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Philco 71 restoration - Printable Version

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Re: Philco 71 restoration - Radioroslyn - 10-31-2011

PS
Not a great idea to run to set with just the 80 tube in it. Without the other tubes in it the power supply is running with small load on it. This can cause voltage at the filter caps to soar very high and short the input cap. Same thing can happen by running the set without the speaker plugged in.
Terry

Re: Philco 71 restoration - ipwizard - 11-01-2011

Thanks.. I found the issue. I had the resistor wired wrong or I should say I had the connection from the 80 tube and the connection to the green/white wire to the speaker wired wrong. nothing is getting hot now at full power in (115vac on variac).

The radio dosen't play though. Kind of stuck as to what to check next but I do need to check voltages yet. I am getting a lot of crackling out of the speaker but I think its the volume control it was very hard to turn and when I turned it the crackling was worse. I have taken that out and apart and cleaned it with deoxit its smooth now just need to re-install. When I touch the grid of the 44 Det Amp with a screw driver I get a buzz if I touch the screw driver the buzz is very loud. Other then that the tuning cap has no affect and touching the grids of the other tubes does nothing.

I did test all the tubes and te only one that was questionable was the 37.

Re: Philco 71 restoration - ipwizard - 11-01-2011

7estatdef Wrote:PS
Not a great idea to run to set with just the 80 tube in it. Without the other tubes in it the power supply is running with small load on it. This can cause voltage at the filter caps to soar very high and short the input cap. Same thing can happen by running the set without the speaker plugged in.
Terry


Thanks for that tip Terry, I had always though it was ok to run just the 80. I will remember that and not do that again..

Re: Philco 71 restoration - Radioroslyn - 11-01-2011

No problem. I'd check the plate and screen grid voltages on the rf amp, mixer, osc, and if amp. If all that looks good check the osc to hear if it is running. Next I'd break out the signal generator and tracer. Then you can hear where the signal is starting and stopping.
Terry 'gte

Re: Philco 71 restoration - ipwizard - 11-03-2011

I made some progress tonight. Some of the resistors I left in were actually way high in value 30-50% high. I swapped the resistors out and put the volume control back in and powered it up. At first I got nothing but that annoying crackling in the speaker which was controlable with the volume control which told me that the output was working.

I started probing around with my tracer on RF and got RF on the RF tube, OSC and IF tubes no audio anywhere. I could hear the crackling in my tracer also. Then I touched the cathode of the 36 again and I heard a loud buzz in the speaker then it started to receive stations. Maybe touching that kick started the OSC? It plays ok has lots of noise (static) and the sound completly drops out every now and then and I can then touch the grip cap of the 36 tube and it starts playing again. 36 tube gone wonky? It tested good.

When it is playing it goes dead when tuned lower then mid 900's (550 - about 940 or so) just get a strange buzzing noise.

Still havent checked voltages ill do that tomorrow. Another wierd thing I noticed when the radio was playing I could not tune another radio to hear the OSC running. It had to be running if its tuning in stations so whats up with that?

Issues still working through:
1. Crackling sound
2. Sound droping out and no audio on tubes until 36 tube pins touched
3. Lots of static when radio is playing
4. cant tune lower then ~940 on the dial.

Re: Philco 71 restoration - Radioroslyn - 11-03-2011

Me thinks your right about the osc coil. Go head and rewind the small portion at the bottom of the coil. That's the feedback winding. It's 25t of small gauge magnet wire 30 to 38g is fine. Looking from the bottom of the coil it is wound clockwise. If you wind it backwards it won't work.
GL
Terry

Re: Philco 71 restoration - codefox1 - 11-03-2011

Agree with all. Replace all the old capacitors before firing this up again lest you lose the power transformer.

Re: Philco 71 restoration - ipwizard - 11-03-2011

codefox1 Wrote:Agree with all. Replace all the old capacitors before firing this up again lest you lose the power transformer.

All the caps have been changed. I rebuilt all the bakelite blocks, and the line bypass caps are safety caps now.

Could the crackling noise be caused by the OSC coil? I can hear it when im tracing RF on the RF tubes. just trying to understand..

Re: Philco 71 restoration - Radioroslyn - 11-03-2011

Well did you rewind the coil? I suppose it may be possible that the noise is coming from the oscillator but I would be more focused on getting it to play than the noise. Sometimes once it's working these little issue go away so I tend to work on the big stuff and let the little stuff fall into place.
Terry

Re: Philco 71 restoration - ipwizard - 11-03-2011

I haven't rewound the coil yet. I was trying to understand more before I go ripping out a coil and rewinding it. Ill pull the coil and take a look at it, I have fixed a couple before by just resoldering the connections, but if thats no good ill rewind.

Thanks

Re: Philco 71 restoration - Radioroslyn - 11-03-2011

Since you gone this far while you've got the generator out put a 260kc modulated signal to the cap of the 36 tube. At this point you should be able to do the IF alignment. You may want to jumper the feedback coil, if it is open it will limit the amount of gain the 36 tube. All this will confirm if all the other stuff is working after the mixer.
Terry

Re: Philco 71 restoration - ipwizard - 11-03-2011

Is it the feed back that keeps the oscliation going?

Re: Philco 71 restoration - Radioroslyn - 11-03-2011

An osc is a amp with a small amount the output fed back to the input. Among homebrewers it all ways seems when building something that is suppose to oscillate it amplifies and vise versa.
How your set does it is a bit of the energy from the cathode is coupled inductively to the plate though the coil. With the cathode coil open two things happen. One no feedback/no oscillation. Two the 36 tube has no dc ground so it won't draw much it any current. So it won't amplify very well either.
Make sense?
Terry 'gte
PS
Short answer is yes.

Re: Philco 71 restoration - ipwizard - 11-03-2011

Makes sence..

I injected a 260kc tone into the grid cap of th 36 and I could hear it on my tracer and through the speaker. When I touched the cathode of the 36 tube the tone was pretty quiet, but when I put the probe on the plate it was louder.

I did take the OSC coil out and I noticed that the secondary of the outside coil is reading high.

According to riders the coil should read:

Primary Inner coil 5.2ohms
Primary outter coil 5.2ohms
Secondary of outter coil 15.7ohms

My coil reads:
Primary Inner coil 5.3ohms
Primary outter coil 5.4ohms
Secondary of outter coil 4.5Kohms

So judging from that guess its time to try my hand at rewinding the secondary (feedback) of the coil. The windings are discolored compared to the primary.

[Image: http://i1129.photobucket.com/albums/m501/ipwizard/Philco%2071/IMAG0743-1.jpg]

[Image: http://i1129.photobucket.com/albums/m501/ipwizard/Philco%2071/IMAG0744-1.jpg]

Re: Philco 71 restoration - ipwizard - 11-03-2011

Well Terry I tell ya you were spot on with the rewind idea. That fixed the crackling issue, and the no audio issue.

It still had lots of static not sure if its my antenna or something else. A couple strong stations come in loud and clear others come in ok but with static.

Going to keep tinkering with it.

Thanks again for talking the rewind tip.