After rewinding the antenna and oscillator coils on this I fired it up but all I get is a faint noise from the speaker. B+ looks good at the 42 plate as well as the 1st and 2nd detector screens. Tried measuring the oscillator voltage on the first detector, pin 4?, but I get +18 to chassis and +25 to B-. Maybe I'm not checking this correctly or I rewound my oscillator primary the wrong direction. I know the audio section is working since touching the 2nd detector grid cap with my finger gives out a loud squeal.

Can a wrongly wound primary on the oscillator coil cause this? Always thought that a wrong way wound coil would give limited reception, mainly on the low end of the dial, but not a completely silent radio.

Jim

Yes, winding direction of the oscillator coil is important. You need positive feedback for oscillation. Try reversing one set of coil wires.

This radio uses a high value cathode resistor for first detector. Schematic I have shows 6,000 ohms. Try going across this resistor with 4.7K. This will help with oscillation.

Hi Jim,
By chance is the 'gimick' capacitor out of place?
Shown here in this post.
http://www.antiqueradios.com/forums/vie ... 6&t=189408

Looks like I'm in for another rewinding on this. Upon inspection I somehow got a cut in the secondary winding plus some overlapping of the windings that I missed

Jim

Although overlapping doesn't look so good it won't stop operation. An open will.

The open part may be fixable so I'll give it a shot. On the primary, how critical is the number of turns (i've got them counted) and the placement of the windings? Do they have to lie right next to each other, can there be some spacing (unintentional) or overlapping?

Norm, what would paralleling a 4.7K resistor across the 6K accomplish?

Secondary fixed and primary rewound the opposite way I had originally wound it. Still no sound. So what else should I look for? All grounds have been checked, resoldered, etc. All the wiring looks good as far as I can trace it out.

Paralleling cathode resistor causes the oscillator to draw more current. If the circuit is on verge of oscillating this would make it operate.

Need to have the oscillator operate. Did you reverse one set of coil wires since it was rewound this time? Number of turns on the tuned side should be close as possible. This determines tracking. The other winding is feedback. Not as critical lower cathode resistance will make up the difference if a few turns short.

Thanks for the explanation Norm. I'll try tacking in the 4.7K resistor this evening.

This thing is ticking me off. Lets start from the beginning. When I first rewound the coil I wound the secondary in a CW direction from the common point and the primary in a CCW direction. No oscillation. Attaching a 4.7K resistor across the 6K, as suggested, did nothing.

Removed the coil and found a cut in the secondary and repaired it. Rewound the primary in a CW direction, even gave it 2 extra turns, and retried. Still nothing. Tacked in the 4.7K resistor and still nothing. The "gimmick" is in place and the wiring is correct.

I do have noise/squeal, whatever, thru the speaker as I bring finger close to the grid of the 77 second detector. I just tried sending a 460KC thru the radio with the generator set on high output using a AM loop placed close to the chassis and nothing. Checking the IF transformer shows all connections/ continuity is good.

Using a transistor radio I tried checking for an oscillator with the transistor radio set around 700KC and turning the tuning cap of the Philco. Nothing. I've already gone trhu and resoldered all connections including redoing all the ground connections off of the tube socket rivots and the B+ is good. All caps and resistors have been replaced.

I got my 77 tubes from a fellow ARF'er and they checked good on my tube tester. I even tried swapping them around in the radio and that proved pointless. I did ask for a picture of the underside of the chassis which ChrisH664 kindly supplied but now that I actually pay attention to it the picture shows that all the tube sockets have been replaced with octals so who knows what the actual wiring should look like.

I'm kinda stuck here and need a good clear picture of the underside of an umolested 84 to compare with and also any advice of what to try next. YES KenG, I won't gut another one again

Probably a good idea. What do you have for cathode, screen and plate voltages on the 77 tube?

My guesses of what you should have:

P Pin 2, approx 200 V (rated 250 V max)
S Pin 3, approx 85 V (rated 100 V max) <-- edited to correct pin number typo
K Pin 5, approx 12 V (2 mA typical cathode current times 6000 ohm cathode resistor)

All voltages measured relative to ground.

If the tube doesn't have the right voltages to work as an amplifier, it certainly won't work as an oscillator. You said you had only 18 volts on the screen at one point. If that is still true there is something seriously wrong at 8 (13K resistor), 9 (0.09 uF cap), or 12 (16K resistor).

Curtis Eickerman

Curtis, pins 4 and 5 are tied together and read +14 volts to B- and +3 volts to chassis. Plkate reads at +205 volts.

Typo, sorry. The Screen is pin 3. What is the voltage there?

3 volts to the chassis says the tube is essentially turned off. The screen voltage might be the answer. The Plate voltage looks OK.

Curtis Eickerman

Screen reads +98 to B- and +86 to chassis.

That voltage is also right, but it's not adding up to a tube that isn't drawing normal cathode current.

With the radio turned off, what is the resistance from the grid cap to the chassis?

By the way, I went back to the 77 transfer curves and the best I can tell, the cathode voltage (assuming the grid is at zero volts) should probably end up at about 4.5 volts with a plate current of about 0.7 mA. The problem is that you have 3 volts which should cause almost 2 mA plate current, but it can't because only 0.5 mA is being drawn by the tube. So the numbers are not adding up unless the grid is floating which is why I asked for a resistance check between the grid and the chassis.

I am referring to the chart here http://scottbecker.net/tube/sheets/021/7/77.pdf With the grid at ground (it is at DC ground through the antenna coil), then cathode voltage will show up in the graph as negative grid voltage (0, -1, -2, -3, -4, -5). That is because grid voltage in tube data sheets is always referenced to the cathode. Since the cathode is 3 V above ground, this makes the grid appear to be -3 V relative to the cathode. Plate voltage is 205 and you can find that along the bottom. Plate current would be read from the left side. Ultimately all the numbers should put you at a single point on the graph and they don't (grid voltage and plate current don't agree with the way the tube should perform).

Curtis Eickerman

The LO runs at the tuned frequency plus the IF frequency. The lowest LO frequency would be 1000 KHz (540 + 460), the highest 2200 KHz (1740 + 460).

RRM

I hate when things change for no apparent reason. Just now I retook the voltage measurements using the cathode as a reference and got 216 volts on the plate, 91 volts on ther screen and -4.2 volts on the control grid of the first detector. So now it looks like the oscillator is running and I can pickup a faint and garbled signal at the upper end of the band.

Still however cannot get a 460KC IF signal to pass thru the radio using he high output setting of my generator and radiating the signal in with a loop antenna placed next to the IF transformer. I know the windings are good as I just rechecked them.

2nd detector readings don't look so good though. Plate is 54 volts, screen 96 volts and control grid is -.45 volts when referenced to the cathode.

Just rechecked the 2nd detector again and now the screen is reading 26 volts so I think I need to redo all of my solder connections, including the grounds again. Probably won't get back to doing this until i get back from vacation so expect about a weeks delay in any updates.

Now that set of numbers is close enough to make sense. Maybe there is a poor socket connection, or the grid cap wire in its clip isn't a good connection, and pushing/pulling things around got something touching again?What if you just directly connect the 460 KHz at the antenna? It will usually blow right through any converter without a wave trap.Might want to check resistor 16 (4 Meg) and make sure it is OK. This is a grid leak detector and is pretty dependent on that resistor with regard to the bias.Agree that there may be connection issues, but it looks like you are on the right track.

Curtis Eickerman

And if you didn't change resistor 16 out yet you might want to lower it to 2 meg. It helped my set out.

http://www.philcorepairbench.com/tips/svctip44.htm

I shelved this thing for awhile because it was really ticked me off. Brought it back out today and finally was able to get a 460KC signal through albeit rather weak. Before all I could get was squeals and whistles.

To my surprise I can now actually pickup a local station but its the only one I can get. Guess it'll take some serious tweaking to get it to play properly but its a step in the right direction. At least I know my rewind of the oscillator coil is good.