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Thread: Collins r388

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  1. #1

    Default Collins r388

    I am newly licensed, and am primarily listening for now, but have run across some problems with my uncles old R388. This was in storage for a number of years, but seems to work pretty well on the shortwave/ ham bands. I have not attempted a complete cleaning, just blown it out and wiped down the easily accessable areas. There are some oddities however, that may or may not be problems. I am unaware what the operating norms are for this radio. In order to pick up anything, it requires rf gain set at 8 or higher. The audio gain is mostly useless, as far as being able to increase intelligibility. The broadcast bands seem to be vary weak compared to the shortwave stations, particuarly 1590 KDAV one of the stronger local stations. I do not have enough experience to really make since of these symptoms and associate them with any particular stage or circuit. Anyone have suggestions on where to start?
    Thank You
    KF5HYQ

  2. #2

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    The R-388/51J-2/51J-3 really requires a higher impedance antenna than that which is supplied by a 50 ohm coaxial cable. The receiver was designed for a 300 ohm balanced feed from the antenna. When a 50 ohm coaxial fed antenna is connected there can be anywhere from about a 6 dB loss in signal strength to over a 20 dB loss in signal strength.

    Especially on the broadcast band, a single wire antenna of a random length will often produce a higher signal strength than using a tuned antenna, especially when the antenna is tuned for one of the amateur radio bands.

    Using a TV 300 ohm to 75 ohm balun often improves the sensitivity greatly. Although not a "perfect" match to 50 ohm coaxial cable the improvement in the impedance matching helps considerably.

    I have both a "civilian" 51J-2 and a military R-388. Both receivers have much improved sensitivity with TV baluns installed. Most TV baluns these days work down to below 5 MHz although, years ago, baluns only had to make it down to 54 MHz.

    Glen, K9STH

  3. #3

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    Thank You, I was under the impression that the military models were built unbalanced from the factory due to the unknown qualities of antennas that might be used. I have been using just a random wire antenna that came attached to an older radio. I will attempt this fix. Thanks again. KF5HYQ

  4. #4

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    HYQ:

    The Collins receivers that were built for use at fixed stations were designed to work with specific load impedances. In the field there might be varying impedance antennas but in a fixed application the antennas generally were permanent and therefore would be "tuned".

    Collins did have antenna tuners for use in applications such as vehicle mounted communications centers, etc. Those tuners were basically like those used by amateur radio operators these days in the fact that they "matched" varying antenna load impedances down to 50 ohms. But, at locations where receivers like the R-388 were used there was generally not even a need for such tuners.

    Remember, it would take a pretty long extension cord to run an R-388 for field operations!

    Glen, K9STH
    Technical Adviser Collins Collectors' Association

  5. #5

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    There are several web sites devoted to the R-388 and R-39x series of radios, a Google will show you.

    There are several known problems with the R-388, both by design and aging that will be covered by the above.

    I have a R-388 and the later 51J4 that Ive overhauled and performed a few mods to be more enjoyable to use. They can become great radios with a little effort.

    Carl
    KM1H

  6. #6
    Join Date
    Oct 2004
    Location
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    Posts
    12,456

    Default

    Quote Originally Posted by KF5HYQ View Post
    The audio gain is mostly useless, as far as being able to increase intelligibility.
    Are you suggesting there is "distortion" in the signal?
    73 de Charles - KC8VWM
    North American QRP CW Club #3159, SKCC# 5752

  7. #7

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    No, it was more like it just raised the volume of the natural hiss...the signal would not become louder though. The nice people at Rad-Comm here in Lubbock ended up giving me a old MFJ active swl antenna that someone has just given them. That helped a lot, it is now usable as a proper reciever, but all the ssb signals sound like one of those chipmunks from Alivin and the Chipmunks. I know that there were not really made for ssb reception, but I had heard that there were ways to manipulate the reciever to get them anyway. I guess I will have to get one of those adapters. I don't really want to go buy another radio just for ssb.

  8. #8

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    HYQ:

    To receive SSB on an unmodified R-388 involves several things, as is with any of the older receivers that do not have a product detector. First of all, turn OFF the AVC. Next, put the selectivity switch in the #2 position. Then turn DOWN the r.f. gain control and turn the audio gain/volume control up. Turn on the BFO and adjust the BFO frequency control about 15 degrees, or so, to one side of the center mark.

    Tune in an SSB signal using the r.f. gain control as the volume control. If you cannot get the signal to "sound right" turn the BFO control about 15 degrees on the other side of the center position. Lower sideband is the convention on the 160-meter, 75-meter, and 40-meter bands. Upper sideband is used on all the remaining bands and on the 60-meter band. You might have to "play around" with the exact setting of the BFO frequency but once you find the "correct" position make a note, or make a small pencil mark on the panel of the receiver so that you can come back to this position.

    For lower sideband on 75-meters and 40-meters the position of the BFO frequency will be on one side of the center position. For upper sideband the position of the BFO frequency should be the same amount on the "other" side of the center (zero beat) position.

    Now for the "fun" part! If you notice there are 2 ranges on the receiver that are marked in red on the dial. Those frequency ranges tune "backwards" from the other 1 MHz segments. This is due to the mixing scheme in the receiver. One of those "backwards" ranges just happens to be the range which covers the 160-meter band. The fact that the receiver tunes in the opposite direction on the 160-meter band inverts the sidebands. Therefore, only on the 160-meter band, tuning lower sideband actually will be in the same position of the BFO when tuning upper sideband on all of the remaining HF amateur bands. Conversely, to tune an upper sideband signal in the 160-meter range requires that the BFO be set to where it would be when receiving lower sideband on the remaining amateur radio bands.

    This same sideband inversion happens in all of the Collins "A" Line amateur band only receivers. That is, on the 75A-2, 75A-3, and 75A-4 receivers the 160-meter band is "backwards" and the sidebands are inverted from the remaining bands. The Collins 75A-1 receiver does not have the 160-meter band and therefore this inversion is not present.

    The AVC in the 51J- series and the R-388 series was not designed for SSB. In fact, the 75A-1, 75A-2, and 75A-3 receivers have the same problem. Only the 75A-4 was designed with SSB operation in mind and the AVC does work on SSB in that receiver. Therefore, you have to turn off the AVC and manually control the gain using the r.f. gain control. You may have to "ride" the r.f. gain control when different stations transmit. This is because often each station will have a significantly different signal strength.

    The "S" meter will not function with the AVC in the off position. That is, it will not work when the meter switch is in the "input" position. However, the "output" position will still give a reading.

    The reason for putting the selectivity switch in the #2 position is to reduce the bandwidth so that it more closely matches that of an SSB signal. Depending on just how well the receiver is aligned, in the #2 selectivity position the bandwidth is between 2 kHz and 2.5 kHz which, it just happens, is the ideal bandwidth for receiving an SSB signal. You can try moving the switch to either the #1 or #3 position and see if the "sound" of the signal is more to your liking. However, it has been my experience that the #2 position works best for SSB in the 51J- series, R-388 series, and the 75A-1, 75A-2, and 75A-3 receivers even though the 75A-3 has a 3.1 kHz mechanical filter. In fact, the mechanical filter that the 75A-4 came with is also 3.1 kHz. But, in today's operation, 3.1 kHz is a bit wide and reducing the bandwidth to between 2.0 kHz and 2.5 kHz works a lot better.

    Once you get the "hang" of tuning in an SSB signal you shouldn't have any more "Donald Duck" problems! Back in the "goode olde dayes", before product detectors became common in amateur radio receivers, most operators had to learn the AVC off, volume control up, and r.f. gain as the volume control in order to receive SSB stations. After a short while this became "automatic" and one didn't even think about how to tune the receiver for SSB.

    My main problem now, since I got my "new" 75A-1 receiver going, is where to put my 51J-2. I have run out of room on my main operating console and I am going to have to move the 51J-2 and put the 75A-1 in its place. If, and when, I get time to rebuild the Collins 32V-2 almost "basket case" that I obtained a number of years ago, I can "pair" the 75A-1 with the 32V-2 and put them on a separate desk. But, the 32V-2 is a VERY "long term" project and I don't expect to get it operating very soon!

    Glen, K9STH

  9. #9

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    Glen, I too have an R-388 that I recently acquired and am looking to have it completely restored. It's in very good shape considering its age. I have done a lot of reading on it to try learning more about it including the manual. It is in my opinion, your response on it is the best and most explanatory piece I have ever read concerning it. Thank you very much for that.

  10. #10

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    I am now a seasoned user of the BFO, in the way you have described, but now mission creep is starting to sink in. When I tune the BFO pitch for one operator, the other is really high or low pitched. Not sure is this is because it is not a product detector or if the frequencies and distances of each operator are messing with my reception. Another odd thing about my set is that the first filter "1" actually seems to increase rf gain, while "0" is more quiet, which is odd because there should not be a filter there. Would adding one of those ssb adapter kits fix the discrepancies between the operators? This radio is turning into my first practical lab with ham gear, but I fear I may run the risk of doing more harm than good. My soldering skill is right down there with my cricket game. non-existent, though I am working on some kits.

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