[ccomer1955]

http://www.nostalgiaair.org/pagesbymodel...006713.pdf

This one looked to be a simple recap, but ended up frustrating me so far. I replaced all caps and most of the resistors by lifting one end, soldering in the replacement, then lifting the other end and soldering in the other end of the replacement. The filter cap arrangement puzzled me, but I ended up with the low voltage 22 uf running with the positive end on the CT and the negative end on the chassis. The FC measures correctly, the output transformer as well, as do the IF coils. When I applied power, the set failed the dim bulb test with a 40 watt bulb just at the 105v mark.

I rather stupidly tried to take no-dim-bulb voltages as quickly as possible at 110v line voltage on my varian and got the following more or less (the 6A7 voltages are suspect to me because I was moving too fast): 80: F=5.2, P=623; 41: heaters=6.5 (I was working quickly, so perhaps the 80 warmed up as I went along. I got the same on the rest of the heaters), 2=275, 3=226, 4= -72, 5=0; 75: 2=106, 3=104, 4= -396 mv, 5=not sure, 6=.9v; 6D6: 2=89.2, the rest are zero; 6A7: 2=216, 3=88, 4=88, 5= -2.2, 6=0; 76: did not measure (remember before when I said "stupid?"). The small voltage cap popped just at that moment. I had used a 20 uf @ 50v for that one and it blew. Don't want to do that again.

The power transformer and other windings throughout seem to have survived without issue, so I then replaced all of the filter caps again with a 22uf 450v, 10uf 500v, and 22 uf 35v (what I had on hand), and I put a small terminal strip in place as a junction for all the filter caps' negative side and the CT so they lie in the chassis with a bit more stability.

I remeasured all my previous work and found a couple of things to set right. I had replaced R42 & R43 (the closest I had for R43 was a 300 ohm resistor), and I resoldered a couple of places where things seemed a bit close to me.There was originally a .05uf Motorola branded cap connecting pin 3 of the 75 to pin 4 of the 6D6. It looked to be factory installed, but I can't seem to find it on the schematic. I replaced it with another before the first catastrophe. I remember the original of this cap was quite burned by the 270 ohm resistor that runs along side it in the chassis (R42) making its value hard to read. I wonder if that is significant to the current problem?

With the changes in place, the set lights up my dim bulb just at about 75v or higher. It seems to dim somewhat after lighting up, but remains lit and gets brighter if I increase the voltage, so I haven't gone any further on voltages.

Originally, the set had an old repair, an 8 uf cap running from the chassis and pin 3 of the 41 tube (which has a red wire that goes to the .1 cap that connects to pin 5 of the 6D6 which I have left out of my repairs. I’m not sure why the 8uf cap was there since the original cap was still in place...hum management, and an indication that something else was wrong?

I also am going to wait until I get a couple of new tubes before trying to power up again since the 41 is marginal at best and the 6A7 could be better. Everything else tests strong including the 75 (all three tests).

So, can any of you recommend a methodology to help me track down the short? This is still not my long suit.

[ccomer1955]

Also I've been tracing the circuits from each pin on each tube today, and I can make sense of all of it EXCEPT that .05 uf capacitor that runs from pin 3 of the 75 to pin 4 of the 6D6. Can any of you pros tell me which cap on the schematic that is? It is the one that was a Motorola cap that was scorched before I replaced it. Somehow, I'm not finding a cap coming off of pin 3 of the 75.

[Radioroslyn]

Well there is a good reason why you don't see it cause it ain't there! Is the suppresser grid tied the cathode and then to the chassis?? Like the schematic sez??
I would suspect that the .05 is just scorched form the heat of the bias resistors. Connecting the suppresser grid of the IF amp to the diode in the 75 tube isn't doing much of anything. I'd tie it to the chassis.

Terry

[ccomer1955]

On the 6D6? Pins 3,4, and 5 of the 6D6 are all tied together and to the chassis from pin 5 (it came that way), but pin 3 of the 75 is connected to the sw antenna coil, the 820K ohm resistor and the IF, and that .05 cap that goes to pin 4 of the 6D6 which I guess does ground it to the chassis, right? So you say either remove that cap or ground it to the chassis? Hmmm. Is the cap C20, then?

[Radioroslyn]

What 20 does is provide a RF ground for the ant coil and the secondary of the IF transformer. If you follow that buss it ends up at the 75 tube diode this provides the - DC voltage which controls the gain of the two circuits (ant and IF).This is called AVC automatic volume control or AGC automatic gain control. The more signal at the anttena the more - voltage that is developed. The more - voltage on the AVC line the less gain the mixer and IF amp will have.
Bottom line is if 20 was shorted the set would work fine but would not have any AVC action. If it  was open then it creates a problem of low gain.

Pin 3 of the 6D6 (screen grid) should have hv on it and NOT be connect to the other pins. If 3 is hooked to other pins that's your short!

Terry

[ccomer1955]

My mistake. It is pins 4,5, and 6 of the 6D6 connected together and grounded to the chassis. Sorry. Old age caught me on the way downstairs to my computer, and I failed to re-read my original post where I have it right. So, as before, pin 3 of the 75 and pin 4 of the 6D6 connected by a .05 cap (C20?). So it looks like that isn't the issue?

I'm pretty positive I have the filter circuit done correctly: 22uf 450v runs positive to negative from pin 2 of the 80 to the CT, 10uf 500v runs positive to negative from pin 3 of the 41 to the CT, and 22 uf 35v runs positive to negative from the chassis to the CT.

Both dial lights light up, and the insulation on their wires looks intact. The on/off/volume pot shows 500K and the homage changes when I turn the knob (measuring from the middle lug to either outside lug). I get good continuity following the pathways for all of the top grid wires. And I can't find a short in the tuning cap, either. I tried the dim bulb test after I cleaned up a few busy solder points, and I still get a soft glow at about 75v that dims a bit, but steadily increases if I increase the voltage. By the time I get to about 100v, the light bulb has a steady moderate glow (not bright, but not dim, either).

If that is all correct about the filter circuit and the attempts to trace a short, then is there a methodology to narrow the search for a short, or should I just continue looking for mis-wiring and crumbling insulation (most of the wiring is cloth-wrapped)? Perhaps adding resistance along a bus and looking for numbers that are too large?

[Steve Davis]

Charlie, what wattage bulb are you using for your dim bulb test?

Steve

[ccomer1955]

40 watt. It's also a clear bulb which means I can see the filament glow a bit easier. I'm not sure that's a good thing. I may be a bit over cautious, but I really don't want to pop another cap or hurt a tranny.

I did one more quick application of voltage...not long enough for a real warm up, but I let the voltage stabilize in stages as I ramped it up on the variac. All in, maybe 5-7 minutes to get to 100v or a bit higher. I only measured the voltage relative to the chassis at the CT (where the filter caps come together), and it seems way too low even without a longer warm up. I measured a -5.5v there...shouldn't it be more like a -25v? I wouldn't think I would make up the extra voltage with a proper warm up. Maybe I should wait to do more investigation until I get new tubes in that don't need a 5 minute or longer warm-up?

[Steve Davis]

I think you need to use a higher wattage bulb, with the radio working normally, a 40 watt bulb will light very bright. With a six tube radio, you need about a 100 watt bulb.

Steve

[ccomer1955]

I tried it with a 60 watt bulb in place and get a dim glow at 100+ volts (it starts around 85v or so). I kept a volt meter on the CT and after a couple of minutes the voltage was -9v and rising when I cut power. I've replaced the 41 with one of the 41s from my 46-280 set and warmup time should be reduced. All of the tubes test in the 70s or better on all tests at this point. I get a growl from the speaker when I touch the ungrounded side of the volume control with a soldering iron, so I'm guessing whatever the issue is, it is not necessarily in the output section.
Around 90 volts on the variac, I get a hum on the speaker that continues as I apply more voltage.

The only voltage other than the CT I took was the plate of the output which was pretty low, but Like I said, I haven't let the set properly warm up since I popped the small electrolytic cap. When I popped the 22f @50v cap previously, I did not replace the 10uf @ 500v or the 22 uf @ 450v caps. Do you think I might want to do that?

Or should I fire up the signal generator and start in with the grid of the 75 or 6D6?

[ccomer1955]

I went ahead with the signal generator and no dim bulb, and the results were encouraging. I was able to produce tone through the oscillator and finally through the antenna. Nice and clear with plenty of volume. Still no signal and noticeable hum, so I put a ground on the set and about 25' of antenna wire and got clear signal on both SW and Broadcast. The hum is still there which I guess explains why the guy in the 50s placed an 8uf e-cap between the chassis and the output.

Voltages with the line running at 115vac are still a bit off: the voltage at the CT is -17vdc and the voltage at the output plate is 270vdc. They should be -25vdc and 285vdc. Could having a 300ohm resistor instead of 270 ohm at the CT affect that voltage?

Correct me if I'm wrong, but the hum shouldn't be there. The reason it's hard to tell is that I have bad electrical interference at my house. Just about all of my radios have bad hum between 1100 and 1600. I'll try again later after the washing machine is done and the televisions are turned off, but I might call this one sort of fixed?

I have to do something more permanent for the antenna and ground. As of now, they are two wires with the black one soldered to the chassis and the white one soldered to a lug on the band switch. I think they had a terminal on the fiberboard back the radio used to have, but I've never seen a picture, so I don't know.