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 3V4 Shows No Short In "A" Position

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Johnny



Posts : 7
Join date : 2010-10-12

PostSubject: 3V4 Shows No Short In "A" Position   Thu Oct 21, 2010 3:22 pm

Hi all, Don't like getting used to a new format but if it's what we have for now I'm very grateful.

I've got an RCA Victor "Strato-World" 3-BX-671. http://www.nostalgiaair.org/PagesByModel/896/M0014896.pdf
Strange looking URL but it's the 1st time i've linked one at this temp site. Hope it works.

The guy I got it from said it was a 7-BX-10 but it has 3-BX-671 plastered all over it and it doesn't have the ballast tube. Anyway when I test the 3V4 audio tube my Heath IT-21 tester tells me that my short light should glow when I switch switches "A" & "G" to the up position. "A" switch does not illuminate my short lamp. Only switch "G". As for how it tests it's showing in the weak area at about 50%.

Should this "short" lamp illuminate with the "A" switch in the up position? I guess I should replace this tube?

Thanks,
Johnny
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radiotechnician



Posts : 35
Join date : 2010-10-12

PostSubject: Re: 3V4 Shows No Short In "A" Position   Thu Oct 21, 2010 5:17 pm

That could indicate 1/2 the filament is burned out. Is the filament setting 1.4 volts? Also would explain 50 % emission. use your ohmeter betwwe pin 5 - 1 and 5-
7 to find out.

de
VA7ASO
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Johnny



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PostSubject: Re: 3V4 Shows No Short In "A" Position   Thu Oct 21, 2010 6:57 pm

Looked at the tube data on this. There's no continuity between pins 1 & 7, 1 & 5 or any other connection to these three pins. I read an open between all three pins. After reading this I went back out and threw it on the tester again. Yes! It is 1.4 volts on the filament setting but this tube doesn't light up at all. Turned out all the lights and in the dark there's nothing there. This would explain the 50% emission.

The guy I got this set from said he was listening to it and a puff of smoke came out and it quit. First thing I thought of is filter caps. I isolated the filter cap can and the caps read as follows.

30ufd 150 volt = 56ufd
60ufd 150 volt = 96ufd
60ufd 350 volt = 94ufd
160ufd 25 volt = 333ufd

There's also a 10ufd in the set that reads about 11.7ufd. I'll be changing them all out anyway but I didn't see anything in the set that appears to be fried. Would the 3V4 tube produce a puff of smoke when the filament went? The tube isn't cracked or broken. The 160ufd is way out of tolerance but I don't think this would have produced a puff of smoke. The can is sealed with no evidence of splitting or cracking.

There is a selenium rectifier in this set. I have no experience with these. How do you check them? I've been told that these parts should always be replaced but I don't even know what to replace them with. Ideas?

Johnny
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tubes4life



Posts : 33
Join date : 2010-10-12
Location : northern Florida

PostSubject: Re: 3V4 Shows No Short In "A" Position   Thu Oct 21, 2010 8:19 pm

Selenium rectifiers were known for producing a puff of smoke when they died. You replace them with an IN4007 diode.

As for the caps, I'd be more worried about shorts, not overcapacity.
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radiotechnician



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PostSubject: Re: 3V4 Shows No Short In "A" Position   Thu Oct 21, 2010 8:29 pm

Open filaments means no emission. Some Heathkit testers rest on 50% for line adjust.


Check the resistors around the 105/120 volt switch. Also loook at the battery/line switch. I remember these radios had a swich operated by plugging the power pulug into slots which switched to batteries. How are the other tubes?Be very carefull working on this radio live.
One slip of a probe can burn all the tubes out.

Seleniums can emit a puff of smoke but if it shorted out those resistors, the 75 ohm section may go first.

Seleniums can be tested however the indication of a weak rectifier was low B+ voltage on the screen of the 3V4. I you use a 1N4007 in its place just add resistance to bring voltages to what the schematic shows.




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easyrider8



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Join date : 2010-10-18

PostSubject: Re: 3V4 Shows No Short In "A" Position   Thu Oct 21, 2010 8:50 pm

If the power is on or has recently been on you can burn out the filaments on the tubes in these sets by pulling them and reinstalling them. You should be more concerned with the electrolytic leakage than the capacity, just replace the electrolytics, don't bother to check them. The paper capacitors should be replaced. You will find a few resistors out of spec which will need to be replaced, after doing this you can do some voltage checks. You may want to replace that !L6 with a 1R5 until you get it up and running, you don't want to fry that one.

Dave
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Johnny



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PostSubject: Re: 3V4 Shows No Short In "A" Position   Thu Oct 21, 2010 10:23 pm

Thanks guys! That's some good info. Not being familiar with this set I'm a little confused on something.

On page 23-60 of the paperwork the "Chassis Top View" shows R33 as a can (or something) on top of the chassis. This can is there but I haven't opened it up yet. Looks pretty simple to get in to. But on the parts list R33 is a wire wound two section resistor with one section being 75 ohms and the other section being 55 ohms. Both 5 watts. R33 is shown on the schematic with one end of the 75 ohm connected to the + side of the selenium SR1. In the chassis there is a big wire wound resistor that connects at this point. This resistor has three legs. Outside leg to center leg reads about 75 ohms and is connected directly to the + side of the SR1. From the center leg of the resistor to the other outer leg it reads about 55 ohms and is connected per the schematic. So far all of the resistors in this set check out pretty good.

I guess in the morning I'll open up this can referred to as "R33" and see whats in it. I do know that the three heavy gauge wires coming out of it go to C57A and C. Reading across those wires I read 1kOhm and reading across to the 15K R30 I read 2kOhm. So that can is obviously R31 and not R33 as stated on the "Chassis Top View". I think I see what's going on. The chassis top view should be labeled R31 and not R33. The parts list is correct. They sure like to throw a guy off some times don't they.

On to the 1N4007 diode. I'll be careful to check voltage to the screen of the 3V4 but how much of a voltage difference does this 1N4007 cause as compared to the original selenium rectifier?

I ordered a new 3V4 and the correct electrolytics that I don't have here. I don't have a 1R5 handy but I may be able to find one at the surplus store tomorrow. I know that 1L6 is a $60 tube.

Johnny



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