[Eric T]

Well, the fuse blew. Don't take much to burn a 1/2 amp. :/

I checked throughout for any shorts. I took the tubes out again, and it still blows the fuse. So I went ahead and disassembled the power transformer and spliced in new wires on the inside, making sure to heat shrink everything as far back as possible.

I checked all windings of the transformer with an ohmmeter, they all seem about right... at least about the same as before.
Edit:
Upon further review, I had inadvertently pushed the On/Off switch on the front (I was using the power strip's switch to cycle power). It's alive again.

Now to figure out where my sig gen cables got packed away during the move...

[Image: http://sphotos-a.xx.fbcdn.net/hphotos-as...8984_n.jpg]

I'll put the dial on once I hear some broadcast noise come through.

[Eric T]

Well, I tried to do the alignment, but there's no radio noise coming through. The audio section is working for sure, and I can see my signal generator output on my o-scope, so that's working.

I guess it's time to review all the basic radio theory that I forgot 20-odd years ago and get busy troubleshooting this beast.

[Eric T]

I appear to have a bad detector tube. All IF, audio, and even the phono amp work fine. I tried to inject modulated RF into the XXL/7A4 tube and get zip. I pull it out and you can hear stuff rattling inside it.

I talked to Jim at Vacuum Tubes, Inc. (great guy, BTW... very nice and very helpful) and had him ship one to my In-Laws house so I can finish this up while I am there this week. I need to do the alignment with the chassis in the cabinet, anyway.

Ron, thanks again for the guidance on the replacement audio output transformer. That Hammond 125C only required very minor mounting hole adjustment and fit right in place. The speaker will hurt your ears even face down on the bench.

Another bit of good news is that the power transformer barely gets warm.

Anyway, I put the dial back on and restrung the bandswitch light and dial indicator.

Here it is:

[Image: http://sphotos-a.xx.fbcdn.net/hphotos-sn...6556_n.jpg]

I may need to find a repop dial plate. The paint is flaking off of this one.

[Eric T]

Another neat thing:

The phono input transformer has an RCA jack on the input side. I am playing music off my phone on this right now with a headphone to RCA cable I had sitting around.

[Ron Ramirez]

Very good, congratulations! Hopefully that 7A4/XXL will get you going again.

Isn't that neat, how Philco provided an RCA (horrors!) jack on their phono matching transformer, that you can now use for your iPod or other MP3 player?

[Eric T]

BTW, I realized that the dial lights alone draw a half amp of current, so I changed to a 1 amp fuse. I may go up to 2 amp when I get the phono changer installed due to the extra load of two more lights and the motor.

[codefox1]

Dial lamps are rated at voltage and amperage. Fiddling around with ohm's law will give you the correct draw of the lamps. Plenty of information on this easily available here or on the web. Hint: 6.3 volts, .25 amps.

[Michael Dennis]

----->

[Eric T]

OK, major problem now.

After working well for about an hour, a full volume growl started and is constant on all bands and phono. The volume control does not affect it, the tone control makes a slight change. I pulled out the 1st audio/2nd detector 7c6 tube and the horrible sound is still there, so I suspect it's somewhere in the audio section.

I'm going to get out the o-scope and see if I can find anything, but I'm also hoping someone has an idea of what might have gone bad here.

[Eric T]

OK, there is a nasty 60 Hz saw-tooth waveform at cap #67 going in to the audio amplifier and it's getting amplified nicely by the push-pull 41s.

Link to schematic in case you're interested:

http://www.nostalgiaair.org/PagesByModel...013487.pdf

EDIT: After much poking and prodding, I still have no idea what has gone wrong. I am still very unfamiliar with vacuum tube electronics. Any help would be very appreciated.

[Eric T]

At this point, I'm guessing the new electrolytic filter caps are bad.

They test OK on my multimeter, but that's made for low voltage stuff.

I also clipped on my meter leads to check the DC output from the filter. As the power is turned on, the voltage rises to 510 VDC briefly before settling down at 260 VDC. Is this normal? Do I need to use 600V caps instead of the specified 500V caps?

[Ron Ramirez]

When you say "at cap #67," do you mean the side connected to the plate of the 7C6, or the other side connecting to one of the 41 grids and the tops of resistors (68 ) and (69)?

With no signal (radio not tuned to any stations), check the waveform at the control grid of the 7C6 (junction of cap (51) and resistor (60) ).

[Ron Ramirez]

Another thing. Input filter (electrolytic) (76) has its positive connected to one of the 5Y4G filaments and the left side of field coil (75)...but the negative should be connected to B-, not to chassis ground. In this case, it's the junction of the power transformer's high voltage secondary center tap and the left side of the 146 ohm section of the "B-C" resistor (77).

[Eric T]

Sorry, I should have been more specific.

I have a 95 volt rough sawtooth shaped 60 hz waveform at the plate of the 7C6. There is no signal at the control grid for the 7C6.

Input filter cap (76) is connected as specified, positive to one of the 5Y4G filaments at the left side of the field coil and negative to the HV secondary center tap left side of 146 ohm resistor.

[Eric T]

I think I have found the culprit.

I had replaced the Candohm resistor with 1 watt resistors. The 146 ohm section (made of a 130 and a 16 ohm resistor) is open. I guess I read the Candohm wattage chart wrong.