Not quite a boatanchor, but definitely in need of help

[N8YX]

I recently fell into (or fell over) a JRC NRD-515 receiver. One might argue it's a poor man's version of the Collins 651-S1, though these rigs always have commanded a decent price.

Mine's in need of help. The knobs look to be coated with many years worth of grud buildup, the RF band pass filter selection circuit (and possibly several band pass filter elements) aren't working properly. On the plus side, it came with the accessory filter board and the OEM 600Hz CW filter. The negative to that is the CW filter appears to not work. It's a Kokusai and their failure mode is well known - as is the fix.

Walt, PE1ABR has been helpful with troubleshooting and providing reference voltage levels in a working unit (he owns several of them). First order of business is to find out what's up with the band pass circuit, then on to the filter. I managed to find a 1000Hz JRC filter for the unit so it'll go in the Aux filter slot.

I'm tempted to mod this thing with a bunch of the more widely known improvements and a couple of my own thrown in for good measure. Such as an IF notch filter and a squelch. Cosmetically, the receiver is a very close cousin to the FT-901/902DM (albeit with black instead of gray knobs) and that's what I'll probably end up pairing it with. I had a chance to buy a complete 515 station (including the transmitter and station power supply) but that setup is a little too pricey at the moment...

[K4PIH]

Knob crud seems to be a universal thing with older gear. All Swan gear comes with knob crud. Some of it is just from crummy fingers but I also think the plastic used promotes mold growth. I take all the knobs and soak then in vinegar then use a tooth brush and antibacterial soap. I've also noticed that high humidity also promotes mold growth.

651-S1, used many in my AF years. Great radio, wish I had one.

[N8YX]

651-S1, used many in my AF years. Great radio, wish I had one.

The one I really wish I had is the 451-S1 It's the receiver counterpart to the KWM-380/HF-380 and as I understand less than 10 were made. Would make for a nice pair of twins on the operating desk.

Back to the -515 momentarily. Appears that I am going to be firing up the desoldering station...

[N8YX]

It's been a while since I started into the -515, so here's a blow-by-blow on what was found.

First, the radio appeared deaf on several bands (verified with a signal generator). The other apparent issue was the failure of the accessory CW filter (or was it? More on this later.) A dialog with Walt, PE1ABR regarding the working of IC61 (a uPB249 ROM on the Synthesizer Board) and its action on IC1 (an SN7445 8-line decoder on the RF Board which controls RF bandpass filter selection) motivated me to test IC1. Several of the decode lines appeared active (low) at once, while others never went low. Removal and replacement was in order.

[N8YX]

JRC must have used the same PC board vendor as Kenwood did, as it's very easy to lift traces while vacuum desoldering. I kept the tool to 700 degrees F but apparently this was too much - one of IC1's pads was partially lifted. Chip-Quik is on the to-get list for the next time I need to desolder anything from the boards.

A socket was fitted, a new SN7445 plugged in, DC resistance checks performed and the RF Board re-installed in the radio. Checking with a signal generator revealed that all ranges showed a meter reading of S9 with 50uV RF applied to the antenna connector. Perfect.

7445.jpg

[N8YX]

Next up was the installation of a 1KHz B/W filter onto the accessory filter board. (As mentioned in the first post, the radio had this board included along with the OEM option 600Hz CW filter.) I left the board out when performing initial RF bandpass filter circuit verification tests, though when I attempted to switch the radio's filter selector to the 2.4KHz spot the receiver went deaf.

Hmmm. That filter is present on the RF Board and even if faulty should pass SOME signal.

Re-installation of the board restored operation, though the 600Hz filter was way down in response. I started poking through the schematic, then looking at the board - both physically and with an ohmmeter. What was actually on the board - and the operation of the filter selector through its associated diode switching matrix - bore little resemblance to the schematic.

Time for an Ian Dury and the Blockheads quote: There Ain't Been Some Half-Clever Bastards. What this one apparently did was to remove some of the chokes present on the filter-select lines, install different types and reroute their outputs to other points on the schematic; this to allow the filter known as "FL4" (on the bottom of the board in the picture) to be the second filter in the selection menu. A Murata 3KHz filter was fitted at the FL4 spot when I received the unit and this had to be removed to allow installation of the 1KHz JRC crystal filter.

SSB-Filter.jpg

Note the yellow arrow. That's actually the stock 2.4KHz B/W SSB filter, which was bad. The CW (600Hz) and RTTY (1000Hz) filters are on the board to the front of the SSB filter.

[N8YX]

I never throw anything away, particularly those assemblies containing useful parts. A scrap TS-930S Signal Board yielded 3 47uH molded chokes; these were installed after the previous owner's handiwork was removed. The lower left arrow shows a stock JRC-spec choke while the upper-right part indicates the choke obtained from the '930 Signal Board:

Filter-Chokes.jpg

[N8YX]

Once the filter switching was determined to be in correct order, failure of the SSB filter (and proper operation of the others) was confirmed. This necessitated another removal of the RF Board, removal of the filter then cleaning, restoration and replacement of everything.

Kokusai filters are known for breakdown of the damping foam used inside them. This link gives all the particulars and presents the reader with a set of instructions on how to recondition one: http://jlkolb.cts.com/site/koku.htm

Sure enough, upon disassembly of mine I found a gooey mess. Careful cleaning and re-packing with closed-cell shipping foam resulted in a working filter with the specified bandwidth characteristics.

The 3KHz Murata which was removed will be used in a separate circuit built onto an accessory board; this after the receiver has been brought to factory spec with an alignment of the internal signal generation stages. At present it's hearing well - even on a 10ft piece of wire in the lab.

[N8YX]

Another R&R of the RF Board, this time to correct what may have killed the Bandpass Filter decoder IC:

Seems that JRC used two back-to-back pairs of diodes as a front-end protection circuit. Devices used were silicon; when you get more than 1.5V RF at the Antenna connector all sorts of interesting and undesirable intermod products appear in the receiver passband. Not uncommon at all if you live close to a MW broadcaster or run the radio in a multi-multi setup.

Counsel at the time was to remove these diodes. In the unit I have, someone did - and in doing so, possibly caused the 7445 bandpass filter decoder to be taken out when a nearby lightning stroke put a sufficient voltage pulse into the set.

I did a bit of research into receiver protectors and the various ways to implement one. Many still utilize a diode (or diodes) somewhere in the current path. One had an NE-2 bulb paralleled with a 1mH inductor; the inductor keeps the DC (static) potential of the antenna low while the neon bulb shorts a voltage spike to ground by way of firing. This didn't seem to be a viable solution for a couple of reasons - one of which being that the addition of the inductor pretty much killed sensitivity of the radio below 2MHz.

What I eventually settled on was a gas discharge tube in parallel with a bidirectional TransZorb. Think of a Zener diode with a very high amperage rating and the ability to conduct in either direction. The decoder IC specifies 30V max switched voltage at its outputs; the other devices (potentially) in the path of a pulse are the 1st mixers - a balanced pair of U310 JFETs. Their gate-to-source maximums are spec'd at -25V. A 13V TransZorb (right arrow) and a 75V Gas Discharge Tube (left arrow) were installed in the original vias. Should the TransZorb ever open after repeated conduction cycles, the GDT will still offer protection from large spikes - and is what Drake, Kenwood et al did in their various equipment lines. Additional measures including switching the feedlines to ground or disconnecting them altogether when storms are forecast are used, and the combination of all should keep the front end free of incident.

Protectors.jpg