[ccomer1955]

http://www.nostalgiaair.org/PagesByModel...013415.pdf

This will be my first candohm build without a live person helping me out, so I have a couple of relatively uninformed questions for this set.

The candohm has three sections with 146 ohms, 31 ohms, and 15 ohms. Can I replace those sections with 150 ohms, 30 ohms, and 15 ohms at 5 watts? or should the 146 ohm section be 10 watts? Also, should my replacement candohm be grounded to the chassis at both ends or at all?

Also, does it matter whichend of the candohm I connect the negative leads of the electrolytics? The schematic seems to indicate the negative leads of the 16uf and 8uf caps ground at the same spot, but in the set, the 8uf and the 12 uf are in the same mute-section tube, so they ground at the same spot. Sorry if that's a dumb question, but I will likely place the wiring as it is now.

Also a mechanical issue: the pushbutton next to the on/off button is stuck in the "on" position. All of the other buttons work as they should, popping back up when another button is pressed. The label for the stuck button is not present, so any information about that one would be appreciated.

Also, any idea why there is a .003 uf 1000v cap on the tone control?

[Jim Koehler]

Candohms can be deceptive. It may seem that a 5 watt replacement would do the job however, what also must be taken into account is the heat sink (read, chassis) that it's attached to.....
That said, what I've grown accustomed to using are the Dale Hi-Watt resistors. They're furnished with mounting tabs so the resistor can be bolted to the chassis. And, since their terminals are on an axial plane, just about the same wire lashup can be used as with the old candohm setup. The drawback is that the Dale units take up a little more space than a candohm would, so the total length of the resistor assy has to be taken into consideration. 
They can be found in most of the values needed and, of course, in wattages desired. 
I finished a GE "colorama" mod 155 and replaced a candohm assy with Dale assy's that fit my needs exactly. It was 14" long!

[Nick3092]

Also, does it matter whichend of the candohm I connect the negative leads of the electrolytics? The schematic seems to indicate the negative leads of the 16uf and 8uf caps ground at the same spot, but in the set, the 8uf and the 12 uf are in the same mute-section tube, so they ground at the same spot. Sorry if that's a dumb question, but I will likely place the wiring as it is now.

The negatives of the 16uF and 8uF both go to chassis ground.  So it will definitely matter which end of the candohm you connect them to.  One end of the candohm goes to chassis ground and the other end goes to the power transformer center tap of the HV winding.  So you want to go to the end that goes to chassis ground (the 15 ohm end).

Although, you don't even need to connect these two caps to the candohm at all.  Since the negatives go to chassis ground, you can put them on any convenient solder point to chassis.  They don't even need to be tied together before going to the chassis.  If it makes it easier to physically wire one cap on one end of the chassis and the other on the other end of the chassis, just tie each negative lead to the closest chassis solder point/tab that connects to the chassis.

You do need to make sure the 12uf is connected to the end of the candohm that connects to the CT winding (the 146 ohm end) though, and not chassis ground.  Otherwise you will have some major hum issues.

[ccomer1955]

Got it. Thanks. One more quick question for the Philco gurus on here:

Is it common for a 1 meg 1/2 watt resistor to drift up to 2 meg? R53 measures 2 meg, and I have replaced it with a 1 meg resistor, but I was thinking that there might be a service bulletin I am not aware of out there.

Also, is 1/2 watt sufficient on the output tubes? R54 measures as the schematic says it should.

[klondike98]

Yes they can drift that much. How was the original marked?

[ccomer1955]

Brown, black, green, silver (i'm pretty sure--I'm profoundly colorblind, so I usually just measure everything). It measures 2.07 meg out of circuit. I have a 1/2 watt 1 meg in now, but I haven't powered up because I've been rebuilding the light sockets (thanks for that tutorial, Ron).

[klondike98]

OK, brown-black-green would be 1 meg and silver is 5% tolerance so the resistor has drifted and was not a run change or modification.

[ccomer1955]

She is singing, again. I have some minor distortion that I can to a degree tune out, but I think perhaps cleaning the bass pot and the volume pot might help that some. It really pulls in all the available stations. I also need to adjust the band switch light so it shines where it should. Now, on to the cabinet.

[klondike98]

[Raleigh]

I don't think you really need 5 watt resistors to replace the candohm in this radio. You can calculate the current through the resistors from data in the schematic, and then you can calculate the power dissipation. Of course, there's nothing wrong with using 5 watt resistors as long as they fit.

From the schematic, the voltage drop across the field coil is 265 - 180 = 85. Using ohm's law, the B current is about 77 mA. Since all of the DC in the radio goes through the field coil, 77 mA is the return current to the transformer center tap through the candohm.

Most, if not all, of the return current goes through the 146 ohm section of the candohm. For simplicity, let's say it all does. The power calculates out to .87 watts. I suspect that very little current goes through the 31 and 15 ohm sections, but if we say all 77 mA does, they'd dissipate .18 and .09 watts, respectively.

You could probably use a 150 ohm 1-watt resistor and half-watters for the other two. For derating, I'd probably double up two 1-watt 75 ohm resistors in series to make the equivalent of a 2-watt 150 ohm, since I don't have many 2-watt resistors lying around. Most likely I'd use 1-watters for the other two, but that's really overkill.

I bought a 42-350 that someone had partly recapped, and he replaced the three-section candohm with a 20 watt and 2 10-watt resistors. Way overkill.

If you still have the chassis out of the cabinet, it would be interesting for you to measure the voltage drop across the 3 sections and see what you get, to see if my calculations are bogus or not. I'm not a EE so I could easily be mistaken.

[ccomer1955]

This article sounds very familiar to me. I wonder if that is the reason I'm hearing hum between stations on my 41-280? I was reading this at work, and it sounded familiar to me. I'll have to look into it when I get home tonight. I'm also thinking I might have the mains hooked up backwards. The hot side goes to the on/off switch, right? And the neutral goes to the transformer primary? After passing through the safety caps in the bakelite box, that is.

http://www.philcoradio.com/tech/hum.htm

[ccomer1955]

I gave it a try last night. Cleaned out all of the tube sockets and each tube pin. I took the 7C6 from the 41-285 and swapped it with the one in the 41-280. And...partial success. The hum isn't gone entirely, but it is much less noticeable.

I found a crack in the speaker cone I had not noticed before, so I patched it. With my finger on the crack gently, the distortion seems to be less noticeable, too, so maybe I'm on the way. I should have the cabinet re-veneered by next week, so I'd like for the chassis to be ready to go by then.

Maybe I'll get a new 7C6 and take it outside to try. My house is an electronically noisy house. ALL of my radios have noisy tuning above 1100. An all electric community.

[ccomer1955]

In working on the 41-285 chassis I have in another thread, I have been doing some comparisons of chassis layouts between my 41-280 and 41-285. I has finally dawned on me that somewhere along the line, someone clipped the CT lead on the output transformer. Could that be the source of the hum I get on this 41-280 set? He clipped it off right at the outer insulating paper cover. Do you think it might be possible for me to make a small cut in the paper at that point and solder a new lead in place? The schematic shows the output transformer CT connecting to the grid of one of the 41 tubes and to the junction of R28 and the electrolytic C27A (16uf).

Looking at a close up of the OT, I can see four leads coming from it. Two go to the plates of the 41s. One goes to the terminal strip beside the OT where it seems to connect to a yellow cloth-covered wire that goes to the speaker. The fourth goes toward the bottom 41, but I will have to look at it when I get home to trace it (and the yellow wire to the speaker). Shouldn't the OT have five leads coming off of it?

[Raleigh]

With the CT lead (black & white) clipped, I don't see how you get hum or any audio at all, since the CT supplies B+ to the output tubes through the transformer primary. Without B+, you get no audio.

Are you sure it is the CT lead (black and white) that is clipped?

The transformer has 5 leads. Check to see if one of the secondaries, a bare wire, is bent back and soldered on to the metal shell of the transformer. If so, it grounds through the rivets that hold it to the chassis. That leaves the four lead wires that you see. The other secondary, which may be a sleeved bare wire, is connected to the terminal strip beside the transformer, where it connects to one of the speaker voice coil wires.

The three primary wires, including the CT, should be connected as you described. I think in the 41-280 the CT wire should connect to pin 3 of the 41 tube near the rear edge of the chassis.

I would try to solder a new lead where the old one was clipped, but before applying power, use your ohm meter to make sure the transformer is good and the connection to your new lead is good. These radios are notorious for open output OT primaries.

[ccomer1955]

You are correct. I took another look at it when I got home, and my confusion came from comparing my 41-280 that has a Philco OT with my 41-285 that has a replacement Thoradson OT.