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Resurrection of a Philips Prestige Valve Radio
I keep getting antique pieces such as Valve Radios, Valve Amplifiers, Valve Spool Tape Recorders etc. For servicing. In fact, this is what I like the most, being my favourite! I got one Philips Prestige Valve Radio 15RB/505/00Z. This belonged to a Public Library at a place around 20KM away, and was abandoned in their attic for many years, as there was none who could service it, until they came to know of me. The cabinet was in a bad condition.
The back cover was missing. The needle was missing. As, usual, my first job was blowing out loose dust, dismantling the unit from inside and do a thorough cleaning with my various brushes. We should never remove the valves from its basis, while cleaning this way, as the dust will get into the base holes. Then I brought it onto my work table for a detailed study. Removed the valves in order to tilt the chassis for bottom view. Did some more minute cleaning and wiping with cloths. The Valve numbers you see below are EZ80 (Dual Rectifier), ECL82 (PreAmp+Amp together), EBF89 (IF amp), ECH81 (Osc and Mixer) and EM84 (Tuning Indicator).
Observed that the Mains Transformer had burn marks and the thermal fuse inserted into the coil is open.
Removed it and gave it for rewinding to my friend, as the primary was short. On doing cold test on the components, noticed that in the B+ rail, 1.5K 5W dropping resistor mounted on the output transformer was cracked and open. Similarly one 18K resistor at the secondary B+ was reading more than 24K.
ESR of the 32+32mfd/450V Capacitor was perfect! One coupling capacitor was found cracked. So, I replaced these three components. Checked the IF transformers for continuity. Noticed that the primary of oscillator IFT (intermediate frequency transformer) was open. The secondary was showing around 7 ohms. In the second IFT (Detector side), both were showing the correct ohms reading,around 7 Ohms. But on the secondary side, the ferrite core was loose. It slid in when I tried turning it. So removed both these IFTS. This is a tough job. The soldering iron cannot be inserted into the area, as there are several criss-cross wiring. So I noted down the wire connections in a slip of paper and cut the wires and removed both IFTs.
The first one you see is a UNIVERSAL IFT released by Philips as a general replacement of all their IFTs. The second one is its original IFT. This indicated that some mechanic had replaced the first IFT. See below to know what is inside the Large IFT. Also side views and the tech details of Universal IFT:
Since I had the circuit diagram of this model, it was easy in tracing the circuit for checking its various stages. I have repaired several Philips Valve Radios with the same circuit diagram.
I dismantled the small IFT and removed its ferrite cores to view the coils.
Removed both the coils one by one and removed the primary and secondary, noting down its length on my table. In this, the primary coil has one primary winding and one secondary winding, with only a few turns. The second coil has only one winding. The length of the second coil wire is slightly more than that of the first coil. Took the coil to a coil winding shop run by my friend, who checked and found it was 44 SWG. He had only 45 SWG. So I took it, came back home and hand wound the wire on its tiny former.
Did the same for the other. Fit the coils back on its place. Soldered the leads to the terminals. Since the wire has enamel coating, first we need to flash a lighted match stick at the terminal, clean it and then apply flux and touch with solder. Otherwise, it will not get soldered properly.Did a dry solder patch on the bottom of the IFT, inserted its cans with its side protection sleeves (to prevent contact of the terminals with the can), and soldered the cans at the bottom. Since the core of the large IFT had lost its grip, I replaced it with a Universal IFT from my antique salvage stock. Both IFTs were soldered with pieces of wire on all the terminals so that I can connect it at the bottom easily. Restored all the connections as per circuit diagram. Got back the mains transformer after rewinding. It has one primary and two secondaries. One secondary will have 240-0-240 winding (for B+ that goes to the Rectifier valve) and the other 0-6V winding (for valve filaments and dial bulbs). The primary is 0-230V. This is a 1 A transformer. Fit the transformer in its place and restored its connections, providing a 2A fuse at the primary in place of the thermal fuse. Rechecked all connections, inserted all valves in its place. Switched it on, and got crackling noise at the speaker, indicating all was well. Injected 455KC/s modulated signal to pin 2 of EBF89 and tuned the corresponding detector IFT for peak output(First secondary and then the primary). Sounded well. Injected the signal to pin2 of ECH81, then tuned that IFT also to the peak. Re-touched the detector IFT and Oscillator IFT, first secondary and then the secondary, to arrive at a perfect peak signal without any distortions. While aligning like this, we have to keep the gang fully closed in MW position. Then connected an aerial and looked for any station. It was giving an excellent performance!
All the bands MW, Short Wave 1, 2 and 3 worked very well! There was no need for any RF alignment! All the valves were working very well! The voltages were also perfect as per the circuit diagram! So, replaced the dial bulbs. (Caution: While checking the Valve Radios, always remove the dial bulbs, as we need to switch it on and off several times, which will blow the bulbs!) Fabricated a needle from a copper wire, by fixing, putting the chassis in, removing for readjusting the length, width, position etc. several times until it was a perfect fit and moved to both sides smoothly.
With the dial plate placed, I readjusted the needle so that it indicated the correct frequency marked on the dial. For this, we need to tune to a known station, and readjust the needle to the frequency marked on the dial. The pictures below are of the fabricated needle and checking its movement position.
Fixed the dial plate and fixed the chassis in the cabinet. Switched on the set, and tuned to a nearby station, and listened to lovely songs!
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company.
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