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Philco 38-7CS Restoration
#1
Hi Guys,

I need a little help with my restoration on a 38-7cs. 

I need clarification on the connections on the volume pot. There are four connections on the pot. On top there are three and then there is one on the bottom side. For the pots on top, the middle one is the wiper on the variable resistor? I'm assuming that the other two are either side of the resistor. Is the bottom lug a hard wired connection off the variable resistor (Connected to the 51,000 ohm resistor?) 

Here's the schematic:
http://www.nostalgiaair.org/Resources/256/M0013256.htm

I think I swap caps on the volume pot and want to make sure.

Also, with the On/Off/Tone Switch, there are also 4 connections: 2 on the back and 2 on the top. I'm assuming one is for on/off and the other pair is for tone. Which one is which? I haven't really looked at it, but I figured I'd ask.  

Thanks

Bernie
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#2
Also, I had to double check. When I compared my radio to some of the pictures online of the under side of similar chassis, I noticed a cap that was missing from mine. I didn't pick it up on the schematic or part number list since I was recapping what was there.  Cap #30 is missing. It looks like it was once there at one time, but its definitely gone now. There looks like there were some small repairs to the radio prior to recapping, but nothing major. This chassis seems to be Rev. 2. I didn't see any note for the removal of this cap. 

What would this do to the radio since its gone?
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#3
Hi Bernie,
Perhaps this will clear it up for you. Looking from the back side end terminal on the left goes to #24, the center (wiper) to #28, and the other end to chassis gnd.

As for the tone/off/on sw two of the connections are to the ac circuit (one to the power transformer and other to the ac cord). The other connections connect to # 38, 40, and chassis gnd.

In regard to #30 it may have been a costs cutting discontinuation, it wouldn't hurt to add in back in. It decouples the grid bias voltage to the 6K5. Without it the 6K5 maybe prone to oscillate.

GL


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"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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#4
Thanks, Terry. 

That's exactly what I was looking for. I'll double check those caps and connections and reply back. That's what I get for not taking pictures before hand. Hopefully this will clear up some of the amplification issues I'm having.
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#5
I went through and started to change out some of the out of tolerance resistors. All caps have been replaced but I'm still get okay reception on lower stations like 710 and 770 but as I go up on the dial I'm getting nothing but static. There is quite a lot of static (white noise). At this rate, any part that I've changed has not improved the reception.

I aligned the radio using an Eico 324 and everything is peaked per the manual. I did get some increase on the voltage of the 6a8 tube and they've come up to were they should be (pin 3 176 vdc, pin 4 105vdc). Prior to some parts replacement, those voltages were about -30vdc. All the other voltages look fairly good. The Plate on the 6K5 is about +15v higher than the schematics call for. The 6K7 is also showing about +10v higher on the Plate as well.

At this point, with the exception of a couple of resistors that are slightly out of tolerance, I'm at a loss with this one at the moment. I do get more gain if I touch the grid cap of the 6a8 tube, but it only increases the gain and doesn't clean up the static noise. Grounding the radio seems to help but only on the lower stations. The tubes did test good and I have a couple NOS in there as well.

Any suggestions?
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#6
>Any suggestions? 
Well yes.  A couple of things are suspect. Do you know if your SG is accurate? Take the If frequency double it and listen for it in a known well calibrated receiver. With the generator set at 470kc listen for the second harmonic at 940kc. This a way to tell if the calibration is good at the If frequency.
Did you see the service note abt the 20 ohm resistor connected to the cathode of the 6A8? Is your 6A8 good ??
In some severe situations rf noise generated by modern devises plugged into the ac mains can wipeout parts of the MW and HF spectrum.
Wouldn't loose any sleep over the voltage differences as you are probably using a modern hi impedance voltmeter, this is normal.
"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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#7
I bought a new 6J5G tube and I replaced the 6A8. There seems to be no apparent change. I realigned it again. I notice that when I try to adjust the tuning condenser (4A) that there's not much of a change. Its very subtle.  All the IFs seems to be working and they are peaked. The ant and oscillator coils seems to be fine as well. I notice that the trim pot for (7A) is a little sensitive.  It picks up stations but still a lot of hiss/static and is only strong on anything below 1000kc. Even with an antenna, nothing is strong above it. 1010 WINS NYC is a very strong station and that barely comes through. 

The signal generator is fairly accurate. 

I went through the remaining resistors. I haven't changed them out yet. There is one hidden in the 2nd IF Can that I haven't gotten to yet. I have to take the can apart. Its a 51,000 ohm resistor. There are about 4-5 left throughout. All the 1.0 M resistors are a little high around 1.2 M and then there's this one that's in the picture. On the schematic, it should be a 1.0 M, but its metering around 1.8M. That can't be right? I didn't see any service bulletin about it being changed and it looks original. The Candohm isn't that far off, but I'll probably rebuild it as well.

I'm still learning the circuits, but where is the AVC on the schematic? 

Thanks


Attached Files Image(s)
   
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#8
Hi,
> a 1.0 M, but its metering around 1.8M. That can't be right?
Yup. I usually don't bother with what the diagram sez just what the color code sez it's suppose to be. As long as it looks to be original not one that has been replaced before. 20% tolerance is what it has in it.

>where is the AVC on the schematic?
It starts @ 3 and goes over to 15 and so on. Ends up over by the volume control.
"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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