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Old 25th Oct 2016, 5:51 pm   #1
6AL5W-Martin
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Default Telefunken LF Radio

I am now restorating one of the biggest and most heavy radios of germany,
the old TELEFUNKEN E108Lw4

We put it with 3 persons on the table

E108 is a low frequency receiver, double conversion super,
receiving from 10kHz up to 1.8MHz,
4 steps selectable bandwiths from +-100Hz to +-4.5kHz,
It have a germanium diode ring mixer, all other is glowing.

This will be a lot of fun to do a fine restoration.
E108 can listen also the Grimeton SAQ

greetings
Martin
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Old 26th Oct 2016, 12:55 pm   #2
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Default Re: Telefunken LF Radio

I hope you can maintain the superb quality of the wiring in photo 5 Martin. It's a thing of beauty I presume the RF Ring Mixer may be similar to the Ring Modulator principle as used in electronic music-[especially in Germany]

Dave W
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Old 26th Oct 2016, 5:20 pm   #3
6AL5W-Martin
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Default Re: Telefunken LF Radio

hello Dave,
my english is not the best, is ring mixer the right word?
In some HAM equipments is this as a part "IE500" or similar, build from 2 broadband transformers and a selected quad of diode.

Here is the mixer from the Telefunken radio, the diode are OA154Q (4pcs)

greetings
Martin
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Old 26th Oct 2016, 6:20 pm   #4
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Default Re: Telefunken LF Radio

Another fine and beautifully crafted set- in Britain, Murphy's B40 (MF/HF) and B41 (LF) sets are thought of as heavyweights but that Telefunken is something else!

I assume that a balanced mixer is near-essential in a set like this going as low as 10kHz in order to keep LO out of the IF?

Good luck with the restoration, don't get a hernia or a crushed disc....!,

Colin
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Old 26th Oct 2016, 8:09 pm   #5
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Default Re: Telefunken LF Radio

The diode ring mixer is normally driven into switching by a powerful local oscillator. This causes a signal in passing from the RF port to the mixer port to be alternately normal, then inverted, then normal again.... etc. This isn't quite perfect, the diodes diodes which are on act as a variable impedance which varies over the LO cycle, and the off diodes act as varactors, giving a variable capacitance effect. On top of this, the diode is a two terminal device, so it is the sum of the LO and RF voltages which determine when it turns on and off. So a strong RF signal modulates the switching times of the diodes, and thus intermodulation happens. On the whole, diode ring mixers still out-perform many other types.

The usual analysis of what goes on is "Sources of intermodulation distortion in diode ring mixers" by Hugh P Walker, published in proceedings of the IERE in the early 1970s. It's an important paper that gets cited quite generally and Hugh together with Guy Douglas pushed the diode ring mixer just about as far as it can go (later written up in Hewlett-Packard Journal April 1982)

Whether a diode ring is a mixer or a modulator and what frequencies are involved is just a matter of what you use it for.

David
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Old 26th Oct 2016, 8:23 pm   #6
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Default Re: Telefunken LF Radio

Wow!

Ja! Ja! Das ist grossen ..... but - wonderbar !!!

What a bit of kit eh?

Good luck with this one Martin.
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Old 26th Oct 2016, 9:28 pm   #7
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Default Re: Telefunken LF Radio

Even with a diode ring mixer that was carefully balanced, in a receiver tuning down to 10kHz, we found it necessary to use a switched crystal filter in the LO feed in order to reduce phase noise sidebands passing through the IF.

Another trick was to run the local oscillator at twice the wanted frequency and employ a divider at the mixer in order to get a very symmetrical LO drive. There's no point in carefully balancing a mixer then driving it with a local oscillator whose mark-space ratio is asymmetric.

David
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Old 27th Oct 2016, 10:28 am   #8
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Default Re: Telefunken LF Radio

It's certainly most interesting to see a mixer of this type (the grand-daddy of all those 8-pin DBMs that became so popular?) in a receiver of this era. There were quite a few sets tuning down to VLF, often of a maritime nature, in the thermionic era but the superhets among them seemed to use classical multigrid mixers (here I use the term in the broader sense of a "multiplicative" device with separate signal and oscillator grids, rather than the specific type so long beloved of US designers in particular). Coverage was also often down to 15kHz, presumably representing a "safe" limit with typical achievable cascaded LC IF passband characteristics between mixer and IF strip?

David, I'm intrigued by this business of crystal filters in the LO drive- does this mean a relatively broad, flat-topped filter but with very tight slope between minimum LO frequency and IF? I'm guessing that it would be switched in below perhaps 30-50kHz signal input frequency, with appropriate characteristics? Apologies for all the questions, and also if I'm grabbing the wrong end of the stick- it's not unknown...

I note that the E108 has several EL84s listed in its Radiomuseum entry- maybe one of these is a PA for LO drive into the mixer? An EL821 would be more "conventional", but there's nothing wrong with using anything that fits the bill just because it's outside the marketed remit,

Colin
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Old 27th Oct 2016, 11:31 am   #9
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Default Re: Telefunken LF Radio

You've just about got it, Colin.

A VLF superhet receiver is going to be an up-converter at its first mixer. The LO tunes from a frequency equal to the IF upwards as the RF tunes from zero hertz upwards.

Let's pick some arbitrary frequencies as an example. 1.4 MHz IF and listening to something on 15kHz.

The LO is therefore at 1.415MHz. Any phase noise on the lower side of the local oscillator and 15kHz out will fall into the IF bandwidth. Mixers aren't perfect and some will leak into the IF, desensitising the receiver with an artificially high noise floor.

Given the usual shape of LO phase noise (except for fractional-N synthesisers whichare worse!) this is why tuning down low gets a rising noise floor. It can't all be blamed on 1/f and flicker noise.

One fix is to put a crystal filter in the LO feed to plant a notch on the IF frequency. It limits how close to zero you can tune, but otherwise improves performance.

The other way is to put in a crystal bandpass filter set a bit higher than the IF so the IF is down the bottom skirt. This kills some noise, and allows the LO through. Again the tuning range is limited. The filter gets switched out as you tune upwards, before the LO falls off the top cutoff. Notch filters need switching out too because they mis-behave away from the notch, or the design is of a notch embedded in a wider bandpass.

There is a crystal filter and switch in the LO feed of the Racal RA1772, and there's one in the HP journal article above in the section by Guy Douglas and a much younger me.

David
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Old 27th Oct 2016, 4:33 pm   #10
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Default Re: Telefunken LF Radio

Many thanks for those explanations. I will try to take this on board. I was particularly pleased with Davids comment that it's a Mixer or a Modulator depending on usage. I had thought that this might be the case [instinctively] but I would have no chance of saying why exactly.

I still have the circuit of an Audio Ring Modulator I was delighted to find in a Library Book in the mid sixties. I tried to knock one up from back to back O/P transformers. Stockhausen was all the rage and Ring Modulator boxes seemed very Sci-Fi and mysterious.
The difficulty there in achieving "balance" seemed to be a positive advantage in the context of the musical experiments taking place though. It's all frequencies in the end I suppose.

Dave W

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Old 27th Oct 2016, 5:20 pm   #11
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Default Re: Telefunken LF Radio

The double conversion of the old radio is the following:

Receive frequency steps.

1.) step: 10kHz to 31kHz
2.) step: 30kHz to 94kHz
3.) step: 91kHz to 233kHz
4.) step: 224kHz to 540kHz
5.) step: 510kHz to 1020kHz
6.) step: 970kHz to 1800kHz

The first IF frequency is related to the step:

steps 1 + 4 + 5 = 80kHz
steps 2 + 3 + 6 = 325kHz

following a crystal oscillator to the second IF of 525kHz
in the second IF it have crystal filters to switch the bandwith.

following BFO + Audio stage

about EL84 tubes:
one is in the tuner, one is in the PSU as regulating, one is the audio pa.

EF85: I found all over EF805s,the SQ version of the EF85
ECC81: I found all over ECC801s, the SQ version of ECC81

it have a calibration oscillator using a crystal what looks like a tube, at 200kHz. The CAL switch simply makes on/off the filament of the triode system what does it.

The input preselection use 4 coils in each band.

The radio have a selection switch for 3 antenna inputs: long wire, 60 ohms and frame.

The sensitivity is, band 1 to 5: 0,5µV for 10dB s/n, band 6 = 0,3µV

there is a switch "Tonsieb", I think this means a special resonance audio filter what is only useful with the BFO.
bec. I am still restorating the BFO not shure how that works exactly...

greetings
Martin
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Old 27th Oct 2016, 6:52 pm   #12
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Default Re: Telefunken LF Radio

The arrangement of the six bands into two different IFs is artful. On some bands it's an up-converter, on others it's a down-converter.

Four tuned circuits for each band should make a tight preselector filter. In fact there may be some careful bandpass tuning to stop the preselector getting too narrow for some signals to pass through without losing some of their higher frequency audio content.

The sensitivity is greater than can generally be used on these frequencies due to atmospheric noise levels.

Quartz crystals shift frequency as they age. Consider an HC6U can which has been soldered together. debris from the soldering contaminates the quartz and thus ageing drift is high. Cold-welded cans are much better. Evacuated glass housings (usually valve-like glass) are even better.

The diode ring will need about 10mW of LO drive power, which doesn't need an EL84, but maybe an EL84 is an easier broadband match to the low-Z mixer?

Mention of Telefunken and special quality valve versions will drive the audiophiles into a frenzy You may need to keep the thing in a bank vault!

David
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Old 27th Oct 2016, 7:31 pm   #13
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Default Re: Telefunken LF Radio

I have seen in my (not complete) manual, the EL84 in the tuner is the 1. oscillator.

audiophiles, yes they are a special problem, some tubes are impossible expensive now, about them... and some of them have no problem to destroy any old radios and test gear to get the tubes inside. I am sorry, audiophiles can`t get anything from me, never.

I have done a mistake.. that is also part of the hobby to do something more then one time
There was to repair a rotating coupling at the BFO. The work is, to remove at first the sub chassis with audio and BFO from the frame, then remove the BFO box from the audiochassis bec. you cannot open the cover from the BFO when it is installed...

So I have done, the mechanical repair is done, close cover from BFO box, put it back in the audio chassis, istall the audio chassis in the frame, allready (except: the BFO dont want to work) so I can remove it again to see what is wrong inside. This will be a workful weekend.

greetings
Martin

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Old 27th Oct 2016, 7:31 pm   #14
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Default Re: Telefunken LF Radio

There are some other units similar to this one which cover the VLF/LF/MF range but were designed to measure the power of channels and pilot tones in Frequency Division Multiplex systems. Some were made to work on cable systems, others handled the baseband multiplexes which were FM'ed onto microwave links and squirted between dishes on 40km hops at 8GHz.

Siemens and R&S made these things with the same sort of battleship construction... Don't try lifting an HP one, either. They make good receivers for the LF amateur bands, but the lack of AGC and precise, switched 10dB steps in gain are not very convenient, but propagation on these frequencies is stable.

David
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Old 27th Oct 2016, 7:46 pm   #15
6AL5W-Martin
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Default Re: Telefunken LF Radio

The radio is back to life

except the BFO dont work, I will remove it again ...
the antenna inputs box at backside must be serviced with oil, there is a rotating C, not possible to rotate that..
and some little things to do.

This radio use a dozen of little bulbs to light up the scale in seperate lines related to the band what is in listening.
why they have not constructed a mechanic to move 2 bulbs upper and lower
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Old 27th Oct 2016, 8:12 pm   #16
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Default Re: Telefunken LF Radio

Unfortunately, there is a "must be valves- at any price!" mindset circulating among some. One could get an excellent 50W semiconductor stereo amp for the price of a single example of certain hallowed valves... but I think I'm missing the jewellry aspect of the craze.

Yes, one could have had a system of indexed chain or cam/lever shifting a lighting carriage up and down. In the lighting aspect, the E108 appears to beat the GEC BRT400 with its ten illumination bulbs- the graduated strip system seems similar but the GEC just has one bulb per scale.

Interesting that the EL84 is the actual local oscillator, fitting in with the mixer drive question. Marconi's CR100 has a notably powerful oscillator (c. 60V/10mA running, pentode strapped anode/suppressor/screen), somewhat more than many but probably nowhere near this one.
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Old 27th Oct 2016, 9:40 pm   #17
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Default Re: Telefunken LF Radio

Some signal generators and other oscillators run a fair bit of power to improve the effective signal to noise ratio... for example the 5763 used in the Marconi TF144H partially this offers the full output power without further amplification and the broadband noise pedestal amplifier chains create. The 5763 is also in the Marconi Q meter drive oscillators.

David
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Old 10th Nov 2016, 5:38 pm   #18
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Default Re: Telefunken LF Radio

E108Lw4 is restorated

Now I must build a frame antenna, the manual says 100µH and a minimum size of one m²

greetings
Martin
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Old 11th Nov 2016, 9:10 pm   #19
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Default Re: Telefunken LF Radio

Martin, have a look at the PA0RDT mini whip, they work really well at LF

Beautiful receiver by the way

Tim M0AFJ
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Old 13th Nov 2016, 9:21 am   #20
6AL5W-Martin
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Default Re: Telefunken LF Radio

hello Tim,

this old radio have a special symmetrical input for a no resonance loop antenna, the preamplification is present and extremly sensitive. So it is a good idea to follow the instructions. I have a little wires galvanik for homebrew silvercoated wires (bath1 = silver following bath2 = isolating paint), so I can build loop antennas in professional quality.
The winding will be done with wires of 15x0.1 silvercoated and paint isolated (Litze)
this gives a low resistance very low noise antenna.

There are 3 inputs the radio can switch between them: sym. loop, 60 ohms and high impedance for long wires. So I can connect all of them both if I need.
My now running restoration is a HFH, there are also 3 loops to build for.

greetings
Martin
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