40-201 Chassis Rewiring - Printable Version
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40-201 Chassis Rewiring - Joeztech - 12-17-2013
Around noon I received my order for hook-up wire from Jameco. I decided to tackle the filament wiring first. Since I had a large array of colors to choose from, I decided to use brown for the filament circuit. That is the most commonly used color for that function in my memory over the years of working on tube type sets. The process was aided by unfastening the audio output transformer and using just one of its mounting tabs to secure it in the corner of the chassis under the power transformer. That left the area around the two no. 42 output tubes and the multiple no. 37 tubes free and clear to work on.
At each tube socket one side of the filament is connected to chassis ground. The other side is the hot feed to the tube and it loops from socket to socket. It makes for steady progression of the rewiring. I clipped the old wire near the tube socket terminal and removed the old wire, used it for measuring a length of new wire with enough extra length to allow for the portion that was clipped off. Then I work at a particular terminal and carefully remove the remaining part of the old wires. I clean the terminal of most of its old solder, then attach the new wires, making sure I have the wire going to the next socket included, then solder both wires. The RF tube was the last in the parallel filament circuit. All filament wires are dressed against the chassis metal as much as possible.
I had to move the AC input filter capacitor block out of the way too, to make the work easier. I finished the rewiring of the filament circuitry around 2:30PM and decided to take a rest from this for the rest of the day. I get tensed up when I work on these tight chassis areas and have to have some wind-down time afterward. Otherwise my muscles get tight and I get a headache etc.
The next step will be to remove the capacitors from the AC input filter block and replace the capacitors down inside. Heat will be used to remove the internal parts that are buried in a tar-like compound. There are good instructions on this website on how to do this type repair or rebuild. I have parts on order from Mouser Electronics for this and other capacitors and resistors in the chassis. I have ordered enough parts to cover all the tubular capacitors and most all the resistors. I have some parts on hand but needed some that I did not have.
I intend to leave the mica capacitors alone unless they prove to be a problem after I begin to test the radio. As a general rule, they are very reliable and most are rated at around 500VDC or better. They have good temperature characteristics and hold their value pretty closely as temperature rises during set warm-up. Sometimes disc ceramic capacitors may be used with special temperature compensation to compensate for capacitance change as the set temperature changes during warm-up. There are also some tubular shaped ceramic capacitors and other shapes that are sometimes used. It is important to understand what type of capacitor is in a given circuit. Consult this forum and others if you have doubts about what kind of part is in a circuit.
It helps me to space out the parts ordering and efforts at working on the set. It gives me more time to think about how I want to approach the restoration and try to find the best way to do it. Basically I try to take one type circuitry to work on at a time. I start at the audio output circuitry and power supply first and work back toward the RF section by steps. I may work on just B+ wiring and coupling capacitors of one stage at a time. Sometimes other parts such as resistors are attached to a terminal on a tube socket or terminal strip and it makes sense to change any parts necessary on that particular node at one time. If for some reason a particular part has to be left unattached at its other end, either make a drawing and/or take a picture, keep notes about why the part was left disconnected and where it is supposed to go. That way when you have to come back to it later you will be able to see, read and understand why you did it that way instead of trying to rely on memory.
Looks like I will be busy for days doing the chassis re-wiring.
Joe
RE: 40-201 Chassis Rewiring - codefox1 - 12-18-2013
Well, sounds like you know what you are doing. So far as colors, as long as you are consistent, it is OK, but many times red is for B+, green for filaments, gray for AC, black for B- or chassis ground, and from there, aren't many conventions that I am aware of.
In any case I don't see a question here, Best.
RE: 40-201 Chassis Rewiring - Joeztech - 12-18-2013
codefox1;
Yes I agree, red for B+. Philco seems to have used some yellow, some green and some red for the filaments and dial lamp leads. I like to use green for safety ground (if present), black or gray for AC, black for ground. Philco did seem to use some white and blue for IF transformer B+ so I stayed with what they originally used except for the B+ input where I did use red. The later convention on filaments that I saw in the 1950s was brown for filaments, so here I stayed consistent with that except for the dial lamp leads where I used pretty much the original colors as best I could tell. Many of the cloth jacketed wires are so faded and changed in color it is really hard to tell. The schematic is sometimes silent on the issue of color of wires.
I decided early on that I was not going to try to duplicate the cloth wiring as I was unable to find the types I wanted, so plastic insulation it is. Since the chassis is already painted and does not look original it did not make sense to me to go to elaborate lengths to try to keep a completely original look to everything under the chassis. Some people would want to and I have no problem with that.
I did some B+ wiring today and stopped to wait for one wire wound dropping resistor to make up for a lower resistance in the field core speaker compared to the original. I is only off about 60-80 ohms depending on which ohm meter I use. I ordered both values so I can try each one to see which gets closest to the voltage shown in the schematic.
Cheers!
Joe