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Old 14th Nov 2018, 2:58 pm   #1
davidgem1406
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Default Dominante A122 FM Tunner.

Hello All,

I need some help here to be able to realign the FM tuner on this Dominante A122. After an extensive search trying to find the information for this alignment and ending up with a total blank I hope somebody can give me the information I need from the circuit diagram.

The range of coverage is from 88MHz to 100MHz UKW band.

I am hoping somebody can tell me what coils and trimmers to adjust for the top and bottom ends of the band, and what frequencies to use.
In total there are 8 adjustments available 3 trimmers and 5 coils.

As far as I can make out C305, C306, TR303, and SP301 set the incoming signal amplitude for the top and bottom ends, whilst the others set the top and bottom end frequencies. But I know not what does top or bottom.

I have included the schematic for the tuner unit. If more circuitry is required I can post as required.

Many thanks,
Regards Dave.
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Old 14th Nov 2018, 3:30 pm   #2
G8HQP Dave
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Default Re: Dominante A122 FM Tunner.

The general rule is that you adjust trimmers at the top of the band and coils at the bottom of the band. Both will affect both ends, so you need to iterate towards the correct settings. The oscillator sets the band; the RF stage then has to track it.

C305 and Sp301 are the RF stage. C309 and Tr303 are the oscillator. C316 is probably a broadly tuned RF adjustment - I'm not sure. Tr302 will also be broad.

Why do you think it needs re-aligning? Has someone fiddled with it?
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Old 14th Nov 2018, 9:32 pm   #3
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hi Dave, thanks for the information, it sounds helpful.
Yes, it has been tweaked. The set was supposed to arrive with a friend having been tweaked to receive the full UK FM band but was not as it should have been.

I tweaked it with some guesswork and had the higher end obtaining all UK stations, but that lost what should have been at the lower end, not a huge concern.
The main problem was some loss of quality that we did manage to improve but only to a point.

We now need to realign back to the manufactured settings, then see where we go from there.

Regards Dave.
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Old 14th Nov 2018, 10:38 pm   #4
kevinaston1
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Default Re: Dominante A122 FM Tunner.

Radio museum https://www.radiomuseum.org/r/funkwe...ante_a122.html
have a service sheet including alignment information. Unless you are a member, you can only download one page at a time.

Kevin
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Old 15th Nov 2018, 1:37 pm   #5
G8HQP Dave
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Default Re: Dominante A122 FM Tunner.

You are unlikely to be able to get the whole of Band 2. You might get a bit more than the original range, but may have to suffer poorer tracking so less sensitivity.
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Old 15th Nov 2018, 10:54 pm   #6
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hello Kevin,
Thanks for the link, I had checked the Radio Museum but I could not tell what the 4 pages were dealing with, they are in German.
I have now downloaded 3 of them and will get the other tomorrow. Once I have all I will try to translate them with Google.

Hello Dave,
The intention, once the alignment has been done, is to make and fit a frequency changer into the RF (Aerial circuit) then feed the output to the tuner.
The changer will be switched in and out using the UKW switch, hopefully, there is as a spare contact set available.

Many thanks
Regards Dave.
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Old 16th Nov 2018, 12:30 am   #7
kevinaston1
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Default Re: Dominante A122 FM Tunner.

The pages are unlikely to be true type fonts, so you will have to run them through an OCR programme to feed the text into Google translate


Kevin
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Old 16th Nov 2018, 12:55 pm   #8
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hi Kevin,
The downloaded pages are graphics.
Having downloaded them I can see nothing that relates to alignment.
I typed the headings into Google translate that resulted in the following:
Pages 1 & 2 are dealing with power supply.
Page 3 is something about sockets.
Page 4 is about an isolating transformer.

Unless there are more pages I cannot access that is all there is.

Regards
Dave.
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Old 16th Nov 2018, 6:48 pm   #9
John Caswell
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Default Re: Dominante A122 FM Tunner.

Hi Dave
First things first,
As this set uses "U" series valves it is bound to be a "live" chassis ie the chassis is connected to the power line and depending which way around the power plug is wired it could be at 240Vac with respect to earth, so BEWARE! Do not connect any signal generator/oscilloscope or what have you directly to the chassis, that way lies a nasty surprise.
If you have/can obtain a 1:1 230Vac isolating transformer of suitable rating, say 100VA, then the situation changes in that as the set is now isolated from the power line you can earth the chassis with impunity. BTW a variac is NOT an isolation transformer. If you cannot get hold of an isolating TX then couple the signal generator to the set with a couple of turns of wire at the end of your generator lead, although from the look of it you can plug the generator into the aerial sockets, but check the isolating capacitors c303/4 are OK first - just in case one is s/c.
As this set looks to be German in origin quite often they would put suitable markings on the scale for RF alignment - I know Grundig always did, and usually there is a mark on the scale for where the pointer should be set at the low frequency end
As the original coverage was 88-100MHz then at a guess the lower alignment point will be around 89-90MHz and the upper alignment point 98-99MHz so try 89MHz and 99MHz
So... at 89MHZ initially adjust Tr303 and at 99MHZ c309, repeating for best results always finishing at 99MHz with c309 so that the scale pointer matches the scale calibration. Check across the scale at various frequencies for reasonable accuracy.
Once this has been done then at 89MHz adjust Tr302/Sp301 for max signal and at 99MHz C305/C316 repeating for best results and finishing once again at the 99MHZ. Do not touch Sp302/303 at all unless this has been tweaked/got at.
Hope this helps bit

John

Last edited by John Caswell; 16th Nov 2018 at 6:53 pm. Reason: Additional blurb
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Old 16th Nov 2018, 9:50 pm   #10
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hello John,
Yes, I understand about live chassis equipment, I am always very careful to see that the AC supply is connected correctly. I have connected test equipment to live chassis sets many times without any problem or mishap, including this particular set. Thanks for the information a heads up never hurts.

Thank you, John, the alignment info is exactly what I am needing, I can now get it back to what it should be.
Sp302/203 as far as I am aware have not been touched.

I will update the result here when completed. At the moment my friend has the set back home with him.

My thanks to all for your input

Best regards,
Dave.
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Old 20th Jan 2019, 4:18 pm   #11
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hello all, I hope you all had a good Christmas and New Year.

Having been away as from December 10 until January 13 it is now time to pick this up again and post the results of our pre, and early, December endeavors, and see what we can do from here.

We used the information posted by John on #9 at 16 Nov 18, the table below shows this.

Or it would if I could paste it here, but I will attach it as an Excell document 001, and a second table 002.

The problem we had was that we could never reach a point where no further change was found.
Having repeated the adjustments time after time a change at one end always resulted in a shift at the other end. A point could never be found where no further change was required.
The result was that the scale never tracked with any accuracy.

We tried higher and lower frequencies for the start points but always ended with the same problem.

Eventually, we decided to set the start points using radio stations at the upper and lower ends of the scale and then check the tracking across the scale using the known stations.

My friend then took the set back to London and checked it out there. The following are the results he sent to me on the 3rd of December. These results are how the set is at the present time. (Table 002)

Hi Dave,
Iím back in London and have set the radio up with the aerial.

Stations receivable, their broadcast frequencies, and the corresponding display frequencies are as the attached Excel file (Shown below).

Lowest receivable is Radio 2 on 89.1 and highest is Classic FM on 101.1, so there is a band-spread of at least 12MHz (close to the design spread of 12.5MHz).

But in terms of the display, that is compressed into a display range from 91.2 to 100.0 or about 9MHz.

So we are pretty much where we go to on Friday.

Iím guessing that the reason we got so bogged down today was perhaps that, as you spotted towards the end of the day, the capacitor has two positions that work at the top end and we unwittingly wedded ourselves to the wrong one.
(My Note: the capacitors are the flat round type that can full rotate in a circle, and will have 2 positions dependant on the direction of rotation that will determine the position used.)

The lower half of the scale is still well out, but the upper half is much closer to the relevant scale markings.

The question is now what do we do to look at correcting this problem and getting things to track correctly.

All suggestions and ideas are very welcome.

Best regards
Dave
Attached Files
File Type: xlsx 001-FM Tuner Alignment.xlsx (9.9 KB, 14 views)
File Type: xlsx 002-Dominante display 3 Dec 2018 (2).xlsx (8.3 KB, 10 views)
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Old 20th Jan 2019, 6:57 pm   #12
Mr 1936
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Default Re: Dominante A122 FM Tunner.

Hi Davidgem.

In FM sets, the local oscillator is designed to be either lower or higher than the RF frequency by an amount equal to the IF frequency, normally 10.7 MHz. Thus if your set is supposed to tune from 88 to 100 MHz, the LO will either tune from 77.3 to 89.3 MHz (Low side) or 98.7 MHz to 110.7 MHz (High side).

Low side working was popular in the early days as it reduces the absolute amount of drift (because the frequency is lower). However, High side working needs a smaller percentage change in LO frequency (12.2 % versus 15.5 %). Once 88 to 108 MHz coverage became a requirement, the smaller tuning percentage was preferred and High side working became more commonplace.

I wonder if it possible that by tweaking too much you have ended up with the LO on the wrong side ? One way to check is to use an independent FM receiver to pick up the weak radiation from the LO, which is between 88 and 108 MHz for some of its tuning range.

Let's assume your set is low side and tunes 88 to 100 MHz. The LO range is 77.3 to 89.3 MHz, a change of 15.5 %. This needs a capacitance swing of 33.5%, for example 30 pF at the top of the band and 40 pF at the bottom.

Now let's try and modify it to tune 88 to 108 MHz. The LO range is 77.3 to 97.3 MHz, a change of 25.9 %. This needs a capacitance swing of 58.4%, for example 25 pF at the top of the band and 40 pF at the bottom. Simply tweaking the LO trimmer isn't going to achieve this on its own, and will tend to just shift the tuning range without making it much wider.

Without resorting to a change in tuning capacitor, you would have to change all the fixed tuning capacitances for smaller ones (assuming stray capacitance permits this) and increase the tuning inductors to compensate. These changes would probably be beyond the alignment adjustment range provided. I would hesitate to do this even with the right test gear, and would of course end up with a set where the tuning scale was meaningless, which seems a pity.

Your idea of a frequency converter ahead of the set seems like a better way.
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Old 20th Jan 2019, 9:17 pm   #13
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hello, Mr 1936,

Thanks very much for your explanation, I did not know anything of that at all.
As you say in your last paragraph about changing the tuning component I would much agree it is way beyond what would be reasonable for us to do.

What we are attempting to do is to bring the alignment back to what it would have been when exiting from the factory. Sometime before our ownership the FM must have been tweaked as it was way of and did not cover the full range 88MHz to 100MHz, but we just can not get it aligned so that it will track reasonably along the scale.

Best regards,
Dave.
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Old 21st Jan 2019, 7:17 pm   #14
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Default Re: Dominante A122 FM Tunner.

There is a lot of very good advice and Dave's alignment instructions perfect (exception Tr 303 to read Tr302) but I wonder if the Grundig 3028 Marlborough Alignment, which uses a very similar circuit, written in English will also help you.

I have attached a copy, sourced from GFGF Schaltplansmmlung NvHR, from Radio & TV Servicing.

Do watch out that the radio chassis is live and follow the recommendations made

Good luck

Chris
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File Type: pdf Grundig_3028Marlborough.pdf (374.1 KB, 32 views)

Last edited by simpsons; 21st Jan 2019 at 7:23 pm.
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Old 22nd Jan 2019, 10:19 pm   #15
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hello Chris,

Thank you very much for this information, it could be just what we need.
I had recently thought about looking at Grundig for alignment detail, mainly because they are a German company and did produce sets similar to the Dominante.

Thanks for the heads up about the live chassis, but I am quite familiar with live chassis items since I worked in R & TV service shops when all chassis were live.

Thanks for the Grundig information. A quick look at the FM tuner circuit and it looks very similar indeed to the Dominante tuner.

We will no doubt be trying the Grundig setup on the Dominante.

Best regards,
Dave.
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Old 19th Apr 2019, 7:52 pm   #16
davidgem1406
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Default Re: Dominante A122 FM Tunner.

Hello all,
Just a quick update on the Dominante FM alignment problem.
We did indeed try the set up procedure as used in the Grundig 3028, we adapted the component numbers to those used in the Dominante and gave it a try.
After a fairly long period trying to obtain some degre of alignment we just abandoned it for the time being.

It seems that nothing will pull this tuner into alignment. Some of the preset capacitors will do nothing, whilst others are so touch that 1/2mm movement will take it right off tune. Some of the coils have very little to no effect.

For now the project has been shelved as we look to find the manufacturers alignment information.

Thank you all for your help and input to this problem, it is much appreciated.

Best regards,
Dave.
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