Hello again: I finished recapping this radio with all new caps except for the high frequency ones...all the paper and all the electrolytics are new. Radio works great on AM but is totally dead on FM.The selector switch contacts all check out.There was one burnt resistor a 3900 ohm from the bottom of the AM oscillator to contact #4 on the mode switch...And after I fired it up on AM the 8200 ohm 2 watt resistor that goes to contact #3 fried...I replaced it with a higher watt resistor that still gets too hot to touch, but on FM that resistor is switched out of the circuit. Can the 6BE6 AM/FM converter work on AM and not on FM? The voltages are high on the 6BE6... plate is 145v should be 135v and G2-G4 is 127v and should be 100v...the other out of range voltage is on the 6AT6 2nd. det AVC...plate should read 40 v and it reads 65 v. All the other tubes are within a few volts on way or the other. All the resistors are within specs and I've rechecked all my solder conections several times.I really need help. Should I retube the suspect tubes with all new? I do not have a tube tester.Thanks for any thing any one can help with.
-Bill

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This set uses two tubes in the FM section that are not used for AM: the 2nd IF amp (6BA6) and the Discriminator (6AL5). If either of these tubes have bad emission you will have a dead FM band.

You can swap around the 1st and 2nd IF tubes and see if that makes a difference (it will disable both bands instead of only FM if the tube that is presently in the 2nd IF socket is inop, since both AM and FM use the 1st IF amp, but only FM uses the 2nd.)

The 6AL5 is a cheap double diode that can be easily bought on eBay or one of the online used tube vendors.

Another possibility is that one of the two FM IF stage transformers has an open winding, and can't couple the signal to the next stage. You can "bug" the FM transformers (with the set unplugged) to figure out if this is the case or not.

The voltages that you quoted are a tad high but not so much that the set wouldn't work unless **something else** is very wrong.

The resistor burning up in AM mode is a mystery that you really need to solve first. It's a fault that could have serious consequences for your safety if you blow it off- replacing the burned-up original resistor with a higher-wattage one is not a whole lot different than putting a penny in a household fuse socket. And who knows what else in the set you are overloading.

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:This set uses two tubes in the FM section that are not used for AM: the 2nd IF amp (6BA6) and the Discriminator (6AL5). If either of these tubes have bad emission you will have a dead FM band.
:
:You can swap around the 1st and 2nd IF tubes and see if that makes a difference (it will disable both bands instead of only FM if the tube that is presently in the 2nd IF socket is inop, since both AM and FM use the 1st IF amp, but only FM uses the 2nd.)
:
:The 6AL5 is a cheap double diode that can be easily bought on eBay or one of the online used tube vendors.
:
:Another possibility is that one of the two FM IF stage transformers has an open winding, and can't couple the signal to the next stage. You can "bug" the FM transformers (with the set unplugged) to figure out if this is the case or not.
:
:The voltages that you quoted are a tad high but not so much that the set wouldn't work unless **something else** is very wrong.
:
:The resistor burning up in AM mode is a mystery that you really need to solve first. It's a fault that could have serious consequences for your safety if you blow it off- replacing the burned-up original resistor with a higher-wattage one is not a whole lot different than putting a penny in a household fuse socket. And who knows what else in the set you are overloading.
:
:Tom: Thanks for the feedback...I already swapped the two 6BA6 tubes with no effect and pulled a good 6AL5 out of a Crosley that works great on FM....I also did contiuity checks on all the IF transformers and the discriminator transformer. That 2 watt resistor running hot I cannot explain unless the output tube is drawing excessive current...but then the voltages on the 6V6 is almost spot on...the 6AT6 is a tad high at 65 volts on the plate when it calls for 40 volts.... could that be the culprit? Someone said that the 6BE6 can sometimes oscillate at AM frequencys but not at FM frequencys. I'm going to replace those 4 tubes...6BE6-6BA6-6BA6-6AT6 and if that does'nt do it I give up....for a while any way.
-Bill
:

::Tom: Thanks for the feedback...I already swapped the two 6BA6 tubes with no effect and pulled a good 6AL5 out of a Crosley that works great on FM....I also did contiuity checks on all the IF transformers and the discriminator transformer. That 2 watt resistor running hot I cannot explain unless the output tube is drawing excessive current...but then the voltages on the 6V6 is almost spot on...the 6AT6 is a tad high at 65 volts on the plate when it calls for 40 volts.... could that be the culprit? Someone said that the 6BE6 can sometimes oscillate at AM frequencys but not at FM frequencys. I'm going to replace those 4 tubes...6BE6-6BA6-6BA6-6AT6 and if that does'nt do it I give up....for a while any way.
:-Bill
::
I don't see how the 6AT6 could stifle FM operation- it is just the AM demodulator/1st audio amp and if it were wonky, you would have no AM.

I guess that it's possible that the mixer tube can't oscillate at the FM IF frequency. There could also be a defect in the oscillator tank circuit- open coil, shorted trimmer cap, damaged tuning cap...

:::Tom: Thanks for the feedback...I already swapped the two 6BA6 tubes with no effect and pulled a good 6AL5 out of a Crosley that works great on FM....I also did contiuity checks on all the IF transformers and the discriminator transformer. That 2 watt resistor running hot I cannot explain unless the output tube is drawing excessive current...but then the voltages on the 6V6 is almost spot on...the 6AT6 is a tad high at 65 volts on the plate when it calls for 40 volts.... could that be the culprit? Someone said that the 6BE6 can sometimes oscillate at AM frequencys but not at FM frequencys. I'm going to replace those 4 tubes...6BE6-6BA6-6BA6-6AT6 and if that does'nt do it I give up....for a while any way.
::-Bill
:::
:I don't see how the 6AT6 could stifle FM operation- it is just the AM demodulator/1st audio amp and if it were wonky, you would have no AM.
:
:I guess that it's possible that the mixer tube can't oscillate at the FM IF frequency. There could also be a defect in the oscillator tank circuit- open coil, shorted trimmer cap, damaged tuning cap...
:
Tom: Thanks again. I checked the trimmer caps for shorts and the variable capacitor, and they check out okay...The oscillator coils for FM are like #10 house wiring as far as thickness..and they are soldered solid. No way they can break. I've double and triple checked every resistor for proper value and all the caps for shorts....The only caps that I have'nt replaced are the high frequency ones that resonate the tuned circuits. All I can do with those is check for shorts. And even if that 6AT6 isn't bad it's only a $2.50 tube. I've spent days on this thing and it does get frustrating.
-Bill

::::Tom: Thanks for the feedback...I already swapped the two 6BA6 tubes with no effect and pulled a good 6AL5 out of a Crosley that works great on FM....I also did contiuity checks on all the IF transformers and the discriminator transformer. That 2 watt resistor running hot I cannot explain unless the output tube is drawing excessive current...but then the voltages on the 6V6 is almost spot on...the 6AT6 is a tad high at 65 volts on the plate when it calls for 40 volts.... could that be the culprit? Someone said that the 6BE6 can sometimes oscillate at AM frequencys but not at FM frequencys. I'm going to replace those 4 tubes...6BE6-6BA6-6BA6-6AT6 and if that does'nt do it I give up....for a while any way.
:::-Bill
::::
::I don't see how the 6AT6 could stifle FM operation- it is just the AM demodulator/1st audio amp and if it were wonky, you would have no AM.
::
::I guess that it's possible that the mixer tube can't oscillate at the FM IF frequency. There could also be a defect in the oscillator tank circuit- open coil, shorted trimmer cap, damaged tuning cap...
::
:Tom: Thanks again. I checked the trimmer caps for shorts and the variable capacitor, and they check out okay...The oscillator coils for FM are like #10 house wiring as far as thickness..and they are soldered solid. No way they can break. I've double and triple checked every resistor for proper value and all the caps for shorts....The only caps that I have'nt replaced are the high frequency ones that resonate the tuned circuits. All I can do with those is check for shorts. And even if that 6AT6 isn't bad it's only a $2.50 tube. I've spent days on this thing and it does get frustrating.
:-Bill
:
Tom: Here's a follow up.....I found a 12BE6 which is the exact same tube as the 6BE6, except for the heater voltage, and plugged it in...It glows and gets hot and it will oscillate on the AM band, but not on FM. Its emmisions are not up to what it would be at 12 volts....would that effect it on FM and cause it to not oscillate ? I'm about at the end of my rope with the time I'm spending on this thing. Thanks again.
-Bill

:
:Tom: Here's a follow up.....I found a 12BE6 which is the exact same tube as the 6BE6, except for the heater voltage, and plugged it in...It glows and gets hot and it will oscillate on the AM band, but not on FM. Its emmisions are not up to what it would be at 12 volts....would that effect it on FM and cause it to not oscillate ? I'm about at the end of my rope with the time I'm spending on this thing. Thanks again.
:-Bill
:
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No way to tell what low emission would do in any given circuit, short of saying that "it won't work right". Sounds like replacing the 6BE6 tube with a known good one is the only thing that you've got left to try.

:
::
::Tom: Here's a follow up.....I found a 12BE6 which is the exact same tube as the 6BE6, except for the heater voltage, and plugged it in...It glows and gets hot and it will oscillate on the AM band, but not on FM. Its emmisions are not up to what it would be at 12 volts....would that effect it on FM and cause it to not oscillate ? I'm about at the end of my rope with the time I'm spending on this thing. Thanks again.
::-Bill
::
:= = = = = = = =
:
:No way to tell what low emission would do in any given circuit, short of saying that "it won't work right". Sounds like replacing the 6BE6 tube with a known good one is the only thing that you've got left to try.
:
Thanks Tom: I've got new tubes on the way...If new tubes do not fix it I will just put it back in the cabinet and recap my Crosley and work on the Coronado next Winter. I'll let you know how the tubes work out. Thanks again.
-Bill