Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
Thank those who helped me come up with a replacement Antenna Coil, that seems to be working.
I'm working on improving the output of the radio, it's very weak.
I've managed to take several voltages at various points, I know these could vary a lot and I wonder if someone with more experience with tube biasing could see if anything is really out of place here.
BTW: Local station WLW is 3 miles away by air, while it is tuned to that the reception will reach a reasonably strong level when the digital voltmeter probe touches pin 6 of the 14B6.
Here is the schematic with voltages added:
[Image: http://bingamon.com/Philco48-230/schemat...ltages.jpg]
Posts: 5,010
Threads: 267
Joined: Nov 2012
City: Bandon
State, Province, Country: OR
Daniel, I merged your new thread discussion of the 48-230 loop antenna into the original thread that you started. We like to keep the topic together as much as possible, it helps with continuity and when folks search for threads on topics they have an interest in.
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
No problem, once I got past the Antenna loop I thought it was a new subject. Nevertheless, it is same radio.
I have some news on this. The resistor R23 to the plate of the 14B6 was way off, over 900K. It was buried deep in the tube socket where get really hot and cooked. I replaced it and the audio is much stronger but it's distorted. Getting closer.
Posts: 7,215
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Looks like #20 open too.
Happy Troubleshooting!
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
I replaced #20, a little better. Also, #25 was over 300 Ohms, replaced it.
Pin 5, strong negative voltage and I put on a 3M resistor and the voltage dropped some and that station was clear as a bell. BUT - Everything else on the dial was distorted. Something going on with the 14B6 diode?
Posts: 7,215
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
This is what I would have a look at. The avc voltage as you tune across a station not WLW . Does it drop more - as the station gets stronger? Since you have a strong station near by it might be driving the avc voltage - all the time. Use a battery powered audio signal source to test the audio stages to determine if the gain is low and if the distortion is coming from the detector.
With #20 bad you had pretty much no screen grid voltage on the mixer and the IF amp. This would make the gain VERY low. Surprised it would do much of anything.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
Yesterday, I accidentally put a 130K resistor on the 50A5 Cathode, didn't work too well. That's one of those moments that describe experiential knowledge of cathode biasing as opposed to mere book or secondhand experience. It was sure quiet, an hour later I realized what I had done but I was out of resistors.
Amazing how the B+ voltage climbed to near 133V - I didn't run it very long at that level.
I put a new 150V in it today and it's back to making sound. Also added a 22uF cathode bypass capacitor in parallel.
At low volume it's fine. When the volume control reaches a certain threshold then the distortions shows up. Like a switch it kicks in. However you can touch pin 6 with the meter probe and the distortion goes away and it seems to work.
I have to agree that it seems that something is going on in the avc/agc. Maybe a I have another wrong resistor somewhere. Bad eyesight, I do almost everything under a lit up desk lens.
The DC voltages below reflect approx. 1/2 volume level and a local station is tuned in.
[Image: http://bingamon.com/Philco48-230/schemat...ltages.jpg]
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
Another question: What is this connector for?
[Image: http://bingamon.com/Philco48-230/connector.jpg]
Posts: 7,215
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
What's it connected to under the chassis?
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
One of them is to that antenna lug to the side and the stator of the tuner.
The other is through a capacitor and resistor to ground
So it's an antenna and isolated ground. I guess the schematic was a bit confusing to me on the issue between chassis ground and ground.
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
Today, I replace resistor #28. Hmm, this chassis to ground resistor has all the cathodes on one side of the resistor and everything else on the other side.
I put a smaller antenna on the set and used the newly discovered antenna connection and it now plays non-distorted but local 50,000 Watt AM station that is 3.5 miles away is basically flooding the radio. I'm going to need to more finely tune the IF and local osc. instead of by ear and hopefully get more stations.
Is it possible that the local osc might be off 455 and on a frequency that is harmonically related to the local station?
Posts: 7,215
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
The lo frequency is simple enough to check. Using a digital communications receiver listen for it at 455kc above the incoming signal (dial reading). So @550kc osc should be @ 1005kc and @ 1600kc it should be @ 2055kc. Close is ok. Loose couple the ant leads so you can the osc signal over to the comm rx.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 12-03-2020, 11:11 PM by Radioroslyn.)
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
I've heterodyned the radio against another radio also listened to tune it but it doesn't "feel" right compared to other radios.
I need to do something more accurate. On Carlson's Lab there is a video that show how much difference precision tuning can make in a receiver.
There may yet be something else going wrong with the set. I believe it may have been exposed to the elements at some point based on the condition the original speaker and transformer were in.
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
Finally, I borrowed a signal generator from work and a frequency counter and was able to tease the IF a bit better. It finally received a station other than WLW.
However, I think the volume control is on it's way out. I haven't tried to take it apart yet.
Posts: 53
Threads: 7
Joined: Nov 2020
City: KINGS MILLS
State, Province, Country: OH
I took apart the volume control today and it answered a lot of questions. The surface that the wiper contacts has the resistive conductive material missing from the 2/3rd mark on up, When you pass that mark the distortion occurs which also means that the wiper and capacitor are the only path, no resistive path to ground. No wonder the IF has been acting strange, it needs a complete path to ground. Also a clue that now makes sense. When trying to tune this, if your screwdriver blade touches the trimmer capacitor in the IF and the edge of the can at same time the reception gets really good -- I suppose that is helping the IF path to ground. Looks like I'm going to need a 500K potentiomer with a 1" shaft with cutout for the knob. Do people usually buy a a solid shaft and cut out/grind the place for the knob?
Users browsing this thread: 1 Guest(s)
|
