I'm part way through the electrical restoration of a 41-255, fixing some previous old repairs, etc., and I see some indications of heat and possible arcing on the multi section Candohm style resistor (pic below) labeled 61 (33-3393) in the power supply section of the schematic (below). The resistance values are close to those listed but I'd like to replace it with discrete resistors, though I need a reality check on power dissipation (no info on the parts list or the 1942 Philco parts catalog...). I've sniffed here and over at the ARF but find varying information.
The filament string has its own winding so not a factor; the 84 rectifier is supposed to be good for ~60ma. Is it safe to assume worst case is 60ma through each section of that multi wire wound? For each section of ~150 ohms, 30 ohms, and 15 ohms, I come up with drops of ~ 9V, 1.8V, and .9V, so I figure dissipation of ~.54w, .1w, and .05w. I already have some 3w and 1 watt resistors on hand and figure that should be adequate. Just looking for a reality check here to make sure I'm not wildly underestimating what I need, since the original resistor seems like a huge overkill for that dissipation. Unless of course Philco just took advantage of what was on hand or perhaps it was easy to adjust the taps for any need, regardless if the required dissipation required was minimal.
I think that you are about right. The entire current of the B+ supply passes through the resistor. It is essentially the cathode resistor for the pair of '41 output tubes. At 180V (The output voltage of the power supply), the current draw across that resistor is about 44 mA. Throw in the current from the other tubes and you approach the max current allowed for the 84 rectifier.
If you believe the RCA Receiving Tube Manual (and there are disparities, such as the cathode resistor being 330 Ohm for 2 '41s in push- pull as opposed to the 192 Ohm total resistance of the Candohm), the voltage drop should be about 14V. Multiplied by 0.06A (max allowed current for '84, RCA says 0.050A, but hey, Sylvania developed the tube, not RCA so we go with your #s). The resistor dissipates 0.8W. However, considering the pic of your Candohm, 3W would be prudent, especially for the 146 Ohm section.
This is a rather interesting radio (I have one). It performs better than the specs. They could have gotten a lot more Audio power out of this model if they used an 80 / 5Y3, but since Philco was a major car radio manufacturer and the '84 was developed for car radios (6V heater, separate cathode sleeve), they likely saved money using this circuit (possibly shows up in a car radio too). Also of interest is how they get the inverted phase for the 2nd '41. It comes from the screen of the 1st '41 instead of from an Inverter Stage. Verify the accuracy of all resistors in that circuit, especially the 3900 Ohm resistor, R56 between the 2 screens and the 2 470 KOhm Grid Resistors, R53 and R54 for minimum distortion.
Hope this helps.
Best Regards,
John, MrFixr55
MrFixr55 Wrote:/SNIP/ It is essentially the cathode resistor for the pair of '41 output tubes / the current draw across that resistor is about 44 mA.
The resistor dissipates 0.8W. However, considering the pic of your Candohm, 3W would be prudent, especially for the 146 Ohm section.
Verify the accuracy of all resistors in that circuit, especially the 3900 Ohm resistor, R56 between the 2 screens and the 2 470 KOhm Grid Resistors, R53 and R54 for minimum distortion.
Thanks for your thoughts and suggestions; they are most helpful. I just happen to have a 3W 150 ohm resistor on hand, so that's what I'll use for the 146 ohm section. None of the resistors in the chassis were ever changed. They're all the porous type and all were out of tolerance. I've already replaced the resistors you mentioned with 1% ones that I had on hand. The previous old work consisted only of snipping some wax/paper caps and wrapping the leads of the new ones around tie points, so a lot of clean up and reworking of those as well as proper electrolytics and line filter capacitors. I'm also making the changes noted in the Philco literature for improved sensitivity and bass compensation. I lucked out on fixing the original pushbutton power switch, which failed due to one wire lead not being soldered to the copper contact inside the switch body.
God Luck with it JTiner, Let us know how it sounds, especially with an outside audio feed. The redio should be a good performer.
Just an update for future reference... I've finished the electrical restoration of this radio and made some measurements (all with 122V AC line) in the power supply section. Overall current draw was ~395ma on the line side. The rectifier output measured 272V (vs 265) and 196V after the filters (vs 180). When measuring across my discrete replacements for the multi section R61, the 150 ohm resistor was dropping 9V, so about 60ma current as expected and just a tad over 1/2 watt. The 33 ohm resistor was similar, 2V, so about 65ma, .13w. When I removed the original multi section, I found that in addition to the apparent flash marks and scorching, the insulating paper was burned away over that same section of the resistor body.
So far performance seems good as far as I can tell using only the loop antenna in my basement (I was able to receive CHU Canada at 3.333MHz last night...). I've performed an alignment and ended up making a chassis solder connection to the riveted mounting plate/ground connection for compensating capacitors 17 and 17A. It appeared 17A was way too touchy mechanically, and soldering took care of that.
Also, regarding the failed pushbutton power switch, I attempted to replace the switch body with a similar one from Lowe's as detailed in a post here on the forum, but ultimately there was enough mechanical difference that the switch only operated correctly intermittently. That may be because of production changes to the switch available from Lowe's or perhaps I missed something. In any case, when I examined the original switch, I found it had failed because the wire lead on one side was never soldered to the copper contact in the switch. I soldered new leads to the contacts and re-installed the original switch. Instead of using epoxy to hold it in place, I soldered two sections of wire into the remaining sections of the original mount rivets so the rivets would stay in place and to act as a guide for sliding the switch body on. When the switch body was back in place, I twisted the wires to hold the switch in place. Some time later, I realized I should have cut the heads off small machine screws and soldered those into the rivets so I could put nuts/washers on, but hey...
Great Work JTiner!! CHU is great for verifying that band works. The programming content is a little boring
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If that contact was supposed to be soldered, you won't be able to coach or discipline the assembly line worker who did it, as the radio is 80 - 82 years old.
I have seen versions of that switch where the connection is made by push in, similar to those cheap version backwire outlets (the ones where springs and not a screw tightened wedge secures the connection)
Great job on the fix!!
I've pretty much finished restoration of this radio, but had a couple of questions that came up after reading some other threads. First, a poster in another thread noted the usual Philco setup for this period was to have the power line filter block before the switch, which was indeed how my set was originally wired. When I replaced the line filter caps and repaired the switch, I changed the wiring to be switch first so that line voltage wasn't continually on the filter caps. Is that generally what most folks do? And why did Philco wire them with the line always across the caps to begin with?
Second, I built a small breadboard version of the external antenna adapter for this radio (connected to the three pin socket on the chassis), but it generally doesn't seem to offer much better performance on BC and low SW than the loop antenna, though it does make a considerable difference on the upper SW band. Is that consistent with others' experience using the external antenna adapter and a ~50' length of wire?
Great question JTiner! I saw the same in the Philco 89 from H&!! that I recently fixed. I supposed that the Bakelite Cap Block used in the 89 served as an easy terminal for the power cord instead of running it up to the switch.
Capacitors conduct AC so there is always some drain (and heat dissipation, which is why one needs to always use "safety" caps for that purpose.
However, I had a 40 year career in BioMedical Field and Phone Customer support and there are commercial filter / input modules that als0 put the filter before the power switch. I think that this is done in TVs also. These EMI Filters last decades without failure. It has been about 45 years since Physics II in College, so I can't tell how many watts a 0.05uF cap draws across a 230V 60Hz Line, but U would doubt that it is much
Considerably more when those paper caps start leaking after 60 years. Poof!!