Above prospect copied from Erres page.
Newest trophy in preserving the unique and precious vintage electronic stuff .
Watching it's photos author wasn't much excited about this tube radio receiver from Nederlands - seller presented it with no special attention as just one more vintage damaged stuff from underroof for eventual repair, Not sure but most probably direct from Nederland. Only one notice turned authors attention - it was presented (also) as Philips. Looking at Philips tube radios he turned his eyes. And next few times consequently he was turning them back.
After few meetings with announcement he said - "I have enough time for checking what may I find in internet about this strange and not fancy looking Philips . Let's loose time". This way he downloaded it's schematic diagramm.
And he got frozen in surprise. Quickly paid required 45 EUR.
Inside dirty cabinet was waiting something he didn't see yet - and I must assure You that he 've seen already hundreds schemas of pre and post war radios. Thought he had seen everything...
"They" packed mono and at the same time stereo end amplifier in one. How could it be? Obviously - if we build two channell tube amplifier for stereo listening then using them pararell for common mono signal the two speakers radiate common mono music - it's simple today's way.
But for Erres company in 1962 it looks that it was too simple and, probably, expensive way. The idea of powerfull mono push pull output at those days was for best radios a market must - on other hand even few customers in those days required stereo in home radios, but slowly it was forced by other producers. To make two complete push pull tube channels would lift the price of this european electronics above sky. Only Japansese producers could afford it with their based on american technology low cost production.
To make two single ended with app. 2 Watt tubes it was not entertaining for the consumer market. Other mono excellent receivers this days had push-pull with pairs of EL, ELL or ECL tubes pumping up above 5 Watts. . So why not to make one channell push-pull using ECL , for example 5Watts and 1,8 Watts stereo with the same circuits and tubes "in case"...? And with this mono push-pull additionally let's make semi- stereo for AM and FM mono radio reception . This is ERRES RA618 - a flagship in 1962 from ERRES - company purchased later on by Philips.
Only ... how to make it in detail?? Originality of this project is big enough to make it for me difficult to explain it in detail for reader. The basic idea is to combine two separate values - push pull output current with end stage common supply current both transformed to output circuit. This is done with windings of output "stereo transformer". Push pull amplifier works in two ways - with phase shift (-180)deg for classic push pull mono operation and "in phase" to obtain above separation of left and right stereo channells. For output was required dedicated set of two special multiwinding cooperating transformers serving both channells and giving balanced outputs for both values and channels. Astonishing solution.
Another rarities of this receiver are: 1) it's amplifier tone control is provided by 3 fixed modifications of negative feedbacks governed by push-buttons.and linear tone "director" potentiometers 2) "loudness corrections" are also incorporated in main left and right feedbacks. Simmilar idea author met only in luxory Stern Rochlitz mono radio "Beethoven" mentioned in "Few thoughts about loudness correction" 3) Separate tubes for IF amplifiers FM (EF85) and for AM (EF89) .
For so called "semi- stereo efect" inside cabinet are used two identical concert speakers and they are switched-over to work in serial with opposite phase. As in this case low tones woud be "compensated" in air , to secure against it, speakers are working in bass range with additional capacity and phase shift so that their volumes are not much damped. Thanks to this advanced solution Erres made demanded so called "semi-stereo sound" with two speakers mounted in front side of radio which was great sonic advantage Other producers at this time did not apply "semi stereo" but instead placed speakers at the sides of cabinet. This produced "like" 3D with space geometry of sound , but for sopranos created problems to listener to be heard from sides . For his reason in luxory radios like Beethoven, Stradivari had to be added tweeter in front, anyway this extra tweeter could not comply with 3D idea and with it's serial capacitor it created phase mess effecting in worsening sopranos. This author experienced by himself. No information about oval 20x15 cm so called "concert" speakers applied in RA618. It looks that they also were produced by Erres company.
It seems that receiver was not in use for many years. Dusty inside, dirty external with corroded contacts of tubes and potentiometers, switches mostly not contacting . Faulty tubes were making feedbacks and terrible cracks in speakers. Author admits it was sweet and tears to rectify it. . Glossy but actually matt caused by dirt cabinet wet cleaned with chemicals , originally glued to wood textile had to be moved by force, then washed. Surprise - it had shrunk sagnificantly after dry up- uu....ps.
Fixing back with maximum tension to cover where possible. Edge damages of textile author masked with gold finishing. Blown scale lighting bulbs replaced, Cleaning and cleaning and... cleaning... Few suspect looking capacitors he had to be changed new. Measuring on and on. The "Director" scale brought back to indicating and lighting . Radio started to work but "with author's activity" FM tuner stopped to receive completly. . Then he discovered the reason why main supply voltages are lowered to 225 VDC instead 295VDC. Step by step was found that mains transformer has working only one of two bridge anode voltage windings . He managed to buy vintage transformer with adequate data and capabilities to spare original one.
The FM range was checked again and troubleshooted - found that too much author's hand force resulted in breaking in printed circuit under FM tuner. Got back to operation and also back to main begining problem existing at FM - terrible irregular cracks when any station was tuned in and very small volume output. It was long way to find out that responsibility was up to last filter - FM detector. One of miniature ceramic capacitors 80 pF in resonance circuit was short. As there is only max few volts and capacitor looked like brand new original , filter never dismantled it is likely that this problem hunted receiver from first day. Mains transformer removed and conected new bigger - some alterations to mechanical fixing had to be done as "new" is turned 90 deg to original, Due to more power the voltages found 10% higher than originally. Applied additional serial resistor to make necessary drop. Still experienced surprises - too much and too boring to say here, untill radio started working up to factory conditions. Finally full alignment of filters done with digital generator and digital oscyloscope.
Radio is equipped with internal FM dipole and very effective long ferrit antenna for Long and Middle Waves. External antenna only for Short Waves is required . Mentioned ferrit antenna was prepeared to be turned to direction of local station after radio was positioned at home. Actually there is lack of local stations and all the joy is to tune to foreign transmitters. In such case hand adjustment of ferrit antenna was uncomfortable. On that reason author made coupled together with FM scale new drive with linkeage making possible turning ferrit antenna from outside about 180deg with FM scale as indication of actual position. This future and still existing very good brand tunning of radio circuits effected in ability to listen (at dawn) to almost all strongest European transmitters- it's "parent" Hilversum from Holland is also present every evening.
You can spot the "banana" type connecting panels (Left, Zero Right) for Plate stereo reproduction, below the same for recording to tape recorder. At left two completes for connecting external two speakers with provided "internal speaker off" contacts. Then one more banana socket set only for mono external speaker. Astonishing.
Original damaged mains transformer before replacement on photo.
Another change author did to external antenna - used additional switch coupled with "orchestra" It made a choice of disconnection it in case when external brings too much buzz on LW and MW, for SW it always has to be "on".
After the main volume knob is pulled up by step and stereo enabled by "stereo" mode key, You can spot down right to volume white lighting indicator for "stereo balance" - unique future which serves to balance of channels volumes with the same volume knob. .
Sound of radio is variable with six independent push buttons "semi- stereo", "director" (allowing tone linear control adequate to icons on the scale), "Speech", "Concert", "Solo" and "Jazz". All of them work excellent improving impressions as intended. Also the input stages for both channels are wired with mentioned already permanent "loudness correction" which works brillant specialy in low volume range. Sound is natural, heavy and juicy. Compairing to modern electronics there is much softness to adore in listening which due to balanced basic range is just pleasant. Checking other Erres radios author noticed that this sound forming circuitry was developed by Erres many years earlier and was applied in many other their radios.
Tubes: ECC85 - FM tuner
ECH81 - AM oscylator and mixer
EF 85 - FM IF
EF 89 - AM IF
EABC80 - AM, FM detection, preamp
EM84 - magic eye
ECL 82 + ECL82 - end amplifiers
EZ 80 - bridge rectifier
Total no of tubes - 17. For comparaison mighty Stradivari 2 used total 20 tubes