Philco radio cap problem - Videokarma.org TV - Video

radio nut · #1 09-24-2009, 09:58 PM

The Philco 41-842 that I needed help getting a diagram for is giving me problems .There is one particular electrolytic cap in the 1LB4 (output tube)heater line that keeps getting very hot. I have replaced it with a new, still tests good cap. The diagram calls for a 100 mf cap 25 volts and the original sectional cap had a 200mf 25 volt in there.I used my digital meter and it does not show ac from the plus or ground of the cap to chassis ground or with my test leads on the plus and ground of it. I show about 4 volts dc from the pos to ground. One side of the tube heater does go to a grid inside the tube.I am lost,since I am not overvolting the electro. and I am not showing AC either ,but if I unsolder one end and test it with my cap checker it shows good.There is also an electro. in the other leg of the heater and it stays cold.The radio is fully recapped, the original speaker is gone. I do not know what the resistance of the output trans. should be so I do not know if it is an issue.I do have a speaker currently hooked up to the secondary of the output Trans. and I have no sound , hum, anything.Any Ideas?

radio nut · #2 09-24-2009, 10:20 PM

I forgot to add that this is an AC/DC/Battery set. The cap in question even gets hot when there is only the rectifier tube in place. The other caps are not getting hot though. This is the second rect. tube I have tried. Both were nos and tested good on a tv-7 tester.It is a 117z6 tube.The only thing that I can think of is the switch is bad that is used when you want to run it with batteries. I do not understand the switch as far as how it is shown and what makes contact when. The reason is the switch to me looks different than the pic on the diagram. The diag. pic shows 3 rows of 4 connections with 2 extra contacts apart from the rest and my switch has 4 rows of 3 connections without the extras.

Chimes · #3 09-24-2009, 11:39 PM

I could not find a listing for a 41-842 but did find a schematic for a 42-482, If you are not running the cap at more than the rated voltage, about the only other thing that could make it get hot is incorrect polarity. Double check it.

rollei35guy · #4 09-25-2009, 06:51 AM

Quote:

Originally Posted by radio nut

I used my digital meter and it does not show ac from the plus or ground of the cap to chassis ground or with my test leads on the plus and ground of it.

Looking at the schematic I believe this set has a 'floating ground'. That is the ground is not on the chassis but at several points that are tied together. Sometimes a solder lug or a tube socket lug, or a combination. There is usually a capacitor and/or high value resistor to chassis to bypass ac line noise. One side of the secondary of the speaker transformer runs to chassis ground too.

These are a bit safer than the notorious hot chassis models. Of course if that capacitor to chassis shorts then all bets are off. There are special caps for that purpose.

Look at the chassis and try to find where the points for this floating ground are.

Are you using an isolation transformer? Recommended regardless of the set.

rollei35guy · #5 09-25-2009, 06:54 AM

Here is a lower res version of the 41-842 schematic from another thread:

http://videokarma.org/showthread.php?t=246027

I have an AC-DC/battery set too so I'm interested in this.

radio nut · #6 09-25-2009, 09:14 AM

Quote:

Originally Posted by rollei35guy

Looking at the schematic I believe this set has a 'floating ground'. That is the ground is not on the chassis but at several points that are tied together. Sometimes a solder lug or a tube socket lug, or a combination. There is usually a capacitor and/or high value resistor to chassis to bypass ac line noise. One side of the secondary of the speaker transformer runs to chassis ground too.

These are a bit safer than the notorious hot chassis models. Of course if that capacitor to chassis shorts then all bets are off. There are special caps for that purpose.

Look at the chassis and try to find where the points for this floating ground are.

Are you using an isolation transformer? Recommended regardless of the set.

The speaker secondary does not currently go to ground. With the original speaker missing the output trans. is just hanging. Could this be part of the problem?

rollei35guy · #7 09-25-2009, 09:35 AM

Quote:

Originally Posted by radio nut

The speaker secondary does not currently go to ground. With the original speaker missing the output trans. is just hanging. Could this be part of the problem?

I don't think so. It may be that one side of the secondary is tied to the transformer mounting. Do you have a scope you can put on the output?

I would first verify the power supply. Check for B+ and filament voltages. And then work forward to the output stages. You still have that hot capacitor that maybe an issue.

rollei35guy · #8 09-25-2009, 10:02 AM

Another possibility you should consider is that there may have been others before you. Changes may have been made and not necessarily the right ones.

You can check against the schematic but that raises the possibility that there may have been production changes. In which case the schematic might not match.

Chimes · #9 09-25-2009, 12:18 PM

What voltage do you measure directly across the capacitor, from positive terminal to negative terminal, not to chassis ground.

radio nut · #10 09-26-2009, 01:18 PM

Quote:

Originally Posted by rollei35guy

I don't think so. It may be that one side of the secondary is tied to the transformer mounting. Do you have a scope you can put on the output?

I would first verify the power supply. Check for B+ and filament voltages. And then work forward to the output stages. You still have that hot capacitor that maybe an issue.

The B+ was 125. the filament voltage on the "hot" cap was 82 volts. well, It is in the filament line and is a 25 volt cap.That also explains why I noticed that I lost a tube in the output.No on the scope for right now, I can borrow one eventually

radio nut · #11 09-26-2009, 01:20 PM

Quote:

Originally Posted by Chimes

What voltage do you measure directly across the capacitor, from positive terminal to negative terminal, not to chassis ground.

82 volts. Gee, Thats alot for a filament line of a 1LB4 tube. The cap is 25 volts like the diagram calls for. So now I think I have to find the fil. voltage problem.

radio nut · #12 09-26-2009, 01:25 PM

Quote:

Originally Posted by rollei35guy

Another possibility you should consider is that there may have been others before you. Changes may have been made and not necessarily the right ones.

You can check against the schematic but that raises the possibility that there may have been production changes. In which case the schematic might not match.

Well, Whomever had this set before me added in a resistor that totally changed the filament line which I put back to the diagram. They also replaced one resistor with a high watt. "brown Bar" resis. and placed it on top of the chassis but did not make sure that the wires going to it did not rub off the coating and short to the chassis. They then also cut out caps completely and put them in a pile in the cabinet. Fun, huh?

radio nut · #13 09-26-2009, 03:10 PM

Well I guess I do not understand one thing. I now know that the tube heater line is getting too much voltage. The diagram shows the output of the rectifier going to a 1,800 ohm res. then to the heater of the 1Lb4. That is the way this set is wired right now. Do I just need to jump up the res. to lower heater volt. or does this tell me that something else is going on?

rollei35guy · #14 09-26-2009, 03:59 PM

Quote:

Originally Posted by radio nut

Well I guess I do not understand one thing. I now know that the tube heater line is getting too much voltage. The diagram shows the output of the rectifier going to a 1,800 ohm res. then to the heater of the 1Lb4. That is the way this set is wired right now. Do I just need to jump up the res. to lower heater volt. or does this tell me that something else is going on?

So inspector, are you feeling a little like Columbo right about now?

Yeah 82 volts on a 1.5 volt filament will shorten the life of a tube. Are you measuring that across the filament?

It looks like all the 1lxx tubes have their filaments in series and there is a voltage divider.

Are you seeing 125 volts at the plug? That might stress things too since the set is designed for 117 volts or so. Do you have a variac so you can back the ac line voltage off a bit?

I'm kind of a 'go-by-the-schematic' unless I'm experimenting. So my feeling is something else is going on. I would continue to trace out the circuit and see what else is different.

A next step might be to check the filament voltages for each of the tubes. But they are in series and you don't want to burn out any more filaments. So maybe from the 1800 ohm resistor side of the last 1lb4 to the 150kohm side of the first 1le3. That should be around 9 volts or so.

Maybe chime will 'chime' in with some other ideas.

When I was in tech school we had a fellow there who shorted out all the resistors on CB to increase the power.

So you may have quite a restoration ahead of you.

radio nut · #15 09-26-2009, 05:20 PM

Yes to the variac. yes again to the 125 line voltage.I will start where you stated. I did notice That at least one of the heater lines did junction to a couple of caps parallel to a IF trans. I wonder if those mica or ceramic caps are bad.I will keep you posted. The 82 volts was across the "hot" cap. If I go from pos of the hot cap to chassis ground I had 5 volts " if " I remember. My printer is dead. I usually print the diagram and then write all over it. Now I keep running to the computer and then back to the set.Right now I am more worried about my radio.