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 Post subject: Blue Spark Shower
PostPosted: Feb Tue 01, 2011 7:53 am 
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Joined: Feb Tue 01, 2011 7:40 am
Posts: 14
Location: Moorpark, CA.
Hey all, this is my official first posting to ARF. I am VERY new to this fantastic hobby/sport/wallet-drain. In fact, I got my first radio in November and I'm hooked!

I'm currently working on an unidentifiable AA5 (probably a mid-thirties Belmont) and having many difficulties. But the problem/question I pose to you experts here is -

I thought I had a bad rectifier (80) because I was not getting any B+voltage at the plates, but the transformer tested fine - using the nifty "light bulb" test I saw online. So I bought a new one today, plugged it in, fired it up, and well.....it looked like a lightning shower of blue sparks inside the new rectifier tube.

Although it was fun to watch, I turned it off very quickly and immediately came to the forum to find out what's been posted about this. I've seen several explanations but none seem to fit. The caps are new, installed correctly, and not shorted. And the transformer seems to be doing it's job correctly.

I would really appreciate any wisdom you guys could impart - possible problems and likely solutions. Like I said I'm very new to this, so please be gentle and use little words :lol: .

Thanks guys!

-Mark


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 Post subject:
PostPosted: Feb Tue 01, 2011 10:47 am 
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Joined: Apr Thu 20, 2006 5:36 pm
Posts: 1527
Location: Holland, MI
If it uses an 80 it's not an AA5. /nitpick

Your Belmont may be identified with a picture and/or a listing of the tube complement. Having a schematic to refer to would help a lot. But the symptom sure sounds like a B+ short to me. If your electrolytics are new and properly installed, then some other miswire or hidden short is the likely cause. Again, a list of the tubes, a photo of the set and probably a few photos of your handiwork will get the ball rolling.


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 Post subject:
PostPosted: Feb Wed 02, 2011 5:42 am 
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Joined: Feb Tue 01, 2011 7:40 am
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Location: Moorpark, CA.
Hi Alan, Thanks for the response! And it's definitely not nitpicky...I could use ALL the information I can get. I tried to upload pics last night but they never showed up. I put them in the "temporary" file like the instructions say, but I can't find them. I'll keep trying, but in the meantime, the tube line up is:

80
42
85 (although it shows75 on the chassis socket)
6D6
6A7

Not sure this is any help and I'm not sure it's even a Belmont, but that was the only make I could find online that showed the same dial and decorative wood inlay that mine has.

As for the sparks, I checked for any shorts I could see and did find where one resistor was touching the chassis. Could this be the cause? When I fired it up, there were no sparks.....but unfortunately, there was no filament lighting up either. Could the new rectifier be blown?

ANY other suggestions of where to go from here are more than welcomed! Thanks again for taking the time!

-Mark


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 Post subject:
PostPosted: Feb Wed 02, 2011 6:52 am 
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Location: Holland, MI
When you say no filament, you mean just the 80? You can test for filament continuity with just a DMM, it's between the two large pins. It should read about 2 ohms.

I gave up on the photo gallery here many years ago; it's much easier to use Photobucket and post the links to the pictures with IMG tags.


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 Post subject:
PostPosted: Feb Wed 02, 2011 7:37 am 
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Joined: Feb Tue 01, 2011 7:40 am
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Location: Moorpark, CA.
Yeah, no filament on the 80 only. The other tubes light up fine. I'm guessing my new "sparkling" rectifier has thrown in the towel. I hate to go buy a new one if the same thing is going to happen. What do you think the chances are that the new one I bought was just bad to begin with?

-Mark


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 Post subject:
PostPosted: Feb Wed 02, 2011 8:55 am 
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Joined: Aug Thu 27, 2009 7:47 am
Posts: 2790
Location: Seattle 98125
You could have a shorted socket, some of the cheaper designs could crack and potentially arc over...I'd check voltages at that socket with the rectifier tube out, between all combinations of pins and make sure you don't have a short still.


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 Post subject:
PostPosted: Feb Wed 02, 2011 4:15 pm 
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Joined: Jun Fri 20, 2008 3:41 pm
Posts: 684
Location: Pueblo West Colorado
Mark,
Welcome!
The type 80 rectifier is a pretty tough tube. It takes a great deal of stress to blow one. My guess is the tube you received took a large mechanical shock. This can cause internal shorts in the tube.
Do you have a tube tester yet? It is good practice to test tubes when received to verify condition before installing them. Some of us old farts have made this mistake a hundred times and learned the hard way...so don't feel bad. It is very easy to install electrolytics in backwards. This too can cause havoc.
I would measure the voltages at the 80 socket with the tube out. If the results are good. Get another 80 and test it when it shows up. If you do not have a tube tester...get one. It will be necessary if you are going to repair many radios. You don't have to get the fanciest one to get started. If you can provide a model number, or pictures...we may be able to be more helpful.
Eric

_________________
"In the valley of the blind...a one eyed man is king..." -Erasmus


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 Post subject:
PostPosted: Feb Wed 02, 2011 5:37 pm 
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Location: Livermore, CA
Mark

Using an ohm meter check resistance between pin #1 of the 80 and chassis. Something may be shorted blowing the 80 tube.

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Norm


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 Post subject:
PostPosted: Feb Wed 02, 2011 7:22 pm 
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Joined: Feb Tue 01, 2011 7:40 am
Posts: 14
Location: Moorpark, CA.
I REALLY appreciate everyone's input on this. It's always hard to ask questions as a newbie for fear of sounding stupid.

On that note :) ...... When checking the resistance between pin #1 and chassis what value would show a short? And an even dumber question - does it matter if I take this measurement with the power on/off?

And, when checking the voltages at the socket with the tube out, should I still expect the plate voltages to be normal?

Eric - thanks for the welcome! I double checked the orientation of the capacitors and they are correct. I definitely want a tube tester, but am saving my pennies for a signal generator and variac first. Is that backwards?

As you may have guessed, I'm not only new to this hobby, I'm pretty new to electronics in general (my background is in hot rods - no capacitors, resistors, AC, high voltage, etc.). I'm currently working my way (slowly) through "Elements of Radio Servicing" and finding it extremely helpful. But it's heavy reading and it's taking some time for these concepts to sink in.

So again, thanks to all for taking the time to help a newbie. I absolutely LOVE this new hobby!

-Mark


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 Post subject:
PostPosted: Feb Wed 02, 2011 8:26 pm 
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Joined: Aug Thu 27, 2009 7:47 am
Posts: 2790
Location: Seattle 98125
Don't try to measure resistance with the radio powered on, you could damage your meter.

You should have an open circuit between pin and chassis; it's definitely shorted if it reads low ohms. Short is no resistance, so close to zero, maybe a handful of ohms if it's a glob of solder or something like that. I'd also check resistances between pins of the socket that aren't normally supposed to be connected with the rectifier out, to make sure it's okay.

HV measured with the rectifier removed will be a little higher than with it installed as the voltage will go a bit high without a load. This is normal. Under load, the voltage drops down to the appropriate level. The filament voltage should be constant with the tube in or out since the tubes will still heat.

It's totally possible that you just got a bad 80, though, hard to tell.

lyve55 wrote:
I REALLY appreciate everyone's input on this. It's always hard to ask questions as a newbie for fear of sounding stupid.

On that note :) ...... When checking the resistance between pin #1 and chassis what value would show a short? And an even dumber question - does it matter if I take this measurement with the power on/off?

And, when checking the voltages at the socket with the tube out, should I still expect the plate voltages to be normal?

Eric - thanks for the welcome! I double checked the orientation of the capacitors and they are correct. I definitely want a tube tester, but am saving my pennies for a signal generator and variac first. Is that backwards?

As you may have guessed, I'm not only new to this hobby, I'm pretty new to electronics in general (my background is in hot rods - no capacitors, resistors, AC, high voltage, etc.). I'm currently working my way (slowly) through "Elements of Radio Servicing" and finding it extremely helpful. But it's heavy reading and it's taking some time for these concepts to sink in.

So again, thanks to all for taking the time to help a newbie. I absolutely LOVE this new hobby!

-Mark


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 Post subject:
PostPosted: Feb Thu 03, 2011 3:57 am 
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Joined: Feb Tue 01, 2011 7:40 am
Posts: 14
Location: Moorpark, CA.
Well I just got home, rushed into the garage and checked the things you guys suggested.

When i checked the resistance from chassis to rectifier pins both heaters and one plate pin show a completely closed circuit - 000 ohms. Only one of the plate pins shows an open circuit.

Just to be sure I didn't have a totally shorted socket, I removed the wires from the tabs and got the same result - totally closed circuit from transformer wires to the chassis.

Am I doing something wrong or is my transformer basically melted inside? I'm confused because it passed the "dim light bulb" test and it sends 6v to the heaters.

-Mark


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 Post subject:
PostPosted: Feb Thu 03, 2011 9:52 am 
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Joined: Aug Thu 27, 2009 7:47 am
Posts: 2790
Location: Seattle 98125
Sounds to me like one side of the heaters ties to chassis. This isn't uncommon, I own several radios. A+ (one side of the low voltage secondary) and B- (center tap of the high voltage secondary) tie together at the chassis.

It really should show a few ohms, but it could just be your meter. I think your transformer is just fine.


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 Post subject:
PostPosted: Feb Thu 03, 2011 12:36 pm 
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Location: Holland, MI
focal, that's only for the 6.3 volt line. An 80 filament is at B+ potential; a connection to the chassis would short it out (causing a shower of sparks....)


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 Post subject:
PostPosted: Feb Thu 03, 2011 6:23 pm 
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Joined: Jan Sun 23, 2011 10:47 pm
Posts: 32
Location: Florida
Mark: Did you make yourself a "light bulb" current limiter?

http://tubenewbie.com/light_bulb_current_limiter.html

This can help save parts if you have a short somewhere.

Also, check out the link below for how to power up the first time. It is a link about guitar amps, but tube power supplies are tube power supplies, so the concepts apply. He is talking about powering up a newly-bulit, never before tested circuit. If you are in the situation where you have never had the circuit running before (you got it dead) sometimes it is easiest to start like it's totally new. He also has some safety info that is good to read if you are new at this.

http://www.paulrubyamps.com/info.html


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 Post subject:
PostPosted: Feb Thu 03, 2011 6:51 pm 
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Joined: Feb Tue 01, 2011 7:40 am
Posts: 14
Location: Moorpark, CA.
David - do you think this means a bad transformer, or a short somewhere in the B+ circuit outside of the transx?

JHow - I did build myself a dim bulb tester and the transformer seemed to check out fine which is making this even more confusing.

Thanks again for everyone's help on this!

-mark


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