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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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18th Apr 2008, 7:56 pm | #21 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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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. |
18th Apr 2008, 10:52 pm | #22 |
Retired Dormant Member
Join Date: Mar 2006
Location: Sheffield, South Yorkshire, UK.
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Re: RA-17 VFO question
Hi Al,
Nice work and a lot accomplished in a relative short space of time. regards Mike |
19th Apr 2008, 6:11 pm | #23 |
Heptode
Join Date: Jan 2003
Location: Tewkesbury, Gloucestershire, UK.
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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,
__________________
Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm |
19th Apr 2008, 7:04 pm | #24 |
Retired Dormant Member
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Location: Bristol, UK.
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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. |
20th Apr 2008, 12:12 am | #25 | |
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Re: RA-17 VFO question
Quote:
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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20th Apr 2008, 2:33 pm | #26 | |
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Join Date: Jun 2006
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Re: RA-17 VFO question
Quote:
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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20th Apr 2008, 4:07 pm | #27 |
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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. |