Spent an afternoon a few months back repairing/modifying my NOrh CD player and thought I’d share the results.
Bought it second hand from the original owner who indicated that the output started humming then quit altogether. Could still use its digital output and it sounded VERY nice with my battery-powered DAC. Its superior sonics pushed me to see if I could fix the analog output problem.
NOrh apparently bought the player from the same Pacific rim manufacturer that made the now-defunct California Audio Labs (CAL) equipment and I understand that this player was a CAL Delta Transport and Alpha 24/96 tubed DAC combined in one cabinet. NOrh was of no help in getting schematics nor was any CAL repair place that I could find; so it was “dig in on my own” time.
All the electronics are mounted on one large board. Looking from the front, there are two power transformers on the rear left. The left-hand transformer is for the digital section and the right-hand transformer is for the analog section.
To the immediate right of the larger transformer is the analog section power supply with a number of three-terminal regulators supplying 2x+15V, -15V, +5V, and an LM317 and pass transistor for the tube regulated B+ (~145V). Turns out that there are little (unmarked) eyelet pads on the PC board where five of the output voltages may be read (all except the regulated filament voltage).
To the front of the analog power supply is the digital power supply, with 3 regulators supplying +5, +7.5 and ~ –33V (for the front panel displays I guess).
I found that the analog section +15 regulator section had shorted input to output putting around +22 V to the op-amps in the analog output section. I bought a new regulator and the chips in the analog section in anticipation of having to replace them all.
In the process of repairing the power supply I found something interesting – a few of the (supposedly) plated-through holes on the double-sided PC board had opened up, disconnecting from the top ground plane so that some of the filter capacitors were effectively out of the circuit. The only way I could fix this situation was to run a jumper ground wire on the bottom side from capacitor to capacitor. If you should ever have to repair a NOrh check ALL component grounds on both power supply sections. I also felt that the +5 volt digital regulator was running way too hot and put a 10 ohm 5 watt power resistor in series with its input to "share the heat".
SO much for the power supply – on to the “bad” analog section that takes up the right-hand side of the main PC board. In the front are two 5532’s, configured in a low pass filter / buffer arrangement. In back of them are two sets of LM741s and TL071s configured I believe in a servo arrangement to keep the output tube cathodes at zero volts thereby eliminating the need for an output coupling capacitor. A muting relay follows the output tubes and by playing a CD, I was able to trace output right up to the relay and determined that for some reason the circuit (consisting of an IC and maybe 5 transistors) that was supposed to pick up the relay wasn’t working. I temporarily bypassed the muting relay and (fortunately) found the analog output to work and sound fine. There appeared to be no untoward pops or clicks during any switching operation so I may disable the muting circuit permanently instead of trying to fix it.
As a final tweak, I installed sockets where the 5532s were and replaced them with OPA2604s. Will try some “better” 12AX7 tubes later. O yeah, as there were no ventilation holes in the cover, I added a few – this IS a tube unit after all.
Cheers
Charles King
