[WallaceRoger]

Unfortunately I haven't tested the tubes, though that would probably be a good idea... I bought the three missing tubes which were sold to me as tested and in good working order, however the ones that came with the radio are unknown. I'm hoping to pick up a tube tester at the ARCI June swap meet.

Thanks Steve, I'll check the voltages.

[jcassity]

Yup,, just took a look at the dog bone resistor refabrication link.. You guys are the Best!

I noticed on my dog bones, the very center is a tad skinny"er" ,, like its hour glass shaped.
using a heat gun on the new plastic exterior tubing would make this happen.
you would need a heat gun adaptor to get your heat out to pin point where you want the shrinking to happen.

nice one !!!

[morzh]

Voltages on tubes could tell you somethinb but to me troubleshooting a radio without having all tubes tested is a waste of time.
My very first rule is: before the power-up test all there is that could be tested.
Includng continuity of the coils and transformers.
Then about 95% of the time the radio will simply come to life and you could proceed straight to the alignment​.

[WallaceRoger]

That makes sense. I think I'll set this aside until I get the tube tester, then start back from there, and then test the coills. The speaker needs to be re-coned anyway. Time to focus on other radios for now.

[Steve Davis]

No, go ahead and check the voltages, especially the plate voltages on the 24s and the 27. If there is no plate voltage on one of the 24s, there is an open winding in one of the RF transformers. If there is no plate voltage on the 27, the interstage transformer has an open primary winding (a common failure). If all the tubes light up, go on the assumption that they are good, at least good enough for the radio to work. You said the radio hums, that tells us the 71s and the 80 are working.

Steve

[WallaceRoger]

The multimeter I have is a cheap piece of junk... it hasn't been working right for days and "ist kaput". Into the garbage.

I bought a Fluke 77 IV from ebay. I'll check voltages and report back once the decent equipment arrives.

[morzh]

Keep in mind, modern DMMs will show somewhat higher voltages in tube devices; it is normal.

[WallaceRoger]

Ok, my Fluke arrived yesterday, so I went ahead and tested voltages. I got ZERO at every single socket, except for the #80 tube.

[morzh]

More specifically....what voltages do you have or have not?
I take it, you are talking of plate/grid voltages?
How bout filaments? Do they all have voltages?

You likely have either filter choke 27 or speaker field coil open.

[WallaceRoger]

Yes, plate voltages. I had never done this before, but I found someone demonstrating it on Youtube. I had the speaker unplugged because that's how he was doing the test, should I plug it in and retest? I didn't bother doing filament tests after the plate voltages came up zero, I figured something was seriously wrong (also I'm not sure how to do it correctly/safely).

I can't do full tests of the tubes yet as I had to order a new NE-51 bulb for the tube tester I bought.

[morzh]

Speaker unplugged....always a bad idea if tested with rectifier in. Never ever do that again (and yes it is the same as having your field open).

Although in 20s case it is not as bad, in some others it could be really bad.

Speaker should be in, and the integrity of output xfmr and the field coil should be confirmed first.
So should be confirmed the absence of shorts across the filaments and B+ voltage lest your radio goes up in smoke.
And.....before following some guy on youtube, you are better off asking here. At least we will try to make you understand your actions and the possible consequences; youtube won't do it.

[WallaceRoger]

Ok, I officially don't know what I'm doing then. I had all of the tubes removed when I did this. It sounds like the video was useless. I pulled all tubes, set the negative probe under the chassis to ground and used the hot (red) to test each socket.

How do I perform a proper plate voltage test? Do I insert all tubes except for the socket I'm testing?

EDIT: I now have a tube tester but I had to order a new NE-51 lamp for it.

[morzh]

Tubes removed...ok then, acceptable.

Still.


1. Establish the field coil and the 27 choke clntinuity.
2. Establish​ the output xfmr primary continuity, both halves to centertap.
3. Assuming the 71A tubes are good (and if truly bad they can kill res#25 1-2 section or output xfmr)
4. WITHOUT the rectifier power the radio and measure the 80 filament voltage: 5V AC.
5. Measure the rest of the filament voltages: 2.5V
6. Measure the ACbetween the two plates of the 80: see the chart but should be around 500V or more.
7. If all passed, power down, put in all tubes, plug in the speaker, connect the meter on DC to B+ and power up. Wait about a minute for the voltage to rise, see it bo around 300+V or so. See the chart for actual value.
8. Measure plate voltages.

Power down.

Watch for smoke and be prepared to power down.

PPS. Hi voltage (plates, B+) can kill you or put you on your butt: be careful.
PPPS. Make sure the speaker cord is good, no shorts, insulation reajages etc.

[WallaceRoger]

OK, here's my progress so far, and thank you for the help.

1:
* Field coil: I did what Terry suggested earlier in the thread and I get 240 ohm and 210 ohm for the first couple pins, the third pin gave 3,230 ohm)
* Part #27 Choke: 436 ohm

Since they weren't zero, I'm guessing continuity is ok (not sure about the one higher reading)?

2:
* Tested output transformer. One side gives me a zero (the one that connects to the 71A closest to the antenna terminal), which means it's open? What's a good replacement part for this?

I stopped at this point. Need some clarification so I don't kill myself.

4: Leave all other tubes (except 80) in for this? Which pins do I use for the filament test (the two larger, the two smaller, one of each, or doesn't matter)? And how do I know which pins are 1,2,3,4 (start bottom right and counterclockwise to bottom left)?
5: Need to know which pins to use for filament test (or if it doesn't matter)
6: Hmm, so I'll use the chassis for ground and which of the pins do I check for the plate voltage? (Is there a handy diagram someplace that shows which is which for various tubes? I do a lot of referring back to reference materials while I'm working).
7: This is done at the power supply? Which point?
8: Since tubes are left in, I'll do this from the underside, correct?

The speaker cord does have some of the wire shroud rubbed off where it was pushed against a wall for many years. Before I do anything else I'll fix that issue.

[Radioroslyn]

#1  Sounds good. Field coil and output transformer should ok.

#2 If you measure the resistance at  the plug with it unplugged from the chassis connecting one of the test leads to one of the pins and go around checking the resistance of the other 3 you should have 2 of the pins that show a resistance around 200-300 ohms. This is the primary of the output transformer. The 3rd measurement will be much higher like 3500 ohms, this is the resistance of the field coil. As per #1 you have this.

*3 Don't know????

*4 See drawing

*5 Depends on what you are looking for? If you want to test for filament continuity (the part that lights up), remove the tube from the set and measure the resistance pins 1 and 4 (on 4 pin tubes) and on 5 pin tubes it's 1 and 5. See *4.
If you are checking to see if you have filament or heater voltage, use your ac volt meter across the pins mentioned above.

*6 Yes. 4pin tubes pin two is plate and this is where your find HV DC also sometimes called B+. The 80 tube has two plates (2 and 3) and these will have HV AC on them.

*7 You can measure the HV at pin 1 or 4 at the 80 tube, - lead at chassis and + at pin 1 or 4.

*8 You can't measure the filament resistance when it is installed in the set as it is in parallel with the filament winding of the power transformer. If you do it will give you a low resistance reading (the winding) and not nessescerally the filament. Other than that yes.

Hope this shed  some light on your questions.