Hey all, I finished recapping and changing resitors in a #18. I was told the radio didn't work before I got it so I didn't precheck anything before the recap as the cord was in poor condition and the chasis appeared to be complete and all original. Looking at the schematic for the 18 model, I keep burning the 200ohm part #7 after about a minute on power up. Tried swapping the R.F. tube, no change. I remove the Det. Ocillator tube #6A7 and then resistor #62 burned, #7 is still ok at this point. Before I fill the house with anymore smoke, help in a direction would be great. Sorry, I don't have a tube tester right now so I am hoping that someone with more knowledge can see something that I am missing. Let me add that I am very confident that no mistakes were made on the recap. Everything was checked and referenced at least three times before I moved on to the next.
According to the diagram you posted part #7, the 200 ohm resistor is a cathode resistor, one end goes to ground the other to the cathodes of the 6A7 and the 78 RF tube, there isn't anything there that would obviously cause it to smoke if it was wired correctly. Part #62, is a 50,000 ohm resistor, it looks like it's paralled with a 2 mf capacitor/condenser, that could be related to it smoking but I'm not sure why, if anything the 2 mf shorting out would burn part #61, the 32,000 ohm resistor since the cap is in series with it and ground.
Regards
Arran
By putting the tubes back in, part 62 no longer burns. It is hard to check the voltage at the cathode resistor since I only had 20-30secs before it burns out. I have double checked the wiring and that part of the circuit is correct per schematic. Only discrepancy I find is the the bakelite cap is 4989-ac rated for .18mf. using one lug on the bakelite, parallel to the 200 ohm resistor. It is superseded to a 4989-odg rated for two .09mf with common ground but using separate lugs on the bakelite. Now what? Hopefully someone can shed some light.
I will add one more thing. Could not having the speaker output plugged into the chassis be causing my issues by overloading the rest of the circuitry? I could try plugging it in and powering up later, just thought I would throw it out there.
(03-12-2012, 08:32 AM)raydog Wrote: I will add one more thing. Could not having the speaker output plugged into the chassis be causing my issues by overloading the rest of the circuitry? I could try plugging it in and powering up later, just thought I would throw it out there.
A radio with an EM speaker should always stay connected to chassis when out of the cabinet, as the field coil ( in your Model 18 ) does double-duty as the second power supply filter choke, as well as the "magnet" for the speaker.
If disconnected, the power supply rectifier and the filter caps can see excessive over voltages, or any resistors and other components, depending on the circuit design.
(03-12-2012, 10:33 AM)Chuck Schwark Wrote: I will add one more thing. Could not having the speaker output plugged into the chassis be causing my issues by overloading the rest of the circuitry? I could try plugging it in and powering up later, just thought I would throw it out there.
A radio with an EM speaker should always stay connected to chassis when out of the cabinet, as the field coil ( in your Model 18 ) does double-duty as the second power supply filter choke, as well as the "magnet" for the speaker.
If disconnected, the power supply rectifier and the filter caps can see excessive over voltages, or any resistors and other components, depending on the circuit design.
Chuck
I knew there had to be a piece of this story that was missing. Never turn your set on without the speaker plugged in. As Chuck sez you can really screw up your power supply and other stuff too!!!
Terry
You really need to convince your resistors to stop smoking lest they die a horrible untimely death.
Seriously, note well advice about speaker field coil and output transformer being hooked up anytime you run the set. If either one is bad (open or shorted) stop. It's only going to get worse if you don't.
I have a few ancient analog DC ammeters to have a look at the current draw. My favorite is the 500 MA. If it goes way up, I rip the plug out and find out why. Got larger and smaller ones too, depends on what I reasonably expect to find. Red terminal to positive side. OK?
Here's an opportunity to do some real detective work!
Start out by disconnecting R7 so it doesn't overheat.
Begin disconnecting B+ wires that enter into this area of the radio.
Obvious choice is the wire that connects to R17 and see if the voltage is still present up to R7. Keep disconnecting wires until the R7 voltage is much improved. Not hard to make a solder bridge, etc.
Thanks guys, although I won't have a chance to try the set with the speaker plugged in for a day or so, it does make total sense. I too figured something was loading down the circuit, never thought about the speaker. Well not out of the woods yet, but one step closer. By the way, only two resistors were harmed during the smoke shows and sure nothing else was damaged. Was real quick cutting the power at the first sign of smoke. I will report back.
(03-12-2012, 08:32 AM)raydog Wrote: I will add one more thing. Could not having the speaker output plugged into the chassis be causing my issues by overloading the rest of the circuitry? I could try plugging it in and powering up later, just thought I would throw it out there.
Now that I look at the schematic, yes it could, if you look at part #61 and part #62 you will notice that with the field coil disconnected they form a path to ground for the B+ supply. As with anything electrical the electrons follow the path of least resistance normally this is the field coil, with the field coil removed it's now that pair of resistors. It's a weird design compared to most sets, normally the output of the first filter cap goes straight to the field coil and the output transformer but these guys added a resistor network on that side of the field coil as well.
Regards
Arran
(This post was last modified: 03-13-2012, 06:55 PM by Arran.)
Hi Arran, it sure sounds like you know your business. That is what caught my attention too, didn't make sense that different resistors would be overloaded by simply removing different tubes. I figured there had to be a common denominator somewhere. A few things came up this week so hopefully I can continue this weekend with plugging in the field coil and move on to fine tuning. Hopefully thats the answer.
I would understand if those #61 and 62 smoked, things being the way you explain. This would take a lot, as the resistors are probably 2-3 watters at least, and the voltage without load, I take it, about 400V or so, and so between the two of them they will dissipate 2W, which is far from making them smoke.
But #7 smoking.....removing the field coil actually removes the power from just about anything, except from res. #22 / 21, which is hi-resistance, and DC-wise foes to one of the Det.osc. grids, so there is no DC path.
No current flows anywhere anymore.
How does that makes #7 smoke, I fail to see, unless, there is some short somewhere, or something else.
A long shot - shorts within coils and within the tuning capacitor may do it, but if this is true, then this would be a good day to go and buy a lottery ticket.
(This post was last modified: 03-17-2012, 06:45 PM by morzh.)
I would understand if those #61 and 62 smoked, things being the way you explain. This would take a lot, as the resistors are probably 2-3 watters at least, and the voltage without load, I take it, about 400V or so, and so between the two of them they will dissipate 2W, which is far from making them smoke.
But #7 smoking.....removing the field coil actually removes the power from just about anything, except from res. #22 / 21, which is hi-resistance, and DC-wise foes to one of the Det.osc. grids, so there is no DC path.
No current flows anywhere anymore.
How does that makes #7 smoke, I fail to see, unless, there is some short somewhere, or something else.
A long shot - shorts within coils and within the tuning capacitor may do it, but if this is true, then this would be a good day to go and buy a lottery ticket.
The schematic and parts list does not give a wattage value on resistors #61 and #62, it also does not mention whether they are wire wound as it does with some other so we would have to assume that they are both carbon composition resistors. From the under chassis parts layout diagram you can see that resistor #61 looks like it may be a 2 Watt but #62 is no more then a 1/2 Watt judging by the physical size shown. In those days it would be rare to find a carbon resistor over two watts anyhow, especially in a circuit that would not normally be dissipating up to 450 volts should the field coil be installed.
As for the part #7 resistor there are two ways that I could see a potential B+ path to ground. The first would be a grid to cathode short in the 6A7 tube, although you would need two grids to short together and to the cathode which seems unlikely. The second way would be for part #19, the .0001 uf capacitor to short out, this seems more likely especially if it's a paper capacitor, not common for a mica to do this but not unheard in a radio this old either. The spike in B+ voltage may hve punched a hole through the dielectric of #19 capacitor, this is assuming that there were no wiring errors and that there wasn't anything causing a short by accident like a lead bumped over or a blob of solder falling in the wrong place like between two pins on the tube socket.
Regards
Arran
Still haven't had a chance to continue on with this yet, have had family here for the weekend. To answer some questions, #62 is a 1/2watt, #61 is a 2w. Part #19 was not replaced as it is a mica. I believed I checked it's integrity but will check again for a short on the mica cap.
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