Racal Dana 9457 Rubidium with intermittent lock
Hi
I have just started refurbishing an old 9475
It did not work at all to start.
I have replaced all the 10 uF 35V caps in the Efratom FRK unit (one
was dead short) and now it powers up, achieves lock in about 7 min and
works fine for about 1/2 hour.
Then the lock light starts to flicker, within an hour it is more off
than on.
When it is on, the lock seems genuine, I see the 10 MHz jump ( I have
it on a scope with the other timebase from a GPSDO at 10 MHz) and it is
stable WRT that. When showing unlocked, the 9475 10MHz off freq by
about 0.05 Hz
Has anyone seen this behaviour before? I cant see any more
electrolytics / tants to change in the FRK . This sort of behaviour is
typical of failing electrolytics.
The actual 9475 is good, it works with a different Efratom module with
all voltages nominal and stable.
The boards in the Efratom FRK don't quite match up with the schematic I
have downloaded, it's similar but not identical.
Thanks in advance
Mark GM4ISM
I looked up that reference online and there is a blog on eevblog about it.
There is a front panel meter. Since the unit locks for a bit what is the
meter showing?
On efc, slowly drifting up or down.
Lamp voltage gives good hint to the lamp output. Is it stable or maybe
slowly dropping off.
Also the lamp could be dropping out and coming back on. They do that near
the EOL.
Best of luck. Looks like a nice unit.
Paul
WB8TSL
On Thu, Oct 12, 2023 at 1:34 AM Mark Hughes via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi
I have just started refurbishing an old 9475
It did not work at all to start.
I have replaced all the 10 uF 35V caps in the Efratom FRK unit (one
was dead short) and now it powers up, achieves lock in about 7 min and
works fine for about 1/2 hour.
Then the lock light starts to flicker, within an hour it is more off
than on.
When it is on, the lock seems genuine, I see the 10 MHz jump ( I have
it on a scope with the other timebase from a GPSDO at 10 MHz) and it is
stable WRT that. When showing unlocked, the 9475 10MHz off freq by
about 0.05 Hz
Has anyone seen this behaviour before? I cant see any more
electrolytics / tants to change in the FRK . This sort of behaviour is
typical of failing electrolytics.
The actual 9475 is good, it works with a different Efratom module with
all voltages nominal and stable.
The boards in the Efratom FRK don't quite match up with the schematic I
have downloaded, it's similar but not identical.
Thanks in advance
Mark GM4ISM
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Mark, in one or two of the 9475s I have fixed, there was a problem with
the LF oscillator in the servo section which free runs at about 8 kHz.
There are some adjust on test Rs that had drifted in value. Page A10 in
my FRK manual. U3 (4060 )should produce an output of 127 Hz from my
notes. If this is off in anyway it will upset the synth. But I see
there is a different version of this board where there is an Op Amp,
also U3 doing the oscillating but at a lower frequency...
Whatever, it needs to be stable.
Dan
Hi Mark,
Have you checked if the xtal heater is working correctly / stable and is not dropping out? If the Xtal oven temperature goes haywire the frequency of the oscillator goes off far enough that Rb lock fails.
Which is no surprise of course.
Today I managed to fix my FRK-L (EUR 15 hamfest grab) which had no less than 3 problems:
1 - the current limiting transistor of the xtal heater was a dead short resulting in a cold oven. Easy find.
2 - the carbon-composite feedback resistor R7 had drifted from 2 Mohm to 3.6 Mohm
Once 1 &2 were fixed I found myself with an oven temperature going past +100 Celsius (au!). There is no thermal fuse (not good).
Failure 3 proved to be opamp U1. Once replaced, the oven temp became stable at 76 Celsius, this temperature is handwritten on the oven assy.
I now have stable Rb lock.
Hth, Wilko
-----Original Message-----
From: paul swed via time-nuts time-nuts@lists.febo.com
Sent: 12 October, 2023 14:42
To: Discussion of precise time and frequency measurement time-nuts@lists.febo.com
Cc: Mark Hughes gm4ism@yahoo.co.uk; paul swed paulswedb@gmail.com
Subject: [time-nuts] Re: Racal Dana 9457 Rubidium with intermittent lock
I looked up that reference online and there is a blog on eevblog about it.
There is a front panel meter. Since the unit locks for a bit what is the meter showing?
On efc, slowly drifting up or down.
Lamp voltage gives good hint to the lamp output. Is it stable or maybe slowly dropping off.
Also the lamp could be dropping out and coming back on. They do that near the EOL.
Best of luck. Looks like a nice unit.
Paul
WB8TSL
On Thu, Oct 12, 2023 at 1:34 AM Mark Hughes via time-nuts < mailto:time-nuts@lists.febo.com time-nuts@lists.febo.com> wrote:
Hi
I have just started refurbishing an old 9475
It did not work at all to start.
I have replaced all the 10 uF 35V caps in the Efratom FRK unit (one
was dead short) and now it powers up, achieves lock in about 7 min
and works fine for about 1/2 hour.
Then the lock light starts to flicker, within an hour it is more off
than on.
When it is on, the lock seems genuine, I see the 10 MHz jump ( I
have it on a scope with the other timebase from a GPSDO at 10 MHz) and it is
stable WRT that. When showing unlocked, the 9475 10MHz off freq by
about 0.05 Hz
Has anyone seen this behaviour before? I cant see any more
electrolytics / tants to change in the FRK . This sort of behaviour
is typical of failing electrolytics.
The actual 9475 is good, it works with a different Efratom module
with all voltages nominal and stable.
The boards in the Efratom FRK don't quite match up with the schematic
I have downloaded, it's similar but not identical.
Thanks in advance
Mark GM4ISM
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