26-07-2020, 06:02 PM
The intermittent LO was a dry joint on HT at a common point on top of the transformer.
MW is now more sensitive, but LW is totally deaf, also the tuning is madly out on LW, which is why it would be bad. The fully closed on MW seems a little high in frequency, but I suspect the hex core is locked with varnish
Careful examination (after measuring RF ferrite rod coils) revealed I'd put back C12 (95 pf) to C3 trimmer thinking I'd removed C1. Curiously C1 stretches across to the RF tag on tuner rather than the near by C3 trimmer. It looked like it was on C9 trimmer. So moved that end to pin 4 of the coil, which of course where it had been removed. The pin 4 also has one end of C8, rather than that going to the LO tag. So 95 pF too much on RF tuning on LW band and lacking the LO padding on LW. Moving that fixed the LW.
Except RTE1 LW and R4 LW very distorted. No better on the reasonably fresh batteries (the PSU is about 1.3V and 84V). I suspected one half filament gone on the DL96, though that normally just reduces the maximum volume and gain. Perfect and as loud as desired on BBC R4 LW and RTE R1 LW. I checked the filament, it's OK, so some other problem on the DL96. So the only "good" valve was the shiny Bentley DK96.
Six paper dielectric caps replaced.
Three wires repaired on the LO coil.
Three valves replaced.
The 16uF 150V electrolytic was fine.
Next is putting it all back in the case, finding a red belt to repurpose as the missing carrying handle and sorting out the M281 Murphy Docking PSU. I'm not even powering that without checking the selenium rectifiers and capacitors.
I had to rebuild my bench battery radio PSU. It developed bad hum. The silicon bridge rectifier had gone faulty. While I was there I replaced the 2 x 1N4007 with 2 x 1N5408. That's the current shunt. There are three capacitors, a resistor and a choke on the LT.
I did photo and draw a sketch before removing the LO coils set. However it's got five tags in use and the circuit diagram vs physical layout is tricky. Access to the RF & LO connections is slightly tricky as the tuning capacitor and LO Coil set are on the base between the wave change switch and the actual main radio.
Slightly spoiled by not having a slow motion drive.
MW is now more sensitive, but LW is totally deaf, also the tuning is madly out on LW, which is why it would be bad. The fully closed on MW seems a little high in frequency, but I suspect the hex core is locked with varnish
Careful examination (after measuring RF ferrite rod coils) revealed I'd put back C12 (95 pf) to C3 trimmer thinking I'd removed C1. Curiously C1 stretches across to the RF tag on tuner rather than the near by C3 trimmer. It looked like it was on C9 trimmer. So moved that end to pin 4 of the coil, which of course where it had been removed. The pin 4 also has one end of C8, rather than that going to the LO tag. So 95 pF too much on RF tuning on LW band and lacking the LO padding on LW. Moving that fixed the LW.
Except RTE1 LW and R4 LW very distorted. No better on the reasonably fresh batteries (the PSU is about 1.3V and 84V). I suspected one half filament gone on the DL96, though that normally just reduces the maximum volume and gain. Perfect and as loud as desired on BBC R4 LW and RTE R1 LW. I checked the filament, it's OK, so some other problem on the DL96. So the only "good" valve was the shiny Bentley DK96.
Six paper dielectric caps replaced.
Three wires repaired on the LO coil.
Three valves replaced.
The 16uF 150V electrolytic was fine.
Next is putting it all back in the case, finding a red belt to repurpose as the missing carrying handle and sorting out the M281 Murphy Docking PSU. I'm not even powering that without checking the selenium rectifiers and capacitors.
I had to rebuild my bench battery radio PSU. It developed bad hum. The silicon bridge rectifier had gone faulty. While I was there I replaced the 2 x 1N4007 with 2 x 1N5408. That's the current shunt. There are three capacitors, a resistor and a choke on the LT.
I did photo and draw a sketch before removing the LO coils set. However it's got five tags in use and the circuit diagram vs physical layout is tricky. Access to the RF & LO connections is slightly tricky as the tuning capacitor and LO Coil set are on the base between the wave change switch and the actual main radio.
Slightly spoiled by not having a slow motion drive.
