The Fine Things are Always Hand Made
I have ahd a nice Phantom for 8 years and finally decided to get into it. 46 caps later, it was working quite well on the bench.
Assembled back in Acusticraft cabinet and BANG POP! Off!
I thought the 8 wire speaker cable had shorted internally, but the output transformer seems to be the issue.Tar melted a little and appears to have spewed a bit around the V C leads. SO.... anyone have a good spare?
12" Magnavox, trans number 18653.
Just to be sure, you have the 12 inch Magnavox speaker with an octal plug to fit an amp using 6V6's output tubes??? And this speaker has two field coils? Your Phantom model has 19 tubes, including the magic eye?
You are sure the fault is the output transformer mounted on the speaker frame and not a field coil?
My experience is about 50 % of that model speaker has a bad output transformer.
The voice coil is 38 ohms and no universal output transformer I know of is suitable.
All the being so, I have a resolution to modify a certain Hammond universal output transformer.
So - from my September 2016 posting to another thread. thus:
"This is my solution. I modified the 125E push-pull Hammond 15 watt transformer from Radio Daze.
Available universal output transformers max out at 15 or 16 ohms including this transformer.
This 125E universal transformer ( lug 1 to lug 6) has a turn ratio of 26:1. I needed 21:1 (measured from another good Scott transformer for this Scott speaker, driven by 6V6 tubes in push-pull). The Scott models Sixteen and earlier Phantoms use the same 12 inch speaker.
For this particular Hammond transformer I chose, there is room between iron laminations and the core to carefully add a layer of wire. After experimentation, I found 12 feet of #22 gage magnet wire connected to the #6 lug and continuing to the RIGHT of lug 6 adjusted the transformer to about the desired 21:1 turn ratio. The new magnet wire is totally exposed.
Used electrical tape on the metal to protect the wire from abrasion while winding the additional wire around the core. Wind the wire fairly tight and neat, and slip 3 or so inches of spaghetti at the end, so the end of the added wire end can be wedged between lug 1 and lug 2 to become a lug #7. Thus: lug 1 and the new lug 7 is about the ratio I wanted.
This transformer will not fit into the original bracket which also holds the speaker terminal strip. So I relocated the original bracket enough so I could mount the new transformer tightly agains the original bracket (after removing the old transformer from the old bracket).
Success - the radio sounds good and the impedance is about right."
I will try to find my photo.
hope 2 photos uploaded.
I am not 100% sure yet. I see some signs of tar around one mounting frame, and with magnifying glass, it looks a little weird around the V C leads, but that may just be age.
Resistance checks seem o.k. I ran some H V from the Eico cap checker from field windings to frame, no leakage.
H V from V C side to frame, no leakage.
No Burned smell or other usual signs. I think the next step is remove chassis, and inspect .
IF I cant locate the issue somewhere, maybe i'll try to post photos of the transformer for you all to look at my tar worries.
Thanks a bunch for the post!
And Yes it is the 12" Magnavox. I did carefully open the bell cover as far as I could and no smoke/smell there.
And my previuos statement was wrong I did not check the field winding, just the windings on the transformer.
IF the field is bad, how do i remove the bell completely ? Will the 4 wire set pull through O.K. without problems?
The 4 wires to the two field coils go through a black grommet that now may be brittle (and stuck) due to age. That grommet is in a short channel to the edge of the bell cover. Just slowly pull the bell cove straight up as you use a large screw driver blade to push the grommet down. BUT you can check both field coils by resistance readings from the octal plug, or from the terminal strip above the output transformer.
Checking resistance to speaker frame is insufficient. Unplug the speaker and take resistance readings at plug.
There are 2 separate field coils. One of them is tapped. I suggest using colored pencils to color code the diagram wires of the speaker wiring to the octal plug. Then follow the diagram as you take resistance readings, thus.
- pin 4 to pin 5 should read about 680 ohms. (the high voltage B+ field coil).
- pin 7 to pin 3 (ground) about 125 ohms (pin 7 is really a tap on the other field coil).
- pin pin 7 to pin 6 about 65 ohms (negative bias for the output tubes).
While you are at it, check the output transformer continuity. pins 1 to 8 and 1 to 2.
On the chance your measurements aren't close, then uncover the speaker terminal strip and repeat the measurements, just incase a sesistanse reading problem is due to a bad connection in the wiring.
If measurements are about right, test your tubes and/or look for problems elsewhere.
Thanks. I did those checks before you had replied.I pulled the bell off (grommet fine and still pliable), and inspected for smell, burns etc on windings .Look fine.
Since I already had the cord and transformer off, I checked the coils at the terminal strip.
Pin4-5 is 600 ohms, no short to ground with 440V on the coil, separate test, see below.
7,6 is 58 ohms, no short to ground.
7,3 is 114 ohms, no short to ground.
I used my Eico cap checker on leakage for the voltage check. Hi V but low current.
I also checked the windings individually by desoldering the common grounds. (for the HV checks)
I put a small battery across voice coil for click check. o.k.
Conclusion is speaker, and field are fine and I may have a problem elsewhere. I suppose I'll recheck the rectifier tubes and audio, then the main chassis.
I really appreciate the help. it will live again.
Good luck. Sometimes a problem is elusive. Keep at it. You will become really familiar with the Phantom. Then if you encounter a Sixteen, 14 tube Masterpiece or even the Super 12, you will feel right at home.
I found the amp and speaker are fine. Problem is in the RF stage.A resistor gets very hot and crackles so I need to find the reason. That noise gets amplified and very loud.
Impressed with the overall construction. I may bid on a Philharmonic next Saturday at an auction. We'll see.
There is probably a very leaky capacitor on the resistor. The resistor itself could be bad, I'd replace the resistor and any caps in the associated circuit.
All caps are new poly mylar 630V quality units.
After getting thrown off course, i finally found my problem. 2 resistors in the RF section were getting very hot. After disconnecting and checking things I found the RF variable cap rotor was at 3.5 ohms to ground, disconnected! Removing the capacitor cover I found 2 more waxy caps that leaked badly at about 50 Volts.Replaced, still 3.5 ohms, inspecting acp all looked fine but I blew it out with canned air and Viola! now up in hte many K ohms. A piece of shaving/solder/ wire or something caused a short, dragging voltage down and overheating the resistors.
All assembled and worked , but oscillator kept dying on high bands. Swapped the 6J5 tube with one of the driver 6J5 and it's o.k. realign and put in cabinet, again, next.
NOW my ?, the Scott manual says use tubes with spiral wound filaments on the drivers, and a couple others to reduce hum. i have hum and it's NOT filters. Any idea what the original tube brand was?
During this era Scott was using Sylvania. A green leaf was the logo on the tube.
Tom Harris said:
Any idea what the original tube brand was?