I finished recapping and rewiring a 42-350. In the end I replaced almost every wire, cap, and resistor in the radio. I took notes before disconnecting anything so I could put it all back right, but my notes weren't good enough, apparently. What happens is that resistor 78 burns up. This resistor connects directly to the center tap of the power transformer secondary and provides bias for 7B5 g1. I used a pair of 1 watt resistors in series to make up the 270 ohms, though I believe the original 270 ohm was a 1 watt.
I've compared all the power supply wiring with the schematic and fixed a mistake or two. No change. The power transformer resistance seems OK. Primary about 11 ohms and the secondary about 260 to 280 on each leg. The filament winding is about .6 ohm.
I am at my wits end and don't know what to look at next. I'd appreciate it if one of you more experienced guys could suggest new troubleshooting steps to me.
I made some measurements after substituting an 1.8K resistor in place of the field coil and a 1 ohm resistor in place of the voice coil, and using a 40W dim bulb tester. I measured about 1.4 ma through resistor 78 and about 18 Volts AC across it. DC voltage across it was almost nothing. There was about 2.7 VAC across the 18K resistor substituting for the field coil, and again DC volts were almost zero.
Resistor 78 measures 297 ohms to ground, just as it should, so there doesn't seem to be a short there.
Any thoughts for what to check next, or any specific wiring error that could make this happen? This radio worked before I got my hands on it, so it makes me sick that I've messed it up.
Raleigh, have you checked the power transformer for a short in the high voltage winding to ground?
Steve
Releigh, the 270 ohm resistor (78) and the 37? ohm resistor (77) in series from ctr tap HV to gnd. 78 goes to XXFM grid via 10 Meg resistor (70) not to 7B5. First grid 7B5 goes directly to ctr tap HV through 76K resistor (64). Check your wiring in this area, appears something is wrong. PL
Raleigh, got correct part numbers thats a 470K (69) not 76K (64) and the series resistor is 27 ohms (77). PL
Guys, thanks for your help on this.
Steve, I checked the power transformer secondary for shorts to ground. At first it looked like one leg was shorted to ground, but I removed the 7Y4 rectifier and the short opened up. I checked the 7Y4. Nothing is shorted to ground, but it looks like pins 1, 6, and 7 are shorted together. Pin 1 goes to pin 8 through the filament of course, and pin 8 is grounded through the socket wiring. In my other 7Y4 all the pins are open. With that tube installed, there isn't any apparent short of the high voltage winding.
I need to replace the burnt resistors and try again with the "good" 7Y4. By the way, I checked all the tubes and found another shorted one. One of the XXL/7A4 had a 70K ohm short between pins 2 and 6.
Planigan, you are right about the wiring on the positive side of resistor #78 (between 78 and 77). I wasn't clear that I meant the transformer side of the resistor biases the 7B5 through the 470K resistor #69. I checked that wiring and it looks good. The center tap, resistor 78, and the 470K #69 are connected to pin 5 of the 7B5 socket, which is an NC. On your recommendation I checked the other side of resistor #78 and that wiring looks OK too. Ditto the other side of 77, and that is also OK, as far as I can tell.
I'm going to try again with the 1.8K resistor substituting for the field coil, just in case. How much B+ current should I expect through that coil? Less than 100ma?
John, I think you have hit on your problem. If one leg of the high voltage winding is shorted to ground, either in the transformer or through the rectifier, you will have an AC voltage on #77 and #78.
Steve
Also check for the possibility of a shorted or leaky rectifier tube socket...
Raliegh, if pin 1, 7 & 6 (heater, cathode, plate) are shorted you have a direct connection of one half your secondary HV AC across the electro cap (76) then through resistor (78) to ground. That should be enough to fry it. Tube must be bad but also check as Ron said and make sure the socket isn't shorted too. PL
I think I've made some progress in the power supply, but I'm not listening to the radio yet.
Basically, after I replaced the bad rectifier and oscillator tubes that had internal shorts, I still had too much current through the 1800 ohm resistor I was using to substitute for the field coil. I suspected I had wired the speaker cable wrong, but I found it confusing to trace it down because the tracers were so faded, so I finally decided to replace with new colored wires.
Once that was done, I set the variac at 117 VAC. The DC voltage at the rectifier and B+ at the output tube plate seemed right, and the current was a steady 43 milliamps through the resistor, so I decided to reconnect the speaker. Now I have some AC hum and static noises in the speaker, but no signal. Tuning through the dial makes no difference, but I get a little pop in the speaker when I touch each plate in the signal path with the meter, but not the oscillator.
B+ readings at each plate were all close to right (5 to 10 volts low or high) EXCEPT the oscillator, which was 51 volts instead of 75. Oddly, the high side of resistor #41 (33K ohm) is about 170 volts, which is close to what I think it should be, but the drop across that resistor is much higher than seems right. That makes me think the oscillator plate current is too high.
I ran out of time last night, but when I get back to it I will look at voltage on the high side of resistor #42 (2200 ohm) and replace the oscillator and see what I can see.
Any other suggestions would be welcome.
Raleigh Wrote:B+ readings at each plate were all close to right (5 to 10 volts low or high) EXCEPT the oscillator, which was 51 volts instead of 75. Oddly, the high side of resistor #41 (33K ohm) is about 170 volts, which is close to what I think it should be, but the drop across that resistor is much higher than seems right. That makes me think the oscillator plate current is too high..
Can't access the schematic at the moment but my 42-327 had similar osc problems. One of the mica caps in the osc circuit was bad.
-Bill
I had just a few minutes this evening, so I took out the oscillator tube to see if it might be shorted. It wasn't, so I put it back in again and turned on the radio. After tuning up and down a little I realized that the volume control was turned way down. When I turned up the volume I got faint stations on the AM band.
I stretched out my workbench antenna and did a quick and dirty "by ear" alignment, which gave me good volume. The band switch was dirty and took some careful wiggling to make contact, so I squirted it with deoxit. I'll wait until it all dries before I turn it on again make any measurements on the oscillator.
I've got more work to do, but the signs are lookin' good!