Oscillator coil help - RCA 35x - Page 1
Forum section: electronics / repair

author: ttx450
author_rank: Regular Contributor
timestamp: January 10, 2022, 11:08:18 pm
content: My original oscillator has a open on one lug. It also had a broken wire that I found and could be repaired on another lug. I purchased a p-c70-osc to replace the original oscillator. I believe that I should use drawing #3, does that look right? Does my hookup/cross wiring look right?Ignore my color markup on the schematic, some colors are not right.Replacement schematic/Old schematic4=6, 5=5, 1=7, 2=8, 3=9 is 3=9 right?Any help is much appreciated.. thks

author: bob91343
author_rank: Super Contributor
timestamp: January 11, 2022, 02:05:39 am
content: The frequency range you want is 455 kHz higher than the input frequency. So it will go from about 1 MHz to 2 MHz.The original circuit uses a gimmick for feedback. Are you going to change that?

author: ttx450
author_rank: Regular Contributor
timestamp: January 13, 2022, 12:40:50 am
content: Quote from: bob91343 on January 11, 2022, 02:05:39 amHi Bob,You are talking 7ish on the schematic? I have not seen one. Here is the original photo of the underside and did not see anything like that or top. That does not mean it was not there. Is there a way to work around this? I am still learning schematics, but if I seen some lose ends or twisted wires I would have hope to remember.On the original oscillator coil, there are 5 lugs. I have connections between 4 lugs at diff points. I dont know how to translate the schematic numbers to the oscillator coil beside verbale. I have 3 drawings and all 3 are labeled different. 1 lug was open, could this be a gimmick in the oscillator?I get around 0.8ohms to pin (6) 12sA7 to ground B- through the coil. It actually goes to Pin (3) on 12SK7 B-. I dont get I have 2 B- points on the oscillator coil? (lower lugs)(correction)One lug of the oscillator coils upper lug goes to the Antenna stator, and through the oscillator coil to B-, 7.3 ohms.Not sure I am explaining this right, still learning along the way.Appreciate any help. Thks(pict was from original chassis when I got it)

author: bob91343
author_rank: Super Contributor
timestamp: January 13, 2022, 03:10:55 am
content: I can't really understand your message. The schematic shows one winding on the oscillator coil that has one end going nowhere. That is the gimmick. It's more of a capacitor than an inductor. It's used to pick off some signal for feedback so it will oscillate. When it does oscillate, it will do so at a frequency that is 455 kHz higher than the dial pointer indicates, in order to generate the intermediate frequency.As is common among manufacturers, the schematic diagram takes some study to discern what they are trying to do. Why did you decide to replace the coil? They seldom fail.

author: ttx450
author_rank: Regular Contributor
timestamp: January 13, 2022, 03:24:35 am
content: Quote from: bob91343 on January 13, 2022, 03:10:55 amI have no experience with these.Why, the one open lug. i.e. the p-c70-osc lugs - 1-2=5ohm, 1-3=6.2ohm, 4-5=2.5ohm. I only get 2 lug points on the old rca, one open lug?

author: bob91343
author_rank: Super Contributor
timestamp: January 13, 2022, 03:49:54 am
content: Please try to be more articulate. I read your messages several times and don't understand. Each sentence should be complete, with a subject and predicate. Do not abbreviate. I don't want to teach you English; I just want to help you make the circuit work.

author: ttx450
author_rank: Regular Contributor
timestamp: January 13, 2022, 11:44:04 pm
content: Quote from: bob91343 on January 13, 2022, 03:49:54 amHi Bob, I will pass on the English lectures (just pretend I am from a different country if that helps). So what do you think about the old oscillator coil with one lug open? Is that the gimmick? Thks

author: bob91343
author_rank: Super Contributor
timestamp: January 14, 2022, 01:37:41 am
content: Look at the diagram to see what the gimmick is. It seems to be a winding that only connects on one end.Which terminal is open? Draw a diagram of what you have. Take pictures. Before clicking on 'Post' reread your message and see if it conveys the meaning you intend, clearly without ambiguity.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 14, 2022, 02:36:08 pm
content: I do not think the part you have is the same as the part you wish to replace.It does not include L5 which is the coil that is open at one end.Your new part is a typical IF transformer. Not the special one in the schematic.I presume L5 is wound on the same coil form.It should be easy to add another winding of just a few turns if you have some enameled wire. You can count the windings of L5 on the old part .I also do not know the L values of the coils on the forms. However it appears that L4 is not symmetric, so I would use 1 and 2 on your new coil for the leads 4 and 5 on the schematic. Ignore lead 3 on the new part or just connect it to lead 2.Does the new coil look anything like the old part? Does it have the approximately the same number of windings?There is another IF coil. Compare this to new part and old part. Count turns.edit: I originally identified the coil with an open lead as L7, I think it is L5.

author: ttx450
author_rank: Regular Contributor
timestamp: January 15, 2022, 12:25:48 am
content: Thanks everyone for there help. This is puzzling to me, as I have several schematic and all are labeled different. If I understood how this is wired (chassis dressing) to schematic I would not be asking for help understanding.The hand drawing is from Ryder's page covering the RCA 35x. How this relates to the coil in the chassis is not clear to me. I can enplane where the leads go.My original RCA oscillator coil has 2 - broke wires. I can fix those, one I broke and the other I found broke. The problem or question is, one lug is open. It goes to the 12SA7 Is that the grid of the 12SA7, its labeled as 06, if I can read it correct.The RCA coil has 3 upper lugs and two lower. Both my 2 lower go to B- which I do not understand. 2 - of the 3 upper lugs go to 12SA7 cathode the other goes to 06 grid? The other goes to Antenna stator.With the one lug open is that the gimmick? Those leads go to the cathode & to the gid? labeled 06 on the schematic. In the Ryder's RCA drawing it goes to cathode and gid.Thanks, appreciate the help.

author: bob91343
author_rank: Super Contributor
timestamp: January 15, 2022, 03:13:09 am
content: Okay here's how it works.The gimmick is the 'open' lug that goes to the grid, marked 3. That is called a gimmick because it's a home made capacitor. It's shown as a coil because it's made of a couple of turns of wire that goes nowhere but provides a small capacitance to the next coil, maybe 2 pF or so.Pins 4 and 5 form a resonanct circuit with the variable capacitor that tunes the oscillator to the desired frequency.Pins 1 and 2 are in the cathode circuit and supply energy to the resonant circuit so the tube can oscillate.The likelihood is that the coil is okay. Why do you suspect it? In what way is the radio not performing?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 15, 2022, 11:24:03 am
content: Think of the "gimmick" as a little antenna feeding a little of the signal back to the grid.Maybe that would help.It is not on your new part. Again, you could put maybe 3-5 turns of wire on the coil and make this component.On your new coil, it has a center tap which is not on the original coil. The original coil does not have a center tap.The center tap is pin 2. So you need to use only 2 of the three taps 1,2 and 3Forget about the "suggested" schematics on your New coil schematic. It is not your circuit.Use the old schematic.IF the broken leads on your OLD coil are just near the lugs, clean the lugs and gently pull one wrap of wire (if you can) off the coil and connect it to the lug.IF you are going to use the NEW coil use (lugs 1 and 2 on new coil schematic) for the "primary" which is connections 5 (plate of tube) and 6 GNDOn the "secondary" (lugs 4 and 5 on new coil schematic) I think the lugs go to the trimmer caps that are on the R lower side of your chassis pic. One of these connections have B- that goes through the coil and will be present on the other side of the coil. Coils are wires after all.The new coil does not have the small third wrappingand if you use the new coil, you will need to make a third winding, the one that is one ended. The "gimmick" or antenna. That one goes to grids of the tube. I think this is grids 1 and 3 which are internally connected in the tube.The new coil (or transformer) is not the same as the old coil.

author: ttx450
author_rank: Regular Contributor
timestamp: January 16, 2022, 05:18:29 pm
content: Quote from: bob91343 on January 15, 2022, 03:13:09 amHi Bob, so the lug is the gimmick? I did not think it was built into the oscillator coil. This radio was a mess and did not work from that day I got it. It looks to have been passed around the ARF members or worked on. This is a learning project for me. Thanks..

author: ttx450
author_rank: Regular Contributor
timestamp: January 16, 2022, 05:29:35 pm
content: Quote from: Wallace Gasiewicz on January 15, 2022, 11:24:03 amHi, I am locked out with my bench Linux machine it seems after a update. Another problem to sort out. I went to login last night and it said I was banned from the site from spam.. work on that today.Thanks for the detailed explanation it make it a lot easier for me to understand. This is my first radio repair and was a mess when I got it and was worked on before. So I am going though the the whole thing rewiring, comparing to schematic, etc. This is my first closeup of how the oscillator coil works and understanding how it is wired. This radio is really a learning project for me as I am going through everything, its not about how fast to get this to play. I enjoy working on different parts and learning about that part of the radio. I very much appreciate you spending the time explaining it clearly to me.I will attempt to fix the two wires they are by the lugs. I might have to add a short wire, I have some thin wire that I made a coil for a crossover. This coil was soldered onto the chassis and wanted to make sure all works before installing it. Once I get this coil sorted out, I can finish dressing the underside and move to the top side. I post pict if I can get back on my bench machine later today. thanks..

author: ttx450
author_rank: Regular Contributor
timestamp: January 16, 2022, 06:05:56 pm
content: Got back on with my bench computer. Thanks to a Admin. It banned my IP, just had to change up my IP address and all is good.Here's pict of work on chassis.

author: ttx450
author_rank: Regular Contributor
timestamp: January 16, 2022, 06:19:47 pm
content: Quote from: Wallace Gasiewicz on January 15, 2022, 11:24:03 amHi, so when I look at the schematic of the new coil, #2 is the center tap. On the old hand drawn schematic (have to many drawings all labeled diff) #3 is the gimmick. To me this is a little confusing looking how the old schematic is drawn. Just trying to understand everything you wrote. Thanks..

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 16, 2022, 08:33:30 pm
content: I think you are correct#2 is the center tap on one coil of the schematic of the new part.On the hand drawn schematic #3 tap is the small antenna like coil that is not present on the new part.