HP 53131a fan / PSU question

[99wombats]

I just acquired an HP 53131a - it has the 3Ghz option but only the basic oscillator option. Anyway I’ve seen people saying how the fan is always running on these units, but on mine it doesn’t seem to - it comes in occasionally but usually only for a few seconds. The unit does make a faint clicking sound every second or so even when in standby. The PSU board seems different to the ones I’ve seen people post pictures of - see attached. Does anyone know if any of these units had some kind of temperature sensing fan controller? Or is there just something wrong with my power supply?

[Venturi962]

Fan bearing may be dirty and clicking is from too low voltage not providing enough grunt for the fan motor to spin.  Probably worth replacing, some alternatives listed here: https://www.eevblog.com/forum/testgear/replace-fan-on-53131a-counter-with-quieter-version/?all

[99wombats]

Ok I’ll have a closer look at the fan. The thing is when it did come on it seemed to ramp up and down to a couple of power levels - it wasn’t just fully on or off. Which made me wonder if it was doing something a bit more intelligent.

[99wombats]

And it was pretty quiet when it was running, didn’t immediately sound like a bearing issue.

[99wombats]

The clicking is indeed the fan starting to spin and then stopping. I’ll investigate further tomorrow. Thanks for the tip.

[Tony_G]

The fan is supposed to run when power is supplied. The timebase should have power regardless of whether the unit is operating or in standby (actually I know the fan should always run and my high stability timebase always has power but I can't remember if the standard timebase always gets power but I assume that is the case as well).

As Venturi says, it is really worth replacing the fan with a quieter one.

TonyG

[99wombats]

Well I took the fan out and it won’t even run from my bench supply so I’m guessing it’s dead - I’ve ordered a Sunon EF40201B3-000U-A99 to replace it, seemed to be the closest in-stock match to some of the options mentioned on the thread Venturi linked.

[thinkfat]

As long as you don't have a OCXO timebase, which I highly recommend you looking into, the fan is not going to do much. It is supposed to keep the inside of the case at a constant temperature. But with only the TCXO timebase, there isn't any cooling necessary when the counter is off.

With an OCXO, however, because it is always on to provide the highest possible stability, the fan would be constantly on as well. So get something silent.

Getting a decent reference oscillator is pretty much mandatory for this counter. It is such a fine, precise instrument, but the built-in TCXO is rubbish.