12-11-2012, 02:20 PM
Transformer running temperatures & related issues. Recently I had to do yet another repair on my Tektronix 475 oscilloscope 
. Post-repair, during a 'soak-test' with the sleeve cover removed, I noticed how hot the mains transformer got: all the indications were (and are) that that temperature is normal: no malfunctions were detected later when the case was re-fitted and a much longer 'soak-test' was run. However, what was even more alarming was just how hot some of the encapsulated bridge rectifiers got (they're not mounted on heat-sinks). For one of those you wouldn't leave your finger on it for too long! Again, that appears to be 'normal' 
. I did some research into this (mainly via the Internet) and it does seem that the PSU section of the 475 is the really weak area in the design: typically the PSU filter capacitors lose capacitance. Now considering the age of the 475, that is fair enough: electrolytics deteriorate with age (especially when fitted next to things that get really hot) - so we replace them: and that's the problem with the 475: component replacement and general maintenance can be extremely difficult. The saving grace with the 475, of course, is that when it is set up, calibrated and working properly, it is a very good 'scope. Just a shame that Tek. crammed a quart into a pint pot - even though a (small) fan is fitted (which runs fairly slowly), this scope still runs remarkably warm - thus shortening component life.
Now compare that to the HP 1715A: a 200 MHz 'scope whose general spec. and performance is comparable to the Tek. 475. Comparing the HP 'scope to the Tek, for the HP, the enclosed volume is 50% greater, the mains transformer is larger; there is a lot more space between the components & pcbs. It runs a lot cooler - and it doesn't have a fan fitted. As a bonus, it is somewhat easier to do maintenance on. The vintage of each is about the same: mid-1970's. My 1715A has proved to be much more reliable than my 475: each have been used in the same environment with about the same usage duty-cycle. For the 1715A, the weight is about the same as a Tek. 475.
Incidentally, both of these 'scopes are displayed in the Gallery.
Al.
. Post-repair, during a 'soak-test' with the sleeve cover removed, I noticed how hot the mains transformer got: all the indications were (and are) that that temperature is normal: no malfunctions were detected later when the case was re-fitted and a much longer 'soak-test' was run. However, what was even more alarming was just how hot some of the encapsulated bridge rectifiers got (they're not mounted on heat-sinks). For one of those you wouldn't leave your finger on it for too long! Again, that appears to be 'normal' . I did some research into this (mainly via the Internet) and it does seem that the PSU section of the 475 is the really weak area in the design: typically the PSU filter capacitors lose capacitance. Now considering the age of the 475, that is fair enough: electrolytics deteriorate with age (especially when fitted next to things that get really hot) - so we replace them: and that's the problem with the 475: component replacement and general maintenance can be extremely difficult. The saving grace with the 475, of course, is that when it is set up, calibrated and working properly, it is a very good 'scope. Just a shame that Tek. crammed a quart into a pint pot - even though a (small) fan is fitted (which runs fairly slowly), this scope still runs remarkably warm - thus shortening component life.Now compare that to the HP 1715A: a 200 MHz 'scope whose general spec. and performance is comparable to the Tek. 475. Comparing the HP 'scope to the Tek, for the HP, the enclosed volume is 50% greater, the mains transformer is larger; there is a lot more space between the components & pcbs. It runs a lot cooler - and it doesn't have a fan fitted. As a bonus, it is somewhat easier to do maintenance on. The vintage of each is about the same: mid-1970's. My 1715A has proved to be much more reliable than my 475: each have been used in the same environment with about the same usage duty-cycle. For the 1715A, the weight is about the same as a Tek. 475.
Incidentally, both of these 'scopes are displayed in the Gallery.
Al.
