The Fine Things are Always Hand Made
I know it’s notoriously difficult to diagnose faults remotely, but I’m a bit stumped and so decided to ask. My philharmonic has developed this problem: essentially, it’s behaving as if it hasn’t got a good aerial when in fact it has- ie poor sensitivity and poor signal to noise ratio. However if I beam a signal to it from an SS tran AMT3000 transmitter, it works very well. Any thoughts?
Have you went through and checked all your tube socket voltages? That's a good starting point.
+1 to Alex' comment, especially the tubes in the front end and AGC circuits.
If voltages are reasonable, the IF alignment is a prime suspect for the symptom you describe. Has the set been aligned and recently checked? Are the variable selectivity rotors well secured to the variable selectivity shaft? Pot metal expansion can cause the variable selectivity rotors to become loose and misaligned even if tightened within the past 10-years.
Hello Norman, the problem is of sudden onset , so probably not an alignment issue?
Perhaps. You should check the variable selectivity rotors. At some point during expansion of the pot metal collars the rotor will come fully loose from the variable selectivity shaft and rotate to a point that gravity dictates rather than a point dictated by the variable selectivity shaft. This condition is a sudden onset problem and I have seen multiple sets where it has happened.
Thanks for your suggestions. I’ve already checked the IF valves and aim to do the others this weekend. Meanwhile, does anyone have an explanation as to why the set works fine with a transmitter, but not the conventional signal? I’d hoped that this might narrow down the problem.
Simple. Energy received at the antenna. Scott receivers will pick up strong signals even when the IF amplifier is well out of alignment but will be noisy and not receive more distant signals well.
Thank you for your help on this, and apologies for my rookie questions! . The set has now gone silent though still lit. I have discovered a burnt out resistor in the audio diode. It’s R9, 1k/1/4 watt. I notice that this resistor, and the other one in the diode (and those in the other diodes) are positioned midway through holes in the side of the aluminium case - half outside, half inside. I’d be grateful to know why they are thus positioned and whether this is critical when replacing.
The resistors were run through holes in the shield to maximize shielding of the diode coil. There are wax-paper capacitors inside the diode shields. They need to be replaced. The resistor that failed most likely did so because a capacitor in the shield had failed.
If you had not recapped the 3 diodes, what about the others out of sight: one inside the big tuning cap cover.
Also, the for the pointer non-FM Philharmonics, the left rear between the expander tubes and the pair if 3 rd audios is a rectangular box. It was 4 caps inside as part of the bass resonator circuit - dismount, pull out the assembly consisting of 4 caps and 2 chokes.
Then the rectangular soldered box below the expander section - holds 7 potted .1 caps to replace.
Thanks Norman and David. The set was restored by or for the previous owner. I’m now trying to address the things that weren’t done then, and your advice helps greatly.
Can I ask - what exactly is the function of the audio diode?