Zenith 6 S 152

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Re: Zenith 6 S 152

Post by Wildcat445 on Sat Mar 26, 2016 11:59 pm

I would put my money on a mica cap somewhere or a tube. You can eliminate the IF transformer by measuring grid voltage before and after the set acts up. If you get positive voltage on the grid following the IF can, then you can suspect a condition like SMD. The transformers in that set do not suffer from SMD as a general rule, as you already know. You could have a gassy tube causing you grief. Or one with an element short that acts up as it heats. I try to run any radio I repair inside a cardboard box for an hour or so. You will be surprised how hot one gets running in a box. Things act up that would not normally cause an issue with normal ventilation. Don't overlook the band switch. I have had my share of grief with those. Many times the issue with band switches is heat related. I believe the fact that the problem started after an alignment may be purely coincidental.

To add to the confusion......I had a little Zenith AA5 one time that the oscillator would not start unless I either changed the IF alignment or replaced the converter tube. Aligning the IF's slightly off peak fixed it. Don't ask me why or how, but it did.

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Re: Zenith 6 S 152

Post by Ed in W. PA on Sun Mar 27, 2016 1:15 am

I will eventually get it. Have traced it down to either 6A8 or IF transformer or associated components. We were always a Buick family having several LeSabre models and one Gran Sport. Unfortunately none of them was a 455 but that Gran Sport was damn quick with the 400 engine. My personal favorite was a 1978 LeSabre , 350 smog engine but all the leg room a guy could want. That car was more comfortable than my 73 Coupe DeVille. Regards, Ed.

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Re: Zenith 6 S 152

Post by Wildcat445 on Sun Mar 27, 2016 1:44 am

You know what you are doing and this may sound like beating a dead horse.  I have been bitten in the butt by band switches that looked good but caused me grief.  Since your problem is in the front end of the radio, I would clean that band switch again with DE-Oxit and then proceed.  The old saying "once bitten, twice shy" applies to me and band switches.  Zenith and Magnavox specialize in having dirty band switches when I am working with one.  I do not remember a time I did not own at least one Buick.  Oddly enough, I was hauled home from the hospital when I was born in my great-grandfather's '38 Packard.  The doctor who delivered me was named Dr. Hudson.  True story.  Very Happy

The following may be a tad off-topic but...... The Buicks I have owned were, for the most part, either straight 8's or vertical valve V-8's. I am particularly fond of the 401 V-8, grandpa thought there was nothing like the 1941 320 straight 8. I am not a fan of the 455 engine. I always found the 400 or 430 to be better performers in almost every category. 455's had oiling problems. They were horrendously thirsty. A well tuned 430 or even a Stage 1 400 would dust the door handles on the typical 455. I owned a '74 455 which was the biggest piece of crap that ever rolled off the assemby line at Flint. I bought a '68 Electra with a 430 brand new and it was as good a runner as any I ever owned.

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Zenith 6 S 152

Post by Ed in W. PA on Sun Mar 27, 2016 7:52 am

I will get it eventually. Up till now only had the scope on G1 of the IF, should follow your advice and check the DC as well. That Gran Sport was a 400( don't think stage 1 or anything like that) and it could hang with anything including the 440's of the day. By the way, the bandswitch is as clean as I can get it.

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Re: Zenith 6 S 152

Post by Wildcat445 on Sun Mar 27, 2016 9:28 pm

You might see if the radio acts up on all bands, or just the broadcast band. That would clue you in to certain mica caps, the bandswitch and coils. If it acts up on all bands, that might make it a bit more challenging to find. You'll get there. Good luck.

Buick did not install the 400 in the big Buicks for some reason. They would run like a scalded dog in a Skylark.

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Re: Zenith 6 S 152

Post by Ed in W. PA on Mon Mar 28, 2016 1:19 am

That's a good thought WC. Didn't work on it today but I did run antenna and ground leads over to that spot on the bench. You are absolutely on the money about the 400 engine. What in the hell were they thinking? So many cars such as our 1970 LeSabre were under powered due to low compression 350 4V engines that GM put in these vehicles. Nice smooth ride but no guts. The difference between the 69 GS and the 70 LeSabre was like night and day. It didn't seem to bother me as much on my 78 LeSabre. I guess by that time I was used to smog engine performance. That 78 was a beautiful car and I KNOW that it had more leg room than my 73 Deville and ran a real lot better than the Caddy. I will check out the SW bands tonight see what we find. Regards, Ed.

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Re: Zenith 6 S 152

Post by Wildcat445 on Mon Mar 28, 2016 1:34 am

Oldsmobile used a two-barrel carb version of their 455 in the 88 series. Pontiac had a two-barrel 400 in the Catalina series. Buick soldiered on with their dinky little 350 in the LeSabre. Why Buick did not offer both two and four barrel 400's in LeSabre is beyond me. The 430 and 455 were available in LeSabre for "severe duty" use (police cars). I have never seen a LeSabre with a 430 and only one, a 1970 with a 455.

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Zenith 6 S 152

Post by Ed in W. PA on Mon Mar 28, 2016 1:47 am

The Pontiac 2V version was better but no treat. Buick had so many engines that were worlds better, but they let them go to seed. What a waste!

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Zenith 6 S 152

Post by Ed in W. PA on Tue Mar 29, 2016 6:28 pm

Since my last post I have the radio playing with the scope, 2 vtvm's, Fluke meter, signal tracer, and signal generator all set up and ready to isolate the problem and guess what--- it won't act up. Just plays fine. Lots of stations on all bands. Probably should let it cook for a few more days and see what happens. I know that as soon as I put this all back together and in the cabinet that will be when it will do its tricks. Regards, Ed.

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Re: Zenith 6 S 152

Post by Wildcat445 on Wed Mar 30, 2016 2:32 am

The problem might be heat-related. What if you put a cardboard box over the chassis and see if that has any effect. The box will hold in heat like the cabinet does, but it allows you to have your equipment hooked up as it runs.

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Zenith 6 S 152

Post by Ed in W. PA on Thu Mar 31, 2016 12:52 am

I have another question or observation to report. While my radio is playing I have noticed that if I clamp my fingers around the wire going between the cap of the 6

I have another observation to report. When I place my fingers around or even near the wire going between the cap lead on the 6A8 converter tube and the vari-cap I get increased amplitude of the signal. I know that a human body can act as an antenna, but I have this connected to my longwire and would think this would be a superior antenna compared to me. The reason I am asking this is because I am wondering if I have some deficiency in the antenna coil or associated circuitry. Any thoughts on this? Thanks, Ed.














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Re: Zenith 6 S 152

Post by Wildcat445 on Thu Mar 31, 2016 11:09 pm

I only have a couple of suggestions. Your longwire antenna may be somewhat directional. You are less directional and may actually help the longwire be a more effective antenna under certain conditions. And the alignment may not be absolutely perfect. No harm is done running a radio with less than perfect alignment. If it operates acceptably otherwise, I would be tempted to leave well enough alone. Good luck.

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Re: Zenith 6 S 152

Post by Bill Cahill on Fri Apr 01, 2016 12:29 pm

I agree with him. And, you are touching the antenna connection. No surprise on that. In the past, for a wire antenna, I made a T connection in the wire, and, ran a second wire in other directions.

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Zenith 6 S 152

Post by Ed in W. PA on Fri Apr 01, 2016 9:41 pm

Thanks guys. WC you are probably right about my longwire as it is not only directional but in need of replacement due to age. As for the alignment, I would bet it is right on the money but now I think I will run through it again just to satisfy myself. Bill, you are probably right as well. I should leave well enough alone, as this set operates quite well the way it is. Thanks again, Ed.

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Re: Zenith 6 S 152

Post by Wildcat445 on Fri Apr 01, 2016 11:41 pm

My longwire antenna is about 100 feet long and is in an "L" shape.  That helps with keeping it from being overly directional, but is still is directional to certain stations.  I tapped into the junction of the legs on the "L" to the radio.  After doing an alignment, and even after you get it absolutely perfect, the alignment can change as the radio warms up and components drift.  I have had to align a radio more than once to get the alignment to "take" and stay where I put it.


Last edited by Wildcat445 on Sat Apr 02, 2016 2:11 am; edited 1 time in total

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Zenith 6 S 152

Post by Ed in W. PA on Sat Apr 02, 2016 12:05 am

I think you might be on to something as far as getting an alignment to "take". I think before I start this I am going to try every 6A8 tube I have then see what develops. I know I am splitting hairs, but I have the time and enjoy the hobby enough to try to get things as right as I can. Thanks for bearing with me, I really appreciate the input. Regards, Ed.

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Re: Zenith 6 S 152

Post by Wildcat445 on Sat Apr 02, 2016 2:10 am

We enjoy helping someone while they are working on their radio.  Almost as much fun as working on my own.  We are always happy to help anyway we can.  And I am glad you got your radio going.  In the way of a suggestion, how about changing out the IF amplifier tubes when you change out the converter tube.  If the IF amps are wonky, that might make the radio drift off station, mimicking a less-than-perfect alignment.  Almost any part that was not changed can be messing up an alignment.

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Zenith 6 S 152

Post by Ed in W. PA on Thu Apr 14, 2016 7:24 pm

I am still having a hard time smoking out the intermittent signal strength problem in this set. The one thing I stumbled on to that seemed to affect the problem is momentarily grounding one side of the broadcast osc. trimmer which on the schematic is marked C24. There are two trimmers on the print marked C24 as this is a pair of trimmers on one mounting assembly. The control that affects it is the one on the lower end of the coil. When the signal drops down the AVC voltage drops from -4.44 to about -2.73 and the station can be heard just at a low level. When it is low, if I take a screwdriver and momentarily touch the adjustment screw on the trimmer while the screwdriver is against the chassis(ground) the volume comes back and the AVC voltage goes back up. I changed C5, a 25mmfd mica cap as a test and will see what happens. WC, if you are reading this post I went to the used furniture store today and had no luck. I believe you told me that Astro Sonic was not what you are looking for, and the three sets they had were all either Astrosonic or SS. Sorry. Regards, Ed.


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Zenith 6 S 152

Post by Ed in W. PA on Sat Apr 23, 2016 12:15 am

Well I thought I had this problem solved until tonight when the set did it's trick again. As I was listening to The Grand Old Opry on WSM, considerable static started to occur followed by a drop in volume and a drop in AVC voltage. This time I just touched the screw on the shortwave osc trimmer ( C24 ) and things returned to normal and stayed there. This is the same symptom-remedy I've had for two weeks now. I have had four different 6A8's in here and three different IF tubes, have cleaned the band switch as well as jumped across the band switch while the set was in reduced signal condition with no success. I also changed a number of mica caps in both the antenna and oscillator areas of the set to no avail. If I touch the Bcast or SW trimmer screws to ground temporarily though the set instantly returns to normal and stays there. I guess the next thing I'll try will be to remove C24 which is both the bcast and sw osc trimmers mounted on a piece of bakelite, measure the capacity and wire separate trimmers in and wait to see if that does it.This last time it went four or five days before it acted up, so this might be a slow process. If any one has any input I would greatly appreciate it. Regards, Ed.

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Re: Zenith 6 S 152

Post by 75X11 on Sat Apr 23, 2016 9:33 am

If it is a problem that occurs as the unit warms up, it can help to warm the suspect area with a hair dryer to expedite the symptom's appearance. I have a Ungar princess heat gun that gives spot application that has revealed a faulty mica on occasion.
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Zenith 6 S 152

Post by Ed in W. PA on Sat Apr 23, 2016 9:57 am

I have one of those small spot heaters ( I think mine is a Wahl ) and have used it and probably half a can of freeze spray but can't seem to have any luck with this approach. Tapping on things, wiggling tubes and such also have no effect. It is a mystery to me why momentarily grounding trimmers on the oscillator coil affects this, I have watched the oscillator waveform and it doesn't change in amplitude or shape when the change occurs. And why would the oscillator affect signal strength? Thanks for the reply, Ed.

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Re: Zenith 6 S 152

Post by 75X11 on Sat Apr 23, 2016 10:57 am

If something were causing such an abrupt change in your volume and avc and you were not having any alteration in your oscillator waveform and the fixed caps were not acting up, I would recheck and do a good clean on all the switches and interlocks in the affected area, such as the switches 7, 8, 9, 11 and 12. I had a number of switches in the last one I did and it took a few re-cleans using a solvent wash then de-oxit to get all of them up to snuff. I'm a little foggy with a cold and fever this morning, so sorry about the delays.
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Zenith 6 S 152

Post by Ed in W. PA on Sat Apr 23, 2016 11:44 am

I have a terrible feeling that I am missing something obvious, as I have done so many times in the past. Not only did I clean these switches but I also placed jumpers across the contacts and tested with no results. It would seem that momentarily changing capacitance, impedance, reactance of oscillator coil is shocking the circuit into operation. When this set is operating properly it sounds excellent and has great sensitivity for a chassis without a RF amp, thus the incentive to fix it right. Thanks for bearing with me, Ed.

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Re: Zenith 6 S 152

Post by 75X11 on Sat Apr 23, 2016 12:34 pm

I guess I would recheck the components in the area that have a ground connection to make sure they are are well grounded or at correct value and not changing with temp.
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Zenith 6 S 152

Post by Ed in W. PA on Sun Apr 24, 2016 2:18 pm

Yesterday I removed the double trimmer assembly marked C24, cleaned the dirt and corrosion from between the mica and reinstalled it. At the sane time I resoldered several connections including ground lugs and checked all of the solder connections I made while replacing components. I got six successful hours of playing time on it last night, but won't be satisfied until it plays a week without a failure. I have the cabinet refinished and new grill cloth installed and can't wait to finish this one as there is a 8 S 463 Zenith up next. Special thanks to Dan Walker. His advice was much needed, carefully followed and greatly appreciated. As I had never refinished anything before his help made the job go a lot easier. Thanks to everyone, Ed.

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