[time-nuts] Racal 9475 Rubidium

Roger Tilsley j.r.tilsley at runbox.com
Wed May 2 22:13:49 EDT 2018

Greetings Paul,

The Racal 9475 does have some inherent shortcomings, the principal one is due to the power supply and the lack of the purpose-designed heat sink for the FRK module.  The unit is built into a “heat-sink case” but to keep the FRK base plate within the specified temperature range Racal had to operate it from a 23.5 V supply.  The power supply originally had a magnificent C-core transformer which produced a rather too-high output voltage consequently the raw d.c. supply was rather high.  The main supply voltage regulator has a rather weird configuration to facilitate heat sinking the pass transistor to the case and this unfortunately precludes using a battery back-up.

What Racal overlooked is that with a linear regulator the total amount of heat dissipated is sensibly constant so the FRK module dissipates less heat than if it were operated at a higher voltage and the PSU pass transistor dissipates more heat.  When the case is opened a well-used 9475 usually stinks of “hot electronics”!  The problem is compounded by the fact that the 5 V logic supply regulator is not adjustable, it is essentially a high current follower from the tap of the R62/R63 potential divider across the 23.5 V main supply rail.

In my experience, FRK modules produce their best performance when operated  from a supply voltage between 27 V and 28 V, selected for individual units but 27.6 V is a good starting figure.  At this voltage the FRK module needs the special heat sink if operated at high ambient temperature.

More recent versions of the 9475 have a good but lower-quality mains transformer (perfectly  adequate for its intended purpose) which has a lower output voltage and this has mitigated the heat dissipation by the PSU pass transistor.

The first thing to do is to monitor the voltage of the 5 V logic supply (between TP2 and TP3) and increase the voltage of the 23.5 V main supply rail (R39) until the 5 V rail is as much above 5 V as you care to risk for the 74-series ICs used.  A safe starting point is 5.2 V but I am wary of going too close to 5.5 V.  I have succeeded in getting several recalcitrant 9475s working properly by this simple ploy of increasing the supply voltage to the FRK module.  In my experience an FRK module operated a with a supply voltage lower than 24 V is prone to failing to lock or intermittent locking after long use.

The complete cure is to increase the value of R36 to produce 5 V with the voltage measured between TP2 and TP3 with the 23.5 V rail wound up to between 27 V and 28 V.  This cures the high internal dissipation problem and considerably improves the performance of the FRK module but the special heat sink is necessary for operation in high ambient temperature or with poor ventilation.

There are a few incidental modifications which I like to do to 9475s.  They were designed when one could have whatever colour of LEDs which one wished provided that the colour was red!  I like green LEDs for the “normal” situation and with the ready availability of LEDs of different colours it is easy to follow the IEC recommendation of orange for “power on” and green for “normal situation”.  The “Rb lock” indicator (LP50) is already green but it is worth changing the “Power on” indicator (LP51) to orange or yellow and I consider it essential to change the “1 MHz” indicators (LP52, LP53, and LP54) to green.  If 4 greens are showing the unit is locked and the outputs are OK which is perceived with a quick glance.

These mods and adjustments are simple and easy to do.  Unfortunately it is not nearly so easy to arrange a battery back up facility so it is wise to operate the unit from a UPS.  The warm-up frequency drift of an FRK module is still detectable 6 weeks or 1000 hours after starting from cold.  The 9475 was apparently intended for supplying the 1 MHz reference frequency for the synthesisers in Naval communications equipment and its drift is sufficiently low for this application within about 2 hours of starting from cold, however Time-nuts are more discerning!

At the time of the introduction of the 9475, Racal were the agents for Efratom but went to great lengths to save the cost of the special heat sink for the FRK module and in my opinion “spoiled the ship for a ha’p’orth of tar” (for US readers ha’p’orth is a colloquial expression for a “half-pennyworth” of old UK money, about 1/250th of 1 US$ at the present rate of exchange).  False economy is also the reason for the 5 V regulator in the 9475, at the time of its design 7805 regulators cost an arm and a leg!

There are not really any other worthwhile mods.  The quality of the 1 MHz outputs is remarkably good considering how they are derived.  In my experience FRK modules usually work satisfactorily if the “lamp voltage” is above 5 V (it should really be above 8 V but rarely is in disposal units).  It is also worthwhile to occasionally tweak the FRK trimmer to keep the oscillator control voltage around mid-range, i.e. 10 - 12 V.

It is not recommended to leave the 9475 meter switch in the oscillator control voltage position, the meter amplifier has too low an input impedance.  Using a high input impedance op-amp as a voltage follower on both the lamp voltage and oscillator control voltage outputs of the FRK module and switching the meter to measure the appropriate output voltage is a mod which some may consider useful - the temperature coefficients of Q12 and Q13 do not cancel and, unlike that of the Q12/Q13 meter amplifier, the high input impedance of the op-amp does not load the oscillator control voltage monitoring output enough to significantly affect the output frequency.

The 9475 is a good “starter” atomic frequency standard, it is reliable and easy to service and the FRK module is also easy to service.  Careful setting up of the FRK module improves its stability.  The older FRK modules found in old 9475s have a high oscillator control voltage which suddenly falls when lock is established while in more recent FRK modules the oscillator control voltage “hunts” up and down until lock is obtained on falling voltage.  Lock is usually obtained in about 5 minutes at normal ambient temperature though the output frequencies still changes slowly for a relatively long time.

I hope that this is helpful.

If any Time-nuts reading this know of a source of not-too-expensive heat sinks for FRK modules, I would be glad to receive details.  I wish to obtain two but I am a pensioner and cannot afford the cost of having them specially manufactured - the cost of the CNC setup kills it stone dead! 

Roger T.

On Tue, 1 May 2018 10:18:35 +0100, "Paul Bicknell" <paul at bicknells.f2s.com> wrote:

> Hi all new member hear could any of you help with the following information
> As I have just bought a Racal 9475 Rubidium and it has problems
> Is there any stock faults ?
> What is the life of the rubidium standard?
> Regards Paul 
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