the low end Tektronix digital scopes of same bandwidth up to 200 MHZ do not use fans?
Do you mean TDS2000, TBS1000 and TDS1000? They are based on 1997's technology. They have no DPO and a short memory. Not a competitor for Rigol DS1000Z or DS2000.
The original question was about why Rigol doesn't engineer their lower end scopes without the cooling fan, which, receives lots of criticism from many reviewers. And yes, I WAS using the Tek TBS and TDS because they are comparable and although they are not as feature rich, they are similar as with respect to functionality for most applications encountered.
As I understand it, they do not use ADCs running at the sampling rate. These scopes store samples in real time in analogue memory, like a bucket brigade if you remember them, and then replay them at the native ADC speed, so they get to use much slower, cheaper and lower power ADCs but also that's why the memory is so limited, it's restricted to the technology in the bucket brigade devices.
Not only are the ADCs slower, but also all the digital stuff around them can be much slower and lower power. Hence, no need for fans.
While I realise that these days they're much maligned, I have an affinity for them because until I came to eevblog the 4ch 200MHz TDS2024B I've had for about ten years was all I ever needed, so I thought.