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


It is currently Jul Wed 24, 2019 3:39 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 55 posts ]  Go to page Previous  1, 2, 3  Next
Author Message
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Wed 12, 2019 9:10 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26034
Location: Annapolis, MD
KD4KO wrote:
I cut the plate wire of the converter tube and applied power; still get moter boating sound.
I did the same to the plate of the IF AMP; still get motor boating sound.

I do not recommend that approach. Disconnecting tube plates can change the DC voltage throughout the set, which in turn can give new symptoms and/or mask the ones you already have.

To test for oscillation stage by stage, tack in a 0.01 capacitor from various tube grids to ground. This kills the gain of the stage.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Wed 12, 2019 11:04 pm 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
I will do as suggested, but I'm trying to understand where the audio comes from, that is applied to thr top of the volume control.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Thu 13, 2019 12:16 am 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
My Schematic printout is hard for my aged eyes to read clearly; however under magnification, I can see what i thought was a crossover from the bottom of IF Transformer A1 was in fact a faint dot connection.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Thu 13, 2019 12:46 am 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
I much prefer to work with a printed circuit with axial lead components than this hand-wired mess.
I would like to temporally disable the AVC circuit, to see if it stops the motor boating.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Thu 13, 2019 12:58 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26034
Location: Annapolis, MD
KD4KO wrote:
My Schematic printout is hard for my aged eyes to read clearly; however under magnification, I can see what i thought was a crossover from the bottom of IF Transformer A1 was in fact a faint dot connection.

The output of the 2nd IF transformer--high side-- goes to one of the diodes in the audio tube. This rectifies the IF signal to have a negative DC component, with the audio superimposed. The volume control is the branch point. AGC DC voltage to the left, and audio through the cap from the wiper

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Thu 13, 2019 10:15 am 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
You explained what I just independently discovered this morning.

https://web.archive.org/web/20101104233 ... ervice.pdf

The above schematic cleared things up for me. I can now see the diode anode of the detector and the capacitor at pin 3 of T2 that strips the RF from the audio.

I remember in 1946, while 12 years old, experimenting with a rectifier tube as a detector, instead of the Galena crystal in my crystal set.

I thought I had discovered something when I placed a capacitor across the earphone connections and notice an increase in the audio level. I sent a letter to a Hugo Gernsback publication about this increase of audio, and was told about the RF being shunted to ground.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Thu 13, 2019 4:42 pm 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
pixellany,
I thank you for your patience with me. You have been very helpful.
I removed the two AVC paths, but I still get the motor boating sound, coming from the L26 secondary.
I'm going to take a long break, and give my eyes a rest.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Fri 14, 2019 3:31 am 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 3930
Location: Littleton, MA
Have you replaced C26, the 12 µF electrolytic?

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Fri 14, 2019 3:36 am 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 3930
Location: Littleton, MA
KD4KO wrote:
In line capacity of C1 A B C are correct, I isolated C1 D and it is 100MFD.

Regardless of their capacitance reading, they should be replaced. They are probably leaky, and may have high ESR. Have you replaced them?

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Fri 14, 2019 11:59 am 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
C26 is shown in schematic;
https://web.archive.org/web/20101104233 ... ervice.pdf

But shown as C2 in schematic ;
https://web.archive.org/web/20101104233 ... ervice.pdf

I cut the positive lead of this capacitor and turned the radio on. It still gives the same sound.
This put-put sound is applied to the VC via pins 4,6 of the secondary of T2.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Fri 14, 2019 12:07 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 3930
Location: Littleton, MA
KD4KO wrote:
I cut the positive lead of this capacitor and turned the radio on. It still gives the same sound.

You need to replace it, not cut it out of the circuit.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Fri 14, 2019 1:51 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26034
Location: Annapolis, MD
First, get the set in the correct configuration--all capacitors connected, all resistors connected, everything the right value, tube shields in place and correctly grounded......etc
Attempt to trouble-shoot only when it is in the right configuration

How about my earlier suggestion about tacking in a small capacitor to kill the gain of a stage?

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Fri 14, 2019 2:25 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 3930
Location: Littleton, MA
pixellany wrote:
First, get the set in the correct configuration--all capacitors connected, all resistors connected, everything the right value, tube shields in place and correctly grounded......etc
Attempt to trouble-shoot only when it is in the right configuration

Have we even established that he's replaced all the electrolytic capacitors? Poor power supply decoupling would be my first guess as the cause of motorboating.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Fri 14, 2019 2:56 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26034
Location: Annapolis, MD
There was discussion at the beginning about the specs for replacement electrolytics, so I assumed it was done.

Indeed---replace all known "problem" parts--all electrolytics and paper/wax capacitors, out of tolerance resistors, etc. before trouble-shooting.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Sat 15, 2019 1:16 am 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
I have ordered five .01uf Metallized polyester 400V axial lead capacitors from eBay.
I also need .001 & .047 axial lead capacitors. any source in mind?
It is a real pain, not being able to go to a local electronic parts store any longer!!!


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Sat 15, 2019 1:20 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26034
Location: Annapolis, MD
Many of us get capacitors from either JustRadios or Sal's---both have ads on the right hand side of the page

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Tue 18, 2019 6:28 pm 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
While waiting for the .01 caps to arrive, I decided to use a 50 volt 1.0 uf Bi-Polarized Cap instead.
Shorting the control grids of V1- V3 have no effect on the motor boating.
By-passing the wiper of the VC, stops the sound as expected, however by-passing the hot end of the VC, has no effect.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Tue 18, 2019 9:01 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5531
Location: Near Brandon, Iowa
The audio output tube is the single biggest consumer of B+ power in this radio (or just about any other tube radio you can name).

Often a "motorboating" noise is caused by the high current demand of the output tube momentarily pulling down the B+ bus voltage, the effect of which "feeds back" throughout the audio section. The result is an audible "pock" sound from the speaker. This feedback path has a time constant: once the B+ bus recovers, the output tube kills it again, and the cycle repeats ad infinitum.

Where the problem appears to be isolated to the audio stage, this is caused by a defect in the B+ supply such that the B+ bus cannot supply enough current to resist having its voltage level pulled down by the demands of the output tube.

Sorry, I didn't bother to carefully follow the thread of what has been done to the set up to this point; but the two "most likely suspects" are the selenium rectifier and the electrolytic filter cap.

One thing to try would be to power the radio off of dry cells, bypassing the AC power supply. A stack of ten 9V transistor-radio batteries plugged together in series works well for the B+ supply; six "D" cells in series will power the filaments for a long time. If the motorboating ceases when running on batteries, that pretty much indicts the AC power supply.


Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Wed 19, 2019 12:57 pm 
Member

Joined: Jun Fri 07, 2019 9:17 pm
Posts: 25
Location: Winter Park, FL
I will pick up the 9v batteries today.
I'm also going to replace the AC filter capacitors.
I would like gut the original, and insert the new caps in its sleeve if possible.


Attachments:
Screen Shot 2019-06-19 at 7.40.28 AM.png
Screen Shot 2019-06-19 at 7.40.28 AM.png [ 406.25 KiB | Viewed 95 times ]
Top
 Profile  
 
 Post subject: Re: Zenith Trans Oceanic L600
PostPosted: Jun Wed 19, 2019 2:31 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5531
Location: Near Brandon, Iowa
I don't like to promote products or services that I've personally not tried, but I've heard good things about this:

https://hayseedhamfest.com/collections/ ... eanic-l600

Yes, it's possible to "cut and gut" the existing cap and restuff it. I've never done it on a TO (I just disconnect the original multicap and install new axial-leaded discrete electrolytics under the chassis).

Take note that the metal shell of the multicap goes to the B- bus, NOT chassis ground.


Top
 Profile  
 
Post New Topic Post Reply  [ 55 posts ]  Moderators: Marcc, Norm Leal Go to page Previous  1, 2, 3  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: westcoastjohn and 13 guests



Search for:
Jump to:  


























Privacy Policy :: Powered by phpBB