R-441A / SRR-13A HF RX has a strange heterodyne

[Opcom]

I've got an an R-441A - AN/SRR-13A, a cool 2-30MHz receiver. No matter what station I tune to, there is always a heterodyne. It is frequency-variable from about 400-2000Hz by the "FREQUENCY VERNIER" knob but does not get to zero beat. When no station is present, there's no heterodyne. This thing works well for AM but I'm mystified about this problem. The diagram shows two BFOs, which is even more confusing, and there is no separate switch to turn them on or off.

Is anyone familiar with this unit and might offer some advice?

Pic. is of one like mine, mine is almost as nice looking and super clean inside.

[Opcom]

schematics - these are from the full 44MB manual on BAMA - -but I extracted just the SRR-13A schematics and tediously stitched the individual pages together so they are much easier to read.

[Rob K2CU]

Excellent job of stitching together the schematic sheets!  The radio is double conversion, first converting to the band 1585KHz  to 1615KHz. then to 200 KHz. The BFO  ( at about 200KHZ) appears to be powered only when either an A1 (CW) reception  position or the FSK position, and not on for AM. So, you should not hear that vernier tuning adjustable heterodyne in AM. Can you define the switch positions , mode, etc under which you find the heterodyne? It also appears that the vernier tuning of the VFO is more like a clarifier type of function to adjust the pitch for desired CW note, and not necessarily going through a zero beat position. Besides selecting the IF filter (Page 2 upper left corner) based on the reception mode, there is also an audio filter (Z1110) for the narrowest of CW modes with a frequency band of  825 to 1175 Hz. There is also the curious "Silencer" circuit which seems to be some sort of squelch, but non functioning for the first four modes (S1101A shorts out the input to the "SIL" amp). It seems that this radio does not know about SSB in the least. The medium bandwidth modes look like they are about 3KHZ wide, at least according to the filter. "A1 Medium" is probably the only one that one might used for SSB.  The issue is whether you could vernier tune the BFO to the skirts of the medium filter. It appears that the BFO is shifted for FSK mode by S1301 contact # 4.

One possibility is that there is a mechanical problem with the gang switching of the "Reception" switch. I examined the BAMA document of the mechanics of the radio.  There are couplings or right angle gearing for the switch shaft to get to the various modules that may have slipped or is not operating.  See the attached pix. I would check that this mechanism is working correctly.

There are so many switch contacts involved with the operation of the various modes that I trust you have really done a good job with the deoxit. 

Otherwise, it is a pretty neat looking radio!

[WQ9E]

The crank arms (that move the various switch decks) are a known failure item.  This site provides additional info and has links to another site detailing crank arm repair:  http://www.eclipseofthechurch.org/w7ekb/glowbugs/Military/SRR_Radios/SRR-13%20Switch%20Arms.htm

[WA2TTP Steve]

I had a similiar problem with a Hallicrafters SX-101 on 75 meters. It turned out to be IF feed thru from a broadcast station on 1650 Khz. I happened to have a MFJ preselector laying around that I put in the antenna line and that cured it. When the receiver was designed the broadcast band didn't go that high.

Good luck,
Steve

[KA3EKH]

The SRR-11, 12 and 13 are possibly some of the worst receivers that the Navy ever bought! They have a tendency to drift, have everything in little sub assemblies that are almost imposable to repair or diagnose short of replacement. And absolute worst of all is the stupid cast levers that are all over the inside of the radio. Be extremely careful if you attempt to tighten them because you can break them by just looking at them! They did have steel replacements but have only seen them once with the majority of the radios still have the stupid cast ones. Possibly one of the levers is broken and your BFO is always on? Also recall that they had some issues with the IF filter assemblies. When the receiver is working it is a good radio, the projected display is fun and it is just a little smaller and lighter then the R-390 that it was intended to replace. At one time I had a couple of the radios, the jumper cable to operate it outside its case and huge amounts of spare parts but sold or traded that stuff all off years ago, tried to block the whole SRR thing out of my mind but seeing your post caused some form of flashback to the seventies and that radio.
RF

[w3jn]

I had a similiar problem with a Hallicrafters SX-101 on 75 meters. It turned out to be IF feed thru from a broadcast station on 1650 Khz. I happened to have a MFJ preselector laying around that I put in the antenna line and that cured it. When the receiver was designed the broadcast band didn't go that high.

Good luck,
Steve

Bingo.  One way to tell for sure is to disconnect the antenna, and feed a signal from a sig gen into the radio.  If the heterodyne is still there, that's it.

[KA3EKH]

Although the SRR-13 sucks in many ways one thing it has is excellent image rejection and shielding.  Also has a band pass filter on the input to notch out BCI so unless your right next door or have a radio that’s been hacked don’t think it will be broadcast band interference. I shipped a thirteen to a friend of mine in Veracruz to use to copy international broadcast back in the eighties because all the local Mexican 50 kW broadcast stations swamped any of the off the shelf SW radios that were available at the time and the thirteen worked  great in that environment .
RF

[Opcom]

The modes in which the BFO malfunction was noticed are the A3 modes and FSK. The A1 modes have presented a challenge because SSB is completely distorted, but RTTY seems OK.

Good calls on the switch levers. Maybe the BFO power is always on for that reason. Its frequency does not change relative to reception switch position. Maybe it is stuck in one place.

The set comes easily out of the case. I do not have the service cable but I can  unscrew the rear bulkhead from the case and attach it to the chassis for tests. Hopefully I can check it further this weekend.

[W7TFO]

Pat, I had one of those RX about 20 years ago.  It had a similar problem, and it was indeed a bad lever arm.

The pot metal thing split thru the clamp portion with a hairline fracture, and was not rotating the shaft but looked OK.

I sold it off after that, as no parts were available to me without buying another complete radio.

73DG

[Opcom]

I read that article. Hopefully the set can be put right without breaking anything. The article said that the switch arms have become brittle with time. I'm no metallurgist and I do not yet understand how that could happen. I have seen a few other cast clamps or fasteners that failed after decades due to the original stress of tightening as evidenced by where they broke, so I believe it but it's still mysterious.

Once the BFO issue is either understood or fixed, I should go through the set using a signal generator and find out what else it needs. The manual is very detailed.

The silencer, which works in this specimen, has got to be the best A.M. squelch ever invented. It actually works when the signal is noisy or weak and doesn't false-activate much at all. Hard to believe its characteristics didn't find their way into every CB radio and most squelch-equipped A.M. ham gear.

The audio chain leaves something to be desired for fidelity and it even has a limiter in line so the audio can seem clipped at times. The positive side of that statement is that the annoyance of a buzzing 60Hz power line noise which plagues me at my location, especially on 75/80M, seems much reduced and the desired signal comes through well. No question it was designed for communications rather than fidelity.

The one I have does not seem to drift. Last night it was still where I left it Saturday on WWV.

As far as image rejection and BC AM overloads, I was getting some BC garbage on the 2-4MHz band. This tells me the set needs some attention.

No de-ox-id has been used yet. The set came back with some working of the controls. I think it will turn out fine once I've gone through it. Now that the weakness of the switch arms is known, it will be worth it to make sure everything moves freely to keep stress off those parts.

[Opcom]

It turns out that the front panel BFO, once aligned, can be set to zero beat the incoming AM carrier and this helped reception of bad signals. I am not sure if that was its purpose but it seems to work that way. The BFO-plate-voltage-control switch is moving but it is probably on the wrong contacts or slipping. I'll have to work on it some more.

Reference to Don's article:
http://amfone.net/Amforum/index.php?topic=27086.0

[KK4YY]

I had one of those years back. Very cool military sound/feel to it. It had an issue with the mode switch that I tracked down to a module from another variant (SRR-11, 12 ?) being installed in the radio. Don't know if it happened at a repair depot or someone just tried to cobble together a working set from hulks (I suspect the latter). Check module part numbers to be sure the right ones are in there. The modules don't all interchange although they may "fit" and the radio may "work".

Don