I purchased this interesting German set a couple of years ago, so it had been awaiting attention for some time. After moving house, we had an ideal position for it to live � on a shelf in an alcove in the lounge, above the fish tank � so it moved straight to the top of the list!
The
set has piano-key waveband selection, and includes SW as well as the usual MW,
LW and VHF. Separate bass and treble controls are provided. The tuning scale
is fairly long and the action is quite slow, allowing accurate tuning. It is,
of course, entirely labelled in German.
The cabinet finish was looking rather tired, and there was some slight damage due to wear-and-tear. The chassis appeared to be largely untouched, although there was some signs that the alignment had been fiddled with.
This is clearly quite a late valve set � I would estimate that it was made in the mid-1960s. The components are good quality, with no wax-paper capacitors to worry about. All the valves, with the possible exception of the magic eye, appear to be the original Telefunken branded components. The magic eye is a Tunsgram. The valve lineup is completely conventional � ECC85, ECH81, EF89, EABC80, EL84, and the set uses a contact-cooled metal full-wave rectifier. The tuning indicator is an EM84 � this is the straight-line type used on many tape recorders, and has a more modern look that fits the style of the set.
Although the set uses a solid metal chassis, much of the circuitry is contained on two PCBs (printed circuit boards). One contains the VHF front-end, and the other contains the complete AM front-end, IF, detection and audio amplification. The output stage is built separately on the chassis. There are also two small PCBs underneath the chassis, containing the tone control components. Behind the piano-key switches are a bunch of components such as the local oscillator coils and capacitors, and a number of wires and components link between this and the PCB. The standard of construction is generally very good, although some sections are difficult to access for servicing.
Repair
The chassis was removed, then the tuning scale was removed for safety. This was a bit tricky as all the piano-keys needed to be pressed simultaneously before it can clear the support bushes around two of the control shafts. After cleaning out all the cobwebs, I carried out some preliminary checks such as measuring the resistance of the mains and output transformer windings. Everything seemed OK. I then connected my capacitor reformer to the main smoothing capacitor, and soon established that this was fine too. I noticed a capacitor that was looking rather sorry for itself, connected between one side of the incoming mains and the chassis. Since I intended to earth the chassis later, I removed the capacitor and did not replace it.
Time for a test. I connected a test meter to the HT, a speaker to the output transformer, and switched on. The HT immediately rose to about 300V (metal rectifier � remember!), and after a few seconds dropped gradually to about 250V. The set appeared to work fairly well on MW, LW and SW, but VHF was dead. In fact with the volume turned up, VHF produced a rustling sound which suggested switch tracking.
However, before I investigated that further there was a more pressing matter to deal with. Being a European set, the voltage selector had options for 110V, 127V and 220V. I was using the 220V setting, but the mains voltage here is around 245V. This sort of overload would be irrelevant with modern equipment, but with valve equipment we need to be more careful. A meter check confirmed that the valve heaters were being run at almost 7V � far to high for 6.3V The generally accepted tolerance is +/- 5%, which gives a maximum of about 6.6V.
A few quick experiments with a handful of wirewound resistors soon established that 120R in series with the mains input dropped about 22V and gave us a heater voltage of 6.4V. The set worked just as well with this resistor in circuit. The actual mounting of the resistor under the chassis was left until after the other work was done, so for now a 10W component was temporarily connected in series with the mains lead using choc-block connectors, and positioned carefully out of the way.
The pots and switches were treated with some contact cleaner before the next test. With the set tuned to a weak station on MW the IF adjustments were carefully trimmed. Although this was not an exact alignment it gave a significant improvement in reception quality. The tuning indicator (which, surprisingly is still bright) reflected the improvement too.
It
was now time to investigate the problem on VHF. There are only five connections
to the VHF front-end, all of which could be easily identified by following through
the wiring. These are HT, heater, ground, IF out and AGC. A signal generator
was connected to the IF out connection, and set to 10.7MHz. Even with the signal
output turned right up the signal strength indicated by the tuning indicator
was minimal. A test meter connected to the tuning indicator showed it was receiving
around about 2V positive on its control grid, whereas it should be several volts
negative depending on the signal strength. Transferring the meter to the small
electrolytic capacitor in the ratio detector showed a good negative voltage,
which could be peaked up nicely by trimming the IF adjustments. This narrowed
down the area of the problem to the waveband switching and a few AGC decoupling
components.
The signal from the ratio detector is connected to a section on the VHF waveband switch via a resistor and a length of screened cable. A further resistor (2M2) is connected from this point to chassis. The switch selects whether this signal, or one from the AM detector, is connected to the tuning indicator and the AF amplifier. The same switch also switches the HT to the VHF front end. It appeared that tracking on this switch was likely, but there were no signs of discolouration. While poking around I found that the rustling sound completely disappeared when the 2M2 resistor was moved. Closer inspection showed that the body of the resistor was pressing against one of the HT switch tags. The voltage across the thin insulated coating on the resistor had become somewhat conductive, causing the problem. I fitted a new resistor, on slightly longer leads to avoid a repeat of the problem.
Although this solved the rustling noise, and allowed the tuning indicator to respond to an IF signal from the signal generator, there was still no reception. I connected a signal generator to the aerial input and tuned it across the VHF band, but even with the level right up there were no results. A replacement ECC85 valve had no effect. Clearly there was a fault in the VHF front-end.
On this set the VHF front-end is contained in a metal box, and the cover is retained with one screw. This allows access to the bottom and sides of the PCB. There were no visible problems so I checked the voltages on the valve pins with a meter. I was expecting the cathodes to be at around 0V, the grids to be slightly negative and the anodes to be 150V or so positive. Everything was correct, apart from one anode, which was at zero. From examination I found that the anode should be connected to HT via a coil and a 39K decoupling resistor. A meter check showed that the resistor was open-circuit. There were no shorts from the other side of the resistor to ground, or anything else to explain the failure, so I fitted a replacement, which solved the problem.
The next job was to set the IF alignment more accurately. Until now I had only been setting it approximately to prove whether things were working. The RF alignment showed no signs of being disturbed so I decided to leave it alone. The VHF alignment was simply set by ear and by watching the tuning indicator with the set tuned to Radio 2. The set was then tried on various other stations on VHF and sounded fine.
The AM alignment was carried out in a similar manner and peaked up very well. However the results were not quite so pleasing � in particular treble seemed to be lacking. This would be fair enough for a cheaper set, but I felt this set should be able to do better. The IF adjustments should probably have been adjusted at slightly different frequencies, giving a flatter IF response, but since I do not have the alignment instructions or a sufficiently accurate signal generator this would be difficult to do properly. Instead I stuck with the alignment-by-ear approach and attempted to slightly detune some of the IF cans in an attempt to improve the treble response without affecting the sensitivity too much. It is easy to detune too much, making the set sound like it is not tuned in properly, but after some experimentation I managed to achieve an alignment which sounded good. Basically all four adjustments (top and bottom of both cans) were peaked, then the two top adjustments were very slightly detuned, one in each direction. The detuning was very slight, just to the point where the magic eye indication dropped fractionally.
The final job on the electrical side was to mount the 120R mains dropper resistor properly. I found an RS dropper section of the correct value. This is rated at 0.3A so it is comfortably under-run in this application. These are round resistors with a fixing hole through the middle. I found a convenient place under the chassis next to the mains transformer and drilled a 4mm diameter hole. The dropper was mounted using a 4mm screw and nut, together with a 12mm long spacer to hold the resistor away from the chassis. The 2-core mains flex was replaced with new 3-core cable, the earth being connected to the chassis.
Cabinet
Although the cabinet looked rather tired, it definitely did not need stripping and refinishing. The original finish was in fairly good condition under the dirt, so a good clean and polish was called for.
The
loudspeaker was removed to prevent damage, and then the dust and cobwebs were
cleaned out before the whole outside was cleaned. I started with foam cleaner,
but although this removed the dirt and grime, it did not remove the paint spots
(why didn't people cover their sets when decorating?). Isopropyl Alcohol was
much more effective here. The top, sides and front bezel are coated with a very
thick coat of clear shiny lacquer � where it is chipped it appears that the
coating is around 0.5mm thick. There were a couple of what appear to be cracks
in the lacquer on the top of the cabinet, but I decided that any attempt to
repair these would probably make matters worse (they would not be visible when
the set is on its shelf anyway). It appears that the coating is so thick and
rigid that it could not cope with the inevitable expansion and contraction of
the wood.
By contrast, the front of the cabinet appears not to have been treated at all, although it probably had a thin covering of a matt finish. This looked rather sad and clearly needed some sort of refinishing. Before starting, I removed the Loewe Opta logo, which was retained with three bent-over pins inside the cabinet. After cleaning the front of the cabinet, I applied two coats of Ronseal Mahogany All-In-One Wood Finish with a soft rag, allowing 24 hours for each coat to dry. This product contains a colourant and protection, and is described as producing a sheen finish. Unlike wax-based finishes, it dries hard and does not need buffing. The product worked well, and bought back some colour and life to the wood, without leaving it too glossy.
I gave the knobs and logo a wash in warm soapy water, and cleaned the outside of the tuning scale with car windscreen cleaner. I also cleaned the piano-key buttons with foam cleaner. These were glued to the switches could not be removed, so they had to be cleaned in-situ. The valves and other chassis components were carefully cleaned where possible. The IF PCB appeared to have been sprayed with a thin coating of wax, which now had a coat of dust stuck to it. I tried to clean this with a toothbrush and Isopropyl Alcohol, but there was too much risk of damaging components so I did not persist with this idea. I did, however, manage to clean the bodies of the resistors and some other components, which makes the whole assembly look more presentable.
Once the cabinet refinishing had thoroughly dried, the only job remaining was to reassemble everything and make sure the finished set worked. Fortunately this was uneventful. The finished set gives very good performance on all bands. As well as looking good above the fish tank, it is also proving useful for listening to our local "Classic Gold" station on MW, a job which it does much better than my hi-fi tuner.
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