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1948 Philco C4608/ Mopar 802 radio
#1
Hi guys,
Its been quite a while since I've posted anything here. Between getting Mom settled in the nursing home and selling her house and catching up on stuff that got put on hold while taking care of her, I haven't had much time to work on radios.
 Well I'm restoring the radio in my 1948 Plymouth, thought it would be a fun project now that winter is coming. I've recapped the whole radio and replaced most of the resistors and I've installed a new solid state vibrator and replaced the missing speaker with a modern equivalent. 
Here are the problems I'm having,
When hooked up to a fully charged 6V. battery (positive ground) and turned on, I immediately get a loud buzzing from the speaker, the tone of the buzz does change a bit as the set warms up but does not go away. I can also vary the tone slightly by rotating the tone control, but no stations are heard. I get static and clicks when touching my meter leads to tube pins, so I'm guessing the amp section is working. I tried hooking up my signal generator and I did hear a faint tone at one point. I was able to vary the loudness slightly by tweaking the IF transformers.
Also my B+ voltages are about 30 volts lower than the schematic lists. As I said I have replaced all the caps including the electrolytics using the same values.
I've just started troubleshooting and I ordered a Sams Photofact for it as the one on Nostalgiaair is kind of confusing to me and hard to read.
I'm not very familiar with old car radios so if anyone out there has any ideas or tips on what I should be looking at, I would GREATLY appreciate it! Thanks!!!!
Kevin
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#2
A couple of things popped out at me. Try removing the rectifier tube and turn it on. Do you still have the buzz??  If so then try removing the vibrator  to hear if that knocks the buzz out. Also noticed  that the resistance of the secondary of the output transformer is pretty high like 30 ohms. Impedance wise that's about 35 ohms. Makes me think that your replacement spkr may not be a very good match if the vc is 8 ohms.

Some of us old timers would use a separate ac power supply to get the radio working and than come back to the power supply to sort the noise issues.

GL
Terry
"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow

Terry
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#3
Thanks for the tips. The buzzing seems to be with the speaker. I was under the illusion that this being a postwar set it would use a PM speaker, but I see it actually does use a field coil speaker. I tried a field coil speaker that is close to what I need that I had laying around and now the buzzing is gone. I can also pass a 455 kHz tone through the set, it's not loud but it's there and I can vary the volume with the control, so it's working some what. Problem is that there was no speaker attached to the radio when I got the car! I thought someone years ago had removed it, but after removing the dashboard from the car I see that the delete cover for the speaker was still installed as if it never had a radio, but there was a radio mounted in the dash. So, I don't know if the previous owner threw a radio in there just for looks or what. I haven't been able to find a correct original speaker so far so I don't know what I'm going to do. The one I'm using now needs a recone and is also too big to use in the car. Again if anyone has any thoughts, please feel free to chime in! Thanks!
Kevin
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#4
I doesn't look like the FC does anything but generate magnetizem for the spkr. Would just use a pm spkr. Some of the modern car spkrs have higher impedance vc than 8 ohms.
"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow

Terry
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#5
Hey everyone,
This may be a dumb question so I apologize Icon_crazy , but how much of a difference would it make if I were to go from a 4 ohm voice coil to say a 6 ohm one? Here's the story. I found a correct original speaker on Ebay and would like to use it if possible. The problem is that when I got it the speaker cone and voice coil were in such bad shape they pretty much disintegrated at the slightest touch. I found a correct replacement cone but am having trouble finding a suitable replacement  4 ohm 1 inch diameter voice coil. I've also tried my hand at making a replacement coil but not having much luck. I just received 2- 6 ohm voice coils I ordered online and physically they are a good fit but they are a higher value than the original. Just wanted to get some input, thanks!
Kevin
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#6
Well....you will increase the load by 50%.
So you will lose the power.
I think it will work ok, but the volume will go down and the full power won't be achieved. It will also move the working point of the amp so it is possible that the clipping will occur before achieving the full power.

Good news - it should work and it shouldn't blow up Icon_smile
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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#7
Thanks Morzh!
I'm going to take a couple of more shots at making my own voice coil before I decide to use the others. Just wanted to know how it would affect performance!
Kevin
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#8
Kevin, You could remove a couple of turns from your new voice coils. Your coils probably wound with two layers (voice coils always have an even number of layers, two four etc.) Remove a couple of turns from each layer.

Steve
M R Radios   C M Tubes
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