Policies  ·  Services Directory  ·  Safety Warnings  ·  Tutorials

Tech Talk

Forum home - Go back to Tech talk

 Nordmende Elektra 58 What is the IF blocking circuit?
« Back · 1 · Next »
 Return to top of page · Post #: 1 · Written at 8:09:08 PM on 28 March 2020.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 129

Hi all,

I am trying to align the above radio, I have translated the german instructions to english. I have connected my generator to the grid of the mixer via a 400 Ohm resistor and 200pF cap and aligned the IF transformers ok but the next step is tripping me up. It says:

"Connect artificial antenna to antenna and earth socket and adjust IF blocking circuit V to a minimum"

The antenna connection is easy but there is no reference to the 'V' adjustment in the drawings, most other adjustments are numbered or lettered.

Can anyone tell me what and where the IF blocking circuit is and what it does.

Thanks, David


 
 Return to top of page · Post #: 2 · Written at 8:26:23 PM on 28 March 2020.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6012

First question is: does it really need to be aligned? If not, don't bother.


 
 Return to top of page · Post #: 3 · Written at 9:33:32 PM on 28 March 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4344

Unfortunately alignment is the biggest cause of "deaf" radio's and is recommended, by most manufacturers if you have been in the RF changing parts.

I am wondering if its like an American "Midwest" I serviced. It has "amplified" AVC. Only radio that I have ever serviced where it actually specified RF voltages. You peak the IF's first with 40uV on the mixer grid and then lowered the voltage and did it again to ensure it was at maximum then you raised the signal to 100uV to set the separate AVC transformer to get minimum signal.

So you may find it is a transformer? This one was an 18 Valve monster. and an Autodyne with a separate Exciter (Oscillator) valve which you removed when calibrating. Weird looks when I fist saw it Four gang tuner. .

One for the RF pre amp; One for the next Rf stage and two for a Colpitts oscillator.

See if you can see an IF type transformer on the circuit with a V rather than a trimmer?


 
 Return to top of page · Post #: 4 · Written at 10:07:31 PM on 28 March 2020.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 129

Hi GTC and Marcc, thanks for your replies.

Yes GTC it needs aligning, I got quite an increase from the IF transformers.

Marcc, thank you, you made me look at the drawing again and I found 'V", it is labeled BFV and has a line above and below the V, I had to look with a magnifying glass to see it and even then it looks like a 'Y'. It is of course roman numeral 5.

It seems to connect the plate of the IF amp to the detector, I will study the schematic tomorrow.

Thank you both for your help, Cheers 😀


 
 Return to top of page · Post #: 5 · Written at 11:08:12 PM on 28 March 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1905

Sounds like it could be a trap tuned to the IF frequency to minimise breakthrough of signals on the IF frequency. Some AWA radios used to have these, for the days when there were marine frequencies just below the broadcast band.


 
 Return to top of page · Post #: 6 · Written at 8:47:39 AM on 29 March 2020.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 129

Thanks Ian, it sounds like it’s something like that.

My initial euphoria was short lived, BFV refers to IF transformer 2. There is BFI, BFII and BFV on the schematic, the letters after ‘BF’ are roman numerals 1, 2 and 5. Strangely there is no BF 3 and 4 but I wonder if they adhere to a numbering system where IF transformer 'BFV' is always IF transformer 2.

There is no correlation between the alignment instructions and the schematic, that is, the adjustment points nomenclature in the text is not reflected in the schematic. There is a drawing of the radio with the adjustment points marked on it but there is no ‘V’ marked. I am beginning to wonder if the alignment instructions are actually for this radio and are not just a generic set that applies to a few radios.

Whatever the answer, there is no IF trap or blocking circuit on this radio IMHO

Thanks for your assistance 🙂


 
 Return to top of page · Post #: 7 · Written at 10:41:03 AM on 29 March 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4344

It may also be a blocking oscillator but I cannot see why? I may be of no help but I could compare it to the Midwest circuit.


 
 Return to top of page · Post #: 8 · Written at 11:20:16 AM on 29 March 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1905

What's the German word you have? "Sperrung"? "Blockierung"?

In any case, if it's an IF trap, which it likely is, its use is optional so it may well have been left out of your radio.

This from a Rodney Champness article on IF amplifiers explains it well:

"
To overcome image problems, an IF in the frequency band just below the broadcast band was sought. However, the frequency band from 405-513kHz had been used by large ships and coastal radio stations since the beginning of the 20th century. This meant that the new IF had to be carefully selected, otherwise marine radio stations could break through into broadcast receivers on the IF frequency.

Obviously, having Morse code transmissions on top of the news or the current popular radio serial would not be well accepted. What's more, it would not be possible to tune the interference out.

The PMG allocated all frequencies for radio transmission services but had not allocated any marine frequencies around 455kHz. As a result, Australia fell into line with the USA which had already adopted 455kHz as the favoured IF frequency.

A number of manufacturers put a series tuned IF trap (455kHz) across the aerial and earth terminals to make doubly sure that interference problems would not occur. At the same time, the gain of the IF amplifiers increased as better low-loss materials became available for constructing IF transformers.
"


 
 Return to top of page · Post #: 9 · Written at 12:15:23 PM on 29 March 2020.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 129

Hi Ian, thanks for that, interesting reading from Mr Champness as usual.

That description fits by the sound of it. ZF-sperrkreis is the word they used.

Anyway, for future reference, I downloaded a number of alignment instructions from other Nordmende radios of the period and found adjustment point 'V' on another model. It is in BF IV transformer can which my radio doesn't have. It's as I thought, generic text instructions and a variable drawing to suit the chassis in question.

Thanks for everyone’s help and the info on IF traps is interesting. Cheers 😀


 
« Back · 1 · Next »
 You need to be a member to post comments on this forum.

Sign In

Username:
Password:
 Keep me logged in.
Do not tick box on a computer with public access.