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Old 2nd Apr 2023, 7:46 pm   #301
Chris55000
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Default Re: Help needed with - Racal Dana 9081

. . .Les–Lawry–Johns had a Gypsy Fortune–Teller earn him "Beware The Blue Tant!" in the 1960s before they even started using them in tellys!

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Old 2nd Apr 2023, 7:58 pm   #302
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Re: Help needed with - Racal Dana 9081
. . .Les–Lawry–Johns had a Gypsy Fortune–Teller earn him "Beware The Blue Tant!" in the 1960s before they even started using them in tellys!

Chris Williams


Hi Chris, I didn't know about that comment, thanks.
My hatred of them goes back a long way, even relatively new Pye Europa's had them go faulty in no time at all (and indeed, they were always blue coloured) I ain't colourist any colour tants deserve to be binned without mercy.
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Old 2nd Apr 2023, 9:00 pm   #303
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Default Re: Help needed with - Racal Dana 9081

I can't recall any failures in the Europas, considering my patch covered lots of ambulance service sets. The occasional polystyrene going intermittent, I think a thousand or 2000pF in the noise squelch circuit?.
The only regular tantalum failure was in the A200 vhf linear amp, from memory a 100uF 16V tantalum across the vehicle battery supply line within. Just charred remains usually, but a 16V working was asking for trouble. Later products (and repairs) had a 25V working voltage.
I must admit, I have had very few tantalums fail, especially considering a lot of repairs and refurbishments that I did back in the 1990s on test gear from the 60s to 80s.
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Old 2nd Apr 2023, 9:35 pm   #304
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Default Re: Help needed with - Racal Dana 9081

The tants in the Europa mic audio stage went bad, mic amp or filtering early on, too long ago to remember exactly, unless I saw a manual then it would come flooding back like a bad dream.

The tants in the PF2 or PF70 was it? they went often too.
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Old 4th Apr 2023, 1:00 pm   #305
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Default Re: Help needed with - Racal Dana 9081

Congratulations on getting it fixed, surprising number of CMOS IC's had bit the dust.

Now the digital side is working, is anything emerging from the output? is the attenuator OK, and do the modulation options appear to be working?
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Old 5th Apr 2023, 8:02 pm   #306
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Default Re: Help needed with - Racal Dana 9081

Hi Peter
It wasn’t, it was working on AM but not FM, but a bit of switch cleaner and replacing C7,C11 on the audio oscillator assembly (two tants you can get to without removing the board).
And it appears to now be working as I have meter movement and needle control but need to investigate a little more to be sure.
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Old 5th Apr 2023, 8:11 pm   #307
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Default Re: Help needed with - Racal Dana 9081

Be aware, one common fault I often had was the DIL 5V reed relay being intermittent contacts. I think it caused FM mod on carrier but no indication on meter, OR, may have been vice versa.

Any I refurbished, I ended up changing this as a matter of course.

Rob
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Old 7th Apr 2023, 7:30 am   #308
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Default Re: Help needed with - Racal Dana 9081

Hi all

Can anyone advise on how to remove the knobs form the Fine / Coarse control, I have removed the little plastic cap and the flathead screw from the middle of the fine control but the knob does not easily come off, i thought at this point i would repost and ask for some advice and do not want to put any unnecessary force on it. Attach a pic of what I am left with after removing the screw from the centre of the fine knob.

Thank robinshack for the info on the 5V DIL will look at that further.

Regards

Mark
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Old 7th Apr 2023, 10:13 am   #309
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Default Re: Help needed with - Racal Dana 9081

Hello Mark,

On mine the knob comes off easily with the nut removed. It then reveals another nut which holds the outer knob in place.

Possibly there is a little bit of corrosion on the collet which is making it stick on the inner shaft. You could try a small drop of penetrating oil to see if that helps, or some gentle heat (think hair drier, nothing more than that, as you don't want to damage the knob).

Paula
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Old 7th Apr 2023, 10:23 am   #310
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Default Re: Help needed with - Racal Dana 9081

Mine is a 9082 and has collet fixing knobs by Sifam. These are Matt black. Yours look the very old type, so probably different.
On mine there is a "nut" with a slot either end used to unscrew it half a turn or so to slacken for removal. When removed, the coarse knob has a nut to slacken.
Sometimes either one can stick even when slackened and I find rotating them round and round eases them enough to slacken. Had to do it with mine just now.
Rob
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Old 7th Apr 2023, 12:50 pm   #311
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Default Re: Help needed with - Racal Dana 9081

Hi Paula, Rob

Many thanks will try the suggestions and let you know if i manage to get it free.
Regards
Mark
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Old 7th Apr 2023, 7:51 pm   #312
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Default Re: Help needed with - Racal Dana 9081

Managed to get it off, had to give it a little WD-40 and let it soak in for a bit, managed to tighten it a little. Thanks again.
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Old 8th Apr 2023, 11:19 am   #313
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Default Re: Help needed with - Racal Dana 9081

Hi all

Bit of a further update and some more test results etc with some more pics:-

These results are for Paula:-
Test 3 Results
Meter now set at zero and running up and down the band needle only flickers when I stop tuning wheel to max of first incriment, so seems to be woirking corectly now.
Test 4
Starting set to 3khz Peak deviaition and needle set to 2.0KHZ
change to 10Khz setting needle settles at 2.1
change to 30khz setting needle moves back to 2.0
change to 100khz setting needle settles at 2.1
change to om rads settign needle stays at 2.1

Test 5
set to 10.00000 after 10 mins meter reads 10.00020
set to 100.00000 after 10 mins meter reads 100.00046

Test 6
Calculate this time to be 31.91% modulation
Attach pic of waveform seen on scope for AM. (TR1.jpg) excuse shadow across scope screen it was from me taking the pic.

I then connected the output to a radio as suggested and could hear a tone all the way down to 0.3 micro volts, from 300uv down to 0.3uv it was getting progressively weaker with each click down of the dial, which i belive to be correct.


I also had to adjust the zero point of the meter using the little black adjuster screw directly underneath the meter, this is shown in pic TR6.jpg and arrowed in white, it is now zero'ed and working fine.

Also attach an overview of all the componets we have changed during this marathon, its a lot! see attachments TR2, TR3 & TR4.jpg's
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Old 8th Apr 2023, 11:28 am   #314
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Default Re: Help needed with - Racal Dana 9081

I also want tyo take the opportunity to thank everyone who has helped and contributed to the thread and ultimately the repair of the 9081, it has been a fantastic effort and I am so pleased with all the help.

A special thanks goes out to Rob and Chris, Peter who i have certainly learnt a lot from about logic and its terminolgy who is extremely knowledgeable on things, last but not least an extra special thank you to Paula who has born the main brunt of my numerous silly questions and misunderstanding of terms etc, theres a lot who would have given up weeks ago but you stuck in there and now the thing is fully working, a real testament to your staying power and kindness.

Thank you all again!!!

I attach one last pic showing all the components together that have been involved in the fix and there was me naively thinking that I would replace a few tants and it would be up and running!

But its been an enjoyable exercise and just goes to show the amount of knowlege and expertise present on this forum.

Happy Easter to you all!

All the best

Mark
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Old 8th Apr 2023, 11:48 am   #315
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Default Re: Help needed with - Racal Dana 9081

Hello Mark,

A well deserved victory and no really expensive parts needed to restore this to working order.

In case anyone is wondering what tests 3, 4, 5 and 6 are about, they were a series of checks to see if there are any other glaring problems, using the 9081's metering/frequency counter where appropriate.

Test 3 was a check of residual FM.
Test 4 checks the attenuator in the meter circuits
Test 5 is the drift when set to fine tune, measured for 10 minutes after a 30 minute warm up.
Test 6 is a check of the modulation depth on AM, using an oscilloscope.

Paula
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Old 8th Apr 2023, 12:42 pm   #316
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Default Re: Help needed with - Racal Dana 9081

Well done Mark and all the helpers.
As well as now having a decent sig gen, you have learnt a lot from this repair, as have I and no doubt others due to the excellent explanations.
This thread will be a valuable asset to anyone else faced with a faulty 9081 I imagine.

If you notice some jitter on the fm indication position of the meter with NO fm mod on, that is quite normal. Some were worse than others I found.

If ever wanting to connect to anything containing a transmitter, be careful not to blow up the attenuator by either an accidental transmit or, the piece of gear developing a fault where it decides to transmit.
There used to be a small accessory box with N plug and socket that fitted on to the output N connector and picked up a dc feed from the little blue terminal adjacent. If it sensed reverse rf power (rf INPUT) it disconnected the attenuator and sounded a buzzer. for the duration.
Quite hard to find now I imagine, though a wanted on here may help??
Another solution I used quite a lot was using a power attenuator of suitable power rating, 20dB (handheld 1-5W rf power) or better still 30dB (mobile rf power, 5-30W) to the output socket.
Rob
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Old 8th Apr 2023, 8:04 pm   #317
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Default Re: Help needed with - Racal Dana 9081

Thanks Paula and Rob, going to celebrate with an easter egg for breakfast tomorrow! 🤣🤣
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Old 8th Apr 2023, 9:28 pm   #318
Chris55000
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Default Re: Help needed with - Racal Dana 9081

I have one of these little devices and will prepare plans for it when my spare time permits!

I would want to keep it for my own personal use as I have two of these to work on and restore, but I will produce adequate documents to enable an equivalent to be built!

Chris Williams
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Old 9th Apr 2023, 10:06 am   #319
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Default Re: Help needed with - Racal Dana 9081

Hi Chris
That will be a great bit of information for all users , thanks for this i look forward to the post.
Regards
Mark
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Old 9th Apr 2023, 10:12 am   #320
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Default Re: Help needed with - Racal Dana 9081

I think you also need to give yourself a pat on the back too Mark, without your diligent approach in following what tests Paula et al suggested you'd never have got this far. Seen it many times over the years here where people ask for help and then just randomly start pulling components out in advance or even in spite of that help.

Anyway, fascinating repair and well done to all involved.

Andrew
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