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knobtwiddler 20th May 2021 9:26 am

Rebooting Racal 383-2 Oscilloscope

A forum member has been extremely kind and let me take a classic 17” Racal 383-2 oscilloscope off his hands. The unit dates from 1972, and he informs me that it was last switched on 10 years ago.

It’s a rare unit, with orange phosphor display. With this in mind, I’m determined to take as much care as possible in getting the unit ‘rebooted’. At this point, I have not connected it to the mains, and have no intention of doing so until I’m convinced I’ve done every bit of necessary research. Of course, with units of this age, you can never be 100% sure something won’t play up when being returned to service, but I’d like to take every step that I can.

The most obvious first step should be to remove the external covers and look for any visual clues around electrolytic caps, i.e. leaking electrolyte or suspicious bulging. I will also look for any obvious humdingers such as RIFA caps (doubt it’ll have them).

What other checks would you do?

I have a large variac (about 7A output from memory), so I can ramp it up slowly – but if it contains switching regulation, I doubt that will help? I ought to add that my field is audio, and the HV / CRT area of electronics is well outside of my expertise.

Going ahead of myself and assuming all is ok: the max. BW of the Racal is 10KHz. I have other scopes and analysers with outputs. I was wondering if I'll be able to look at signals above this frequency, if I feed it from these... What about a Thurlby DSA524 ?

Any suggestions gladly received. TIA!

dave cox 20th May 2021 10:08 am

Re: Rebooting Racal 383-2 Oscilloscope
Check it has the correct fuses!
Check it has the correct mains voltage selected.
Check it is correctly earthed, connect to an RCD and switch on but be ready to switch it off :)


chriswood1900 20th May 2021 1:48 pm

Re: Rebooting Racal 383-2 Oscilloscope
It is certainly worth trying the Thurlby DS524, the large screen scope will need a horizontal sweep speed of 50usecs/div with a trigger input I did some experiments with a Leader large screen display and a DS524 but I needed a external horiz timebase using a function generator.
In the spec Thurlby says you need 5Mhz bandwidth on the scope but worth a try after all experimenting is fun!

factory 20th May 2021 5:50 pm

Re: Rebooting Racal 383-2 Oscilloscope
You may want something to monitor the current draw as you bring it up.
Also keep the intensity low in case the timebase decides not to run to avoid screen burn.

Found a older version of the 383 large screen oscilloscope from Airmec (taken over by Racal) here;

And another older Airmec large screen scope here;

WME_Bill mentions he has a manual for the Airmec 279 here (post 6);
No idea how much difference there will be in all these older models.


knobtwiddler 21st May 2021 6:38 pm

Re: Rebooting Racal 383-2 Oscilloscope
Thank you, all - much appreciated. I will be making a checklist and observing each point. The advice re: screen burn is particularly important, as it won't be easy to find a replacement 17" orange phosphor screen!


knobtwiddler 21st May 2021 6:41 pm

Re: Rebooting Racal 383-2 Oscilloscope
>You may want something to monitor the current draw as you bring it up

I have a current monitor. Are you advocating bringing it up on the variac, whilst watching draw? Does anyone think there could be any risk involved? With linear PSUs I think variacs are a great idea, but I am nervy with items that might have switching supplies. My understanding is that a lot of TVs from this period had switching PSUs (they are not a modern invention!). Is there a chance that the Racal from 1972 might?

factory 21st May 2021 7:02 pm

Re: Rebooting Racal 383-2 Oscilloscope
Probably not if it's a switching based PSU, what does it use? Most old oscilloscopes I've seen don't, but at least one tube based Tek scope does.
Monitoring the current should make it more obvious if something decides to short and you can switch it off before smoke escapes.


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