Grundig stereo decoder transformer connections - UK Vintage Radio Repair and Restoration Discussion Forum

brightsparkey · 23rd Apr 2018, 11:47 am

My Grundig stereo 60's radio, which was thrown around by Hermes, suffered damage to the stereo decoder module.

The transformer in the anode side of the first triode became ejected from its base, severing all four wires (see the snip of the schematic attached)

The bobbin has two windings, one takes two layers of the bobbin and the other only one. There are four wires in total.

I have guessed which winding is which and reconnected it. It more or less works, but the relay doesn't close (i.e. switch to stereo mode) until around 450uV of RF signal. Also no amount of adjusting the core in the transformer affects the sensitivity of the decoder.

Does anyone have a similar decoder to inspect, so they can tell me which coil is which? The lower (closer to the PCB) is the one with more turns.

The decoder schematic is labelled 'Stereo-Decoder 4U' and the set is a 4070U which is a US export model. I notice that some values are different from the schematic, so it may have been modified for UK use at some point.

Many thanks,

Kevin.

German Dalek · 23rd Apr 2018, 12:09 pm

If you don`t get help during the next week, I will search in my big stock for it
and I shall give you the info you want.

Best regards,
German Dalek

orbanp1 · 23rd Apr 2018, 2:13 pm

Hi Kevin,

hifiengine has a service manual on the Grundig stereo decoders, it also describes the one you have:
https://www.hifiengine.com/manual_li...-decoder.shtml

Peter

brightsparkey · 24th Apr 2018, 11:52 am

Hi Peter, unfortunately not the one I have. The link is to a transistor decoder of much later vintage. Mine has a single ecc83 dual triode and some OA81 diodes. Its a reasonable match for the decoder schematic that's on the radiomuseum.org website along with the radio.

Another useful site for manuals and info. though, - Thank-you.

I could just rewire it the other way around and see what happens, but its very fiddly and the wires are delicate..

Kevin.

orbanp1 · 24th Apr 2018, 2:10 pm

Hi Kevin,

That document, starting at pg. (pdf) 60, describes the tube based stereo decoders.
They cover Decoders 6, 5, and 4. They seem to differ only in the mode of switchover, from automatic (with tube based or transistorized control circuits) to manual, the decoder circuits seem to be the same.
They also describe measurement and adjustment data.
Check it out!

Peter

Biggles · 24th Apr 2018, 2:44 pm

At a guess, the primary will be the one with the two layers on the bobbin, assuming it is matching the relatively high impedance of the valve anode circuit to the lower impedance side with the diodes from what I can see of the diagram.
Alan.

brightsparkey · 24th Apr 2018, 2:58 pm

Hi Peter - to my shame, I didnt scroll to the end - yes it looks like there is a page for the 4u decoder with a complete set of instructions for tuning - very useful - I'll start translating them!

Apologies for missing it the first time!

Kevin.

G8HQP Dave · 24th Apr 2018, 3:01 pm

Yes, I would expect the winding with the greater number of turns to be the 38kHz tuned circuit in the anode. Did the capacitor survive its journey?

brightsparkey · 24th Apr 2018, 3:16 pm

Alan,

that was my first guess and is how its wired for at the moment. Now I have some setup instruction I can work out if its correct. The manual also gives the schematic for the stereo-mono relay switch circuit which I didn't have in my copy from the radio museum.

most useful thank-you.

Kevin.

brightsparkey · 24th Apr 2018, 3:18 pm

Yes the cap was still attached to the base of the transformer. In addition there is a resistor across it - as I say the schematic doesn't tally perfectly.. this seems to be common - the SABA I repaired had some different values fitted than on the schematic - original SABA parts too.

Kevin.

ms660 · 24th Apr 2018, 3:22 pm

The alignment of the 6 has been translated here, there might be similarities to the 4 that might be of help:

https://www.radiomuseum.org/forumdat...ecoder%206.pdf

Lawrence.

G8HQP Dave · 26th Apr 2018, 11:42 am

I just thought: the switch to stereo mode depends on the level of the 19kHz pilot tone, not the 38kHz L-R signal. Check the other transformer shown in your circuit extract, and the two diodes which act as a frequency doubler.

brightsparkey · 9th May 2018, 4:12 pm

"I just thought: the switch to stereo mode depends on the level of the 19kHz pilot tone, not the 38kHz L-R signal. Check the other transformer shown in your circuit extract, and the two diodes which act as a frequency doubler. "

Sorry for the delayed response - just getting back to this. I've tried to understand the circuit design - its not entirely simple. In fact its elegantly cunning!

In the setup instructions the coil 104 which is the transformer that was damaged is referred to as adjusting the 'sidebands'.

From the circuit I think the function may be to feedback the non-19kHz signals to the top side of the 19kHz tuned transformer so that the output of the doubler is less affected by the audio. Its tuning is therefore partly to align the phase of that feedback signal. In the instructions it says to tune for a sharp null in the signal at the top of the 19kHz transformer.

I have constructed a test signal generator (I'm lucky to have access to a modern arbitrary waveform generator which can make nice FM-multiplex signals for me) so am now in a position to run some tests and get up close and personal with the circuit on the 'scope - will report back.

Kevin.

G8HQP Dave · 10th May 2018, 2:34 pm

The transformer you highlighted passes the stereo difference signal (DSB on a suppressed 38kHz carrier) to the mixer (0A81 diodes on the extreme right of your circuit extract). If the phase is wrong then you get poor stereo separation, but this should not affect whether the stereo indicator lights. That depends on the level of the 19kHz pilot tone (lower transformer in same anode) or the 38kHz carrier (transformer above second triode).

brightsparkey · 11th May 2018, 1:28 pm

Ok Got it - I was reading the circuit the wrong direction - so it feeds the DSB signal into the mixer like the "RF" port with the "LO" at 38kHz fed from the other transformer - this makes sense now.

I have adjusted the slug in the repaired transformer according to the setup for a sharp null in the 300Hz test tone. So I think its the right way around (otherwise it wouldn't tune up right. <Releif>

I also adjusted the main 19kHz transformer and attempted to adjust the 38kHz mixer transformer which was stuck and wont budge so will have to stay where it is.

The result is much improved performance and it switches at the 180uV specified in the setup instructions - so you were quite correct - it was the 19kHz that was off tune. I suspect it was 'got at' as there is candle wax on the top of the box and some of the sticky label covers have been disturbed..

Its much happier now - result !

Now I can reassemble the radio and gear up for the case work, except for the scale lamps which have become a nightmare - back to the other thread for that saga...

Thanks for the nudge in the right direction Dave - the circuit now makes sense to me! I should have known better as a dyed in the wool RF engineer!

Kevin.

G8HQP Dave · 11th May 2018, 3:09 pm

Glad you have got it working. If the 38kHz phasing is wrong then you will get reduced stereo separation. Fortunately you can to some extent compensate for phase errors in the coil you can't adjust by tweaking the coil you can adjust.

23rd Apr 2018, 11:47 am	#1
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Grundig stereo decoder transformer connections My Grundig stereo 60's radio, which was thrown around by Hermes, suffered damage to the stereo decoder module. The transformer in the anode side of the first triode became ejected from its base, severing all four wires (see the snip of the schematic attached) The bobbin has two windings, one takes two layers of the bobbin and the other only one. There are four wires in total. I have guessed which winding is which and reconnected it. It more or less works, but the relay doesn't close (i.e. switch to stereo mode) until around 450uV of RF signal. Also no amount of adjusting the core in the transformer affects the sensitivity of the decoder. Does anyone have a similar decoder to inspect, so they can tell me which coil is which? The lower (closer to the PCB) is the one with more turns. The decoder schematic is labelled 'Stereo-Decoder 4U' and the set is a 4070U which is a US export model. I notice that some values are different from the schematic, so it may have been modified for UK use at some point. Many thanks, Kevin. Attached Thumbnails

23rd Apr 2018, 12:09 pm	#2
German Dalek Hexode Join Date: Jan 2011 Location: Düsseldorf, Germany. Posts: 368	Re: Grundig stereo decoder transformer connections If you don`t get help during the next week, I will search in my big stock for it and I shall give you the info you want. Best regards, German Dalek __________________ And now something completly different: MARC BOLAN, he was/is the real king of Pop Music!

23rd Apr 2018, 2:13 pm	#3
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Grundig stereo decoder transformer connections Hi Kevin, hifiengine has a service manual on the Grundig stereo decoders, it also describes the one you have: https://www.hifiengine.com/manual_li...-decoder.shtml Peter

24th Apr 2018, 11:52 am	#4
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Re: Grundig stereo decoder transformer connections Hi Peter, unfortunately not the one I have. The link is to a transistor decoder of much later vintage. Mine has a single ecc83 dual triode and some OA81 diodes. Its a reasonable match for the decoder schematic that's on the radiomuseum.org website along with the radio. Another useful site for manuals and info. though, - Thank-you. I could just rewire it the other way around and see what happens, but its very fiddly and the wires are delicate.. Kevin.

24th Apr 2018, 2:10 pm	#5
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Grundig stereo decoder transformer connections Hi Kevin, That document, starting at pg. (pdf) 60, describes the tube based stereo decoders. They cover Decoders 6, 5, and 4. They seem to differ only in the mode of switchover, from automatic (with tube based or transistorized control circuits) to manual, the decoder circuits seem to be the same. They also describe measurement and adjustment data. Check it out! Peter

24th Apr 2018, 2:44 pm	#6
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Grundig stereo decoder transformer connections At a guess, the primary will be the one with the two layers on the bobbin, assuming it is matching the relatively high impedance of the valve anode circuit to the lower impedance side with the diodes from what I can see of the diagram. Alan.

24th Apr 2018, 2:58 pm	#7
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Re: Grundig stereo decoder transformer connections Hi Peter - to my shame, I didnt scroll to the end - yes it looks like there is a page for the 4u decoder with a complete set of instructions for tuning - very useful - I'll start translating them! Apologies for missing it the first time! Kevin.

24th Apr 2018, 3:01 pm	#8
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Grundig stereo decoder transformer connections Yes, I would expect the winding with the greater number of turns to be the 38kHz tuned circuit in the anode. Did the capacitor survive its journey?

24th Apr 2018, 3:16 pm	#9
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Re: Grundig stereo decoder transformer connections Alan, that was my first guess and is how its wired for at the moment. Now I have some setup instruction I can work out if its correct. The manual also gives the schematic for the stereo-mono relay switch circuit which I didn't have in my copy from the radio museum. most useful thank-you. Kevin.

24th Apr 2018, 3:18 pm	#10
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Re: Grundig stereo decoder transformer connections Yes the cap was still attached to the base of the transformer. In addition there is a resistor across it - as I say the schematic doesn't tally perfectly.. this seems to be common - the SABA I repaired had some different values fitted than on the schematic - original SABA parts too. Kevin.

24th Apr 2018, 3:22 pm	#11
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Grundig stereo decoder transformer connections The alignment of the 6 has been translated here, there might be similarities to the 4 that might be of help: https://www.radiomuseum.org/forumdat...ecoder%206.pdf Lawrence.

26th Apr 2018, 11:42 am	#12
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Grundig stereo decoder transformer connections I just thought: the switch to stereo mode depends on the level of the 19kHz pilot tone, not the 38kHz L-R signal. Check the other transformer shown in your circuit extract, and the two diodes which act as a frequency doubler.

9th May 2018, 4:12 pm	#13
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Re: Grundig stereo decoder transformer connections "I just thought: the switch to stereo mode depends on the level of the 19kHz pilot tone, not the 38kHz L-R signal. Check the other transformer shown in your circuit extract, and the two diodes which act as a frequency doubler. " Sorry for the delayed response - just getting back to this. I've tried to understand the circuit design - its not entirely simple. In fact its elegantly cunning! In the setup instructions the coil 104 which is the transformer that was damaged is referred to as adjusting the 'sidebands'. From the circuit I think the function may be to feedback the non-19kHz signals to the top side of the 19kHz tuned transformer so that the output of the doubler is less affected by the audio. Its tuning is therefore partly to align the phase of that feedback signal. In the instructions it says to tune for a sharp null in the signal at the top of the 19kHz transformer. I have constructed a test signal generator (I'm lucky to have access to a modern arbitrary waveform generator which can make nice FM-multiplex signals for me) so am now in a position to run some tests and get up close and personal with the circuit on the 'scope - will report back. Kevin.

10th May 2018, 2:34 pm	#14
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Grundig stereo decoder transformer connections The transformer you highlighted passes the stereo difference signal (DSB on a suppressed 38kHz carrier) to the mixer (0A81 diodes on the extreme right of your circuit extract). If the phase is wrong then you get poor stereo separation, but this should not affect whether the stereo indicator lights. That depends on the level of the 19kHz pilot tone (lower transformer in same anode) or the 38kHz carrier (transformer above second triode).

11th May 2018, 1:28 pm	#15
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Re: Grundig stereo decoder transformer connections Ok Got it - I was reading the circuit the wrong direction - so it feeds the DSB signal into the mixer like the "RF" port with the "LO" at 38kHz fed from the other transformer - this makes sense now. I have adjusted the slug in the repaired transformer according to the setup for a sharp null in the 300Hz test tone. So I think its the right way around (otherwise it wouldn't tune up right. <Releif> I also adjusted the main 19kHz transformer and attempted to adjust the 38kHz mixer transformer which was stuck and wont budge so will have to stay where it is. The result is much improved performance and it switches at the 180uV specified in the setup instructions - so you were quite correct - it was the 19kHz that was off tune. I suspect it was 'got at' as there is candle wax on the top of the box and some of the sticky label covers have been disturbed.. Its much happier now - result ! Now I can reassemble the radio and gear up for the case work, except for the scale lamps which have become a nightmare - back to the other thread for that saga... Thanks for the nudge in the right direction Dave - the circuit now makes sense to me! I should have known better as a dyed in the wool RF engineer! Kevin.

11th May 2018, 3:09 pm	#16
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Grundig stereo decoder transformer connections Glad you have got it working. If the 38kHz phasing is wrong then you will get reduced stereo separation. Fortunately you can to some extent compensate for phase errors in the coil you can't adjust by tweaking the coil you can adjust.