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Thread: Not quite a boatanchor, but definitely in need of help

  1. #11
    Volcano Tamer W3WN's Avatar
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    Times like this, I sure wish you took side jobs.
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  2. #12
    Master Navigator K4PIH's Avatar
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    I like the GDT use.
    "Don't put it on the plate if you can't eat it!"

  3. #13
    Administrator N8YX's Avatar
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    Got Rob's kit of parts in last week and reworked the RF Board. One of the additions was an IF buffer board in place of C291 - the AM detector coupling capacitor. This helps improve detector linearity on all modes and is especially useful if you're going to run an external detector for synchronous AM (SAM).

    I plan to monitor the receiver IF outputs with a scope, so the boards will prevent loading the IF down when one is connected.

    A few assorted eBay transactions yielded two Yaesu FRT-7700 preselectors plus some connectors. These were originally marketed for use with their FRG-7700 receiver and have two flying-lead pigtails (one for MW broadcast band and the other for SW) to connect to the parent receiver's push-terminal antenna jacks. Job One was to swap these out for an RCA male-terminated run of RG-174 (SW range) and a female-terminated version of same (BC range). Luckily, two of the preselectors side-by-side are exactly the width of an NRD-515 so I can put both on the 10-12M lineup's middle radio.

    The reason for this...as with many dual-conversion receivers, the problem of images arises. Not a big deal if you're far away from high-powered emitters, but one of the 10M Gang is right down the road and occasionally goes QRO. A passive preselector will help reduce image interference.

    So far, so good. When my order of RCA female connectors arrives I'm going to modify the NSD-NRD transceive cable and put a flying lead on the receiver side's RX Mute line so the secondary receiver can also be muted upon transmit.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  4. #14
    Administrator N8YX's Avatar
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    Another update to the ongoing repair saga:

    I'm a perfectionist when it comes to gear performance. If something isn't quite right I'll doggedly pursue it until the equipment in question is operating at its design spec. Receiver #2 (newest arrival in the lineup) worked as I'd expect one of these to operate. Prior to me acquiring the set, it was sent to Rob Sherwood for his modifications and a thorough alignment, Thus, I consider it to be the "baseline" against which the performance of any other NRD-515 should be compared - and this has been verified on the test bench.

    Running the first and the Sherwood-modded receivers side by side, I noticed that the original was still a bit deaf. Chancing across an eBay auction for another one, I got it ("for parts or not working - No Returns") for not much more than the two accessory JRC filters (included in the unit) usually fetch. It was advertised as having very low sensitivity above 16MHz.

    Never hurts to have another example to compare to, right? When I unpacked #3 and put it on the bench, I found it to be able to adequately hear a 2uV, 1KHz-modulated signal on the upper HF region but both of the Kokusai mechanical filters (2.4KHz/600Hz) included with the unit required repair. Once this was done I performed the Sherwood mods and installed one of his AM pre-detector buffer boards in the vias left by the removal of C291. Note that this is a well-documented modification, but since Rob had the boards on-hand I got a couple rather than simply rolling my own.

    After all the work was completed, the Signal Board was returned to the receiver and a full alignment performed. This is where things get a little hinky as compared to the procedure listed in the service manual. One of the major complaints about this receiver is the absolute non-linearity of its S-meter. Another is that it isn't as sensitive as compared to, say, a TS-940S or later modern rig. If you follow the service manual, you probably won't get either aspect totally right. Through a bit of empirical tinkering and adjustment, I came up with a way to get the S-meter to track fairly accurately. The big surprise came yesterday and this morning after I'd put the newest acquisition in the place of the original. It hears things that #2 doesn't. This may be due to the addition of the buffer board, but I think more of it is due to the way the IF Gain adjustments were made.

    Where does this leave #1 - the first receiver I'd acquired and restored? I applied the procedure to it and while the meter tracks accurately now, sensitivity is still "meh". I'd noticed the AGC motorboating when the set was first powered up with the RF Gain set at maximum - and I'm now of the opinion that the AGC subsystem is unstable, thus reducing sensitivity. I'm going to replace every electrolytic capacitor on the Signal Board as a starting point. #2 has had this done and is stable, though #3 appears to have the OEM capacitors on the board and works quite well.

    I'll write the process up and take a few pics of #1's adjustment points when that repair is successful - just in case this bites another JRC owner. Walter, PE1ABR suggested a few things to check but most involved the synthesizer and VCO sections of the radio - and those circuits appear to be operating properly.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  5. #15
    Master Navigator K4PIH's Avatar
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    When I desolder on sensitive boards, I use solder wick and just enough heat to make it draw.
    "Don't put it on the plate if you can't eat it!"

  6. #16
    Administrator N8YX's Avatar
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    Quote Originally Posted by K4PIH View Post
    When I desolder on sensitive boards, I use solder wick and just enough heat to make it draw.
    You need a vacuum desoldering station to take the components off anything JRC built. The vias are plated through and eyelets are present on both sides of the board. No more that 670 degrees tip temperature, either - or you'll start to lift the foils on the solder side.

    A DIP component gets into a whole 'nother level of frustration here. I highly advise cutting all the pins, removing one at a time then installing a high quality socket.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

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