Hello, Working on this radio and have completed the recap including ecaps. I have noticed there is no plate voltage on the 75 tube. There is no plate voltage at the positive end of the second 8mf cap. I have either made a error in my notes or I failed to write something down. Anyway just how does the second 8mf cap get it's high voltage. I see nothing on the schematic that shows me that. There is high voltage on the first 8mf cap. There is plate voltage on the other tubes.
There is an error in the sch: the second cap has to be connected to the top pin of the #37 field coil. The missed the dot.
And so is the bottom pin of the #28 resistor 70k.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 10-10-2024, 05:23 PM by morzh.)
Thank you for the reply. So I did make a mistake that being believing the schematic. I always verify with the schematic. I was pretty sure my notes were wrong when looking at the schematic but they were not. When I made the connection radio came to life. Must say it plays pretty good. Now for the alignment and get it in the box.
More than one Philco sch has that mistake; it is a known one and popular at that: missing the tiedot from a plate resistor to the rail while connecting to the cap.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Before a rewire, I always make it a habit of tracing every connection and cross referencing the schematic. I will then recreate the schematic in full using CAD software (I like kiCAD, because it is free). I actually missed one tie dot myself while I was doing this for my 49-906, in the IF section. Those little dots are pesky!
In my recreated schematics, I will also add a semicircle to indicate one trace passing over top of another WITHOUT a connected node. Common practice is to do this to the horizontal trace. It’s a lot of work, but I hope it helps out the next guy to be working on my sets! I also create revisions and update parts that need to be replaced. I once needed to replace an output transformer with the closest model I could find from Hammond, for instance. Another time I needed to select a different value for a plate resistor because I couldn’t find the value called out on the Philco schematic. I got one within 10% tolerance, and marked it up on my schematic.
I realize that this is all probably overkill, but I find that it really helps me when I may go months at a time before needing to go back to the same schematic. A computerized schematic is also much more legible.
The closest I come to that is do some crude drawings of parts I replace or take out for cleaning. I don't start to question things until I start the rebuild process. I'm not sure I could even reproduce a schematic from looking at the wiring in the radio, I'd probably miss some wires or connections. I do admire people who can and do this. I have found times when Philco uses resistors that are not available any more so I just use the closest value and then measure to get closer to what was there.
When I just started with this hobby, I had the two of the first radios unmolested.
So I did not exactly re-wire, but I needed to remove some stuff, and that sometimes involved multiple disconnects. Despite myself being an EE, I ihad no experience in the tube radios, and I did not know whether to trust or to not trust the schematics, so I made hand-drawn sketches of the wiring. Which, when putting it back together, I then would compare to the schematic.
I noticed, the two were not always equivalent, but then those were not necessarily mistakes, there were variants, codes, service notes etc. So that did not bother me.
Then with the 60/66 series (I think) I encountered that missing tiedot. That was not a hiccuo with me, as I understood that would be a mistake to follow that schematic. But I also understood that there are many folks in this hobby who are novices, who are still learning, and who, even if knowledgeable in some electronics, would not necessarily be knowledgeable enough in tube circuitry. Plus we are accustomed to trust the printed documentation.
Tthis said, I never saw a 60 or 66 (which are more or less the same radio) that really needed rewiring. Short of those that spend their life in wet environment and the wire rotted out, most times this old non-rubber wiring is good. Even if stiff sometimes. I do not touch it, and it looks good too.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Like with Mike I have also run into the experience of an error in the diagram, though I can't remember in which radio, I think it was with a part value like a capacitor where they put the decimal in the wrong place, or a resistor where they left out a zero. I have also run into a factory wiring error, most recently in a Phonola radio from 1937, they tied the wrong two pins together on the 6K7 IF amplifier tube, the suppressor grid to chassis ground through the tube shield pin, it was supposed to be connected to the cathode and it is in most sets. This may explain why there was evidence of the set being in and out of the service shop until the 1970s when it was retired, and why the IF amplifier tube was a more recent replacement, suppressor grid being more negative then the cathode probably did not help the performance of the set, or tube life. Someone on the assembly line in 1937 got clockwise and counter clockwise mixed up, pin 8 and pin 1, pin 8 is the cathode, pin 1 is the tube shield.
Here is a diagram of a similar set, the main difference is that the one I was working with had a magic eye tube, so maybe it was a 772-S.
We could also mention the factory miswire of the tweeters on 38-690. Or the fact that even with this fixed, one could not hear them due to the crossover cap being too small.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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