07-11-2023, 11:57 AM
Risen to the top of the pile, a Pilot Blue Peter wireless. 4 wavebands, M, L, SW, & Trawler. A set I had not seen previously.
An unusually shaped light wood cabinet with a back panel secured by only turnbuckles. All original knobs. Two core figure of eight mains flex.
Aerial and Earth wires hanging loose out of the back with labels on them, another novel Pilot idiosyncrasy, there are others as we will see.
A quick inspection revealed a hefty mains transformer, good size output transformer on a decent sized loudspeaker.
Very odd valve line up, 6K8GT mixer/oscillator (Octal), 7H7 IF amplifier (B8B), 6Q7GT audio amplifier and detector/AGC (Octal), 6V6G audio output (Octal) and 5Z4G full wave rectifier (Octal). The 6V6G is probably replacing a 6V6GT valve. Removed chassis from cabinet and cleaned up spider carcasses and dust
Tests showed the output transformer to be intact, clicks from the speaker, no mains shorts, all valve heaters intact, dial bulbs blown and the 2 cord drives intact. No obvious bad wiring, a couple of untidy repairs, and a burnt screen grid resistor on the 6K8GT valve. This was replaced along with it's waxed paper decoupling capacitor which was assumed to be leaky.
With all valves removed power was applied for a while to prove that the mains transformer was serviceable.
Valves back in, power up on a limiter ( only 40w ), looks normal so gave it the beans. All seems OK, adding the aerial gave reception on all wavebands.
Checking voltages reveal that the output coupling capacitor was as usual leaking, overdriving the valve, but also that the HT voltage was way too high, over 420v. One of the "repairs" was a 32mF capacitor on the rectifier cathode to chassis as a reservoir capacitor, not on the circuit diagram. As this set unusually has a swinging smoothing choke on the cathode this capacitor should not be there. Removing it restored a sensible HT voltage.
Replacing the coupling capacitor removed the positive g1 voltage on the output valve, to the relief of the valve and mains and output transformers. This high HT was probably the reason for the burnt screen grid resistor combined with a slightly leaking decoupling capacitor.
Checking the other wax capacitors I felt it prudent to replace the screen grid capacitor on the 7H7 too, the top cut tone capacitor from the output valve anode to the tone control and the tone capacitor across the output transformer primary. All the rest are at low voltage and were not causing any problems so were left.
The set now performed very well. Noticing the negative feedback shorting plug on the chassis rear I pulled it out to be surprised by loud positive feedback uncontrolled by the volume control. The output valve was oscillating violently. The cause of course was that the previous repairer had in error reversed the wires on the output transformer primary when he had reinstalled the chassis. Reversing these removed the positive feedback and restored normality. I wonder how many of these sets owners ever removed the NFB shorting plug, the sound with NFB is surprisingly less quality and at lower volume.
Another wireless saved, whatever am I to do with them all?
An unusually shaped light wood cabinet with a back panel secured by only turnbuckles. All original knobs. Two core figure of eight mains flex.
Aerial and Earth wires hanging loose out of the back with labels on them, another novel Pilot idiosyncrasy, there are others as we will see.
A quick inspection revealed a hefty mains transformer, good size output transformer on a decent sized loudspeaker.
Very odd valve line up, 6K8GT mixer/oscillator (Octal), 7H7 IF amplifier (B8B), 6Q7GT audio amplifier and detector/AGC (Octal), 6V6G audio output (Octal) and 5Z4G full wave rectifier (Octal). The 6V6G is probably replacing a 6V6GT valve. Removed chassis from cabinet and cleaned up spider carcasses and dust
Tests showed the output transformer to be intact, clicks from the speaker, no mains shorts, all valve heaters intact, dial bulbs blown and the 2 cord drives intact. No obvious bad wiring, a couple of untidy repairs, and a burnt screen grid resistor on the 6K8GT valve. This was replaced along with it's waxed paper decoupling capacitor which was assumed to be leaky.
With all valves removed power was applied for a while to prove that the mains transformer was serviceable.
Valves back in, power up on a limiter ( only 40w ), looks normal so gave it the beans. All seems OK, adding the aerial gave reception on all wavebands.
Checking voltages reveal that the output coupling capacitor was as usual leaking, overdriving the valve, but also that the HT voltage was way too high, over 420v. One of the "repairs" was a 32mF capacitor on the rectifier cathode to chassis as a reservoir capacitor, not on the circuit diagram. As this set unusually has a swinging smoothing choke on the cathode this capacitor should not be there. Removing it restored a sensible HT voltage.
Replacing the coupling capacitor removed the positive g1 voltage on the output valve, to the relief of the valve and mains and output transformers. This high HT was probably the reason for the burnt screen grid resistor combined with a slightly leaking decoupling capacitor.
Checking the other wax capacitors I felt it prudent to replace the screen grid capacitor on the 7H7 too, the top cut tone capacitor from the output valve anode to the tone control and the tone capacitor across the output transformer primary. All the rest are at low voltage and were not causing any problems so were left.
The set now performed very well. Noticing the negative feedback shorting plug on the chassis rear I pulled it out to be surprised by loud positive feedback uncontrolled by the volume control. The output valve was oscillating violently. The cause of course was that the previous repairer had in error reversed the wires on the output transformer primary when he had reinstalled the chassis. Reversing these removed the positive feedback and restored normality. I wonder how many of these sets owners ever removed the NFB shorting plug, the sound with NFB is surprisingly less quality and at lower volume.
Another wireless saved, whatever am I to do with them all?
Boater Sam.
