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Philips Novosonic Valve Radio Restored
I had got one Philips Novosonic Valve Radio in a very bad condition for a possible restoration from the same place where I had got another Philips Valve Radio a few weeks back. The cabinet was all in bad shape: No back cover, rat had cut many wires underneath including the power cord and dial cord, plywood at top and bottom were exposed and in layers, cone paper of speakers were moth eaten, medium wave coil was cut, but had all the valves in place.
First and foremost, as always is the case, I did a thorough cleaning of the inside with blower (watching a lot of paper pieces and other particles normally used by birds for building nest, flying away from underneath and sides), removed the chassis out, and did one more cleaning with blower and various brushes, and made it ‘qualified’ for entry into my service room! All blowing and cleaning should be done without removing the valves. Otherwise the dust will go into the valve base holes and it will be extremely difficult then to clean it.
Since, I had the circuit diagram of this model with me, I removed all the hanging rat bitten wires by cutting these at the soldered points. The locking springs of the Bass, Treble and Pick Up Push Button Switches were rusted and spoiled. So I removed these wires also completely deciding to disconnect these. Then removed the valves one by one; EM84 (Tuning Indicator), EZ80 (Double Rectifier), EL84 (Power Amp), EBC81 (PreAmp), EF89 (Intermediate Frequency) and ECH81 (Mixer and oscillator). Cleaned the pins of these valves with metal brush and made it shining. Gently wiped the dust from the valves ensuring that the valve numbers printed on it are not erased. Sprayed WD40 liberally on the valve bases, piano band switches simultaneously pressing the buttons one by one for effecting a self-clean of contact points, volume control, spindle joints of the Gang and its clutch wheels (ensuring that not even one drop goes inside the gang, lest it should affect the capacitance affecting RF frequency range. We should also ensure that it does not enter RF coils) and also the moving wheels of the dial cord. Allowed this to dry off overnight.
Connected a new mains cord and checked for continuity of the primary of the mains transformer. It was OK. Checked the secondary, both HT and 6V for the filaments and dial bulb. It was also OK. Connected the wires to the mains and checked the AC voltages that should be present at Pin 1 and 7 of EZ80 valve. It was reading 240V each. Checked ESR of the 50mfd+50mfd Can Capacitor. It was Ok. Switched off mains, inserted EZ80 valve and checked the DC output at pin 3. It was very low, only around 110V. Suspected problem with the Valve. Replaced the valve with a new one from my stock. Same reading. So, I disconnected all the loads from the B+ and Secondary and checked. It was still low. So, I knew the culprit is the can capacitor, which is shorting when fed with voltage. So, cut the wires from the bottom and connected two 47/450 capacitors at the bottom.
Connected to mains and checked the DC Voltage at Pin 3 of EZ80, it was showing 300V DC (A good valve, indeed!). Disconnected the mains, discharged the capacitors and checked the continuity at the primary and secondary of the audio output transformer. It was ok. Checked the resistors in the B+ line. Found a few resistors burnt, value far above and saw one cracked coupling capacitor. Replaced all these. Checked all other resistors and found these to be ok. Look at all the trouble making components and mess making wires removed:
Inserted the EBC81 (preamp) and EL84 (Power Amp) valves. Connected a pair of long wires at the output transformer secondary for connecting to a test speaker. Switched on, waited, and got the consoling and pleasant audio hum at the speakers. Touched the hot pin of the Volume Control and got a crackling noise, which means everything is ok in the audio stage. Switched off and waited for the B+ to get discharged. Checked the primary and secondary of both IFTS. Found the primary of Oscillator IFT was open. Removed the IFT, opened it, removed the coil and rewound it by hand as I did last time (Please see my earlier article on Philips Valve Radio Repair). Refit the IFT. Switched on and heard bursting and crackling noises, and noticed that ECH81 valve was arcing inside. Switched off immediately. Removed the valve. Switched on and again got the same noise. First I thought it was in some coupling capacitors or on feed back circuit. So isolated these one by one, but with the same result. So, my attention went to the valve base. Switched on and observed the valve base from underneath after switching off the table lamp. I could see visible arcing between pin 7 and 8. Checked voltage at pin 7. See what I saw, the meter was showing 223V DC present at pin 7, which should not be!
Switched off. Checked continuity between these two pins. It was infinite. Isolated all connected at pin 7 and checked again. The arcing was still there. So, concluded that the valve base is the culprit. Removed it and see how it looked:
It was arcing only when the voltage was present though there was no continuity on cold test, because there was the dark burn marks between the two pins. This type of valve bases have layers and are very cheap and prone to develop leaks between pins, mainly due to loose contacts at the valve pins. So, replaced the valve base with a new one from my stock, and fixed it with mounting screws, in place of the rivet holes of the old valve base:
Rewired its connections at the bottom as per circuit diagram, restored all connections removed for isolation purpose. Restored the MW antenna coil connections. Switched on. The set was perfect. Waited for about half an hour to allow the set to warm up. Selected MW on the band switch, manually turned the gang to its fully closed position (all metal leaves inside). Tuned the Detector IFT by feeding 455Khz modulated signal via .1mfd/400V capacitor, at pin two of EF89, first secondary and then primary for a peak output. Then fed the same signal at pin 2 of ECH81 valve and tuned the Oscillator IFT in the same way, secondary first and then the primary, readjusted the detector IFT once again and again the Oscillator IFT and ensured peak output. Sealed the adjusting screws on top of the IFTS with carpenter wax. So IF Tuning was over. Switched off my computer, UPS and the tube lights in my room. Went to the other rooms and switched off the Tube Lights there also, changing all lights to CFL. Otherwise Radio will pick up 50hz signals from these and we may not be able to tune to any stations. Connected an external aerial, turned the gang manually, and I was very happy to tune to various stations crystal clear in all the bands! There was no need for any RF tuning at all! The output was so amazingly awesome! The funny thing is that both the speakers, with most of the paper missing, gave a very good output! (Anyhow I got the paper patched up later by my friend who is running a speaker repair shop. He pasted a paper on top the old one, without disturbing the spider, and fixed the outer on the round frame)
Now the next job was to tie the dial cord! This is a challenge to any technician! Luckily the circuit diagram showed the diagram of this too! But the problem was that the first part of the thin wire was cut to several pieces by rat and I could not find a replacement flexible wire in my stock. We cannot use any copper wire, as it will wear out soon and get cut. So I decided to forgo one part of the guiding outer sleeve, which has very narrow hole, and replaced it with a tough sleeve. May be due to my experience, the whole thing worked just fine with the first attempt itself! We need to ensure that when we turn the tuning spindle left, the needle should move to the left, and vice versa, at the same time ensuring that the Gang opens and closes at the places where the frequencies are marked on the dial! The frequency will be low when the gang is fully closed, so the needle should be at the extreme end where the MW frequency is around 455KC/s (about 450 Meter Band). When the gang is fully open, the needle should be around 1600KC/s (Below 200 Meter Band). When the IFT is tuned to 455Khz at the lowest capacitance, the IFT oscillations will be 50Hz lower than the tuned frequency, preventing overlapping of stations, and preventing howling, whistling etc. Those of you who are interested in learning more about RF/IF and alignment, working etc., please visit this website: http://www.vintage-radio.com/repair-restore-information/valve_alignment.html and http://www.vintage-radio.com/repair-restore-information/valve_if-rf-stages.html
Replaced the front panel dial. Replaced the dial bulb, which I had removed earlier in order not to blow it off during my several on/off attempts. Placed the needle in the appropriate place after tuning to a known station and looking at the dial. Fit the chassis into the cabinet, through the front side. Switched on and the Radio was back in working condition!
Unfortunately, once again, this customer was also not having the budget for remaking the cabinet and provide a back cover! He said he would fabricate one back cover himself from any cardboard! So, I had no other go, except to deliver the Radio in this condition! What else once can do? Anyhow, another job well done, though I had to spend several hours and sit for days together to complete this work! At least, I can claim that I spoiled the attempts of a naughty rat to throw the Radio to Electronic Waste Basket!
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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