It is quite rarely that I go looking for such information as I can usually find it on my own, but I'm stumped.
I'm in need of the correct service information (hopefully schematic), but at the very minimum, the internal fuse rating, for a very early McIntosh MC275. This is the first version, but I seem to have an early revision as the earliest schematics I can find omit the fuse I speak of which appears in the HT circuit just prior to the bridge rectifier. The fuse in mine is a 3.2A slow-blow and this seems incomprehensibly oversized for such a thing; thus I seek the original McIntosh specification on that fuse, but it would be nice to find the exact schematic.
The unit I have is below 500E1 in serial and the earliest documentation I can find is 600E1 but that doesn't include fuse or schematic information. The earliest schematic I can find is 10001 and on in serial numbers.
I would be quite grateful to anyone who can locate the original data!
I'm in need of the correct service information (hopefully schematic), but at the very minimum, the internal fuse rating, for a very early McIntosh MC275. This is the first version, but I seem to have an early revision as the earliest schematics I can find omit the fuse I speak of which appears in the HT circuit just prior to the bridge rectifier. The fuse in mine is a 3.2A slow-blow and this seems incomprehensibly oversized for such a thing; thus I seek the original McIntosh specification on that fuse, but it would be nice to find the exact schematic.
The unit I have is below 500E1 in serial and the earliest documentation I can find is 600E1 but that doesn't include fuse or schematic information. The earliest schematic I can find is 10001 and on in serial numbers.
I would be quite grateful to anyone who can locate the original data!
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Further to this, I've noticed that McIntosh' serial numbers were in two different numbering schemes it seems; such that one of the manuals I located specifies that it covers units 10001 to 10147 and 100E1 and above. So I'm learning more, except about the fuse.
I bet you've been searching carefully enough that none of us can get more information than you already have, unless he's the first hand owner of such a device and also owns the original documentation/service manual/schematics.
3.2 A slo-blow appears to be the correct value of a primary fuse in the 230 Vac world. As a guess, I'd replace the plate supply fuse with a 0.8 A slo-blow one. Anyway, the fact that McIntosh chose to omit this fuse in later editions may indicate that almost no bad experiences were reported to them with this part of the PSU. Hence, you also could leave the 3.2 A fuse alone.
Best regards!
3.2 A slo-blow appears to be the correct value of a primary fuse in the 230 Vac world. As a guess, I'd replace the plate supply fuse with a 0.8 A slo-blow one. Anyway, the fact that McIntosh chose to omit this fuse in later editions may indicate that almost no bad experiences were reported to them with this part of the PSU. Hence, you also could leave the 3.2 A fuse alone.
Best regards!
I took measurements to determine the current draw in this portion of the circuit and under a worst-case operating condition of square wave at full output on both channels, the current is 2.8A RMS, so really it does appear that this is the correct value of fuse, although it seems strange still that it should be a slow-blow. Obviously a square wave is not an output scenario one expects in audio, but as a theoretical maximum it is quite sensible to choose this fuse as it was chosen; especially given the power factor as it is prior to the capacitors.
After discovering that the MC240 also uses such a fuse, I went looking and found that all the schematics I discovered also showed no fuse in that area.... a nasty pattern is arising here!
It also seems that, given the numbers specified in many of the manuals, this amplifier is not that early; although it is the "first mark" as they say (the one with an octal plug for CV outputs and so on).
After discovering that the MC240 also uses such a fuse, I went looking and found that all the schematics I discovered also showed no fuse in that area.... a nasty pattern is arising here!
It also seems that, given the numbers specified in many of the manuals, this amplifier is not that early; although it is the "first mark" as they say (the one with an octal plug for CV outputs and so on).
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At standby, it's on the order of 320V. The standby current is about 450mA. Don't forget that a lot of this current is reactive (out of phase with voltage) as the fuse is between the rectifier and the capacitors.
EDIT: Sorry, the fuse is between the transformer and the rectifier; but this does not matter, the power factor is still going to be the same low value.
EDIT: Sorry, the fuse is between the transformer and the rectifier; but this does not matter, the power factor is still going to be the same low value.
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320 V times 2.8 A means 896VA, plus heater supply power. Wow, how long did the power transformer cope with that?
Best regards!
Best regards!
That is only a short term test condition. But consider that under normal operation, if you were to operate sinusoidally at max power you'd still see half as much VA, give or take. That is just life for power transformers operating into rectifiers and capacitors. There is a good reason why large tube transmitters used to use choke input filters!!
In audio, the average power is so substantially lower than the peaks ever are, that things can be quite undersized in a sense.
In audio, the average power is so substantially lower than the peaks ever are, that things can be quite undersized in a sense.
If we consider that the line fuse is indeed 5A, right away we know we can get 600VA indefinitely on a 120V line. Being a slow-blow fuse, we can allow a great deal more for some seconds; surely shorter than the time of the temp-rise in the transformer 🙂 Obviously the designer(s) expected the transformer would at least handle anything the properly selected line fuse would allow.
Certainly it must be possible, but I'd expect the changes to have more to do with external safety requirements (for example in Canada a lot of equipment that would otherwise have external fuses had internal only and nothing accessible from the outside). Also all photos of the innards of MC275 and related models have the same fuse inside! But nothing on any of the schematics...