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. On Thu, May 3, 2018 at 3:13 AM, Roger Tilsley <[email protected]> wrote: > 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. > > > On Tue, 1 May 2018 10:18:35 +0100, "Paul Bicknell" <[email protected]> > 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 > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
