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Old 17th Feb 2019, 11:44 pm   #1
Dugfirtree
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Default Racal RA17C?

Greetings,

I have a Racal RA17C and it works great up to 16 MHz and then only static, I can hear the 100 kHz xtal oscillator at 0 on each band but no RF gets Through above 16 MHz. Looking in the schematic I do not see any signal paths that changes at 16 MHz. Also, I notice my S meter is not as responsive on 14MHz band as it is on the lower Freq. bands. The Xtal Calibrator only has a strong signal at 0 kHz on each MHz band, the only other xtal Cal signal that can be heard is at 200 kHz on each band. Shouldn't I hear a strong Xtal Cal signal at every 100 kHz? Any suggestions?

regards,
Kurk
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Old 18th Feb 2019, 4:42 am   #2
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Default Re: Racal RA17C?

With the current sunspot conditions, there may not be much above 16MHz to hear.

In a healthy RA17 with no input connected and the calibrator off, rotating the MHz knob through its range should give peaks in the background noise pretty much as each black segment on the MHz scale passes its pointer. As you turn the knob, you get a chuffing sound from the noise peaks.

This is a surprisingly good test of the front end, it shows that the Wadley converter is working and that you have sufficient gain to hear the noise of the RF stage.

If it's not just band conditions and things are going wrong i the receiver it points to the Wadley converter which is a rather clever block converter which takes a chosen 1MHz slice of the RF input, translates its frequency and presents it to a 2 to 3 MHz tuneable IF. If I remember rightly, the 100kHz calibrator is fed into the 2 to 3 MHz section, not the 1 to 30MHz front-end.

The 100kHz calibrator is derived from the 1MHz master crystal oscillator by means of a single-valve injection locked oscillator. These go out of adjustment and make for rather strange calibrator results.

These sets are of an age where many resistors are going high in value and paper capacitors are going leaky and these factors seem to create the majority of faults.

It's a long time since I last pulled in at Denny's for breakfast before heading up Fountaingrove Parkway...

David
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Old 18th Feb 2019, 12:53 pm   #3
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Arrow Re: Racal RA17C?

Quote:
Originally Posted by Dugfirtree View Post
I have a Racal RA17C and it works great up to 16 MHz and then only static. I can hear the 100 kHz xtal oscillator at 0 on each band but no RF gets through above 16 MHz.
Re the last bit: "but no RF gets through above 16 MHz."
Connect your Racal up to a decent signal generator and confirm that before doing anything else. Using that S.G., compare the sensitivity at, say, 3 MHz, 10 MHz, 16 MHz and 21 MHz.
Then please come back here and tell us your findings.

Al.
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Old 18th Feb 2019, 2:19 pm   #4
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Default Re: Racal RA17C?

When you try to get signals above 16MHz, what position is the antenna range switch set to?
Wideband or 6-30MHz?

Do you adjust the antenna tuning setting?

Have you checked the switch contacts and components on the antenna range switch?
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Old 18th Feb 2019, 7:41 pm   #5
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Default Re: Racal RA17C?

Greetings,

I tested the minimum signal I could receive using my HP8640B sig Gen, it has an attenuator /adjustable power level down to -140 dBm.
I get about
-125 to -130 dB from 1 to 5 MHz,
-120 dBm 5 to 12 MHz.
-115 dBm 12 to 14 MHz.
-110 dBm 14 to 18 MHz.
-100 dBm 18 to 29 MHz
So it looks like a gradual roll-off.
Does not matter if antenna SW is in wideband or 16-30 position, bandwidth set to 8 kHz for tests. I can compare to my IC7300 and it does fairly well below 14 MHz. But I see about S5 signals this weekend on 21 MHz (CW contest on 15 Meters) with IC7300 and cannot detect with RA17. I have all the Caps and Resistors on order to restore maybe its time to replace all the paper Caps and drifty resistors. I already replaced every Cap and Resistor in the Power supply and audio output circuits. Now no more Hum.

Kurk
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Old 19th Feb 2019, 2:09 am   #6
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Default Re: Racal RA17C?

There should be a large sensitivity difference between wideband and
tuned input, of course, you have to retune the input for even small
changes (100 kHZ) in the tuned positions.

I had no problem with the RF in my RA17, it just works, so
I can't help.

Getting the calibrator downconverter to work properly was a horrible task:
the manual instructions did not work. It worked fine if it started,
but would not self-start. I just had to play with it, using
a scope to check waveforms. I finally got it to work reliably.
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Old 19th Feb 2019, 3:51 am   #7
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Default Re: Racal RA17C?

I finally received a SSB signal from Texas on 18 MHz today, so I think a lot of my worries are just band conditions and maybe it does have some gain roll-off at higher Frequencies that could be improved with a total rebuild. The Antenna tune is very sharp and hard to find with no signals on the bands. I do get a gain improvement when not in wideband and can find the correct tune position. I guess I will start the rest of the re-build with the Calibrator downconverter. However, the 1 MHz oscillator output seems kind of low at only 1.4v p-p?

Kurk
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Old 19th Feb 2019, 5:15 pm   #8
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Default Re: Racal RA17C?

Based on my experience, sounds as though the 1 MHz xtal osc. / harmonic generator / 37.5 MHz amp. needs attention. Replace caps. and high ohmic value resistors. Then carefully check the alignment of the 37.5 MHz amp., adjusting as required. The alignment of the 37.5 MHz band-pass filter is critical. I've never found it necessary to replace any of the valves in that chain.

My notebook for the RA-17 records that
(a) For the 1 MHz xtal. O/P, a level of about 5v. peak-peak at PL2 / PL3 gives the best overall performance & stability.
(b) The signal level to the harmonic generator is critical. Insufficient drive produces reduced gain at the higher frequencies.

Al.

Last edited by Skywave; 19th Feb 2019 at 5:23 pm.
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Old 19th Feb 2019, 6:49 pm   #9
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Default Re: Racal RA17C?

Go for the 1MHz oscillator level first. With enough drive the divider for the calibrator may come to life.

David
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Old 20th Feb 2019, 1:13 am   #10
Dugfirtree
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Default Re: Racal RA17C?

Wow, thanks for all the great advice, you guys are an Encyclopedia of information. Yes, I measured the 1 MHz drive on PL2 (cable to Xtal Cal. removed) and RP B-N-C PL3. Both measured 1.27 v p-p. This is a wonderful receiver, I use it with a Johnson Valiant Transmitter of the same era for a total tube station, that I put on the air for SKCC contacts. I am sure I will have a few more question in the future. Thanks for the trail.


Kurk
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Old 20th Feb 2019, 6:52 pm   #11
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Arrow Re: Racal RA17C?

Hi Kurk,
Just a quick line to state that there are several people on this forum who have had a lot of experience maintaining the RA-17 variants, so if you do run into problems, just ask!
As David (Radio Wrangler) has mentioned, many of these receivers will now need a serious overhaul, generally concentrating of high ohmic value resistors and capacitors - and it is not unknown to find the odd low value mica capacitor that is faulty now and again. The area that usually needs most work is the 2nd. VFO: it's not unusual to discover that it needs a wholesale replacement of every R and C. Why? Because there is no ventilation for that VFO box - so the components simply get gradually 'cooked' over the years.

Al.
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Old 20th Feb 2019, 10:56 pm   #12
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Default Re: Racal RA17C?

In one of those odd coincidences, we had a visitor up at the radio club this evening, someone I'd not met for a lot of years, the chap who bought my Racal RA17 in about 1991.

Peter, and Bruce who was along with him to help deliver an AR88LF, both read this forum and thought they'd worked out this must be me! yup, they're right.

THe RA117 is the version made for the Americas with real BNC sockets and an extra IF frequency and 1 watt of audio. I bought mine new unused, sealed, from John's Radio in Birkenshaw. All the castings gleamed. It had never been opened.

David
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Old 21st Feb 2019, 3:53 am   #13
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Default Re: Racal RA17C?

Thanks Al,
I guess the 2nd VFO will be next to rebuild. This Radio has some very tight cavities, thanks for the advice, I have a little Cap/multi checker that I use with clip leads that is remarkably accurate if I can solder suck and open one lead of the Cap I can get a good reading.

I have been looking for a RA117, they also have a good antenna switch built-in. Switching the 250VDC on the back of my RA17C to mute for xmit is hard on relays.

I also Have a Racal MA197BU Preselector/protection unit. However, I cannot find any documentation on it. It works great on some bands and not so good on other bands.
and I can see where someone has backed out some of the slugs almost all the way out.
I have been trying to find the tuning/alignment procedure for it?

Kurk
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Old 27th Feb 2019, 12:58 am   #14
Dugfirtree
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Default Re: Racal RA17C?

Greetings,
I replaced 15 resistors and 10 Capacitors in the 1 MHz osc, Harm Gen & 37.5 MHz Amps circuits. Now the 1 MHz signal is 4.8v p-p. I adjusted L2 but ended up in same spot. I did not align the 37.5 Bandpass filter. I also changed all res & Caps in the Calibrator down-converter. However, the reciever is performing awesome. I get -138dB MDS at 1 MHz and get this value all the way to 26 MHz band ( -139 on 7 MHz band). at 27 MHz it drops to -127dBm, 28 MHz up to -134dBm and at 29 MHz is -125dB. So the RA17C is receiving better than it ever has since I owned. I also adjusted the calibrator slugs
and now I get a good Cal sig every 100 KHz and in the center of adjustment range at all places. Now I do not know if I should finish the rebuild or use it? Now the only thing wrong with it is the Limiter does not work, nothing happens when I turn it on. It now has the sensitivity of some of my new Transceivers. Should I keep rebuilding or should I use?
Regards,
Kurk
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Old 27th Feb 2019, 8:14 am   #15
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Default Re: Racal RA17C?

It sounds like you've done a thorough job in the main trouble areas where components get the hardest life. The same paper capacitors and family of resistors are used throughout so you'll see slower degradation and failures as components in easier areas age more slowly.

Use and enjoy, or get it to a better state of reliability than it had when new in time for the next sunspot cycle?

The answer may be to flip a coin. There is no certain answer.

David
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Old 27th Feb 2019, 1:17 pm   #16
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Default Re: Racal RA17C?

The noise limiter will only 'work' on pulse-type interference and those pulses need to have fairly short rise-times and be quite large in terms of relative amplitude. The limiter will have no effect on general background 'mush'. The N/L in every comms. Rx. I have ever operated behaves same way.

Al.
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Old 28th Feb 2019, 2:12 am   #17
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Default Re: Racal RA17C?

Thanks for the info. I also have a Collins R390A and the Noise limiter on it is almost as good as a Audio DSP, but I guess its a different circuit, my R390A also has close to same MDS, but has bad IMD distortion on strong signals that the RA17C does not. Also, it is not as ergo friendly as the RA17C.

Kurk
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Old 28th Feb 2019, 3:20 pm   #18
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Default Re: Racal RA17C?

There was an article in the International Journal of Solid State Circuits by P R Rafuse (Of MIT fame) about a ring-of-DMOS FETs where he compared that large signal handling of his mixer against the R390 which did not come out at all well.

Ironically, Racal did an underwhelming job of changing the RA17 receivers to transistors and decided to really do it well, and they chose the Rafuse mixer for their RA1772 which is an excellent receiver and room heater.

David
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Old 1st Mar 2019, 10:37 pm   #19
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Default Re: Racal RA17C?

"Also, it (the R390) is not as ergo friendly as the RA17C."
Yes, I have heard folks who used the RA17 professionally say how much they liked it as an 'operators radio'. Full marks to Racal who had little experience of designing receivers at the time.
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Old 2nd Mar 2019, 9:02 am   #20
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Default Re: Racal RA17C?

The one ergonomic advantage of the R390 is that with the Racal, you have to keep peaking its input preselector filter yourself. Collins provided cams to do it. CRM/R6A and AR8516L also have linked preselection.

David
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