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Old 18th Apr 2008, 7:56 pm   #21
Skywave
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Thumbs up Re: RA-17 VFO question

18-04-08; update.

Noise limiter (NL) now working; problem was bad solder joint.
The write-up in the manual on this part is VG & really helped the fault-find process.

This overhaul - which certainly developed into much more than I imagined - is now complete. It is my sincere hope that some of my findings - as reported above - will be of use to other fixers in the future.

"Thank you" to those members who contributed - as above.

Al / Skywave.
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Old 18th Apr 2008, 10:52 pm   #22
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Default Re: RA-17 VFO question

Hi Al,

Nice work and a lot accomplished in a relative short space of time.

regards
Mike
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Old 19th Apr 2008, 6:11 pm   #23
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Default Re: RA-17 VFO question

Al,

Thanks for the excellent write up at the end of your overhaul. I might attempt something similar one day and it will be extremely useful.

What is your opinion of the NL now that you have it working correctly? I find mine unusable (RA17 C-12) due to the massive distortion it introduces. However, I have never checked it electrically so possibly it has a fault. Would be interested in any pointers.

Regards,
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Old 19th Apr 2008, 7:04 pm   #24
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Default Re: RA-17 VFO question

It's useful to have these things written up and to bounce ideas around.

Going through an RA17 can certainly cause some head scratching and while the fault finding procedures in the manual are useful, you have to use the equipment you have, such as a scope rather than a CT471 RF voltmeter and so, your mileage may vary. Also, they were written with sets a few years old in mind, not 40+ years old.

Anyway, congratulations on a successful conclusion.

Pete.
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Old 20th Apr 2008, 12:12 am   #25
Skywave
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Arrow Re: RA-17 VFO question

Quote:
Originally Posted by Keith View Post
Al,

What is your opinion of the NL now that you have it working correctly? I find mine unusable (RA17 C-12) due to the massive distortion it introduces. However, I have never checked it electrically so possibly it has a fault. Would be interested in any pointers.
Regards,
Hi Keith -
I must say that the cct. description in the manual is VG - it very clearly explains how it is meant to work; it's not difficult to understand.

Overall, the NL is good. I have met better; I have met worse. It is a peak-clipper type of noise limiter and works by blanking the audio when a noise pulse appears that is above a pre-determined threshold. This threshold is not user-adjustable.

I checked mine out as follows -
I tuned into a fairly strong BC station at about 5.5 MHz and examined the signals at various points in the NL cct. using a 'scope: the waveforms at various points were as predicted by the manual.
Now, as it just so happens, the o/head lighting in the shack is provided by flourescent strip lights - and when they are switched on from cold, make wonderful broad-band noise generators. You could clearly see the spikes on the 'scope when the RX was just tuned to the edge of the side-band on the BC station. When the NL was switched in, you could then clearly see that the the noise spikes were being chopped off.
Secondly, using my SG (HP 8640B) running AM, as the modulation %-age was slowly increased you could see the recovered audio being clipped at about (and above) the 30% mod. level - which is what the manual says it should do.

If yours is faulty, I suggest that you check the two 1-Meg. & the 1M5 Rs and the time-constant C in the NL. The cct. diag. & manual shows what I am referring to here.

Good luck with it!

Al / Skywave.
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Old 20th Apr 2008, 2:33 pm   #26
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Thumbs up Re: RA-17 VFO question

Quote:
Originally Posted by Keith View Post
Al,
Thanks for the excellent write up at the end of your overhaul. I might attempt something similar one day and it will be extremely useful.
Regards,
An 'overhaul' was not what was originally planned: the original idea was to try to fix the lack of sensitivity: 1st. VFO box also contains the RF amp. Replacing the RF amp. valve helped, but there it became apparent that there was more to it than that. The RA-17 is such a big, heavy lump - and complicated - that once it was on the bench I soon realised that to make the 'investment' in effort worthwhile, (recalling an earlier comment from Pete, XTC, about resistors changing value), that a thorough check-out seemed in order. From that point, the job simply mushroomed.

As regards my write-up, my personal opinion on this Forum is that when I put out a shout for help & receive helpful replies, it seems appropriate to inform those helpers of the result of thier contributions. Therefore, some sort of report with comments to help other restorers seems in order. My report was much longer than I would have liked - but the RA-17 is a sophisticated beast & my attempts to prune it down to a sensible level ran the risk of losing important detail.

To conclude - recent off-air tests are rewarding.
Using the same aerial ("T" LW, 30ft. vertical, 50ft. + 50 ft. horizontal top), I now find that I have to make regular use of the aerial input attenuator to avoid RF overload! Previously, the IF & AF gain controls had to almost at max. - with the attenuator at zero atten. Measurements now indicate a SNR of 10dB for 1uV (or less) + 30% AM RF input from 1 to 30 MHz.

Al / Skywave.
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Old 20th Apr 2008, 4:07 pm   #27
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Default Re: RA-17 VFO question

Keith,

Is it time to start a new thread on the RA17 NL and your problems with it?

I'm happy to contribute with measurements and tests etc. at the risk of finding out things I didn't really want to know. Mine appear to work and I'm happy with them, but I haven't tested them.

But, as Al/Skywave says, when you start testing these things and really looking at them, it can just turn out to be a huge undertaking, rewarding 'though.

Pete.
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