[flatheadjr]

I've been working on this model 144 for a while now and I finally have rebuilt a by pass filter capacitor and replaced the electrolytics and a few other questionable caps with many more to still replace. I am still not getting anything out of this unit. I have the voltage at the rectifier, filiment voltages are good but I am not reading any other voltages. First where is the best place to get a good ground, and why am I not getting any voltages? I started at the rectifier and then went to the output tube. The output is a type 42 tube, would no voltage mean bad tube, it lights up but I would guess no emissions would create no voltage? I don't have availability to a tube tester.

[Arran]

 In most of these pre war Philcos the most negative point is the center tap of the high voltage secondary of the power transformer, so you should be able to take voltage readings between that and most other points, except for the tube heaters.
Regards
Arran

[morzh]

If you have voltage on rectifier and are measuring it (relative to what? centertap or the chassis?) relative to the centertap, you might have resistors #30 open (either one, 263 or 21 Ohm), or the field coil open, or the choke #70, or all of the above.

[flatheadjr]

I wasn't really sure where to check the voltage from so I just assumed the chassis was ground. I'll have to check from the center tap on the transformer. What's the best way to check the choke #70? The #30 resistor is under the type 78 tube, isn't that part of the audio?

[morzh]

Well, you can check from the chassis, most voltages are measured relative to it anyway, it was unclear to me how the measurement was done.
If it was to the chassis as you said, and you see the rectifier voltage but not B+ - check continuity of the field coil and that other choke.

[flatheadjr]

The only field coil that I see on the schematic is on the speaker, is there one on the chassis and what number is it? Do I have to disconnect the filter choke to get a reading? And on the filter choke (#70) do I just have to show continuity through it or should there be resistance and if so how much? The schematic doesn't give a resistance reading for the choke filter.

[morzh]

Field coil is field coil, it is only one and it is on the speaker.
Measure both across that and across the other choke.
It should be some meaningful number about from a few hundred ohm to a kiloohm or two. Not tens or hundreds of kohms or open, not short either.

No you do not have to unsolder either one, just ohm it out.

[flatheadjr]

Okay so I get about 2.3k ohm on the choke, hopefully that's about right, and half that on the the field coil. So no opens and no mega ohms. However I got to thinking about it and it may help to have the speaker connected to the chassis. But if measurements look right and after I have things connected what else should I look for?

[morzh]

You mean your speaker (with the field coil) is not connected?

[flatheadjr]

Yeah I didn't have it connected.

[morzh]

What voltages beyond the rectifier do you expect to see if your speaker and thereby your field coil are not connected?
Look at the sch, familiarise yourself with it, try to understand it at the minimum basic level.
Don't power your radio until you do. Bad idea.

[flatheadjr]

I understand the diagram fairly well, I expected to find voltages somewhat close to the ones listed in diagram with the tube socket voltages. I figured since the chassis is ground everything should be read relative to that. I'm not used to working with both ac and dc in one location, I usually work with old car radios that start with 6V dc. I've rebuilt the bypass capacitor which was completely missing and the electrolytics have completely been replaced. I have 2 old ones to replace yet, close to the power supply and a couple in the audio section I'll do those tomorrow and check again. All the bake lights were bypassed and replaced with micas long ago so those should be okay.

[Raleigh]

The field coil on the speaker is part of the power supply filter. It does double duty generating the electrostatic field for the speaker. If your speaker is not connected, it breaks the high voltage path, so you won't get any DC high voltage at any of the tubes, only at the rectifier cathode and the positive of input filter cap (usually.) The schematics can be confusing, as they usually show the field coil in the power supply section, but it is actually on the speaker. Sounds like you have another choke, too, which might add a bit of confusion.

Check the continuity across the output transformer primary. If it is good, connect your speaker, power up, and you should have plate voltage at all tubes. If not, start with the input cap (the one connected directly to the rectifier cathode) and trace the high voltage path with a dc voltmeter through the field coil, the other choke, both sides of the output transformer primary to the plate of the 42 amp tube. Referenced to the chassis ground, you should see a voltage drop at each test point. Whatever amount the schematic says.

Let us know what happens.

It's a good idea to limit the AC current with a variac or dim bulb tester the first time you power up, just in case of shorts.

[flatheadjr]

I am getting small amounts of resistance on the field coil, I checked it with 2 different meters and it's close to the schematic, it says 660 ohms and I read around 674 so I figured close enough. (Sorry I must have fat fingered the numbers)

[Raleigh]

I'm not sure that's close enough; it's 40% low, so it's got a short in there somewhere (assuming it is the right part--you might want to check the part number of the speaker to be sure.) For now, it's what you have to live with. But make sure you don't have any continuity between the field coil and any metal part of the speaker.

Your schematic might have voltage numbers at various points. Some do, some don't. If so, you can use them to calculate the expected B+ current and then use the voltage drop across the field coil, the choke, or the output transformer to calculate what it actually is.

The main thing now is to make sure you have B+ to the plates of the tubes, and to make sure it is within range of the published voltages, since that was the original problem you had. Once you get past that problem, you can move on to the next one.

You might have one or more resistors in series between the center tap of the power transformer and the negative end of the output filter cap, which is probably grounded to the chassis. The purpose of those is to drop the center tap below the chassis ground and generate negative voltages to bias the grids and maybe the cathodes of some of the tubes. If one or more of those resistors is open, you won't have a return path for B current, so check those resistors if they're shown on the schematic. Alternatively, the center tap might be connected directly to ground, in which case, ignore the above.

Once you've established continuity from the rectifier cathode through the field coil, choke, output transformer, and bias resistors (if present), if you don't get B+ at the output tube plate, or if it is very low, you've probably got a short somewhere, so turn the radio off and use your ohm meter and the schematic to find it. Otherwise something is liable to smoke. If you monitor the voltage drop across the field coil when you power up, you can see if it's in the right ball park very quickly.