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Old 3rd Jan 2017, 11:02 am   #1
George G4EUF
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Angry Racal RA17L.

Hello All
Happy New Year. I am currently working again on my RA17L rx, having now repaired my HP8601A sweeper and realigned the 37.5 MHz filter after replacing all the SM caps within, however, I think I have read within the group that it is possible to replace those in the 40mHz filter without touching the trimmers and of course I am aware of the frequent warnings "don't touch this filter!!!
I am now receiving sigs on the 20m amateur band but for some unknown reason other bands appear to be all but dead apart from loads of audio??
Any comments please.
73 de George G4EUF.
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Old 3rd Jan 2017, 11:13 am   #2
Skywave
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Default Re: Ra17l

I don't understand your comment "other bands appear to be all but dead apart from loads of audio". What is that "audio"? Since you state that the other bands are 'dead', that "audio", presumably, is not demodulated r.f. signals. Are you simply referring to the noise produced in the receiver?

Al.
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Old 4th Jan 2017, 12:16 am   #3
Colinaps
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Default Re: Racal RA17L.

Hi George,

There are many things that can render an old Racal deaf - the filter is just one!

If you're getting a decent noise peak tuning through each MC/s mark, the filter is letting enough through that you should be able to hear something, band conditions permitting.

If your sweeper is working reasonably well and you can arrange marker pips, you should be able to set up the 40MHz filter.

It's fiddly, but not that fiddly.

It's worth checking the preselector with the sweeper as well.

Cheers,

Colin.
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Old 4th Jan 2017, 9:41 am   #4
George G4EUF
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Default Re: Racal RA17L.

My thanks to you both for your replies, perhaps I should have worded it a bit better and stated that there is a lot of receiver noise rather loads of audio.
I do not get a peak in noise at each meg point only at zero where there is a definite peak.
Again thank you.
Regards.
George.
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Old 4th Jan 2017, 1:49 pm   #5
Skywave
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Arrow Re: Racal RA17L.

Ah! Now I understand: thank you. The drive level from the 37.5 MHz chain is critical. (I've often held the opinion that an additional stage of gain in that chain is really required). Since that is where you have recently done some work, I suggest you check the signal voltages in that chain at various points. And it is also obviously necessary that its alignment is correct, including its bandwidth. The air-spaced trimmers in that filter are very interactive, especially the ones at each end.

I would also check the operation of the 1 MHz crystal oscillator. A lack of amplitude from that stage can also cause the problem you have.

Al.
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Old 4th Jan 2017, 8:26 pm   #6
Colinaps
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Default Re: Racal RA17L.

Waving a scope probe with a short tail around the harmonic generator valve (with the valve cover off) should show you a very short 1MHz pulse with all sorts of harmonic mess going on between spikes. C.
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Old 5th Jan 2017, 8:57 pm   #7
George G4EUF
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Default Re: Racal RA17L.

Hello again both,
Have now had a chance to check the o/p voltage at the 1mHz o/p and I am getting around 2.2 volts at exactly 1mHz so I am pretty sure that is somewhere where it should be according to the manual. Another test I have done is to remove V9 and with the VVM at T/P 3 at each mHz point I get around 2.5-2.6 volts which would suggest to that the first VFO, harmonic mixer and the 37.5 MHz filter are doing what it is supposed to do.
Something I am unsure about is that when aligning the 37.5 meg filter Racal say that you must shunt the VVM with a 12pf cap, the I/P capacitance of my Marconi VVM is 1.5pf so I have used a mall ceramic trimmer cap adjusted to 10.5pf to shunt the RF probe. What Racal don't make clear is that you should or should not use this shunt when measuring for peaks on each mHz point from TP3 with V9 removed, however, the voltages mentioned above are pretty well consistent on the 1st VFO dial at each point.
Colin,
I will try your test with a scope probe.
My thanks to you both again.
Regards.
George.
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Old 6th Jan 2017, 11:51 am   #8
Richard - F4VPR
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Default Re: Racal RA17L.

Quote:
Originally Posted by Colinaps View Post
If your sweeper is working reasonably well and you can arrange marker pips, you should be able to set up the 40MHz filter.

It's fiddly, but not that fiddly.
There is one trick that I hope isn't a suck eggs time. . . . . .

A long time ago now as newly qualified Radio Techs at GCHQ pre "donut" days we were given racks of RA17's to re-align using a Polyskop 1.

Having totally fouled up the filter alignment, the old hands showed us that by taking the filter screen lid off you could easily see discolouration witness marks on the trimmer stator vanes that allows all the trimmers to be reset as they were for another alignment attempt!

R.
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Old 6th Jan 2017, 3:39 pm   #9
rayfarrar
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Default Re: Racal RA17L.

Quote:
What Racal don't make clear is that you should or should not use this shunt when measuring for peaks on each mHz point from TP3 with V9 removed


Yes you should shunt the probe when doing this. Are you using Issue 2 of the Operating and Maintenance Instructions?

Regards

Ray
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Old 6th Jan 2017, 3:52 pm   #10
George G4EUF
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Default Re: Racal RA17L.

Hello Richard, Bonjour mon ami,
Thank you for that suggestion, I have not even attempted to look at the 40mHz filter so I am unsure as yet whether thy have been "tweaked" or not. I will take the covers off and have a look. Sometime ago someone in the group suggested that it is possible to change all the SM caps in the filter without disturbing the trimmer caps, that does of course assume the replacements are of exactly the same capacitance, A old friend of mine, G8JFJ, many moons ago did tell me that the Lemco SMs in the Racal,s were noted for being unreliable, but you have to take into consideration the age of these receivers and I would suggest that many capacitors and resistors would have changed over the years. In my case I have changed a lot of both.
Thank you again for you suggestion.
Perhaps work you on the air sometime.
Auvoir.
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Old 6th Jan 2017, 4:04 pm   #11
George G4EUF
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Default Re: Racal RA17L.

Hello Richard,
Well that clears that one up then, thank you, yes I am using issue 2 of the manual, I will go back again then with the shunt cap in and see if my figures are repeatable.
Thanks Ray.
Like the picture of the 1st VFO dial !!
George.
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Old 6th Jan 2017, 7:59 pm   #12
George G4EUF
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Default Re: Racal RA17L.

My apologies Ray, think I have got Richard on the brain, must be my age !!!George.
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Old 6th Jan 2017, 11:15 pm   #13
Aubade65
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Default Re: Racal RA17L.

If you do want to setup the 40Mhz filter there is a description of the process including the response curve of the filter in EMER724 on the VMARS site
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Old 8th Jan 2017, 10:17 am   #14
George G4EUF
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Default Re: Racal RA17L.

Hello Aubade 65,
Thank you for that, have now downloaded the paperwork and find it a lot more detailed than the manual itself! very useful, I am a member of VMARS so did not feel guilty about printing it out. Many thanks again.
George.
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Old 8th Jan 2017, 10:47 pm   #15
Skywave
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Arrow Re: Racal RA17L.

The 40 MHz B/P filter: I'm sure you'll do this, but I'd like to mention it anyway. Before you replace or adjust anything in that filter, check its response first. It may not need adjustment.

I have read your findings with the 37.5 MHz chain. Nevertheless, I would check (and replace, if necessary) any high-valued resistors and any capacitors in the value range 0.5 to 0.01 µF. The bandwidth, its corner freqs., the flatness of that filter and its output drive level are all critical.

Al.
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Old 9th Jan 2017, 8:46 pm   #16
George G4EUF
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Default Re: Racal RA17L.

Many thanks Al,
I will get back to you on that one, I have just replaced the "film" scale on the beast, I had a spare VFO2 in the loft, the original looked a bit "tatty"
Regarding the replacement of resistors and caps, I have systematically changed many having had any number that had changed value and caps that looked a bit sick.
Thank you again.
George.
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Old 10th Jan 2017, 12:10 am   #17
Skywave
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Arrow Re: Racal RA17L.

Hi George. As you mentioned the 2nd. VFO, did you open it up and check the condition of the components within? On account of the inadequate ventilation of that box, most of the components within it have usually been reduced to near toast! It's not unusual for a complete rebuild to be required.

And yes: replacing the film scale can be fun, eh?

Best of luck with it; perseverance usually wins with the RA-17 (and its variants).

Al.
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