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Model 48-808, C201 looks like a resistor - Printable Version

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RE: Model 48-808, C201 looks like a resistor - jzeferino - 03-10-2020

I did get it, thanks!

Just need to find a way to secure it on the chassis as it is longer than the original.

   

I still have to use the surviving part of R100, so a little fabrication is due.

Kind regards.
Joao

RE: Model 48-808, C201 looks like a resistor - jzeferino - 03-10-2020

It's been some time but there has been some progress.

   

I left the capacitor can undisturbed and moved the 4 electrolytics to other locations. I also tried to leave original components as much as I could but they just crumbled apart as soon as I touched them.

Even micas had to go.

   

Now most tubes have broken filaments. Can someone recommend suppliers for 35Y4, 50A5, 14B6 and 14Q7, preferably tested?

Thanks a lot.
Joao

RE: Model 48-808, C201 looks like a resistor - mikethedruid - 03-10-2020

I have actually had very good luck buying tubes from eBay. That's where I buy most of mine. Most of the listings will say if they have been tested, and even give the type of tester, and the results.

RE: Model 48-808, C201 looks like a resistor - Stall-U-Rated - 03-11-2020

findatube.com is my go to place.. Icon_biggrin

Steve

RE: Model 48-808, C201 looks like a resistor - jzeferino - 05-20-2020

Hello! Hope everyone is OK.

So the tubes finally arrived. 

The set turns on but hardly produces any sound at all. The tuned station can be heard but it's very, very faint.

Voltages are similar to the ones listed in the schematic except for 50A5's pin 7. It should be negative but I read about 8V DC.

   

Also, 50A5's pin 6 should be 0V DC but I read 3V DC. Every component had to be changed as the originals crumbled away. I did test every component before installing, but who knows...

Any help appreciated.

Thanks.
JZ

RE: Model 48-808, C201 looks like a resistor - Radioroslyn - 05-20-2020

Hi JZ,
Your fine. The cathode is going to be positive when measuring from the b- rail (- probe on the - rail). Note the ecap there if it was -on the cathode the cap wouldn't like it at all! If you connect the meter - lead to the cathode and the + to the control grid you will see the - bias voltage (the -6 or so).
If you touch the grid pin of the 14B6 the spkr should buzz like mad. If it does I would go on to the alignment. If not you may want to check the spkr, the voice coil maybe rusted to the metal pole inside. This makes cone pretty much frozen in place.

Something that looks that good can't have much wrong!!!!

RE: Model 48-808, C201 looks like a resistor - jzeferino - 05-20-2020

Hi, Terry. Thanks for the fast reply.

It looks like the speaker has had better days, so I'll start from there.

   

The output transformer has continuity on both sides. If I'd want to use a 4 ohm speaker to test, would I replace the current speaker on these leads?

   

Thanks again.
JZ

RE: Model 48-808, C201 looks like a resistor - jzeferino - 05-22-2020

Just an update, I used an output transformer and speaker from another Philco waiting its turn to be restored and the issue persists.

Sending a 600Hz sine signal through the output transformer makes the speaker sing so I'll be looking for issues from the aerial onwards.

Thanks.

JZ

RE: Model 48-808, C201 looks like a resistor - jzeferino - 06-05-2020

This one baffles me.

The set does demodulate the input signal and plays back a tone, but it's exactly twice the original frequency.

For example, if the original pre-modulated input is a sine wave at 600Hz, the result is also a sine wave but at 1200Hz.

Most passive components have been changed by now, and the result is the same even with other tubes. Right now I have C405 reading 75 pF instead of 56 pF as per the schematic. Should it be changed?

Also, the "canned" R and C components at the 2nd IF transformer have not yet been checked or replaced. 

   

Any comments or remarks appreciated.

All the best.
JZ

RE: Model 48-808, C201 looks like a resistor - Radioroslyn - 06-05-2020

Hi JZ,
>For example, if the original pre-modulated input is a sine wave at 600Hz, the result is also a sine wave but at 1200Hz.

Where are you injecting this 600cy signal? How do you know that it's going in @ 600cys and coming out @1200cys?
Are you using a scope to determine the frequency? Does the signal SOUND like 1200cys coming from the speaker?

RE: Model 48-808, C201 looks like a resistor - jzeferino - 06-05-2020

Hi, Terry.

This happens while trying the AM alignment. I'm using a 600Hz sine wave to modulate a 455kHz carrier at 30%. The input goes in the tuning capacitor stator, and the scope is connected to the speaker leads.

I was expecting to get the same 600Hz sine wave at the speaker terminals. The sine wave is there, but at 1200Hz. The pitch sounds much higher.

Same result with a dummy load replacing the speaker.

Trying the same method on other radios produces the expected results, so it shouldn't be related to the signal generator or scope.

Thanks
JZ

RE: Model 48-808, C201 looks like a resistor - Radioroslyn - 06-05-2020

I can't think of anything that would cause the modulating frequency to shift to double it's original.
A fault in the modulator or some sort of odd mixing of the mains frequency and the modulator. Have
you tried the adjust the IF transformers? Perhaps the IF stage is oscillating.
I would ignore it for now and see if aligning the IF makes it go away. Or maybe it's trousers are just
toooooo tight.[Image: https://philcoradio.com/phorum/images/smilies/icon_lol.gif]

RE: Model 48-808, C201 looks like a resistor - jzeferino - 06-08-2020

Icon_lol Icon_lol

Went ahead and aligned it. It works!  Music

But I can't get my mind off the frequency doubling. If I just set the signal generator to output a sine at 600 Hz and plug that to a standalone speaker I get the correspondent low pitch sound and I can confirm the frequency using the scope. If I use that same sine wave to modulate a carrier (say 455kHz, IF of this set), not only the scope reads a 1,2MHz sine at speaker terminals but the sound the set plays has a much higher pitch. I'm sure this will have an interesting explanation.

On to the final details:

Thanks!

JZ

RE: Model 48-808, C201 looks like a resistor - Radioroslyn - 06-08-2020

Hi JZ
Glad to hear you got it working again. As for the dial cord you might try look in a sewing shop for some thick thread. Here in the States you can get dial cord but I don't know if it makes sense to pay the shipping over to Portugal.
The speaker is another issue as the modern ones have such a large magnets it's difficult to mount the output transformer. I think fabrication is going to be the way to go. I'm not sure if the magnet will effect the electron flow inside the tube if placed close enough. You can try ebay but it's hard to say what you'll get in the end.
GL.

RE: Model 48-808, C201 looks like a resistor - jzeferino - 09-05-2020

Hello, everyone.

It's done!

   

Regarding the previous issues:

1- About the frequency differences, yet another problem between the chair and the signal generator keypad. The AM modulated signal at the antenna didn't really show the expected pattern on the scope. It turned out I was swapping sine wave frequency and modulating frequency inputs... All good now, set properly aligned.

2- Dial cord replaced. Had never done it before. Between finding the correct type and length of cord and the ordeal to set it properly (in one of the attempts, turning the knob left would make the pointer go right and vice versa...), I personally find it one of the most patience draining tasks of all times. But so rewarding once it's done.

2- Regarding the speaker, a suitable replacement just couldn't be found. The speaker pole did have some rust on it, which was preventing the voice coil from moving freely. It was cleaned as best as possible with compressed air to the point that no further scratching would be noticeable. The cone itself had some tears that were addressed with speaker glue. It's working satisfactory, for the time being.

I'd like to thank everyone for the great support and encouragement. Here it is in action:

 [Video: https://youtu.be/0O_uxKeQ09g]

On to the next one!

All the best!
JZ