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


It is currently Feb Sun 17, 2019 6:30 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 26 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Fri 03, 2018 10:41 pm 
Member
User avatar

Joined: Aug Mon 08, 2011 4:19 am
Posts: 354
Location: Indianapolis, Indiana, USA
My Zenith 6-S-128 has low volume. I have replaced all paper caps, out of tolerance resistors and took apart and cleaned the volume control. I did voltage checks at tube pins and all were good. When I cleaned the volume control, the radio volume went from just barely audible to a low whisper.

When I did an alignment per Zenith instructions and volume at full, there was not much response when aligning the 1st IF. The 2nd IF was able to be peaked. When I did step 3, no signal tone was heard from the speaker. Step 3: change signal generator leads to the antenna and ground terminals of the receiver. After I did the alignment, no stations were received.

Some information: the screw that holds the mica down on the Antenna portion of the tuning condenser was not correct. I found the closest match possible and replaced it.

Any help is appreciated!

Joe


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Fri 03, 2018 11:07 pm 
Member

Joined: Nov Tue 14, 2017 5:09 am
Posts: 1168
Location: Austin, Texas
It could be a lot of things. Any time you can't get a good IF peak it indicates a significant problem. You might try removing the can from the first IF transformer to see if it looks OK. If it has melted wax drips, it is probably bad. If you just want the radio to play, you can often get by with resistor-capacitor coupling to replace a bad IF winding.

Have you verified that the sensitivity control is working correctly? I think you should align in the high sensitivity position with the 6K7 cathode shorted to ground. I didn't see anything about that setting but I may have missed it.

Jay


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 3:12 pm 
Member
User avatar

Joined: Feb Sat 10, 2018 4:58 pm
Posts: 654
Location: Livermore, CA
Joe, if I am reading your description correctly, the IF alignment "sort of" worked, but you got nothing when injecting an RF signal directly into the antenna. This could indicate a problem with the RF section, such as an open antenna coil, loose wire, inadvertent signal grounding, flaky switch contacts, etc.

Also, your description below has me a bit puzzled:
Quote:
Some information: the screw that holds the mica down on the Antenna portion of the tuning condenser was not correct. I found the closest match possible and replaced it.

It sounds like you a describing a trimmer capacitor located on the main tuning capacitor, which would likely be part of the RF alignment for the radio, and the screw should thus not simply be tightened all the way. The same would be true for other trimmer capacitors in the radio.

Hope this is of some help.

-Bryan


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 3:44 pm 
Member

Joined: Mar Sat 14, 2009 5:56 pm
Posts: 4345
Location: RI, 02885
Has your "gimmick" been placed from the tuning cap to the 6A8G grid cap lead? I had a similar issue on my 6S152 and that fixed it.

_________________
I'm not a hoarder, I'm a caretaker of scarce commodities


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 10:52 pm 
Member
User avatar

Joined: Aug Mon 08, 2011 4:19 am
Posts: 354
Location: Indianapolis, Indiana, USA
Thanks for the replies.
JnTX- I did the alignment with High Sensitivity selection on. When I switched to local there was a noticeable response. I did look in the 1st IF can with flashlight. There was no melted wax seen. I need to take it apart for a closer look.

Old Rad Lab- See photo for the not original screw in the antenna section of the tuner cap. I did not have one of the hardboard washers like seen in the osc section of the tuner. I used a metal washer on the antenna section. When I loosened this screw, the radio peaked then went back down again.

Bugman- See photo. Is this the "gimmick" you are referring to?


Joe


Attachments:
Zenith 6-S-128 tuner cap with notes.jpg
Zenith 6-S-128 tuner cap with notes.jpg [ 226.79 KiB | Viewed 425 times ]
Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 11:06 pm 
Member

Joined: Nov Tue 14, 2017 5:09 am
Posts: 1168
Location: Austin, Texas
Old Rad Lab wrote:
Joe, if I am reading your description correctly, the IF alignment "sort of" worked, but you got nothing when injecting an RF signal directly into the antenna. This could indicate a problem with the RF section, such as an open antenna coil, loose wire, inadvertent signal grounding, flaky switch contacts, etc.

-Bryan


The instructions to move to the antenna terminals is with the generator at 456KHz. It's for setting the IF trap. If the trap is correct, there should be no response when the generator is moved.

Jay


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 11:12 pm 
Member
User avatar

Joined: Jan Tue 23, 2007 10:23 pm
Posts: 5260
Location: Chicago IL
Hmmm. I usually don't hop into the electrical mechanical forum to give advice but here are my thoughts.

Since you said your radio has low volume, I would assume that it's actually receiving stations but just isn't very loud? Are the stations that are being received spread out correctly across the dial? This is an important thing to distinguish. To discard anything within the audio stage, which I don't think is your issue by the sound of it, what happens when you inject a signal directly into the potentionmeter of the volume control? Is it loud and clear here?

I don't know the exact alignment procedure for your particular radio but I assume during the IF alignment you injected the IF signal into the mixer tube, specifically one of it's grids. In this case if the IF wouldn't tune then I would suspect more an issue with the IF stage itself and not the prior RF stage, especially if the tube voltages are checking out. If the IF transformer is the kind with a tunable secondary and a tunable primary, was the peaking issue on both sides? Did you notice the signal picking up a bit at one of the extremes of the IF's tuning range? IE with the screw or log all the way down or up?

EDIT: I just re-read that after the alignment you no longer had any stations. Were there stations prior to the alignment and did they conform to what I asked? IE spread correctly across the dial etc? If you didn't check before try checking now by transmitting some signals with your signal generator and seeing if they more or less correctly fall where they're supposed to on the dial. Check all over the dial.

_________________
-Carlos


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 11:29 pm 
Member
User avatar

Joined: Aug Mon 08, 2011 4:19 am
Posts: 354
Location: Indianapolis, Indiana, USA
JnTX- In my other radio alignments, they have not called to move the signal generator to the antenna terminals. What you say makes sense- since there was no response (signal noise from speaker), then IF trap ok.

tube42- before the alignment, I only got the 2 strongest local stations (one 1280am) at a whisper of volume. I checked for stations all along the dial.


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 11:34 pm 
Member
User avatar

Joined: Jan Tue 23, 2007 10:23 pm
Posts: 5260
Location: Chicago IL
If the two strongest stations you heard were on the correct place on the dial then I would more so shift my suspicions to the first IF stage and not the RF stage. I would be curious to see what is on the mixer tube using an oscilloscope and seeing if signals are present there while tuning across the dial. Try again using your RF generator and transmitting signals over a long piece of wire. Again, the correct placement and band spread of the signals will give us a good glue as to what the RF stage/mixer is doing.

What about the IF tuning question? Did you notice the IF transformer trying to peak when at one of the extremes of it's tunable range?

_________________
-Carlos


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sat 04, 2018 11:49 pm 
Member

Joined: Mar Sat 14, 2009 5:56 pm
Posts: 4345
Location: RI, 02885
roguethistle wrote:

Bugman- See photo. Is this the "gimmick" you are referring to?


Joe


Yes, I thought it should mentioned in case you hadn't encountered it before. All is well. As mentioned above, look to the IF's. A signal tracer would be helpful in isolating the section the signal is being suppressed.

_________________
I'm not a hoarder, I'm a caretaker of scarce commodities


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 12:10 am 
Member
User avatar

Joined: Feb Sat 10, 2018 4:58 pm
Posts: 654
Location: Livermore, CA
Joe, thanks for clarifying with the picture of your fix for the trimmer cap on the main tuning gang. That should work, and it sounds like you can peak it.

I don't have a copy of the alignment instructions, but I'm guessing it follows a typical sequence of events such as:
1. Inject a modulated IF signal (at 456 kHz for this radio) at the grid of the IF amp tube and peak the 2nd IF.
2. Inject a modulated IF signal at the grid of the Det-Osc tube and peak the first IF.
3. Inject a modulated IF signal into the antenna and adjust the wavetrap (IF trap) for lowest signal.
4. Inject a modulated signal at 1600 kHz with the dial set to 1600 and align the oscillator for maximum response.
5. Somewhere around 600 to 700 kHz, you would inject a signal at this frequency and adjust any padders, rocking as needed.

Was this the general process you followed? If so, you should be good there, but I still wouldn't rule out a problem with the antenna coil (open or short), bandswitch, etc.

If you inject a modulated signal into the radio's antenna at a middle frequency of the dial (1000 kHz or so), can you tune around and hear this signal?

-Bryan


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 1:17 am 
Member
User avatar

Joined: Aug Mon 08, 2011 4:19 am
Posts: 354
Location: Indianapolis, Indiana, USA
tube42- I had the 1st if screws turned all the way in, and also turned all the way out. That made no difference. The out meter moved only ever so slightly if at all.

Old Rad Lab- yes, that was the general alignment instruction. at step 3, manual said signal gen attach to antenna. there was no signal heard from speaker. Only slight movement from my simpson 262 output meter if any at all.

As best that I can recall, when I set signal generator to say 1000kc and radio to 1000kc, the radio did not send signal tone to speaker.
I need to hook up the signal generator again and try again later tonight.

Thanks for your help,

Joe


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 2:17 am 
Member

Joined: Oct Sun 05, 2008 6:47 am
Posts: 1197
Location: Bothell, WA
Probably a dumb question, but I have to ask. I believe this radio needs tube shields. Are they all in place?

_________________
Chris

Looking for mid to late 1930s Canadian Rogers, Majestic, and DeForest Crosley radios


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 2:46 am 
Member
User avatar

Joined: Aug Mon 08, 2011 4:19 am
Posts: 354
Location: Indianapolis, Indiana, USA
cwysong ,
I removed the glass tubes that had shields and replaced them with metal tubes. I do not believe the metal tubes need tube shields.

Joe


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 5:07 am 
Member
User avatar

Joined: Feb Sat 10, 2018 4:58 pm
Posts: 654
Location: Livermore, CA
You are right, metal tubes don't need tube shields, but that is true only if pin 1 of the tube socket is grounded to the chassis. For a radio made to use glass tubes, the nominally unused socket pin 1 may not be grounded, or worse yet, may be used as a convenient tie point to connect several components which may have voltage on them. If the latter is true, the outside of the metal tubes are not only not grounded, but potentially have voltage on them.

One other quick note: If you get a chance to inject a 1000 kHz modulated signal into your antenna, make sure you tune the radio dial all across the band to see if you can pick up the signal, i.e., not just at 1000 kHz on the dial. If your alignment is way off, you might pick up the 1000 kHz signal generator tone quite some distance from that frequency as indicated by the dial pointer.

Carry on!

-Bryan


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 7:45 pm 
Member
User avatar

Joined: Aug Mon 08, 2011 4:19 am
Posts: 354
Location: Indianapolis, Indiana, USA
Old Rad Lab- I did another alignment attempt last night per your notes from the above post. I place the tube shields on 6k7 and 6f6 tubes even though they are metal. I used my am transistor radio to verify signal from signal generator.
1- signal to pin 3 of 6k7. I got signal across all dial.
2- signal sent to grid of det/osc (6a8 tube) pin 4- no signal; pin 5 no signal; grid cap- no signal
3- signal sent to antenna- adj wave trap (if trap)- no signal
4- signal to 1600khz and dial set to 1600khz- no signal; also at 1000khz and across whole dial- no signal
5- 600 to 700khz- no signal
Did I hook the signal generator to the correct pins of the 6a8 tube?
I took apart the 1st IF can and the coil looked good.

Joe


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 8:20 pm 
Member
User avatar

Joined: Feb Sat 10, 2018 4:58 pm
Posts: 654
Location: Livermore, CA
Joe, thanks, and this is progress. I don't have a copy of the alignment instructions for a 6-S-128, but I'm going to assume that my generalized instructions given earlier are close enough to start with. Let's walk through what you observed:

Step 1: You injected a 456 kHz signal into the signal grid (the cap on top) of the 6K7 and got a signal in the speaker no matter where the dial is set. That's good, and it means your 6K7 IF amplifier tube and everything beyond that is working, at least well enough to proceed.

Step 2: You injected a 456 kHz signal into the signal grid (the cap on top) of the 6A8 Det/Osc. tube and got no signal. Pins 4 and 5 of this tube aren't relevant to this test, but the fact you got no signal when injecting into the cap on top of the tube means you either have an issue with the tube, or with the first IF transformer between the 6A8 and the 6K7.

Actually, in looking back through this post, I think I know what the problem is. When you replaced the (insulating) fiber washer on the trimmer cap on the main condenser with a metal washer, that screw and washer are now shorting the metal tab to the frame of the condenser, thereby grounding your signal. Try removing the screw and metal washer entirely, and just let the metal tab "float" over the mica insulator. In that way, it won't be grounding that metal tab. Please try this and see what happens.

-Bryan


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 8:49 pm 
Member
User avatar

Joined: Aug Mon 08, 2011 4:19 am
Posts: 354
Location: Indianapolis, Indiana, USA
Old Rad Lab- ok, I took off the washer and bolt on the antenna portion of the tuner condenser. I got one channel, but the sound is barely audible on the speaker. I did check the 6a8 tube- tested 770 good on my eico 625 tube tester. Yesterday as I was opening the 1st IF can, I noticed a trimmer with bolt that was very loose. this trimmer is inside the chassis, and the alignment procedure page calls it the broadcast padder (this trimmer goes through to the top of the chassis and is adjustable from the top of the chassis). It has mica sheets under the bolt. Before I did yesterdays alignment, I screwed it in just a tad to firm it up. I am not sure what part number this is as I could not find it on the schematic.
Yes, this is progress.
Thanks,
Joe


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 9:53 pm 
Member
User avatar

Joined: Feb Sat 10, 2018 4:58 pm
Posts: 654
Location: Livermore, CA
Yep, progress! If you now inject a 456 kHz signal into the grid cap of the 6A8 Det/Osc. tube, can you hear it? If so, then I think your issue is probably mostly alignment, as I believe you already confirmed that the oscillator is working. If you still can't hear this signal, there's more work to be done.

One question: What kind of antenna are you using?

And one thought: The trimmer capacitors, the ones with a mica insulator sheet between two plates of metal, don't necessarily need to have their screws tight, or even snug. If they are working properly, the act of screwing the screw in and out is changing the distance between the two metal parts and thereby "trimming" the capacitance. The mica sheet is just there as an insulator (well, technically a dielectric, I guess). So, when adjusting the trimmers when aligning the radio, they might end up in a snug position, but they might also end up in a fairly loose position. Just the way these work, and don't worry about the screw falling out.

-Bryan


Top
 Profile  
 
 Post subject: Re: Need help with Zenith 6-S-128 low volume
PostPosted: Aug Sun 05, 2018 10:11 pm 
Member
User avatar

Joined: Feb Sat 10, 2018 4:58 pm
Posts: 654
Location: Livermore, CA
PS: If you have a copy of Riders volume 7, the page near the end, "Zenith 7-7" has the location of the various trimmers and padders. Basically, the wave trap trimmer (part #2 on the schematic) is on the rear panel of the chassis, the broadcast padder is between the 1st IF transformer can and the main tuning condenser, and the shortwave and broadcast trimmers are near the 6A8 tube.

PPS: With the screw and washer removed from the trimmer as previously, can you now hear any modulated RF signals in the radio, across the dial, such as 1000 kHz or so?

-Bryan


Top
 Profile  
 
Post New Topic Post Reply  [ 26 posts ]  Moderators: Marcc, Norm Leal Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Bedfordman, Exabot [Bot], Google [Bot], madsowry, nash, thunderbird281 and 11 guests



Search for:
Jump to:  


























Privacy Policy :: Powered by phpBB