I have a Motorola B3E from 61. It's AM/FM and the AM works just fine, but FM I get nothing. Sometimes I'll get static throughout the band but then it will go completely silent. I plan to recap this radio, but was curious if anyone knew what could be causing this problem with only the FM band. The only schematic I have been able to find is in DJVU format here: http://techpreservation.dyndns.org/schematics/61/61_Motorola_B3W_102.djvu
Other than this there doesn't seem to be much info out there on this radio. Thanks in advance for any help.
Mike
:I have a Motorola B3E from 61. It's AM/FM and the AM works just fine, but FM I get nothing. Sometimes I'll get static throughout the band but then it will go completely silent. I plan to recap this radio, but was curious if anyone knew what could be causing this problem with only the FM band. The only schematic I have been able to find is in DJVU format here: http://techpreservation.dyndns.org/schematics/61/61_Motorola_B3W_102.djvu
I would first suspect the FM RF amplifier andd mixer tube, V1A&B. That is the only tube that is for FM only. To get AM, all of the other tubes must be working.
Lewis
:Other than this there doesn't seem to be much info out there on this radio. Thanks in advance for any help.
:
:Mike
73's de Edd
Mike
Mike
Wouldn’t you just know it…I skimped on my schema layout right to the edge of the tuner layout, so all that I might have showing
Is the 1st I.F. transformer of the system JUST barely showing.
Now, since I am the BLIND-DEAF-DUMB person in the knowledge of the units physical minutiae, how about the following details of the internals of the dedicated pot metal F.M. tuner ?
I can not positively confirm the mechanics of the design within the tuner but the pics seem to show either two slugs that are tied to a treadle such that they move into the osc and RF portions together as the mechanics of tuning across the FM band end up inserting and retracting them from within their coils. Another construction mode is the use of a closed loop of dial cord or wire such that the slugs are center imbedded around the wire / cord and then each slug moves in or out of the coils during the tuning process as the dial cord is moved.
What seems to throw me is your mentioning multiple turns of “fine” wire involved. I would certainly only expect there being but a mere few turns of wire required at the 88-100 spectrum, and the norm is for that “wire” to actually be in the form of wide and thick copper / brass banding wound around their coil form so that the RF surfactal area “skin” characteristics would be enhancing R.F. performance at the frequencies involved, as well as improved mechanical characteristics when subjected to vibration.
Their companion internal tuning slugs would be minimally requisant of permeability , with their composition being of minimal powdered iron composition and mainly, a dispersive inert filler materiel. Heck, even brass tuning slug compositions could be utilized via the utilization of a diminutive / decreased inductive tuning effect.
IF your problem actually IS those slugs mentioned, you would be hard pressed to find slugs of those minimally inductive characteristics, any substitution otherwise, would not be able to get the same identical spread in tuning across the FM dial so that the dial scale logging will still be in agreement with the received stations.
Now my final thought might be to two other things in relation to your statement:
Now IF that unit uses an external screw terminal(s) for use of an external or wire antenna connection, there actually might BE an antenna input coil made with fine wire, and if it took a lightning hit, accidentally, it certainly could do a number on that coil.
I can’t relate it to being a slug tuned coil design, however, with the previous construction being the norm.
The last thought would be the coil in question actually being the 1st I.F. transformer, which is actually contained within the tuner housing, and may be open construction and dependent upon the tuners shielding for itself , as compared to its own typical alumin-i-yum-yum housing that is typically seen on I.F.’s. A lot of those tuners were of foreign construction, and the I.F. slugs adjustment orifices were small internal square holes. Could a previous “diddler” using an ill fitting jewelers screwdriver have chewed up and cracked the slug…easy to do on a square hole inset as compared to a hex hole with its dedicated alignment tool profile. Even the conventional standard cut / molded cross slot adjustment is more trouble free from tool damage.
73's de Edd
Thanks for the help thus far. I don't know exactly what happened to that coil, but according to the schematics it is the FM antenna coil. It's hard to tell how many windings there were due to the condition of the coil. There aren't many windings at present, but there are multiple stray strands sprouting at random from what is left of the windings. The core is the square slot iron core as you mentioned. I have the correct tool for this core, but it does little good being that it is "stripped out" and disintegrating. Oddly enough everything else in the FM tuner section looks untouched. My best guess by the looks of it is that someone with limited skill and patience took a crack at making an adjustment of some kind and this is the result. Now to fix the problem: Would a spare coil from a different brand parts radio of the same era function correctly in this radio? I have an old GE laying around that I could steal it from. I haven't looked at the GE coil yet, but I would assume that it it would be similar. Thanks,
Mike
Most ‘honnable’ apologies for the dee-lay, seems like your reposting flew down the strings in a big hurry, and remained unseen by me until just now.
I pulled up a finely detailed referencing of that chassis and have found the RF front end of the unit to be utilizing a quite broadly tuned RF amplifier, which might give a bit of leeway on the degree of criticality of the damaged area.
I was certainly hoping that the problem child was the internally contained 1st 10.7 mhz IF transformer, which is a common staple and could easily be worked around.
Consult the provided reference and confirm for sure that the unit that you have damaged is in reality, the RF input antenna coil for the unit and not the 10.7 IF unit.
The next confirmation would be the checking out , both visually and ohmically, to see if there is a possibility that the secondary of the two windings might still be intact.
There is the primary portion on the left that couples in the 88-108 RF from either a 75ohm input or 300 ohm balanced line input perspective. Our critical portion is on the right where I have the A-B mark up references where that inductance along with the tube grid capacitance , is desired to be roughly resonant at 90Mhz. Minor initial alignment tweaking was intended to be accomplished with its NOW decimated internal slug.
The other point of interest would be the lower left bottom corner where the broadband RF makes its entry, the unit was primarily considering its FM band RF to be from a clamped on coupling to the AC line cord, as is shown at the right top quadrant of that info. Additionally, if in a fringe area, it was also making both 75-300 ohm inputs from an external antenna input being available. Now the thing that probably answers the nature of the failure mode on the unit is the set of Cap / Ristors that are series inserted in the RF supply line, shown at the left corner. This provides isolation of the antenna terminals for an otherwise hot chassis set, with FM RF just flying thru the 470 pf ceramic cap sections.
Also, if connected to an external antenna, and if that set took a direct / nearby lightning hit, it would have to arc over across those caps to be able to get to the primary of the antenna coil. If subjected to such, typically the coil wires , under close / magnified examination would tend to have some copper ball ended portions.
However, sounds like your unit possibly just might have had the antenna coil “ damaged” by a previous DELMER PEE NUMBNUTTZ type of character that used a jewelers screwdriver within the units micro square cored slug…I just can’t fully visualize the manner of the coil additionally getting its damage ???
Now getting down to seeing if that puppy will whine again now…without your feedback being forthcoming yet.
If you found that the secondary of the antenna being open….. that could be worked around initially by the solder tacking in of a physically small…possibly short leaded also… 50—68—75—91—100—120 ohm..(Whatcha got? )carbon composition resistor to fill in for the secondary of the antenna coil between reference points [A]---[B].
While inside the tuner housing, also route out a moderately fine gauge insulated wire that would permit its passage, externally out of the tuner housing via an existant hole in the case, one end being tacked to [A] and the outside lead free to accept a clip on test lead(s).
Reassemble the unit and then clip on 3 feet or so of wire for an external antenna input and then fire up the set and confirm that it might now be bringing in the first FM being produced by it in decades.
If it progresses to the condition of there being some FM reception, then the ant coil repair aspect can be resolved.
Thumbnail reference:
73's de Edd