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Racal 9475 Rubidium
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Paul Bicknell
2018-05-01 09:18:35 UTC
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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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Angus via time-nuts
2018-05-01 12:37:07 UTC
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Hi,

There's a copy of the manual at: http://www.ko4bb.com

What are the symptoms?

Angus.
Post by Paul Bicknell
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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David C. Partridge
2018-05-01 13:03:45 UTC
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Most obvious issue - is the Rb "bulb" coated on the inside with a metallic deposit - if so it's time to use a heat gun to move it back to where it belongs.

David

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From: time-nuts [mailto:time-nuts-***@febo.com] On Behalf Of Paul Bicknell
Sent: 01 May 2018 10:19
To: time-***@febo.com
Subject: [time-nuts] Racal 9475 Rubidium

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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Bob kb8tq
2018-05-01 13:16:36 UTC
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Hi

Ok, quick intro to Rb standards:

On an Rb you have a light bulb. It’s a really weird bulb but a bulb none the less.
On most (but not all ) designs the bulb has a finite life. Various improvements
over the years have stretched out the life. Just when which outfits did which
improvements …. who knows ….

Rb’s need to run hot. They have a couple of heated zones inside the physics
package. Heat and electronics are not a good combo. Various designs have
issues on the stuff inside the hot zones.

In addition to all this, they have crystal oscillators that drift. Once the oscillator
drifts far enough, the device isn’t going to lock up.

That’s a quick, very non-specific to your unit, list.

Bob
Post by Paul Bicknell
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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Jerry Hancock
2018-05-01 15:35:55 UTC
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They also suffer from all the typical electronic component age related faults. I had 5 units that had four bad capacitors each, for instance, and after replacing the caps, they run well.
Regards,

Jerry
Post by Bob kb8tq
Hi
On an Rb you have a light bulb. It’s a really weird bulb but a bulb none the less.
On most (but not all ) designs the bulb has a finite life. Various improvements
over the years have stretched out the life. Just when which outfits did which
improvements …. who knows ….
Rb’s need to run hot. They have a couple of heated zones inside the physics
package. Heat and electronics are not a good combo. Various designs have
issues on the stuff inside the hot zones.
In addition to all this, they have crystal oscillators that drift. Once the oscillator
drifts far enough, the device isn’t going to lock up.
That’s a quick, very non-specific to your unit, list.
Bob
Post by Paul Bicknell
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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and follow the instructions there.
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Dan Rae
2018-05-01 15:20:31 UTC
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Post by Paul Bicknell
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?
The Racal manual for this only covers the support stuff.  Your problem
is almost certainly with the Rb module; these were some variety of FRK,
either FRK-L or -H.  These are repairable without much difficulty, but
obviously you will need the manual for it.  In the FRK there is a
crystal that will have aged, and a free running audio oscillator around
8 kHz in the servo unit that may well need attention.  I did fix mine
some ten or more years ago and since then it has worked when needed.   I
wouldn't worry about the lamp yet before checking the rest of it.

Dan
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Roger Tilsley
2018-05-03 02:13:49 UTC
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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!

Regards,
Roger T.
Post by Paul Bicknell
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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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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Adrian Godwin
2018-05-03 14:38:00 UTC
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What sort of heatsink does it need ? Do you need the original part or would
a lightly-machined generic module do ?

There are some lovely heatsinks in obsolete (server quality) PCs. Huge
chunks of copper with a fine fin structure and a matched fan. Not all of
them are noisy.
Post by Roger Tilsley
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!
Regards,
Roger T.
Post by Paul Bicknell
Hi all new member hear could any of you help with the following
information
Post by Paul Bicknell
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
_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
Post by Paul Bicknell
and follow the instructions there.
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Dan Rae
2018-05-03 15:37:37 UTC
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Post by Adrian Godwin
What sort of heatsink does it need ? Do you need the original part or would
a lightly-machined generic module do ?
At the time, at least in the UK, it was a very common extrusion, often
used for power supplies etc., and in fact I see two on the back of big
Astron supplies under my bench.  I had some in my "junk box" that even
had the mounting holes in the correct places.   If anyone needs it I can
take photographs of the back of my 9475 with the heatsink fitted.

Dan
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Adrian Godwin
2018-05-03 15:44:47 UTC
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Like this one ?

Yes, very common - I'm sure something suitable could be found, if not in my
junkbox then at an amateur radio sale.

Loading Image...
Post by Dan Rae
Post by Adrian Godwin
What sort of heatsink does it need ? Do you need the original part or would
a lightly-machined generic module do ?
At the time, at least in the UK, it was a very common extrusion, often
used for power supplies etc., and in fact I see two on the back of big
Astron supplies under my bench. I had some in my "junk box" that even had
the mounting holes in the correct places. If anyone needs it I can take
photographs of the back of my 9475 with the heatsink fitted.
Dan
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Paul Bicknell
2018-05-03 17:35:33 UTC
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Hi Dan
Regarding a photographs of the back of your 9475 as my unit dues not have a
heat sing and it is serial number B002 so your offer would be useful

Regards Paul B

-----Original Message-----
From: time-nuts [mailto:time-nuts-***@febo.com] On Behalf Of Dan Rae
Sent: 03 May 2018 16:38
To: time-***@febo.com
Subject: Re: [time-nuts] Racal 9475 Rubidium
Post by Adrian Godwin
What sort of heatsink does it need ? Do you need the original part or would
a lightly-machined generic module do ?
At the time, at least in the UK, it was a very common extrusion, often
used for power supplies etc., and in fact I see two on the back of big
Astron supplies under my bench.  I had some in my "junk box" that even
had the mounting holes in the correct places.   If anyone needs it I can
take photographs of the back of my 9475 with the heatsink fitted.

Dan
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Nick Sayer via time-nuts
2018-05-03 16:40:17 UTC
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Post by Roger Tilsley
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.
Would there be any value to designing a 24 -> 27.5 volt boost converter that one could use to power it from a more conventional supply? How much current would it need?
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Bob kb8tq
2018-05-03 16:55:19 UTC
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Hi

You *really* need to keep the baseplate of the FRK series devices down. Their reliability drops quite a bit
as you go from 40 to 50 to 60 C on the baseplate. I would avoid boosting the input voltage without a very
good heatsink ( or heatsink + fan ) setup.

I’ve …. ummm ….. cooked … a number of FRK’s over the years.

Bob
Post by Nick Sayer via time-nuts
Post by Roger Tilsley
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.
Would there be any value to designing a 24 -> 27.5 volt boost converter that one could use to power it from a more conventional supply? How much current would it need?
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Dan Rae
2018-05-03 18:10:16 UTC
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Post by Bob kb8tq
Hi
You *really* need to keep the baseplate of the FRK series devices down. Their reliability drops quite a bit
as you go from 40 to 50 to 60 C on the baseplate. I would avoid boosting the input voltage without a very
good heatsink ( or heatsink + fan ) setup.
I just measured mine after about two hours of warm up, the case sides
(which also are a sort of sink) measure around 33C, and the rear heat
sink around 34C in a fairly chilly (17C :^) ) southern California room. 
I don't think it's ever going to cook the FRK which has been working
when needed, as I just discovered, since 2001.

Dan
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