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Old 5th Aug 2018, 8:00 am   #41
Radio1950
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Thanks Jeremy,

problem fixed.
Clean as a whistle.
More tomorrow.

It's beer-o'clock.
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Old 5th Aug 2018, 8:27 am   #42
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Quote:
Originally Posted by G0HZU_JMR View Post
There is the VCO coil itself plus some inductors in the VCO circuit for the Digital VFO and these can act as pickup devices for stray magnetic fields. Is there any way you might be operating the radio on top of or near something with a large magnetic field? Maybe next to a large PSU/transformer or a charger or fan?.
Wasn't one of John Thorpe's receivers prone to this? Possibly the 7030? In the back of my mind I have a vague recollection of warnings about stacking this radio with other equipments.
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Old 5th Aug 2018, 8:40 am   #43
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

It's a big problem with spectrum analysers. They use magnetically tuned YIG oscillators and even with a few layers of shields....

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Old 7th Aug 2018, 4:25 am   #44
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

For the reader in a hurry.
It was probably 50 Hz mains induced into the DC supply earth system via one lead with lug on a suspect earth connection using one of the mains transformer attaching screws.

Now for the longer story.
I’ll go through this now as I actually remember carrying it out, so that other owners may just find it helpful to see the intermediate steps.
Please keep in mind that whilst I was attempting to find this problem, I was always conscious of regarding this RX as a heritage set, and I was very reluctant to change or replace anything unnecessarily.
This was made worse for me, in that this FM effect is small, and unnoticeable on received SSB speech.

The comments from Jeremy heavily implicated 50 Hz and possibly 100Hz sawtooth, so I concentrated on probable causes.
Good suggestions regarding nearby 50Hz influences, but no. I have seen CROs affected by this.
At one stage I even moved the RX and test gear into another room. No change in FM effect.

I tried to provoke a change in the FM effect on the CRO, by using appropriately sized pieces of thin sheet steel around the power transformer as shields. No change.

I tried the same, around mains wiring, and other parts of the RX case and chassis.
No change.

I tried again with a small ferrite bead on the end of a bamboo kebab stick, positioning it near the ferrite slugs in the top of various inductors and the small decoupling chokes, eg near the VCO L28. It affected the VCO temporarily until loop relock, but not the amount of FM.
No change.

Again, in the back of my mind was the fact that the importing agent had modified the mains wiring of this set, and I had a deep look at what was done, to see if somehow this might be the cause.

This probably only affects Australian sets, but is included for completeness.
Originally, mains was wired in shielded twin from the rear panel voltage selector to a PCB connector P8/J8 which mated with the LHS of the PCB behind the front panel, then ran in tracks along the whole PCB to the ON/OFF toggle switch S1 on the other side of the panel. The agent cut the small tinned wire jumpers from the switch terminals to the PCB, then ran a new mains cable, in better shielded twin, directly from the voltage selector and transformer to the switch S1 terminals. This bypassed the LHS connectors and the front panel PCB.

Dreadful design, from a present day perspective.

The mains wiring was removed entirely from P8. Unnecessary wiring (for other voltages) was also cut from the voltage selector to the transformer, as the thin wiring was not really compliant with Australian mains “appliance” requirements.

The agent did a good job really, and the mod in my opinion was carried out to “Best Commercial Standard”.

I could not fault the mod, in respect of being an FM cause.

I removed the mains bypass ceramic capacitors at the mains voltage selector; all three.
No change.

I removed an MOV across the mains socket, added by our agent; not original. No change.

I unsoldered the mains from the transformer primary, and connected a “death lead” directly to the transformer.
No change.


Back to the PSU and Regulator PCB.

I checked the PS Regulator PCB, and earthing, and although I had reservations about the main earth routing, and the practice of mostly using the chassis frame as earth, it seemed OK.

I checked each diode bridge OK.

The PSU ripple on the two 15 V supplies measured about 6 mV RMS on my HP427 meter, which has AC response down to 20 Hz. Some of this was probably “bench hash”.

Similar on the 5 V supplies, about 8 mV RMS. All correlated on a CRO.

I then made a breakthrough.


I left the mains unplugged, and substituted a bench “quad” regulated linear power supply which gave 10 V DC and 20 V DC, and a common negative , and connected to the output of the two diode bridges, CD1, CD2, which should all be safe. I just clipped onto C1 positive with +20 Volts, and onto C5 positive with +12 Volts. The original regulators were thus still in circuit, and idling along.

There was no FM.

But this could mean that the NRD515 Regulator Board still was the cause of the problem.

Restored to mains operation with FM present, and continued, but I made a tactical error here.

Very, very reluctantly, I snipped out the regulator output electro capacitors, one by one, C4, C13, C8, as I have experienced rare problems with high capacitance on the output of linear regulators, and these were 100 mfd, a bit high really, and with possible instability.
No change.

Again reluctantly, I replaced the main capacitors, C1, C5, even though they checked OK with ESR, and I had previously noticed no change with new capacitors temporarily bridged across them.
No change.

I changed all three 78xx regulators, using tested new good quality ST brand devices, and careful attention to heat sinking, as the +15V IC7, 7815, is running at 700 mA, and gets quite warm.
No change.

I had carried all this out, mindful that very small mains or ripple voltages may be at fault.

I resolved to remove the mains transformer, and whilst removed a bit from the RX chassis, and with wiring still attached, I would reorientate it with respect to the original position, and see if the FM varied with orientation.

There was no FM.

Nothing I could do would bring it back.

Transformer primary winding measured more than 50 Megohms to frame at 750 V DC.

Bolted it all back in.

No FM.

I have restored the RX to normal, and removed the additional 10 megohm resistor I had across C316 in the VCO.

This fault had something to do with how JRC wired the earth for the regulator board. It went from “TP9” on the PCB to a screw and underneath the mains transformer on one corner.
I can hear you all now “sighing”.

There are no nuts for the four transformer screws, just two metal threaded strips.
There was also the shield earth from the cable running 10 V AC to the S Meter lamp under another transformer screw.
It gets worse.

The transformer sits on top of a thin square of plastic coated sheet steel material glued to the chassis. And,... like most transformers, this one was lacquered, so it is possible that the earth lead lug was between the lacquer and the coated sheet steel, and maybe only made a poor contact with the screw.

I have shifted the DC Reg earth to a screw on the rear panel.

Now I really do not know who did what here, JRC or the importing agent, and they both would have tested the receiver as OK before delivery.

The NRD515 DC and signal earthing is not quite best design. It has all the signs of optimisation for board modularisation. All PC board earths, including the DC regulator, rely on screws holding the PCB earth lands onto brass standoffs. Worse, the undersides of the PCBs have solder on lands around the screw holes, which deform and can leave a loose contact.
The PCBs do have thin earth wire connections, but the paths are not direct, and are somewhat all over the place.

The transformer itself appears well made with some magnetic shielding around the laminations, and a copper sheet shorting strap for strays.

Having a look at RX2, and it has a small amount of FM also. The transformer mounting and lugs are much the same as in my RX1.

This second receiver is not mine, and borrowed for test only, with no changes to be made. I did some probing around the same transformer area, but could not find anything, without removing the transformer, which is a moderate size job anyway. The owner is travelling and not contactable at present.
I may have an update later.

My tactical error?
I could have used two separate external AC sources, 10 and 18 Volts, instead of the receiver mains transformer. Alas, this is a hindsight thing, but which would have kept my DC regulator PCB original. It would have meant removing four wires from the Reg PCB, and I was averse to this at the time.


Photo 1 is 1000Hz AF Out CRO trace before fix. After fix, this is a very clean sine wave.
Photo 2 is PS Reg circuit. The regs are 78xx, C1 4700, C5 6800.
Photo 3 is transformer position in chassis.
Photo 4 is a classic dry joint on DC Reg PCB upper RHS; but of no consequence, other side is good.
Attachment 5 WAV file for Jeremy. NRD515 RX1 10MHz5 SSB 1090 Hz 5 secs post fix WAV as “PDF”
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Old 7th Aug 2018, 4:28 am   #45
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

The receiver RX1 now has quite a clean AF output for SSB.
There may be an update to the story later.

Photo 1 is a “screen save” of a spectrum plot of RX AF output, 1090 Hz, from the program Audacity.
The AF has some Harmonic Distortion which may or may not be normal. The low freq components are probably a mix of RX hum, test equipment hookup residuals, and bench hash. These components look worse than it is. They are a well down in level. Cannot hear it at all.

The free Audacity program “spectrum analyser” view is useful, probably not as good as Jeremys.

The Audacity view is with processing post recording; it is not a live view.

I can also see the AF second harmonic OK with a Siemens D2055 selective voltmeter (from the old FDM days!) monitoring the IF output at 455 KHz.


I started this “quick look” at my NRD515 as a precursor to renovating another heritage receiver, an Eddystone 1830.
Maybe soon.

For Damien
Photo 2 is a memory lane photo for you, NRD515 on top of my Eddystone 1830!
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Old 7th Aug 2018, 7:00 am   #46
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Ah, the photo of the transformer out of the set shows a classic no-no unless other precautions have been taken.

The transformer is held together by bolts through holes through the lamination stack. Part of the flux in each limb of the lams therefore cuts each bolt and there will be a small 50Hz voltage induced along each bolt, but it will be from a very low source impedance and can flow a lot of current if there is any path from one end of a bolt to the other.

This causes trouble if tthe ends of the bolts are connected together, even by contact with the topmost and bottom most lams. It can act as a shorted (partial) turn.

Good practice is to fit plastic bushes to one end of each bolt so only one end is connected.

I see that the ends of the pair of bolts away from the mounting have a lovely force spreading strap between them... also doing duty as a shorting strap! Metal washers, not insulating bushes.... would the other end of these bolts be treated the same? This isn't too bad, if the lams are well varnished and don't short one end to the other.

Much worse is the mounting end. the EMF induced along the bolts may have a path onto one mounting L-bracket, through the receiver chassis, up the other L bracket and to the other end of the bolt. The through-lams bolts ought to have insulation bushes at one end, or one L bracket ought to be insulated from the chassis.. That insulator pad on the chassis is only good if the chassis to L bracket screws are also insulated - those on one L bracket.

The problem is that the through-lams screws are hot, end versus end and can couple large circulating currents into the mechanical structure. They have no place in being used as chassis connections. One end of one being used as a safety ground is needed to make the transformer safe, but the others should be floated at one end.

Add in the fierce sensitivity of VCO tuning lines and you have trouble. A loop running 1kHz ref freq won't have much loop gain at 50Hz to fight it with.

The insulating pad looks like JRC knew about things in this area and may have cured or avoided such problems, but the people doing the mods may not have been aware.

VERY good fault finding and diagnosis! I remember how much trouble it was the first time I came across shorted transformer mounting bolts.

Another one is to beware the amount of field radiated by 50Hz fans!

David
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Old 7th Aug 2018, 10:37 am   #47
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Well, I for one have learnt a huge amount from this thread - well done all.

Nice pix of the JRC & Eddystone. See my shack pix for mine and a 990R. The latter is great for airband AM and FM broadcast, but not much else due to it's wideband filters.

Here are a couple of pix of my JRC's internals showing the transformer for comparison with yours.

The one I once used in anger was built into a console and I think run from DC.
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Old 7th Aug 2018, 12:07 pm   #48
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Not only is this the first NRD 515/Phasing thread on the Forum, it has been a very interesting read [well the bits I understand anyway] and it will be a useful resource for the future. Just to report back on the, unusual, offer I made to the Forum [in post 17] I did get exactly the right response from another Member. He has the technical background that I lack and experience of two later NRD sets. He was prepared to make the trek [and it really is a trek for him] to the South Coast and we have agreed a sale. The whole kit and caboodle will be collected in September.

I know we always like to hear about outcomes on here. Thanks again to Radio 1950 for starting things off in the first place and [inadvertently] the whole chain of events that has assisted me. I'm also grateful to anyone who has offered advice, particularly David [p20] and Damien [p11 onwards]. Getting things sorted was interesting. I'd kept the gear clean and dry for over two decades [the main units were wrapped in the Bury Times and Daily Mail from November 2002] I'd put everything together much more efficiently than I had thought and located the Rx/Tx/PSU/Spkr/Memory unit and Controller + original paperwork and the relevant Manuals immediately. I've never seen anyone refer to having BOTH the Rx and Tx before and the potential purchaser was able to point out that the linking cable has a non-standard plug at one end, making it somewhat crucial. I was about to embark on a prolonged search at both ends of the country when I thought to double check
The Tx was at the bottom of the box and under the Mic [below ancient newspaper I found that TWO leads were plugged into it-too obvious] so full steam ahead!

The new owner tells me he will begin a slow regime of examination, monitoring and restoring to function this elaborate setup. I'm hopeful he may be able to report back on that here in due course but of course, that will be up to him.
I only wish that the Widow of the original owner was still with us. She and he would have been pleased to know these classic items have found a home where they will be used and appreciated.

Dave W

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Old 7th Aug 2018, 12:22 pm   #49
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Just a thought...

That copper tape wrap on the transformer goes around the winding and inside the limbs, do it's a Faraday screen, it can't have its ends connected together else it'd be one hell of a shorted turn. So it stops capacitive coupling from the windings to its neighbourhood.

Some transformers have a copper flux leakage strap around the whole thing, over the outside of the limbs and everything. This IS soldered together and shorted-turns the flux escaping from the transformer, but not the flux within the transformer.

This whole thread is going swimmingly. There are a few areas where the 50Hz sideband business can be attacked fairly easily.

David
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Old 7th Aug 2018, 5:42 pm   #50
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

What a teriffic read this thread has been, very entertaining! Radio 1950's tenacity and clear write-up of the final cure are commendable. And that 1830 is a thing of beauty!
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Old 7th Aug 2018, 6:13 pm   #51
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

A very interesting read . There are some very knowledgeable people on here, I have a 535G and find it overall a good receiver but not without it's niggles
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Old 7th Aug 2018, 7:54 pm   #52
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

It looks like some good progress has been made

I had a quick look at the latest wav file and it shows that the 50Hz modulation level is much lower. However, there is now some 50Hz baseband 'hum' on the signal as you already spotted and this looks like it has a square wave spectrum as it looks like 50 Hz with a lot of odd order harmonics.

The PLL loop response is also different with a wider loop bandwidth and I guess this is due to your retrofit of the original loop filter parts. However, RX2 doesn't have this wider loop BW? There now looks to be some noise peaking in the RX1 loop response and the loop BW is beginning to look a bit wide with less damping. It's beginning to look quite wide wrt the 1kHz phase detector frequency and if it gets much wider or shows less damping (as N changes across the tuning range of the VCO) then it could mean the loop stability could become marginal in parts of the tuning range of the Digital VFO. The 1kHz reference sidebands also look to be lower now. However, the wider loop BW does mean the loop is now able to cancel/correct the 50Hz pickup as an error signal. The 150Hz terms are now the highest and this is because the loop can't correct this higher frequency out as an error signal. So I think a tiny part of the improvement is also due to the loop filter changes.

In terms of the 50Hz warble, it does look and sound a lot better and it would be interesting to see if the transformer arrangement in the second receiver can be improved if you get the opportunity to look at it.
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Old 7th Aug 2018, 8:41 pm   #53
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Hope this isn't too alarmist, but here's a doctored version of your plot to show what it would look like if the loop was about to go unstable at its natural frequency. Just like a regenerative receiver that was adjusted to the verge of oscillation to get both gain and selectivity, the PLL noise response peaks up to a narrow/noisy gain spike at the natural frequency of the system if the loop heads towards instability and the version below is doctored to show how a PLL looks when right on the limit of going unstable.

Yours is still some way off this but it doesn't take much in terms of a change in division ratio or a change in VCO tuning gain to rapidly hit this problem. So I'm not sure if your latest loop response is how it should be. Sometimes the combination of the VCO noise response and the choice of loop bandwidth can give a false impression of an under damped system but yours does look to me to be a bit under damped.
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Old 7th Aug 2018, 9:01 pm   #54
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

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A very interesting read . There are some very knowledgeable people on here, I have a 535G and find it overall a good receiver but not without it's niggles
I recall John Wilson doing a write-up of either the 535 or the 545 which used DSP in the 'back end' and found AM to be plagued by monkey chatter from adjacent channels. I must see if I can find it in the archives.
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Old 7th Aug 2018, 9:16 pm   #55
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

I think as a final check it would be worth taking a receiver wav file at the top and the bottom of the Digital VFO range to see if the loop response changes much across the range of the Digital VFO.

One thing in its favour is that the maximum division ratio N corresponds with the lowest VCO frequency (because of the mixer in the loop) and I'd expect the VCO tuning gain (in MHz/V) to slowly increase towards the lower end of its frequency range. So there will be some cancellation of the effect of increasing N here I think. But it's still worth having a look at both ends of the range to make sure the small amount of loop peaking doesn't worsen enough to cause a problem across the tuning range.
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Old 7th Aug 2018, 11:03 pm   #56
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

That would be most worthwhile.
Jeremy's plot of the stable version still shows a fair bit of noise peaking (the noise peak centres are a dead give-away to the loop bandwidth, and the number of dB of peak height suggest that the loop phase margin is a bit lower than I'd go for. But it's a difficult compromise. You really can't have everything you want - well not all at the same time.

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Old 8th Aug 2018, 1:20 am   #57
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

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I recall John Wilson doing a write-up of either the 535 or the 545 which used DSP in the 'back end' and found AM to be plagued by monkey chatter from adjacent channels. I must see if I can find it in the archives.
I had a 545 at one point, but can't say I noticed that AM problem. Maybe I had a later build. The DSP is quite good, given the age of the technology. I only ever use it lightly to take the edge off noise anyway.

The 545 is a very pretty radio, if we can use that term

Another one I regret selling ...

And a pix of the mess inside the 515's Kokusai filters, before repacking with some cotton wool.
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Old 8th Aug 2018, 8:05 am   #58
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Thanks very much again Jeremy,

no time for more at present. Back soon.

Damien,
In the interim.
Can you please have a look at the attached photo of my RX (in its original state, just a bit pulled apart), and check if your NRD515 has that same thin black earth wire and lug on the transformer mount screw?
You can see it from the top with maybe a torch, and with only the top cover removed.

Goes to "TP9" ("ETH") on the DC Reg PCB?
Thanks.

I found this photo from an earlier operation, changing the path of mains cable to the ON/OFF switch on the front panel.
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Old 8th Aug 2018, 11:11 am   #59
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Quote:
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Quote:
Originally Posted by Andrew2 View Post
I recall John Wilson doing a write-up of either the 535 or the 545 which used DSP in the 'back end' and found AM to be plagued by monkey chatter from adjacent channels. I must see if I can find it in the archives.
I had a 545 at one point, but can't say I noticed that AM problem. Maybe I had a later build. The DSP is quite good, given the age of the technology. I only ever use it lightly to take the edge off noise anyway.

The 545 is a very pretty radio, if we can use that term

Another one I regret selling ...

And a pix of the mess inside the 515's Kokusai filters, before repacking with some cotton wool.
OK Damien
I've got into one of my 'I must find this just to satisfy myself' moods. I haven't found JW's 545 review yet, but I did come across this:

Date: Sat, 12 May 2001 11:15:32 -0600
From: Rob & Terri Sherwood <rob@sherweng.com
Subject: Re: Prem-Rx: RE: Great prem-rx posts
A comment on the JRC 545. When it first came out, likely a pre-production unit that I tested for
Passport, the garbage that one heard tuning around a clean test signal in the plus / minus 20 kHz range
was staggering. There were hundreds of tweets and multiple hets, and an AGC that was going nuts. (By
the way, the test generator was at that time an HP 8640A, a very clean cavity tuned oscillator divided
by N to produce the HF test signal.) If the test signal was over 50 dB above the noise floor of the radio,
the garbage from the DSP was so bad I wondered how JRC could contemplate shipping the product.
After giving them a heads up, they modified something in the DSP to limit the stop-band signal so that
its artifacts were different. I am not saying significantly better, but different. Instead of hundreds of
tweets, the stopband garbage just became noise to a larger extent. To help this mess, we typically
replace the DSP protection filter at 455 kHz with an 8 kHz filter instead of the 18 kHz filter that comes
stock. Does this help plus/minus 4 kHz? No. But it does provide 40 dB additional rejection at plus /
minus 5 kHz, and does help when having to cope with an adjacent-channel signal that is stronger than
the desired signal by 50 dB or more.


As you say, later units may have been given some attention in this area. BTW, I recall an article in Radcom about re-furbing the Kokusai filters, very interesting.
__________________
Andy G1HBE.
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Old 8th Aug 2018, 1:21 pm   #60
deliverance
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Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Useful info Andy .
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