[Masonparker]

Just recapped a Stewart-Warner A72T3 the AM portion of the radio works great.  The FM side has nothing just a low low  steady buzz.  During the initial investigation I noticed the 12H6 tube was replaced with a 6H6 tube  I have since ordered the correct tube but the radio plays great with the 6H6.   I looked into SMD but I don't see where they used silver mica in this radio. Any help would be greatly appreciated

Thanks

[Paul Philco322]

Have you tested all the tubes?

Paul

[David]

The set contains several 12BA6 tubes try moving them around and see what happens. Unless you have a extra then  sub for the one near the ant first.

[RodB]

First off, SMD typically affects AM and has popping or loud, intermittent static symptoms. (No static on FM - Steely Dan)

Double check part 85, 5 mfd 50 volt electrolytic cap. The negative must be connected to pin 3 and positive to pin 4 (B-) of the 12H6. This can be a 4.7mfd or 5mfd cap, nothing else.

If you don't have an FM generator an AM generator will do in a pinch. Just use an unmodulated 10.7MHz carrier to the FM antenna with a high impedance meter attached to 6AQ6 pin 6 to read a negative voltage. This voltage is the AGC voltage and varies depending on signal strength, which can be changed from the signal generator. If you get the AGC to vary with signal strength from the antenna the FM oscillator is probably not working. Otherwise one of the FM tubes may be bad; it's been my experience the FM RF amplifier. Double check any parts you replaced in the FM section. Do not change any of the settings until after you find and fix the culprit.

[morzh]

I am not sure how a 6H6 tube can work instead of 12H6 as the filament of 6H6 is for 6V and there should be 12V there which should quickly dispatch with the 6H6 tube.

[Masonparker]

Thanks  here is what I Have

Paul - Tested all the tubes with two different testers all tubes were OK on both testers.

David - switched the 12BA6 tubes around no change.  I don't have and extra one to substitute

Rob - I attached a PDF picture of how it's installed and I checked my pre-photos and the installation of Item 85 matches the pre-photo.  Also you will notice I used an 8 mfd.
I will order the 4.7 and 5.  Should i still move the (+) to pin 4.  Looking at the schematic the pre-photo matches the schematic.

Morzh - not sure how the 6H6 is working but it is


So the 12BA6 FM RF, 12BA6 FM 2nd IF  and the 12H6 are the tubes associated with the FM circuit, correct??

and the 12BE6

[Radioroslyn]

You really need to have a 12H6 rather than a 6H6 in your set. The 6H6 draws twice the heater current and unbalances the heater string.

[David]

Someone in the past may have worked on the series string to get the 6H6 to work. Posting a link to a old post were I posted a link to aids to war time servicing.  https://philcoradio.com/phorum/showthrea...0#pid78180

[MrFixr55]

Researched this before I saw Terry's comment, +1 on Terry's comment. The radio works without burning out the 6H6 because it is a series string and 6V is not going to make much of a difference. The bigger issue is that a 6H6 heater draws 0.3A. A 12H6 and all the other tubes in your radio draw 0.15A. So even though this is a 6V tube instead of a 12V tube, it is not heating up. Since the 12H6 is the FM Detector and not part of the AM signal path, failure of the tube to heat will definitely cause no sound for FM. AM is working because the AM section detector is the diode section of the 6AV6 2nd Detector / 1st Audio tube. Hope this helps! Good Luck!!

[Masonparker]

thanks it does help and the 12H6 should be here this week. I will let you know what happens after I install it. Also thanks for the article.

[morzh]

Terry


I understand now (after looking at the sch) why the 6H6 does not blow up - the tubes are serialized, not parallelled.

So the resistance of the 6H6 is 20 ohms (6V/0.3A) and 12H6 is 80 Ohm (12V/0.15A), so at more or less the same current the 6H6 tube instead of getting 6V will get 1/4 volts it needs. Well, a bit more as the current is raised a little. Let's sai 1/3, that is 0.1A. So the tube will be working, but really way under the specs. Plus the voltage on the rest of the filaments will be higher which will stress them a bit.


So, I am sure the radio will work that much better with 12H6 in it.

[Masonparker]

The 12H6 came today and the radio works a lot better on AM  sound is louder and the stations are stronger.  Bad news still no FM   but a have a louder buzz now.

[Radioroslyn]

>If you don't have an FM generator an AM generator will do in a pinch. Just use an unmodulated 10.7MHz carrier to the FM antenna with a high impedance meter attached to 6AQ6 pin 6 to read a negative voltage. This voltage is the AGC voltage and varies depending on signal strength, which can be changed from the signal generator. If you get the AGC to vary with signal strength from the antenna the FM oscillator is probably not working. Otherwise one of the FM tubes may be bad; it's been my experience the FM RF amplifier. Double check any parts you replaced in the FM section. Do not change any of the settings until after you find and fix the culprit.

What were the results?

[Masonparker]

It's been while since I worked on this radio.

I checked all the tubes again and they tested all ok. So I started checking all the circuits to make sure everything was correct when I noticed two plates were touching on the tuning condenser. After I straighten out the plates the FM works but only getting two stations. The two stations come in strong but only two.

I also noticed when I turn on the radio the bulb gets very bright then dims down. I measured the voltage at the bulb and its 2.9 V

Schematic is up in the very 1st post