Schwabenland Receiver Repair Survey
Stocknumber: Ln 21021
Since 2014, we own a Schwabenland RX, obtained from France.
The seller has told me that the 'Band Switch Handle' does not respond (cannot be rotated at all).
Page initiated on 27 December 2017
Status: 11/12 January 2018
It proved to be a mechanical failure, where someone had started with solving the problem long time ago, but apparently was encountering too many obstacles.
However, I could not foresee that other problems might arose; therefore I took no detailed photographs. With the support and ingenuity of Hans Goulooze it was ultimately managed to get the mechanical section operating again. The real problem proved to be - that crease (lubrication) has become far too sticky; after, say, 75 years. Maybe hampering, the narrow tolerances between the band-switch-shaft and its tubular guiding-hose. For it we had do remove the tuning scale entirely. A nuisance, the tiny screws (1.9 mm), which could only be made to rotate - by heating them up by means of a quite heavy solder-iron; think of several hundred watts. By the way, this successful procedure (technique) was entirely Hans Goulooze's idea. However, it worked out perfectly for all concerned screws; 16 in total. Even then some force had to be put in, as to separate the band-switch-shaft from the guiding hose! Also the bandwidth tuning suffered problems - but here 'Kontakt 88' oil cured the nuisance.
Only afterwards, I realised that we should have documented it by taking photos; too late!
Viewing our Schwabenland after the "band-switch" troubles had been solved
A picture taken from the left-hand side
The screening plate is lacking.
Luckily we possess a second Schwabenland RX lacking its housing. Thus, in case of essential problems, we possess spares.
Viewing just a bit more from the rear section as well
Looking at the quartz-filter (up) and audio-band-pass-filter section (down)
Clear, like all Lorenz products, its construction is rather neat and sound!
The quartz-filter is internally having an exceptional band-with-tuning provision, as to by-pass Kauter's quartz-filter patents, in the possession of Telefunken.
Kauter's tuneable quartz-patent relied on de-turning in opposite manner the in- and output loading coils; by means of differentially tuned capacitors. Whereas Lorenz - de-tuned by means of a dust-cores loaded Bakelite arm; the moving arm is pivoted in its centre. By this means the two coil sections could accordingly be proportionally (de-)tuned. Resulting in a variable band-width response.
As to understand with what we are dealing:
The Ln 21021 Schwabenland schematic
As to approach it in PDF, please click at it as to activate the link
Please notice the green resistor having some solder-line material on its contact-cap. Its value was 50 kΩ; after replacement some irregular frequency jumps were cured
But frequency tuning responded still in an irregular manner.
We therefore have to approach the tuning capacitors; covered by a rectangular plate
Please look carefully at this photo
Might it be - that the two left-hand side rotor sections being mounted outside the centre of their stator groves? (AOB. a bit more towards the left-hand side of the stator slit)
I must admit, that when these photos had been taken, we didn't notice any kind of irregularity!
The downside, we have to remove its cover plate again.
The screws having 1.9 mm threat, which is always a bit tricky, in particular when space is quite cramp.
I also bore in mind, that the previous owner had told me: that he obtained this receiver somewhere in the mid 1960s (when he lived in The Hague). But found out that the ceramic tuning condenser-shaft was broken. However, he was rather proud that he once solved the problems. Might this be the reason for why the two RF tuning sections being ill adjusted?
And if so, how can we cure the problem, without damaging the fragile ceramic shaft?
Measuring and comparing the various voltages, I found that g2 of the second IF amplifier stage was quite low (19 V). After bridging the resistor between + and the screen grid, whilst still a resistor is loading it against ground.
The screen grid voltage jumped up to 95 V, a bit higher than in the first IF circuit. What might have occurred, is, that the blocking capacitor has reset itself. It is of the type having pertinax contact strips. Over the years suffering from faulty dielectrics, caused by hygroscopic dielectric wax ; like do almost all Allied wartime blocking capacitors (and not only these). Albeit, that in the foregoing days German military industry relied on hermetically sealed-off types; which even nowadays full-fill their genuine specifications! But its manufacture was quite more elaborate, and hence, more expensive.
Next: approaching first the tuning capacitors again
On 1 January 2018
As just mentioned at the end of the foregoing chapter, today we should first access the tuning capacitor again.
We therefore have to remove the covering plate again.
One matter is luckily clear, that the foregoing photos of the same device caused, what the Germans call: eine optische Täuschung
It is evident, that the the previously doubted correct adjustment of the rotor-plates versus the centre of the stator slots isn't actually true.
We may therefore relieved consider - that we do not have to dismantle the tuning-capacitor device.
On the other hand, there are quite many indications that matters have been approached after manufacturing it. The indications are visible due to signs of poor screw-drivers handling. Only once? No, for quite many occasion. Try, critically, to find out yourself for witch occasions.
As to find out whether there is a chance of frequency adjustments for: Llow and Chigh, it was discovered that the trimmer for Chigh did not responded correctly
There is no way around, we must dismantle all 32 coil-boxes! Again Hans Gouwlooze had to assist with his screw-heating-technique.
But after carefully approaching the 4 screws it went all well
It is clear, that the coil unit has to be detached first from the small mounting frame.
The trimmers were fixed by means of two Al screws.
There doesn't exist much space for manoeuvring our trimmer device
The typical appearance of such trimmer types
Invented in Germany, most likely introduced by the famous Hescho Company.
Only later in the war, these trimmer types were adopted within some British wartime communication gear.
Its great advantage, its flatness as well as the choice of specs; such as Tc and capacitor range.
Please look careful just at the border between the metal screw with attached solder and the deposited silver-layer
Under a magnifier-glass it can just be noticed that there exists a tiny slit.
These trimmer types suffer for many decades, because these had been never touched (rotated).
When force being put at the crew the stuck ceramic disk will not move and the soft soldering breaks.
When you face such a problem: take a screwdriver and put some weak torque at it; at the same time touching with you soldering-iron the trimmer-screw + screwdriver, so that the screw will warm-up a bit. At a certain moment you might hear or feel a soft click and all moves (rotates) again smoothly. Before approaching it this way, look first whether you can determine into what direction torque should maintained; bear in mind that such device can only be rotated over 180°.
Never try to cure it by using regular solder tin, but use types having a higher silver content. Sometimes, it might help using 'old type' solder-paste. I never managed to get it as nice as the genuine trimmer. But it operates nevertheless sound again.
Hans Goulooze told me, that heating the trimmer up near to the melting solder melting point of silver-solder is very helpful.
Therefore we should use our heating pistol. On 11/12 January 2018
On 11/12 January 2018
The last week we have repaired the oscillator trimmer belonging to Band (range) III
Which was shown in the FuBl 2 - EBl3F Survey.
This week we dedicate some time to the trimmers belonging to the ranges V and VII
I would like to show the followed procedure in a more dynamic way.
Last week I have aligned the Frequency scale for the lowest (L side) and the highest (C side) of Band V, after we have repaired its C-side trimmer within its according coil box; be haven't documented it
However, switching it on yesterday and an irregular noise and checking the valve currents, it appeared that valve 3 didn't consumed its regular current.
Please notice the most rear contact of the triple block capacitor, which blocks the g2 voltage
However, I have soldered the contact, but bearing in mind the reputation of these types of hermetically sealed-off, I suppose that the resistor connected onto the HT line might be faulty.
But, for the time being the receiver sensitivity is back; let us wait and see how it in the future behaves.
But let us now follow how Hans Goulooze supported me with removing the four screws with which the coil boxes being hold (fixed) within the coil-turret.
Every screw necessitates such treatment
Sometimes being repeated, before with some careful force clicking sound is noticed when the screw threat allows it turning outwards.
It demands great care, before a defect trimmer can be detached
We do not need much imagination to, to understand what our problem is
It is evident that the black (oxide) surface has to be cleaned first
For this occasion Hans brought in a far better soldering flux called: collodium
Film 251 (270) Hans Goulooze has cleaned the silvered surface first. He uses, for this occasion, a so-called collodium solution. The main trick - introduced by Hans, is heating up the trimmer ceramic just below the tin-silver solder melting point (approx. 210° C). At the end of soldering and after temperature has decreased, first Hans measured whether a low resistance between the trimmer electrode and the centre screw does exist. Thereafter he checked whether the trimmer disk can be rotated easily.
Afterwards, this trimmer had been remounted inside its coil arrangement, it operated sound. As did, by the way, the other one too.
A problem arose, that a wartime alignment document noticed that both band-ends should be chosen 3 degrees from each scale end.
But what scale end? Because the actual scale length is much broader (wider) than what being engraved into the scale disc; and further markings are none existing.
I therefore took the third engraved scale-markings from the left- and right-hand side towards the centre.
In our situation, which might be valid for other serial-numbers as well.
I 1530 kHz (L-end) 2050 (C-end)
II 2130 kHz (L-end) 2970 (C-end)
III 3030 kHz (L-end) 4170 (C-end) This trimmer had been repaired last week
IV 4230 kHz (L-End) 6070 (C-end)
V 6030 kHz (L-end) 8670 (C-end) Has successfully been repaired yesterday
VI 8660 kHz (L-end) 12340 (C-end)
VII 12160 kHz (L-end) 17960 (C-end) Has successfully been repaired yesterday
VIII 17260 kHz (L-end) 25620 (C-end)
All bands (ranges) has been aligned in respect to frequency
The accompanied sensitivity felt sufficient, and therefore I haven't touched the according RF stages.
However: it has to be noticed, that the Köln E 52 receiver is of a better class.
On the other hand is coming into military service might have been "the best possible solution available".
Rather curious, in Summer 1943, a full description of the Schwabenland receiver appeared within a Radio Mentor issue (including its integral schematic). Which appearance is for military apparatus, most unluckily.
Heft 7/8 July/August 1943
By the way - a nice example of German wartime radio periodicals.
In my personal perception a good example.
It has to be noticed though, that Radio Mentor, for what ever reason, must have had good contacts - or might have possessed large paper stocks.
Up to the very end their magazines were hardly reflecting poor wartime paper quality.
As reflects, for example, the Funkschau magazine of the same period.
What might have counted, it was also available abroad, and in this respect might have been used for a kind of non-political propaganda; "Aushängeschild".
Maybe, investigating why the BFO harmonics at the various calibration points are, on higher ranges (bands), so weak?
To be continued in due course
By Arthur O. Bauer