A customer and good friend, Alan Sykes, ex BBC guy brought this little gem in for a sort out. As Alan is a nice guy I agreed to do this. Normally, I would have hid in the air conditioning duct work, pretending to be dead. You have to understand there is a lot going on with this set in a tiny space. LW, MW, SW and VHF, all crammed into a very tight space. As usual, all the paper capdensors were electrically, having morphed into 200K resistors, so had to be replaced. Picking away like keyhole surgery, carefully snipping around. Real headache. Brain surgeons think they have it bad, nothing compared with one of these sets. The main problem with these AD-DC sets, apart from the extra heating given off but the dropper resistor network, is the output stage. Coupling capacitors and associated parts all fail upsetting the bias conditions of the UL41 output valve. As a result, it lives a life with the choke half out, creating excessive anode current, distortion, runs hot and eventually ends up with heater-cathode leaks. This gives a nasty hum that cannot be got rid of. You initially blame the reservoir capadensors, but no amount of farads will rid you of the hum. The cathode of the valve has a sweet link to the mains AC supply. Same thing happens with the BUSH DAC90s. Eventually after a skip load of waxy replacement capdensors and a few resistors that had wondered way outside their limits, got it up and running. A nice new-old UL41 fitted meant we had little hum. Being an AC-DC set as mentioned, the HT is halfwave rectified, so a small amount of low 25 Hz hum will be present. No point in adding higher value smoothing capdensors as this will stress the poor HT rectifier with more ripple current, which will ultimately lead it to need counseling. I was brave enough to try a little alignment, but these I.F slugs can be hard to move from their 60 year old sleep. All in all it sounded very good on all bands. A real punchy sound from such a tiny baby..
