My love hate relationship with my old Mission Cyrus One amp hit an all time low when it refused to power up.
I quickly established that the mains input fuse had blown. The replacement also immediately blew. This could be serious. Time to look inside.
Opening up I expected, and indeed hoped, to see a dilapidated smoothing capacitor or two. All is well here. The main caps measured out with a high DC resistance and looked fine.
Maybe something else is drawing lots of current? The next easiest way to check is by removing the internal fuses to isolate the power amplifier. Again the input fuse blew when power applied. I’m getting worried now. For one thing I’m running out of spare fuses. Of more concern is that the evidence is starting to point at the transformer as the failing component.
The quickest way to establish if this is so is to disconnect the transformer from the rest of the circuit. I cut the wires to the PCB. Fresh fuse, power on, dead fuse. It’s looking like a dead transformer. The primary measures at about 18 ohms DC, but that doesn’t tell me much because the AC impedance at 50Hz will be much higher.
The power supply is often what separates the men from the boys in the world of power amplifiers. Cyrus uses a simple circuit design, but very well executed using high quality components and excellent mechanical design. The transformer is an example of this, being supplied by the long gone Scottish manufacturer Holden and Fisher, who also supplied Naim Audio.
The electrical specification is clearly documented on the transformer, as are the wingdings.
To complete the spec I need the dimensions, as shown above. getting this right is crucial. There is not much space in the Cyrus. the transformer is a very snug fit.
I couldn’t find transformers with 22.3V secondaries, but 20 or 24 are readily available. 20V should work OK. 24V would need a bit of maths to make sure I don’t blow anything up. The bigger problem is the dimensions. Most 100VA transformers are shorter and wider. About 40mm x 95mm. That isn’t going to fit. With standard off the shelf transformer unsuitable, I’m looking at serious money for a replacement, even if I can find one.
Another option is to go down the external transformer route. The mission Cyrus (Two) is designed so that you can attach an external power supply, the PSX. In this configuration the internal transformer only feeds the phono pre-amp, so I would be able to substitute a much smaller internal transformer which would fit inside.
Before I spend serious money going down that route, I’ll have one last go at powering up. This time I’ll go straight into a 13A plug with a 5 amp fuse. Hmm. This time the fuse doesn’t blow. Furthermore I’m measuring 22V AC on the secondaries with my Fluke meter.
I re connected the chassis mains socket and tried again. Fuse blew! I’m going to run out of spare 1.0 amp fuses soon. Hang on. Fuses aren’t specified to decimal places. maybe I’ve got the package upside down? Are they actually 0.1A fuses..
OK. Nothing more to see here.. ID10T problem.