Question: I have a Motorola 77XM21.
I replaced all caps, replaced bad tubes. Antenna reconnected to specs.
I haven't aligned the set yet.
AM very strong. No FM.
This is my first early AM/FM radio.
Any ideas??
Thanks!
Mark
:Question: I have a Motorola 77XM21.
:I replaced all caps, replaced bad tubes. Antenna reconnected to specs.
:I haven't aligned the set yet.
:AM very strong. No FM.
:
:This is my first early AM/FM radio.
:
:Any ideas??
:
One nice thing about the schematic shown, it does give out some voltages. First off do check over your work.
Use a magnifier glass, as sometimes it looks good till you take a BIG look at it. I see it has three 12BA6 tubes. Swap them around. If none of that is the problem, start checking voltage in the F.M. mode. I would maybe start at V-5 and around T-7.
I like your radio though. Nice swing dial, and classic looks for that vintage. And by the print, should be a very good working and sounding radio
:
:
Hows about a few preliminary analytical evaluations:
Just for signal insurance trail about 10-20 feet of hook up wire from FM antenna terminal 1 and run around the room.
Flip bandswitch between AM-FM positions ~ 10 or so times for mechanical cleaning.
You said AM was OK, so initially, lets leave in AM and take a a low loading measuring instrumentation. . . . like a digital
voltmeter and connect its main probe to pin 2 of the 12BA7 ---negative probe to ground and confirm that it is
reading its ~ negative 8 volts as referenced to the schematic. Then switch to FM and see if that bands osc negative voltage
is about half of that prior read voltage on AM .
If no reading at all, tune to the low freq end of the FM band to see if it pops in.
Nothing there ? check your screen voltage at pin 12BA7 pin 1 to see what the basic B+ supply voltage to the whole 12BA7 is.
If somewhat below that ~97 VDC... ye olde selenium rectifer . . E1 . . .may be slouching in performance with age and need
replacement with an 1N4007 diode . .and appropriate dropping resistor in series to reduce the output of that more
efficient solid state rectifier, down to the original derived voltage values on the schematic.
If that proves to be the case, and that restores FM, that is it.
BUT, should you have found the specified neg osc voltage on that grid in FM mode, then move on down to V4 12BA6
the 2ND FM IF / Limiter and monitor it and tune across the FM BAND and expect a rising and declining DC negative
voltage as a station would be tuned into and then off from.Up in the 10's and 20's of volts on strong stations.
If this proves to be the case, BUT you are hearing no audio from the stations, then we will need to evaluate the FM detection
stage. . . . .but. . . .having touched so many situations already. . . . . .
Standing by for feedback.. . . .
73's de Edd
:
:
:
:
:
:
:
:
:Hows about a few preliminary analytical evaluations:
:
:
:Just for signal insurance trail about 10-20 feet of hook up wire from FM antenna terminal 1 and run around the room.
:
:Flip bandswitch between AM-FM positions ~ 10 or so times for mechanical cleaning.
:
:You said AM was OK, so initially, lets leave in AM and take a a low loading measuring instrumentation. . . . like a digital
:voltmeter and connect its main probe to pin 2 of the 12BA7 ---negative probe to ground and confirm that it is
:reading its ~ negative 8 volts as referenced to the schematic. Then switch to FM and see if that bands osc negative voltage
: is about half of that prior read voltage on AM .
:
:If no reading at all, tune to the low freq end of the FM band to see if it pops in.
:
:Nothing there ? check your screen voltage at pin 12BA7 pin 1 to see what the basic B+ supply voltage to the whole 12BA7 is.
:
:If somewhat below that ~97 VDC... ye olde selenium rectifer . . E1 . . .may be slouching in performance with age and need
:replacement with an 1N4007 diode . .and appropriate dropping resistor in series to reduce the output of that more
: efficient solid state rectifier, down to the original derived voltage values on the schematic.
:
:If that proves to be the case, and that restores FM, that is it.
:
:
:
:BUT, should you have found the specified neg osc voltage on that grid in FM mode, then move on down to V4 12BA6
: the 2ND FM IF / Limiter . . .pin 1 . .. and monitor it and tune across the FM BAND and expect a rising and declining DC negative
:voltage as a station would be tuned into and then off from.Up in the 10's and 20's of volts on strong stations.
:
:If this proves to be the case, BUT you are hearing no audio from the stations, then we will need to evaluate the FM detection
: stage. . . . .but. . . .having touched so many situations already. . . . . .
:
:
:Standing by for feedback.. . . .
:
:
:
:73's de Edd
:
:
:
:
I'll retest the radio . I'll let you know what the reults are.
I've never seen a radio with with the 3 tunning shafts. According to the
schematic, the shafts are FM variable I-F trim, FM osc. trim and FM ant. trim
Thanks!
Mark
::Question: I have a Motorola 77XM21.
::AM very strong. No FM.
In that design, tuning of the 3 HF permeability-tuned tanks is critical. You may need to overpower it to get a starting point for adjustments. Also IIRC, the "caps" are mica washers, and even tho the whole thing is relatively low-Z, it's good to chip off any flux residue which may have gone lossy by absorbing moisture over time, especially if it bridges the mica disks.
I'm in the testing process.
Question: This radio had a resistive line-cord. The cord was spliced with a two wire cord. I cliped the two prongs of the cord to my meter.
the reading was 94. This is low.
I read when you replace the resistive line-cord with the standard two wire you should use an in-line power resistor .
Your thoughts.
Thanks!
Mark
:I'm in the testing process.
:
:Question: This radio had a resistive line-cord. The cord was spliced with a two wire cord. I cliped the two prongs of the cord to my meter.
:the reading was 94. This is low.
:
:I read when you replace the resistive line-cord with the standard two wire you should use an in-line power resistor .
:
:Your thoughts.
:
:Thanks!
:
:Mark
Check this sight under References-Tips and training.
You will find an index with ways to this.
Strange. . . .on consulting the sets schematic. . . .mine eyes are not seeing a resistive line cord being used on that set, at all.
I would think possibly that some do-it–yer selfer just might have used an old ironing cord for replacement on a radio. . . .in not having anything else at hand.
BUT that certainly would not present a 94 ohm resistive loss passing thru it.
Looking at the tube series filament string, they total up in the order of 117 volts , which would have been the norm on AC line voltage in ye olden times.
A final thought on a line cord resistance might have been its use overseas on a 220 v source, but that lines AC plug type on its end should confirm that possibility.
73's de Edd