[time-nuts] HP 5328A fan - suspected issue

[Robert Atkinson]

Hi Brett,
In the 5328A the unregulated 25V DC supply and OCXO run whenever the unit is connected to power. The on/off switch controls the main circuits power supply. The triac (Q12) and opto-coupler (U6) turn on the fan when the 3.5V DC supply is present (its only active wthe the unit switched on). The oprimal solution would be to use a 24V DC fan running from the raw DC (junction of F1 & R1) with it's negative lead switched to ground by a NPN transistor whose base is connected to the 3.5V DC (with a series resistor of course. I'd remove Q12 and U6 and all the associated 115V AC wiring. you can then pick up the 3.5V from the pad where U6 LED anode was connected). If you can only find a 12V fan just put a 12V zener in series with the fan (anode to fan cathode to F1). To slow the fan and reduce noise use a higher voltage zener.
Also consider one of the variable speed temeperature controlled fans with remote sensor. Put the sensor out of the direct airflow.

HTH,
Robert G8RPI.





________________________________
 From: Brett Owen Rees <breree at gmail.com>
To: time-nuts at febo.com 
Sent: Friday, 17 January 2014, 23:03
Subject: [time-nuts] HP 5328A fan - suspected issue
 

Hi all,

This is my first post to this list. I recently acquired three HP 5328A
counters and have been using one in my shack. They all seem to have fan
issues, with the fans not running in two of them and in the third unit a
resistor near the fan starts to smoke if I turn it on. All of the fans look
identical, being 110V AC units.

So, I have some questions:
- should the fan run all of the time or is it on a thermostat
- does the smoking resistor mean that the fan is seized
- can I replace it with a 5V/12V computer fan. If so, can I safely tap DC
off the power supply and will the fan introduce any electrical noise? I am
240V here so sourcing a fan 110V fan locally may be difficult

The counter I have running seems ok without the fan running. It is the pick
of the bunch with the OCXO, 525MHz and DVM option. Being the best
oscillator I have currently, I have checked it against WWV and it seems ok.
I also used it as an alignment  reference for a funcube dongle receiver
(with tcxo) and am receiving WSPR spots on frequency, so I think it is
good. I am slowly getting the time-nuts bug and am building a GPSDO with a
Morion MV89A ocxo ...

Many Thanks es 73
Brett VK6EZ


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