[EricS]

How can I test an 84 rectifier tube when I dont have a tube tester? I have been looking online and havent found much to help. Is it just easier to buy another one on ebay for $10?

[morzh]

And it is 50/50 chance the fuse will still blow.

Eric

you gotta learn how to measure things or you will get quickly frustrated and give up.

The easiest way to check the tube, very basic, is buzz between the

1) Between pins 4 and 2, and then 4 and 3. There should be a clean open, NO conductivity whatsoever. Discard if there is any.
2) Between 1 and 4, and then 5 and 4. Same as above.
3) Between pins 1 and 5. There should be some measurable resistance in Ohms. If open, discard.

In case 1 or 2 are bd, there is a good chance this is your problem with the fuse.


http://www.radiomuseum.org/tubes/tube_84.html

[EricS]

Thanks for the link - this is helpful! I'm new to tubes, all of my experience is in solid state and I'm finding this to be quite different.

I have zero ohms (complete connectivity) among pins 4 and 2, 4 and 3, 1 and 4, 5 and 4. The same is true for pins 1 and 5. Seems like a dead tube. I'll get another tube.

Thank you!
Eric

[jerryhawthorne]

Morzh is dead on. Perhaps easier as I suggested earlier is to pull the dang tube and check again the resistance from the + of the first cap to ground. If it goes really high, the tube is your problem as it should be very high. If it remains low (shorted) your problem is not the tube. You HAVE to get the resistance on that cap to ground very high. It is either the tube which you can check by just pulling it and measuring resistance from that cap to ground, or a short near near the tube socket or from the filter cap to the plug for the speaker. That cap goes to the speaker plug to add the field coil to the filtering network and then returns to the second cap. If the speaker is unplugged it indicates the problem is very localized between the cathode of the rectifier to the 12mfd cap on to that plug going to the speaker.
Make sense? Look at the schematic that shows the field coil for the speaker following that first cap. What it does not show or indicated is that is part of the speaker. Hence, going through the wiring to the speaker, through that coil and returning to go to the second filter cap.
Looking forward to hearing your resistance reading with the tube out. Make certain your meter is set properly at perhaps 10K full scale with positive to the cap and negative to ground.
Jerry

[morzh]

Well, there you go. Yes, a dead tube is the first usual suspect.
The load short is the second (pretty much 'cause they are lined up left to right in the sch).

However this does not relieve you of the duty to check the load for a short, which is measured across the first electrolytic or, which is the same, pin 4 of the 84 socket and the transformer centertap (or you can use the chassis, then it will be 200 or so Ohm instead of short).

[morzh]

Jerry

My aproach is always "check everything". Even if you have a shorted load, it does not mean your tube is good. Gosh knows, it might be the shorted tube that caused the shorted load. Or vice versa. Not likely, but not impossible. Better check it than blow another fuse and lose another tube / cap.

This is why I still want Eric to inspect the load.

[jerryhawthorne]

Mike, I think we are both asking the same things. The + of the first cap to ground or center tap should read very high, one about 200 ohms higher than the other. Pull the tube and see if they read very high in several hundred K of resistance. At this point it's a moot point which point to check at. They are both reading terribly low. Enough to pop a fuse for sure. I would like a response Eric is resistance reading from that cap to ground (easier to find than the center tap) with the tube pulled. We shall both stand by.
Eric, Mike (Morzh) is a ton smarter than me on this stuff. I just try to put it to a level that I understand.
Good luck, Jerry

PS: Eric, don't keep adding fuses or power until such time you resolve the "short".

[EricS]

Hi guys, sorry for the silence - I've been traveling since last week. I purchased a NOS 84 tube and it measures properly with my meter - filament is good and all other pin combinations are open.

After inserting the tube and connecting the speaker, the measurement you asked me to make from Pin 4 of the rectifier to chassis ground measures completely open instead of the 200 ohms you specified. All four wires on the speaker harness are unbroken and show good continuity. The speaker coil measures about 5 ohms and the field coil measures about 1000 ohms.

With the 84 tube pulled, there is no connection (infinite resistance) from pin 4 to ground. The same is true from the pos terminal of the second cap to ground.

Any ideas on what to check next? I'm starting to think that a tube tester would be a good idea...

Eric

[morzh]

Next if you are done recapping etc you can put a V-meter across the rectifier output, turn it on, and see if
1) High volage (about 350-400V, shooting up to 450V) develops, and
2) Nothing glows red, smokes, crackles etc.

Then you could start seeing if the radio is alive etc.

[EricS]

Thanks for the protocol, Morzh. Should the other tubes be installed when I power up? I'm thinking I want to put my light bulb inline with the ac cord to make sure it doesn't glow brightly still.

[EricS]

OK, with the speaker disconnected, I get 343v from the rectifier and all tubes are glowing! No crackles, no funny smells, etc. i kept the light bulb in series with the AC mains and it stayed completely dark during power up. Next, I'll connect the speaker and see if I can get any reception.

So, if all of this was caused by a bad rectifier, what used to happen when there was no fuse and the rectifier shorted? It seems that the radio might be more fragile than the old screw-in fuses for the house outlets.

[EricS]

If I have measured correctly, the power supply to the the pilot light is 6.75v AC. I have never come across a low-voltage AC lamp before. Is this a specialty item, or will a 6v DC bulb work here?

[morzh]

Incandescent lamps are not AC or DC, they are RMS value. Which is either AC or DC.

Your figures look fine, but you are better of with the speaker as this will discharge your lytics when you turn it off, as without it B+ does not get to the tubes and the lytics stay charged after turn-off, and to the higher value (possibly even exceeding their WV rating) too!

Your 6.75 V should be 6.3V but this is likely due to your outlet being 120V and not 110V.

I think you are good to go.

[EricS]

I replaced the dial lamp, restrung the dial cord, and put the chassis back in the cabinet. With fingers crossed, I fired it up for the first time. All of the tubes light up, the 7B7 IF tube is the most dim, almost can't see the glow from it.

Lots of motorboating through the speaker. I was able to tune in one weak station and hear recognizable words, but that was about it. The frequency of the motorboating noise changes with the tuning. Ran through the tuning push buttons and the AM, SW, and police bands. More of the same- sounded like a bad soundtrack to a 50's sci-fi flick.

Does this indicate that the next step is to perform an alignment that I've heard other people mention?

Little by little...

[morzh]

Its a good sign. No smoke and some reception.


Make sure
- you have all caps changed, tubular/lytics/others if present, no exceptions other than micas
- all tubes checked
- resistors confirmed and changed if necessary.