[Radio1950]

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”