I posted this on the other forum but I want to also put it here to get as much exposure as possible.
This A.B. chassis has 4 bands; 3 SW and BC. It has a band switch shaft which has a round wheel on it (like a pulley);Y on picture. It connects to a small color wheel (X on picture) via a belt. When the band switch is rotated to a different band, it is supposed to turn the color wheel so the appropriate color shows up in a small window on the dial (Z on 2nd picture). For example, when the band switch is tuned to the "red" band (6-18 MHz), red is supposed to be displayed in the small window. As you can see in the picture of the dial, it is mostly red but some blue is showing. This is a good example; sometimes one color shows, sometimes two colors show, and sometimes they are completely "off" from the correct band because of slippage.
I believe originally there was a metal braided belt of some kind which controlled this wheel. However, it is now missing and I'm trying to figure out some way to replace it.
I have tried a rubber O ring, highly tensioned, but that has not proven to be successful. The lower band switch "pulley" is smooth brass and the color wheel is mounted on a hub which is also smooth metal. The O ring simply won't "grab" either so that it turns correctly. I have put some rosin solution on it trying to help its "grabbing power" but that was also futile.
I am wondering if any one has a suggestion what might be a possible solution. I have considered trying to super glue some sandpaper to the color wheel hub in order to gain some traction. But this is impossible to do on the lower (band switch) end of it because it is so narrow---just wide enough to accept a small diameter O ring. So it would continue to slip there even if the sandpaper idea worked on the top end.
Any ideas?
(This post was last modified: 06-25-2013, 02:41 PM by Art Hoch.)
Art, is there any chance of using a double wrap of the dial string on each pulley to increase the friction? I assume there is some spring on the line to hold tension?
Jerry
The bottom wheel is not a pulley; simply a stationary round wheel with concave grooves on it. It only turns slightly at the same time the band switch knob/shaft is turned to a different band. I don't know if the original setup had any spring or not. But it would seem to need one if regular dial string is used. That's an idea anyway.
Art, with that little movement, sounds like you need something stiffer than just dial cord. Without a tensioning spring on the dial cord, not much of a chance of it working. Probably more drag than just turning a tuning cap and with the slight movement, you need some real tension, no lag or stretching of the cord. Try a stiff spring on the line. Interesting problem, not knowing what was there originally.
Jerry
I will have a look at the band indicator on my Northern Electric chassis downstairs, it's basically an American Bosch clone. I haven't really looked at it in years but the dial is very similar in layout to your set.
Regards
Arran
I wonder if the drive belt was actually a phosphor bronze band like the ones used on early Atwater Kent radios. Is there a hole in the drive surface of the pulley to fasten the belt with a screw? There seems to be some sort of hole visible in the photo.
Here is a source for the Atwater Kent belts. Perhaps one of these could be modified or used for the purpose?
No, there is not a hole in the color wheel hub but there is what I would call a small round "tit" sticking up. It would seem that whatever kind of a belt was used, it would have had an end loop that fastened on to it ---maybe slipped (hooked) over it---somehow. But it would also seem as though a loop would have come off as the wheel rotated through its cycle. Perhaps it was wrapped around the hub a couple of times as dial strings often are wrapped around a shaft. Just don't know!!
I checked my Northern Electric chassis, the American Bosch clone, and while it isn't exactly the same, three bands instead of four, it uses a simialr sort of band indicator. They used a length of heavy duty (thick) dial cord that was wrapped around the hub of the indicator and a pulley of some sort on the band switch. I didn't pull the chassis out but there does not seem to be a provision for using a belt on mine, no tensioner pulley or groove to speak of.
Regards
Arran
don't have one to look at, but any mechanism of yore that uses a dial string has a spring to allow for eventual stretching. Several turns of the string were usually needed on the small turning shaft, and yes, roughing it up a bit with fine abrasive after removing all grease helps a lot with traction.
Putting this on hold until I receive definitive information/pictures/manual telling me what total parts were used. I cannot see where one cable and one spring will operate this wheel. Guess it will have to suffer a partial fix for a while. I wish somebody had a Bosch service manual to share!!!
(This post was last modified: 07-09-2013, 07:23 PM by Art Hoch.)
Flash!!!! Got the band switch connected to the color wheel. Scratched out some of my hair I think. I finally got some bronze cable and figured out the directions/turns to make it turn correctly. The spring is pretty "heavy duty." Perhaps you can see. That's the good news.
The bad news--In this long process I killed the radio. Can't get it to play on any of the 4 bands like it was before I started on this part of it. All I'm getting is a loud hum when touching the grid on the 6F5 tube. *Sigh* Otherwise silent. Back to the drawing board.
On my very similar Northern Electric Bosch clone chassis they used a heavy duty dial cord in much the same way as the bronze cable you used, it looked like it wrapped around the set screw on the hub of the coulored wheel. As to why the set stopped working I don't think it's related to the new cable you added unless the cable is physically stopping the band switch from hitting the detant for each band position. The only thing I can figure is that you may have disturbed something like a grid cap or a component lead about to go, that is unless the set hasn't been overhauled yet.
Regards
Arran
Aaran.
No, I agree. Don't think the cabling has anything to do with the set dying. I believe that it used a metal cable originally to change the color wheel. I have been checking continuity and just haven't found it yet. Swapped out tubes so it's not that either. Just need to keep on keepin' on!
I think you will be very happy with this set, as I said Northern Electric produced a clone of most of the Bosch models in the mid 1930s and they seem to be decently designed. My chassis has a replacement Hammond power transformer, which may have been a weak point in the N.E versions, but it's very similar to yours except for the band count, three verses four bands, and mine has a shadowmeter.
Regards
Arran
(This post was last modified: 07-22-2013, 09:04 PM by Arran.)
Arran,
I'm already happy with the set. It's been playing for several years. Just don't know what happened to it while I was working on it. I was being gentle but perhaps not gentle enough!
