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 Post subject: New fall project: Comet Pro
PostPosted: Nov Mon 23, 2020 12:39 am 
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Location: Wood River, Ill.
I have had this Hammarlund Comet Pro for a little while. It came with a full compliment of coils; AA thru EE plus the XX coil set. The condition is excellent, cosmetically and electrically, it appears to have had some work done. I have purchased from this seller before and had no doubt to his claims that it was functional, so was in no hurry to get it on the bench. (The bench was occupied with other work.)

I finally lugged it down the stairs to the shop and put it on the bench.
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The front panel and the case appear to have been repainted. The paint job looks very good, and some extra holes in the panel have been patched so they are not visible. The chassis is clean, both on top and bottom. Electrically, the filter caps have been changed out, and the originals, or period replacements have been cut out of circuit and left in place. The new filters were neatly placed under the chassis. When asked, the seller stated that he did not know if the receiver had been recapped, so I am guessing he did not do the work that was done. The paper/wax caps, that exist, are somewhat questionable; kind of looking like they have been restuffed. (There is no wax present on the outside.) The 4 rectangular metal cap cans, mounted on the top side of the chassis, are also questionable. The wire leads do not appear to be factory soldered in place. The two metal cans on the bottom side are also questionable. I suppose I am going to have to do some investigation on these, and there were no notes left behind by the last guy.
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The first thing I see is that the main filter caps are all supposed to be 8 mf, and two are 47 mf, with the 3rd being 22 mf. I am thinking the input, to the filter should be the original 8 mf. According to my RCA tube manual, the input should be a 4mf. I am seeing 303 VDC at the input to the filter and 241 VDC at the plate of the 2A5. I could see using something a little larger than the original 8 mf, but 47 mf seems very high.

The manual also indicates that the speaker output transformer is looking for a 4000 ohm speaker. I am wondering what others use for a match between the speaker and the 4000 ohm secondary on the output transformer. I have used 70v to VC transformers on similar receivers in the past, and am just wondering what others are using. I do not have an original speaker for this unit.

My first impressions: Tuning is different! This may take some getting used to. I found an manual on line with the tuning charts, so will begin doing some learning here. Otherwise, I am getting good strong signals, at least on the BC bands with coil set DD.

Thanks for looking.

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Mon 23, 2020 6:16 am 
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Location: Santa Cruz, California
I have one of these. It's a slightly earlier version than yours in that it doesn't have whatever those two knobs on the far right of your set's front panel controls. The unusual tuning with these is a common remark.

For a speaker, I use the one that's a match to my circa '35 National HRO. The originals were 10" Magnavox units with an oversized horseshoe magnet in a cube cabinet and a plain circular opening. They seem to be very rare and probably sell on Ebay for more than the radio. If I was in your shoes I'd probably look for any late 20s table top set that had a standalone permanent magnet speaker and use that for now. Think matching speakers from Atwater Kent 55's, Radiola 17/18's, Crosley Musicone's, etc. Any impedance mismatch shouldn't make for much of a difference.

The late Alan Douglas once answered a question I had regarding these speakers on here as well. It might still be archived if you do a search.

You have a nice looking example. I've seen so many that have been butchered in the past.

Chris


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Mon 23, 2020 2:05 pm 
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Hi Chris, a great looking NEW toy, congratulations.

That's a very nice looking radio, and it works to boot. I've never seen a comet pro in the flesh, so to speak, and it would be nice to see one of those in person.

You mentioned coil sets, are those the two large cans behind the dials? What is it that is odd about the "tuning" of these receivers?

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Mon 23, 2020 2:35 pm 
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Location: Morris Plains, N.J. 07950
These are neat receivers, Chris. They're historically significant because they were one of the first, if not the first, commercially available superhets designed for SW use. Here are a few things I learned (some the hard way):

1. It's a good idea to reduce input voltage with a variac. On one of mine, the power transformer smoked after the set was recapped. I suspect the problem was modern line voltage.

2. Hammarlund made several variations, so it's important to make sure you have the right manual and schematic. The things to watch for are tube lineup, the front-panel controls and features (e.g., presence or absence of AVC and Crystal Filter). Schematics are available in Rider's, on BAMA and on Al Klase's page.
http://www.skywaves.ar88.net/commrx/Hammarlumd/Comet/Comet_Pro.html
Match what you have with the appropriate manual/schematic.

3. The rectangular multi-cap cans on top of the chassis are wired strangely. I forget the details, but I recall I had to study the schematic to get the connections right. There's plenty of room to put replacements under the chassis.

4. As for the speaker, I believe different variations had different speaker requirements, so the correct schematic/manual will tell you what you need.

5. As for the tuning scale, I got a little notebook, and I recorded the settings for landmarks on each band. That way, tuning is simply extrapolating between known points. It seems awkward, but don't forget that the people who used these sets when they were new had cut their teeth on the 0-100 scales on the three-dial TRFs of the 1920s.


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Tue 24, 2020 3:28 am 
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Location: Wood River, Ill.
Thank You Mike for your kind comments. Thank You Joe for your information and advice.

Mike, it is a three dialer...sort of. The dial left of center sets the oscillator, the center dial is the band spread, and the third is the WL (wave length, I think.) Obviously I have a lot to learn. (But that's the fun part for me.) As I understand it, the center dial selects the frequency within the band width of the coil set in use. The oscillator is adjusted to bring in the station, and can be adjusted to place the IF above or below the station frequency. The WL dial fine tunes it, as far as I can tell. I have not operated it for very long, as I am not sure of the state of the capacitors, at this time, and don't want to push it too far. I am sure I will know more later, once I feel comfortable with it.
This is what is under the shield cans:
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And one of the two coils:
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Joe: I am operating it on a variac, set to 110v. I will be putting together a bucking transformer one of these days to run these old rigs on. I am sort of used to a three dial TRF, after cutting my teeth on an Atwater Kent earlier this year. I find this a little easier to use, but have not quite figured out the WL dial, as you can see in my reply to Mike, above. This one appears to follow the November, 1933 version, as far as I can tell. It has the crystal filter, but does not have the AVC option. The AF tube is the 2A5, but I am still working my way thru the schematic and wiring. I am a little concerned with the high value of the filter caps; being too high. I think I am going to change them back to the original values, or at least the input filter.

I have not yet decided how I am going to treat the capacitor cans on top of the chassis. I suspect they may have been re-stuffed with new components, but I want to see what is in them and their condition.

Regarding speakers, it appears that it requires a transformer with a 4000 ohm primary. So far, I have been using a 70 volt constant voltage matching transformer with a 3.2 ohm speaker, here in the shop. It appears to be working well. I found a set of tuning charts that covers most of the coils that I have, as soon as I figure them out. I will set up a logging chart for this guy, much in the same way as I have done with the old AK model 20.

Thank You for the link to the other site. That is one I have not run across yet.

I have to say that one of the things I really enjoy is researching and reading about some of this old technology and the means,by which, they worked thru the issues and problems they ran across as the technology bloomed, back in the golden age of radio.

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Tue 24, 2020 10:23 am 
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Location: Puyallup, WA 98371
Chris,
That's a very clean example! I'm envious of your collection of coils. Yours is a later production model- it doesn't look like it has the BFO pitch control under the cabinet lid. "Later" is a relative term- these are all pretty early. Sounds like you know what you're doing, running it on 110 and all. I'd check the rest of the caps at your earliest convenience, as I suspect you will :)

I have a nice one too, but it's too well preserved. Totally original and "unmolested". I haven't been able to bring myself to electrically restore it. Whether I re-stuff or not, I'd still be altering it, and as they say, it's only original once. Fortunately I have quite a backlog of radios that are also of great interest to me, but haven't led such a sheltered life...

Thanks for sharing yours here.

Dale


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Tue 24, 2020 4:23 pm 
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The Comet Pro truly has character and I love how the original instruction manual refers to its ability to track the HFO on either side of the frequency to avoid image issues.

Although the National SW-3 is a very capable receiver, I bet the early ham ops and SWLs who upgraded to this early superhet were amazed at the relative ease of tuning compared to managing even the better regens of the time.

Bill Orr provides a nice explanation and history of this receiver in Ham Radio Horizons of December 1978 starting on page 42 and available here: https://worldradiohistory.com/Archive-D ... 2-1978.pdf

Rodger WQ9E


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Wed 25, 2020 11:35 pm 
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Hi Dale,

I debated whether or not to restore this radio until I opened it up and saw that the filter caps had been replaced under the chassis. Finding that they had, I figured it was no longer original. Just acquiring the radio is not enough for me. It is the workings and operation of the old technology that interest me the most. I just don't want to do any harm to what is already there.

Although the BFO tuning rod is not visible in my photos, it is located on top of the BFO transformer. This unit uses the 2A5 output tube. I believe they refer to this as the 3rd version of the Comet Pro line.

Hi Rodger,

Thanks for the link to the Bill Orr article. It was an interesting read, which I will go through again. The article, along with the owner's manual I have, will help to understand the tuning of this receiver.

I have found, what I think are two interesting components under the chassis. I believe these are some form of capacitor. There are two brass strips riveted to a phenolic plate. Is this some early form of the "gimik" capacitor that Hallicrafters and others used? They are not both the same. On one, the two brass strips are "L" shaped, and on the other, they are just two flat brass strips. I am asking out of curiosity.



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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Wed 25, 2020 11:49 pm 
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They look like an attempt to make a low value fixed capacitor. A gimmick capacitor, however, is variable - adjustable by twisting or untwisting the two wires.

-Chuck


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Thu 26, 2020 7:07 pm 
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I found another one of the low value capacitors under the BFO can. So far one, coming off the oscillator plate to the "WG" coil, has a marked value of .0000006 Mfd, another is coming from the BFO plate to the 2nd IF primary, and it's value is not listed. I just find them interesting, as I have not seen caps like these before.

I changed out the 47 MFD filter caps that a previous owner had installed, and put in the original 8MFD caps. I thought the B+ would go down, with that change, but it came up about 6 volts. :roll: I was expecting the opposite. Things are still running cool, though. I am not seeing any hot spots, yet. I am still powering thru the variac, set for 110 volts.

I hope everyone has a good Thanksgiving.

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Thu 26, 2020 9:09 pm 
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Here's my Comet Pro, from a 2018 Silicon Valley flea market. Came with all coils other than the highest frequency set. This one has had some cobbling on the front panel with added switches and terminals but the price was right.

It is in the queue for restoration at the moment.


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Thu 26, 2020 9:58 pm 
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I just found this article. Appendix A deals with the Comet Pro and has some suggestions for restuffing the caps.

http://www.radioblvd.com/hammarlund_super-pro-part3.htm


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Fri 27, 2020 3:30 am 
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Thanks, and same to you and your's, stay well.

I'd guess it draws a hundred watts or so, which would make a bucking transformer "in the middle" of a short power cord. An easy accessory that looks something like this.

Image

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Nov Fri 27, 2020 8:37 pm 
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Joe, Thank You for the link. I have had that site on my favorites list, and have looked at the article before, but had not gotten that far in my reading.

Mike, Thank You for the suggestion. I have a couple of transformers on hand that I was going to build into bucking transformers, but have not yet found a suitable chassis or enclosure to put them in. It would be nice if someone built a bucking transformer similar to the isolation transformer you have pictured. I have a autotransformer, with selectable taps, that I picked up at a hamfest a few years ago. It is a Crestrol CT-1 Line Voltage Regulator and subsequent internet searches have yielded nothing. I suspect that it was a close out that one of the vendors was selling. It works well with my old Zenith console.

I have been experimenting with various filter caps. Using an original as specified 8 MFD in each of the filter positions yielded higher voltages than expected, and more hum in the audio than I would like. I ended up keeping the 8 MFD at the input to the filter, and a 20 MFD in the 2nd, and an 8 MFD in the 3rd. I installed a 10 MFD in the bias supply for the 2A5. Doing so yielded voltages on the plate, and grid of the 2A5 that are almost exactly what the RCA tube manual calls for. Having nothing else to go by, I thought that this should be close enough. I am measuring 305 VDC at the filter input. I hope this is satisfactory.

I pulled one of the "bathtub" caps and found that it has been re-stuffed with new capacitors. Looking closely at the capacitor cans mounted on the chassis top side, it looks like they have been re-stuffed, as well. The wire coming from them appears to be of modern manufacture, cloth insulated, with a plastic inner insulation. I don't think I will walk away from this one, while operating, though. I removed the 1.5 amp fuse that was in the fuse holder and replaced with a 3/4 amp fuse. The current draw, while operating, is right around a half amp, perhaps a little more. I may have fused it too tightly, but a fuse is cheap insurance.

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Dec Wed 02, 2020 1:48 am 
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Joe Connor wrote:
These are neat receivers, Chris. They're historically significant because they were one of the first, if not the first, commercially available superhets designed for SW use. Here are a few things I learned (some the hard way):

3. The rectangular multi-cap cans on top of the chassis are wired strangely. I forget the details, but I recall I had to study the schematic to get the connections right. There's plenty of room to put replacements under the chassis.


I'll second that statement. Mine was an earlier version and those caps were a challenge, I don't remember the specifics but the way the caps are wired (3 in each one I think) is NOT the way one would assume. You have to pay very careful attention to what is happening in the schematic.

When you asked the question I knew there was some sort of challenge involving caps, and when I saw the picture it came back.

Interesting radio. As I recall I used a small PM speaker with an xfrmr. The AK 55 speaker has a field coil and will not work.

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Dec Wed 02, 2020 1:22 pm 
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About everyone loves the Comet Pro.

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Dec Wed 02, 2020 3:33 pm 
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N9whh wrote:
Hi Dale,



I have found, what I think are two interesting components under the chassis. I believe these are some form of capacitor. There are two brass strips riveted to a phenolic plate. Is this some early form of the "gimik" capacitor that Hallicrafters and others used? They are not both the same. On one, the two brass strips are "L" shaped, and on the other, they are just two flat brass strips. I am asking out of curiosity.



Those are mica capacitors. You should be able to determine their values from the schematic.

Pete

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Dec Thu 03, 2020 4:29 am 
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I worked from home today, and figured it would be a good time to give it a good tryout over an extended period of time. Once tuned in on the BC band on the EE coil set (250-550 m), I set it to one of our St. Louis stations that I frequently listen to. I listened all day without touching the tuning, only reducing the RF gain for phone calls etc. It played virtually all day without any attention, on my part, playing for roughly 9-10 hours. I had no issues, nothing got overly warm, no drift, etc. It behaved itself very well!

Joe: I can see where the canned caps on top of the chassis might be a challenge, indeed. I believe that the previous owner may have already re-stuffed them. I pulled the two under the chassis, and they had been. The wire leads going into the caps has been replaced, so without seeing any evidence of a problem, I decided to leave well enough alone.

I am sitting here listening to WSM, out of Nashville, drift in and out, (mostly in). I have been logging stations on the two BC coils (DD & EE), and am getting the hang of tuning. It is different! :D Of course it doesn't hurt to have a digital radio handy to figure out what I am listening to, either.

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 Post subject: Re: New fall project: Comet Pro
PostPosted: Dec Thu 03, 2020 4:38 am 
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I am glad that it is working well for you Chris and the slightly different tuning system will soon be second nature to you. The Comet Pro is a surprisingly good performer for such an early superhet that covers a very wide frequency range. As expected, image rejection is poor (and basically non-existent on the highest range) but as noted in the operating instructions you can track the oscillator on either side to get rid of a troublesome image response. Or you can "cheat" and use an external preselector to tame the images.

Personally I like running these receivers as designed to experience what they were like from the factory and image response is part of their personality. Even with the new issue of image response, it was still a huge step over the simple regens that many hams were using. You could tune across the range without having to ride the "regen throttle" and you don't have to worry about the bandwidth/gain trade-off in choosing the regen setting.

Rodger WQ9E


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 Post subject: Re: New fall project: Comet Pro
PostPosted: Dec Fri 04, 2020 5:07 pm 
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I don't think there is much more that I can do with this receiver, other than hook it up to a real antenna and play with it for a while. I just placed an order with McMaster Carr for fasteners, and included some black oxide screws to replace the bright plated screws the previous owner installed on the front panel.

As Rodger suggests much of the fun of owning and working on one of these old rigs is experiencing the features and nuances of these older rigs and learning how they worked with them back in the day. It is equivalent to my Model A Ford, which I keep as original as possible, because much of the fun of driving such a machine is the experience itself. Now I don't drive it on the interstate, at night, or in heavy traffic, but I do drive it. With it, I did cheat a little, by installing turn signals, because most people don't know hand signals...many people think I am waving at them when I do use them. :roll:

I will see how it does once I get it up in the shack and get connected to an antenna.

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