Well the little Laney Cub12R gets the Zeta Services this week’s award for the most stupid amplifier to service. On the outside, looks all pretty and sweet. Customer said it was blowing fuses and didn’t work.
With this little girl fully undressed on the bench, so signs of fuse blowing, the original mains fuse, 250mA was in tact. Nice to see a standard set up with no silly ICs, JFETs. The FX board however was tucked away hiding. Powered up with a current limiter, watching the power being consumed from the mains. A signal and speaker connected, we got good audio with about 30 watts being taken from the supply. Then, audio slowly vanished and the power consumption down to 9 Watts. HT was still there. Bit of testing around showed that there was no heater 6.3V to the valves. The first thing I did was to glare at the mains transformer that looked like it had not reached puberty yet. Yes, there was 6.3VAC at the connector. Long story short, Laney had specified a PTC in series with the heater supply, I guess to prevent heater flash in the 3 x ECC83s-12AX7s. Nice touch, well done, thanks, good engineering. This device was duff, so replaced and all was good. BUT, The main board is held in place with 4 spacers screwed to the chassis. It seems that the valve bases were fitted to the chassis first, then the board fitted and soldered after. So, if the board has to come out for re-work, everything has to be unsoldered. Nightmare. The board is populated with normal grown up resistors and other bits, but also loads of tiny SMD components that cannot be reached. This is all nicely cooked via a couple of EL84s and 12AX7s under chassis. WHY make things so awkward?