National NC183D weirdity
I have been working on a 183D for a few weeks. Changed out all the caps, and at least half the resistors (I did if they were 10% or more off). It's a decent looking low time unit with very little wear on the gears. All new tubes except for the 5U4. I've gotten to power up time, and here's what happens.
With unit in standby
A. with the 5u4 out, I can bring the variac up all the way and all the tubes heat up like they're spose to. 6.3vac on all of them
B. with the 5u4 in, as I get toward 100-105vac with the variac, there is about 100v coming out of the 5U4 and through the filter choke. At some point around 110-115 on the variac, there is a flash in the 5U4, and the fuse is blown. The first time I did it, I had all the tubes in and I think the OB2 regulator flashed also, it's probably blown. I took the tubes out that get b+ even in standby-- audio out and the two oscillators, and BFO, and it did the same thing.
I've carefully checked everything I can think of, and it all looks good. from the B+ out of the choke, I measure several megohm resistance to ground (with those same tubes out). I know these have a reputation of bad power transformers, and maybe that's what's happening? Or maybe the 5U4 is bad? I'm thinking of making a plug in diode rectifier with a 200 ohm resistor or so in series, and disconnecting the B+ from the 5v winding. It seems I've read somewhere that the 5v winding gets shorted to the B+ inside the transformer and that's what causes the transformers to melt down and go bad.
That's all I can think of. Any help would be appreciated.
Last year I replaced the power filter caps on an NC-155 and had the 5U4 light up - as in the parts that are not supposed to be lighting up - and blow out. Next one did the same thing, although I caught it and shut it down before I lost the tube.
Problem was a short associated with my cap replacement effort. I did not have the exact replacement caps so I had to get creative with more modern ones and that was the reason for the problem.
Also, as part of that effort I tested a number of "NOS" and used electrolytic caps I had on hand. I do not have a cap tester but instead charged them up and noted their decay time. Allmost all of them were bad, and one had worked fine for about 30 min but went bad the next time I turned on the radio. So if you used some New Old Stock caps they might very well be bad.
So check very carefully for shorts, and if you can isolate the filter caps from the rest of the set and charge them up from a separate power supply that would be good, too. Start out at 12VDC and go up to close to the rated voltage. You can note the decay time and see if they are good that way. The one I ended up using shocked me when I was installing it - and it had been sitting on the bench after the test for close to an hour after I did the charge test!
And change out the 5U4 as well. The OB2 probably is good.
As for using a solid state rectifier, I bought a ready made 5U4 replacement many years ago and tried it out in my NC-155. It worked but the voltage was so much higher that I smoked a resistor when I left the set on Standby mode while cooking dinner one evening.
Very interesting that you check capacitors by discharging them. You can actually check the exact value of a capacitor this way. I have a capacitor checker, but it only will test up to 200uf. I restore stereo gear and they have some really large caps, 10-20,000uf. If you charge them up with a 9volt battery, then put a 10k resistor across the cap and measure how many seconds it takes to discharge to 37% of the intial voltage and multiply that by 100uf, you will have the measured capacitance value.
I'm not surprised that your old caps did not work out. Old caps are junk as far as I'm concerned, unless they're film, ceramic, oil, etc. electrolytics are good for 15 years of so, and that's all.
On the National 183D, I got it working. I think it is the power transformer gone bad, which is supposedly pretty common on the 183D. Something about the 5v winding that powers the 5U4. Anyway, I completely disconnected the 5v winding, and made up a SS rectifier on a tube base. I used 2 1N7001's and a 10 watt 200 ohm dropping resistor. I'm well aware of those plug in SS 5U4 replacements you can buy, and they give you too much voltage. The 5U4 usually drops about 30-40 volts across it. The resulting B+ I have now is about 7 volts under normal, which is fine with me. I don't like the B+ coming on right away at power up, so I reworked the standby switch so that it is now in the center tap ground of the B+ supply. The standby switch as it was stock only interrupted the B+ to some of the tubes. This way, if I power up in standby, the B+ is completely off and then I can turn it on once warm up is done. I'm also adding a 1 amp thermistor in the AC line, which will give about a 3 second soft startup when I turn on the power, giving those filaments a gentle start to the warmup process. Supposedly, this will greatly extend tube life. The thermistor will also drop the AC about 3 volts, which is great, because my mains here are just over 120, which will bring it down real close to 117v.
So, since my fix worked, I'm assuming the power transformer was going bad. hopefully the filament and B+ windings will last a lot longer. LOL
And you were right about the 0B2, it is fine.