Weltron T3000 FM-tuner with 2MHz offset - UK Vintage Radio Repair and Restoration Discussion Forum

Apfelmus · 22nd May 2022, 12:36 pm

Hi all,

I have one of these "bookshelf-tuners" on the bench right now. Reception-wise its working fine, good distortion, sensitivity and stereo reception. Some information, including a circuit diagram, can be found here:
https://www.radiomuseum.org/r/weltron_t_3000.html

BUT: the frequency alignment is about 2MHz off! The tuner receives between 89 and 111 MHz! The frontend of this tuner is a blackbox in the circuit diagram. I was trying any of the slugs to see if detuning will move the offset around, but no luck at all. Even turning the slug marked "OSC" did not make any difference!

Somewhere I read these frontends have been used in car-stereos, due to size constraints. I cannot find any information on these. The slugs have the following labels (see the blown-up pic): OSC, FOT, FIF, AIFT (2x).

What do these labels mean? And why does turning the OSC coil does nothing? Could there be something broken, althoug reception is in spec?

cheers
Martin

ms660 · 22nd May 2022, 12:48 pm

Looks like the AM Osc. to me in the 2nd photo.

Lawrence.

Radio Wrangler · 22nd May 2022, 3:37 pm

I'd guess that the permeability tuner sections are for the AM band(s) and that VHF/FM might be by varactor diodes and a pot. There seem to be TOKO-looking coil assemblies under that screen. Going by the labelling FM and AM IFTs, and likely the FM osc and RF coils.

A car front-end of that period would need both FM and AM bands.

David

ms660 · 22nd May 2022, 4:59 pm

Apparently there's six permeability tuning coils, the three on top for AM and three below them for VHF.

The schematic on radiomuseum is marked up as a Weltron T 3100:

https://www.radiomuseum.org/r/weltron_t_3000.html

Lawrence.

Apfelmus · 22nd May 2022, 5:02 pm

What is meant by "permeability" tuning coils?

You mean I have to lift the frontend off the PCB and there should be access to the FM alignments?

cheers
Martin

ms660 · 22nd May 2022, 5:10 pm

Quote:

Originally Posted by Apfelmus

What is meant by "permeability" tuning coils?

You mean I have to lift the frontend off the PCB and there should be access to the FM alignments?

cheers
Martin

Permeability tuning is tuning by varying the inductance of the tuning circuit as opposed to the more common method of varying the capacitance.

I've no idea what's what so far as any adjustments to those coils or which coil does what.

Lawrence.

paulsherwin · 22nd May 2022, 5:12 pm

Permeability tuning works by moving the core of a coil rather than padding it with a variable capacitor or varactor. The technique was quite common in car radios of that era, but is unusual in a HiFi tuner.

Apfelmus · 22nd May 2022, 6:35 pm

The slug labled "FOT", with the blue mark, seems to be a trimmer cap. I didn't try this one yet...

So with permeability tuning, all the alignment of the FM frontend should be done by the adjustment screws in front of the coils? There are 6 coils, the adjustment screws are sealed and you cannot access them when built in.

What could possibly cause such a large offset in the frontend, with the tuner still working well?

cheers
Martin

ms660 · 22nd May 2022, 6:43 pm

In my first post I did suggest FOT but deleted that bit as I wasn't sure, but you never know, FM Osc. Trimmer.

Lawrence.

Apfelmus · 22nd May 2022, 6:55 pm

Quote:

Originally Posted by ms660

I wasn't sure, but you never know, FM Osc. Trimmer.

And you're perfectly right: less than 10° counterclockwise and the tuner was perfectly aligned again, frequency-wise. 87-109MHz tuning range!

Thanks to all of you!

cheers
Martin

Apfelmus · 4th Jun 2022, 2:16 pm

Hi all,

a little followup after having solved the oscillator alignment thanks to all of you!

I was checking bandshapes on the IF part, although there is nothing to align as its all Quartz filtered. But the coil named "FIF" does influence the shape. Yet, when I align FIF for maximum level, sensitivity is getting worse than when FIF is adjusted off-peak.

That seems weird, so for closer examination I connected my wobbler on the antenna input and checked bandshape at the output of the frontend, labled FM IF, (see first pic, top trace) and also at the output of the IF amplifier, pin 8 (bottom trace).

In the first pic the shape of FIF is clean and centred to the low side of 10,7MHz. The output of the IF amp is centred to 10,7MHz. In the secon pic FIF is aligned on 10,7MHz. So with the FIF properly aligned, the bandshape of the FIF gets weird and I get a lot more noise on the IF amp output without a significant change in bandshape.

The third pic shows the same situation (FIF aligned on 10,7MHz) but with a very low RF input to the tuner. The 10,7MHz signal is barely noticeable due to the noise. The strange thing is that the s/n ratio gets better when FIF is misaligned, as in the first pic.

Anybody has an idea how this can happen?

cheers
Martin

22nd May 2022, 12:36 pm	#1
Apfelmus Pentode Join Date: Sep 2014 Location: Stuttgart, Germany Posts: 176	Weltron T3000 FM-tuner with 2MHz offset Hi all, I have one of these "bookshelf-tuners" on the bench right now. Reception-wise its working fine, good distortion, sensitivity and stereo reception. Some information, including a circuit diagram, can be found here: https://www.radiomuseum.org/r/weltron_t_3000.html BUT: the frequency alignment is about 2MHz off! The tuner receives between 89 and 111 MHz! The frontend of this tuner is a blackbox in the circuit diagram. I was trying any of the slugs to see if detuning will move the offset around, but no luck at all. Even turning the slug marked "OSC" did not make any difference! Somewhere I read these frontends have been used in car-stereos, due to size constraints. I cannot find any information on these. The slugs have the following labels (see the blown-up pic): OSC, FOT, FIF, AIFT (2x). What do these labels mean? And why does turning the OSC coil does nothing? Could there be something broken, althoug reception is in spec? cheers Martin Attached Thumbnails

22nd May 2022, 12:48 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Weltron T3000 FM-tuner with 2MHz offset Looks like the AM Osc. to me in the 2nd photo. Lawrence. Last edited by ms660; 22nd May 2022 at 1:09 pm.

22nd May 2022, 3:37 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,867	Re: Weltron T3000 FM-tuner with 2MHz offset I'd guess that the permeability tuner sections are for the AM band(s) and that VHF/FM might be by varactor diodes and a pot. There seem to be TOKO-looking coil assemblies under that screen. Going by the labelling FM and AM IFTs, and likely the FM osc and RF coils. A car front-end of that period would need both FM and AM bands. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

22nd May 2022, 4:59 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Weltron T3000 FM-tuner with 2MHz offset Apparently there's six permeability tuning coils, the three on top for AM and three below them for VHF. The schematic on radiomuseum is marked up as a Weltron T 3100: https://www.radiomuseum.org/r/weltron_t_3000.html Lawrence.

22nd May 2022, 5:02 pm	#5
Apfelmus Pentode Join Date: Sep 2014 Location: Stuttgart, Germany Posts: 176	Re: Weltron T3000 FM-tuner with 2MHz offset What is meant by "permeability" tuning coils? You mean I have to lift the frontend off the PCB and there should be access to the FM alignments? cheers Martin

22nd May 2022, 5:12 pm	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,937	Re: Weltron T3000 FM-tuner with 2MHz offset Permeability tuning works by moving the core of a coil rather than padding it with a variable capacitor or varactor. The technique was quite common in car radios of that era, but is unusual in a HiFi tuner.

22nd May 2022, 6:35 pm	#8
Apfelmus Pentode Join Date: Sep 2014 Location: Stuttgart, Germany Posts: 176	Re: Weltron T3000 FM-tuner with 2MHz offset The slug labled "FOT", with the blue mark, seems to be a trimmer cap. I didn't try this one yet... So with permeability tuning, all the alignment of the FM frontend should be done by the adjustment screws in front of the coils? There are 6 coils, the adjustment screws are sealed and you cannot access them when built in. What could possibly cause such a large offset in the frontend, with the tuner still working well? cheers Martin

22nd May 2022, 6:43 pm	#9
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Weltron T3000 FM-tuner with 2MHz offset In my first post I did suggest FOT but deleted that bit as I wasn't sure, but you never know, FM Osc. Trimmer. Lawrence.