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MAC 500 - Temperature


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As a bit of a winter project I have been restoring a pair of old MAC 500's however now that they are all back together again I am concerned about the surface temperature of the heads when they have been running for over half an hour or so. Both units show that they are exceeding +100C according to the internal temperature monitor and this evening I have measured the surface temperature of the lamp back plate with a thermocouple and this has stabilised around 80 dec C at an ambient room temperature of 22 deg C.


I will add at this point that all of the internals including fans have been cleaned thoroughly and are virtually spotless and both fans are running ok in the heads. Both units have also been fitted with brand new lamps and even after the units have been running for a while there are no signs of any errors or other side effects.


This prompted me to measure the supply voltage on the head fans and I found that these are running on 6V yet the fan ratings are 12V so it seems that they are running only at half speed. Reading through both the service and owners manual there is no reference to being able to control the speed of the fans however a past post on Blue Room seems to infer that the fan speed can be affected by setting the head to studio mode. Setting the head to 'Stud' mode however has no affect on the fan speed, it just optimised the effects.


Can anyone advise me if there is an option for manually increasing the fan speed or does the unit control this for itself? If possible I would prefer to fun the fans at a faster speed in order to reduce the head temperature. Noise levels from the unit are not an issue as they are to be used in a live music environment.


There is a post script to this in that when I was making some voltage measurements on the second head the probe accidentally shorted the +12ve fan supply to the temperature sensor and this has caused errors on the second unit with bTER and HTER being displayed. This has caused the head fans to be supplied with 12V and so running much faster. I suspect I have taken out the 74HCT132D device unless anyone knows any different?


P.P.S - I have just discovered after lamping off and removing the head cover that the fans are actually running at full speed with a +12ve supply and that when the internal temperature sensor drops below around 55 deg C the fan speed / supply voltage drops again, so this has answered my first question. However I am still quite surprised by the high temperature of the back plate at a fairly nominal ambient temperature.


If anyone can help me on the second issue I would greatly appreciate it.





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Do Mac 500s have the option to calibrate the heat sensor? I know the 918s (all the functions of a 500, but in a scanner) do, but their motherboards are different. It could be that an un-calibrated sensor is causing your Macs to register the temperature as being lower than it actually is, and therefore running the fans at a lower speed for too long.
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