Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Mar Sun 29, 2020 7:22 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 20 posts ] 
Author Message
 Post subject: RCA Model 2X61...Fading problem
PostPosted: Feb Tue 11, 2020 5:55 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 780
Location: new hyde park ny usa
Hi all......Plays well for 10 minutes then fades out R12 1200oHM 1 WATT resistor gets red hot. Voltage on 35l6 on plate and pin 4 drop over 10v. All caps have been replaced. All help appreciated. Thanks AL


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Tue 11, 2020 6:25 pm 
Member

Joined: Apr Fri 20, 2018 6:55 am
Posts: 83
Location: San Rafael, CA. 94903
Hey Al,
Is C19B correctly installed? If R12 is 'red hot' it seems like something is shorted on that line that it feeds.
"Voltage on 35l6 on plate and pin 4 drop over 10v." Do you mean they read 10V lower than what is noted on the schematic?
73,
-marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Tue 11, 2020 8:00 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 37752
Location: Livermore, CA
Al

R12 will be hot but not red hot. Do you have 83 volts on the left side of this resistor as seen in the schematic?

http://www.nostalgiaair.org/pagesbymode ... 014894.pdf

_________________
Norm


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Tue 11, 2020 11:55 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 780
Location: new hyde park ny usa
I should have been more clearer.....When radio is playing well voltages on pin 4 (left side of resistor) is 83v and plate volt is 118v. When she fades voltages are 73v on pin 4 and 108v on plate. When resistor is tested cold it is 1200 ohms. Electro's are correct. Thanks AL


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Wed 12, 2020 12:10 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 37752
Location: Livermore, CA
Al

Seems like the resistor has proper voltage. Resistor will be hot in operation but not red hot.. Newer resistors are smaller so will be higher in temperature. Using a higher wattage resistor will spread out the heat so it won't seem as hot.

_________________
Norm


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Wed 12, 2020 12:59 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10918
Location: Victoria, Australia
+1 on Norm's comment: I squared R applies. Looking at the circuit I would suggest R12 is right on the red line at 1Watt and any increase in line voltage or variance in current draw upwards is going to tip it over the edge.

If there are no other problems with load, 2Watt, or a 5Watt WW would be more appropriate.

Marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Wed 12, 2020 3:59 am 
Member

Joined: Dec Mon 23, 2019 5:52 pm
Posts: 31
It's been my experience that the only part that can heat up in 10 minutes to cause trouble is a tube. Can you replace the 35L6 when the problem occurs with one that you have pre-heated in a tube checker? Or at least monitor the lower voltages and see if they jump up when the tube is removed before the rectifier filament goes out.


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Wed 12, 2020 7:34 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10918
Location: Victoria, Australia
If the resistor is a problem one would half expect the voltage to drop load-side & increase on the rectifier side.

Marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Wed 12, 2020 11:35 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 780
Location: new hyde park ny usa
I replaced resistor with 7w, and replaced 35l6 with same results....there is nothing more annoying than fading! Al


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Thu 13, 2020 1:06 am 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 1537
Location: Cromwell, Connecticut
In my opinion, a 7 watt resistor is not fixing the problem. There were thousands of AA5 and 6 sets with a 1or 2 watt resistor in this position and they worked fine

Since you have fading and voltage dropping, or current draw going up, it’s time to look deeper. I would start to look at those slug tuned IF cans on the 83 volt buss. They could have SMD and internally leaking or shorting.

This is not easy, but can be verified. I believe member Electricboyo had a write up about how to test these transformers for leakage. After supper, I’ll look for it.

Edit:

On page 2 of this post, EB mentions using an external power supply to check for B+ leakage . You can also measure the AVC voltage to see if it creeps up when the signal fades.

https://antiqueradios.com/forums/viewtopic.php?f=5&t=362257&start=20

Hope this helps.

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Thu 13, 2020 2:35 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 780
Location: new hyde park ny usa
I thought about SMD, but it don't have that usual crackling noise when fade occurs. She don't even give me 10 minutes of good play anymore fades in a couple minutes. All of the plate voltages are 10v below normal except 35l6 at 107v


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Thu 13, 2020 2:50 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10918
Location: Victoria, Australia
Ok we have eliminated r12. I am not confident at this point with SMD being the cause. As that lists metal tubes: Pin one is normally the body earth. Check all of them to ensure that nothing other than earth (B-) is on them. It could be a bad filter cap? I have given up on 450V types for too many fails.

IF primary can be checked by disconnecting B+ to the can (tubes out) then use an insulation tester at 250V to apply power. But!

A method if you think it "B" wiring: As that has no obvious voltage divider across "B" . While the tubes are out Disconnect B+ and the electrolytic from the 83V rail at R12. Measure resistance. Can't see a reason for any. At this point in this situation, I get the reformer in the bench PSU (stepped voltages and regulator will lock up @ 15mA. Reformer was mainly built for finding new dud Ecaps) and apply voltage whist monitoring it.

If there is a problem; Starting at the end of the rail one disconnects B+ from the node (perhaps the IF primary) and advances toward the power source until the fault disappears at which point you have the area where the fault is. If its a serious short a low wattage battery lamp as a barretter with its correct voltage may be more appropriate.

I have had new Ecaps get to a voltage and then start conducting at a hyperbolic rate, so never think that being new means its good.

Marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Thu 13, 2020 2:58 am 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 1537
Location: Cromwell, Connecticut
All of the voltages in the stages preceding the 35L6 are fed through the resistor that gets hot. So the likely place to start is on the 83 volt buss. The 12SQ7 tube is fed through a total of 330K so not likely the issue.

You can measure the cathode voltages on the the two 12SK7 tubes when it fades. Should be .7 volts per Schematic. If one goes a lot higher, then that stage is drawing too much current. Leaky, shorted, or gassy tubes can be at fault. Even the 12SA7 can have an issue.

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Thu 13, 2020 3:10 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10918
Location: Victoria, Australia
I do like the fact that they listed cathode current as the voltage across rk can be determined for each tube with rk fitted. Do be aware that any LV bypass cap across rk can be punched through if rk goes open.

rk= cathode self bias resistor.

Marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Thu 13, 2020 11:51 am 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 1537
Location: Cromwell, Connecticut
We are assuming that all paper caps were replaced. Based on this, if the yellow caps were used, is it possible one was touched by the soldering iron leaving a small hidden melted spot. These caps can short or be compromised and could be an issue.

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Thu 13, 2020 12:09 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10918
Location: Victoria, Australia
Tbone wrote:
We are assuming that all paper caps were replaced. Based on this, if the yellow caps were used, is it possible one was touched by the soldering iron leaving a small hidden melted spot. These caps can short or be compromised and could be an issue.


Very true, but apart from a visual check and comparing everything to the schematic, or layout diagram: There has to be a safe non destructive way of assessment applied to determine where the fault is without initially applying power from utility supply (Mains). If the tubes can be tested using a tube tester: do it.

Also support the caps: If they are still Waxed Paper there is a place for them immediately, along with all aged electrolytic ones: Bin.

Marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Fri 14, 2020 1:37 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 780
Location: new hyde park ny usa
Problem solved!! c10 was one of those yellow .1 caps, that was indeed touched by soldering iron on the unseen bottom! It was mentioned in one of the posts and proved to be culprit. Thanks to all who gave their time and brain power to solve problem. AL


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Fri 14, 2020 5:31 am 
Member

Joined: Apr Fri 20, 2018 6:55 am
Posts: 83
Location: San Rafael, CA. 94903
Good work Al.
Perseverance, perseverance, perseverance!!!
73,
-marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Fri 14, 2020 11:06 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10918
Location: Victoria, Australia
That's the thing one likes about a dead short, Its often the easiest thing to find and without much equipment.

Marc


Top
 Profile  
 
 Post subject: Re: RCA Model 2X61...Fading problem
PostPosted: Feb Fri 14, 2020 6:53 pm 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 1537
Location: Cromwell, Connecticut
That's good news. At the worst it cost you extra time and a 39 cent part.

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
Post New Topic Post Reply  [ 20 posts ]  Moderators: Marcc, Norm Leal

All times are UTC [ DST ]


Who is online

Users browsing this forum: JnTX, Marko582, Popov, westcoastjohn and 14 guests



Search for:
Jump to:  
































-->


Privacy Policy :: Powered by phpBB