Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Early B&W and Projection TV

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 02-28-2019, 11:17 PM
Phil Phil is offline
VideoKarma Member
 
Join Date: Aug 2016
Posts: 59
My nightmare set: RCA 9T240

I've been working on a RCA 9T240 that has proven a real challenge. The set had been worked on previously by someone who made a mess of it. First of all the power supply was nothing like the schematic indicated. There were resistors that weren't supposed to be there and connections where none should exist. I started by putting the power supply back as it should be according to the schematic. I then made a list of every connection to the various power supply buses (two handwritten pages) and verified everything was connected as it should be. Of course, all paper capacitors and electrolytics were replaced and every resistor checked with any out of tolerance being replaced.

At this point I had a very out of focus picture which was way off the left side of the screen and terribly out of focus. HV was 6.5 KV. It seemed the flyback might be bad so I ring tested it with a Sencore VA62 and it tested bad. (see also my post in the test equipment section for more on this) I obtained a HVO-10 Merit universal flyback and in the process of installing it found that basically every connection to a terminal strip in the HV cage was wrong. I disconnected every wire from that terminal strip and reconnected it according to the schematic. After doing that I had a normal sweep and proper high voltage. The linearity was poor, but at least there was a picture. In the meantime I bought a HVO-3, which is a direct replacement for the original part. I installed it tonight and had an out of focus picture for about 15 seconds until it died. At that point I reinstalled the HVO-10 and now have a picture again.

Another issue I havent mentioned before is that the vertical osc. was way off frequency. There is supposed to be, according to both Sams and Riders, a 1.5M resistor from the moving contact on the vertical hold control to the rest of the circuit. It had a 4.7M when I started working on it. I had replaced it with a 1.5M. I have checked the circuit over and over and it all seems to be per the schematic. I replaced the 1.5M with a 3.9M and now the vertical with sync. This seems strange. Something must not be right. The vertical linearity is very poor as you can see in the attached image. Any ideas? It would seem the need to change that resistor would be a clue? Perhaps the vertical osc. transformer has problems?

Any thoughts would be appreciated.
Attached Images
File Type: jpg IMG_20190228_203701593.jpg (73.8 KB, 64 views)
Reply With Quote
  #2  
Old 03-01-2019, 08:29 AM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 10,498
You may want to look for other revisions of the schematic. Some sets had dozens of production changes over multi-year production runs sometimes setting original 'wrong value' parts to match the schematic reverts a set to a less ideal version of the circuit than what it left the factory with.
__________________
Tom C.

What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4

Reading between the scan lines since the mid 2000's.
Reply With Quote
  #3  
Old 03-01-2019, 08:38 AM
timmy's Avatar
timmy timmy is offline
VideoKarma Member
 
Join Date: Jan 2011
Location: ferndale ny
Posts: 2,119
Wow I would agree with electronic m on checking other run schematics as how can so much be different in a given schematic. I had a similar problem with a teletone 7 inch, where resistors were present in the set but showed nothing on the schematic along with others sections were different from the schematic, only to find out that infact there was another schematic where changes were made in the run.

Last edited by timmy; 03-01-2019 at 08:41 AM.
Reply With Quote
  #4  
Old 03-01-2019, 03:14 PM
DavGoodlin's Avatar
DavGoodlin DavGoodlin is offline
Motorola Minion
 
Join Date: Aug 2011
Location: near Strasburg PA
Posts: 2,879
Hi Phil, Where do you live? I have a 9T240 that I think is all original and I need to downsize in a way helpful to other collectors.
__________________
"When resistors increase in value, they're worthless"
-Dave G
Reply With Quote
  #5  
Old 03-01-2019, 03:14 PM
zeno zeno is offline
VideoKarma Member
 
Join Date: Mar 2013
Location: New Hampshire
Posts: 3,286
Checked the Sams index there is only one folder listed for the
9T240. There are NO PCB's ( production change bulletin ) listed.
PCB's are only as good as the tech that files them. Usually one
page that tends to get lost. Someone with OEM manuals may
be able to help but considering the large amount of questionable
work I doubt its not just changes. Looks more like a butcher job.
Stuff like this drove us nuts. Nothing worse than finding someone
elses mistake & the customer almost always lied ( its never been fixed ) !

For now you have it down to a vert problem so if you havnt put NOS
brand name tubes in the V osc & out then look for mistakes.

73 Zeno
LFOD !
Reply With Quote
Audiokarma
  #6  
Old 03-04-2019, 09:49 PM
Phil Phil is offline
VideoKarma Member
 
Join Date: Aug 2016
Posts: 59
It was one of my first thoughts there was a change during production but haven't been able to find any other schematics. The riders lists a few changes in parts, whereas the Sam's doesn't, but not the resistor in the vertical osc. section. I have found a few mistakes in the Sam's. For one thing according to the Sam's tube placement chart the audio input tube is a 6AU6, whereas it is actually a 6AV6, a very different tube. Also the way the vertical osc. is wired according to the Sam's will not work. The riders again is correct. I found a well hidden 56k that measured 80k replacing it helped the vertical linearity a good deal. Now I'm trying to determine why I have no audio. Feeding a signal into the input of the sound IF gives an appropriate output so the sound carrier must be lost between the tuner and first two IF stages. The cores in the upper section of the unshielded IF transformers are ruined. Apparently they were stuck and the previous person who butchered this thing so much anlso broke them and made the top of the three of them perfectly flat. I will try replacing these from my parts chassis tommorow and see if I can get any hint of sound carrier through it then.
The vertical linearity is still not great. I will try a few other 6K6 tubes and see if any if them do better.
Reply With Quote
  #7  
Old 03-04-2019, 10:32 PM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 10,498
The sound is probably similar to my 8TS30...I had to align the audio on it. I tried the sam's non-sweep generator procedure (since I don't have a working 20MHz sweep gen), and while I could peak the system on freq, the audio carrier from the tuner wasn't on freq (thus no sound). Eventually, I started repeating peaking the system off freq at several KHz intervals above and below correct freq until I found a point where I could get a bit of audio from my agile modulator at some fine tuning setting. I then peaked the IF to the program audio by ear, tuned the generator to the freq I peaked the sound IF at and peaked the IF again...Those big unshielded coils on/near the tuner did need adjustment in the process.
__________________
Tom C.

What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4

Reading between the scan lines since the mid 2000's.
Reply With Quote
  #8  
Old 03-06-2019, 10:46 PM
Penthode's Avatar
Penthode Penthode is offline
VideoKarma Member
 
Join Date: Jun 2009
Location: Kitchener/Waterloo Ontario Canada
Posts: 700
Quote:
Originally Posted by Electronic M View Post
... Eventually, I started repeating peaking the system off freq at several KHz intervals and and below correct freq until I found a point where I could get a bit of audio from my agile modulator at some fine tuning setting. I then peaked the IF to the program audio by ear, tuned the generator to the freq I peaked the sound IF at and peaked the IF again...Those big unshielded coils on/near the tuner did need adjustment in the process.
I was taken by your comments. Changing the if tuning to find the sound? That is certainly not what you are supposed to do!

To do the job properly, you must have a calibrated frequency generator. Then you must sweep the if. That is the only way to do the alignment properly. The equipment to do a proper job is cheap and is readily available on eBay.

These pre 1950 sets are of split sound design. You use your calibrated fixed 21.25MHz generator to initially get the sound if into the ballpark region and the sweep for proper bandwidth and discriminator symmetry.

You adjust the tuner local oscillator (fine tuning) setting to provide the correct audio if from the tuner. The fine tuning on these split sound sets are to always tune for best sound. This is the problem for if you do not coordinate the proper alignment of the video and audio ifs together, the best picture will not match best sound.

The alignment of the earlier RCAs can be quite a challenge. But when properly done, the RCA sets provide incredibly good pictures and sound.

Those big unshieided coils you mentioned, are they on the top? These are the traps. There are two adjacent channel sound, two adjacent channel video and two co-channel traps in the 9t240 video if. These must be properly aligned to achieve proper video and sound.

I have been aligning these sets for 50 years now and I believe with a bit of study with practise and perseverance anyone can be do it. It requires a bit of study and asking questions. And forget about using the Sams notes on these early sets. They are riddled with errors. Use the Riders as they are copied directly from the RCA Service notes. Just adhere to the step by step method in Riders and you will be fine. There are lots of other old publications on line which will provide some help.

Last edited by Penthode; 03-07-2019 at 08:19 AM.
Reply With Quote
  #9  
Old 03-07-2019, 06:38 PM
Electronic M's Avatar
Electronic M Electronic M is offline
M is for Memory
 
Join Date: Jan 2011
Location: Pewaukee/Delafield Wi
Posts: 10,498
Quote:
Originally Posted by Penthode View Post
I was taken by your comments. Changing the if tuning to find the sound? That is certainly not what you are supposed to do!

To do the job properly, you must have a calibrated frequency generator. Then you must sweep the if. That is the only way to do the alignment properly. The equipment to do a proper job is cheap and is readily available on eBay.

These pre 1950 sets are of split sound design. You use your calibrated fixed 21.25MHz generator to initially get the sound if into the ballpark region and the sweep for proper bandwidth and discriminator symmetry.

You adjust the tuner local oscillator (fine tuning) setting to provide the correct audio if from the tuner. The fine tuning on these split sound sets are to always tune for best sound. This is the problem for if you do not coordinate the proper alignment of the video and audio ifs together, the best picture will not match best sound.

The alignment of the earlier RCAs can be quite a challenge. But when properly done, the RCA sets provide incredibly good pictures and sound.

Those big unshieided coils you mentioned, are they on the top? These are the traps. There are two adjacent channel sound, two adjacent channel video and two co-channel traps in the 9t240 video if. These must be properly aligned to achieve proper video and sound.

I have been aligning these sets for 50 years now and I believe with a bit of study with practise and perseverance anyone can be do it. It requires a bit of study and asking questions. And forget about using the Sams notes on these early sets. They are riddled with errors. Use the Riders as they are copied directly from the RCA Service notes. Just adhere to the step by step method in Riders and you will be fine. There are lots of other old publications on line which will provide some help.
I don't own a 21MHz sweep generator (and many TV repair shops in 1948 didn't either thus why the service lit had a procedure to align the IFs WITHOUT a sweep gen).

Your talking to probably the biggest test gear cheapskate on VK. I haven't spent more than $6 a pop on the last 10 soldering irons I've used...And not much more on anything else I have...Also I have never done a sweeping alignment on anything, and never have needed it to get satisfactory operation. Also, every 21MHz sweep rig I've seen is tube thus needing a recap to even work, and obviously woefully out of cal. Show me a freshly calibrated 21MHz solid state TV sweep/marker generator with cables for under $40 and I'll show you a chicken with a full set of teeth.

My 8TS30 (I'm not sure why you think it was a 9t240) which I did that sound work on had a great picture and absolutely NO sound after recap, replacing off tolerance resistors, tube swaps, etc...Before I adjusted it I used my AM generator (the modulator on it is crude enough for the 400Hz audio mot to make some FM detectable modulation) to find the sound IF freq and it was WAY WAY off (so I had nothing to lose). Once my adaptation of service lit procedure got sound to pass I peaked the IF, set finetuning for best picture and repeaked the IF. I couldn't have done a better job with the equipment I had, and may even have been able to do worse with a sweep gen.

The coils were above chassis, but the set was an 8TS30. Those coils probably were the traps...I looked them up on the schematic and knew what they were/how I needed to deal with them when I was working on that set, but that was probably over 6 months ago, and memory has faded.

Someday when I have the money/gear and time I'll pickup sweep alignmet....I actually have been buying some dirt cheap 44MHz gear, but I haven't had time to use it...I have few enough 21MHz sets that it ain't worth gearing up for those.
__________________
Tom C.

What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4

Reading between the scan lines since the mid 2000's.

Last edited by Electronic M; 03-07-2019 at 07:55 PM.
Reply With Quote
  #10  
Old 03-07-2019, 10:35 PM
Penthode's Avatar
Penthode Penthode is offline
VideoKarma Member
 
Join Date: Jun 2009
Location: Kitchener/Waterloo Ontario Canada
Posts: 700
There isn't much difference in the alignment between the 8ts30 and 9t240. Both are split sound sets. The main difference is the sound takeoff is from the tuner in the 8ts30 and after the 2nd vif in the 9t240. It is not so much having the sweep generator for 21.25MHz. It is the accuracy of the frequency.

In a split sound set, you must tune for best sound with the operational fine tuning control. This means the sound carrier must be precisely set for correct fm sound demodulation and the video carrier must sit exactly 4.5MHz away at the 50% on the response slope to achieve optimum picture and sound. You can only obtain this adequately by sweep alignment.

As for the tools, when I was younger I had no money for test equipment. I used a standard fixed frequency signal generator for the sweep marker I calibrated off of WWV and other sources. The sweep generator was $2 and the scope was free. Of course some repair was required but once I was assured it was working well enough, the alignment was a snap. And I can assure you as best as you can compromise in trying to get sound and picture, the difference between this and a properly aligned set is like night and day!

But first I am curious: when you fine tune your 8ts30, as you rotate the fine tuning knob from on side to the other, you should start out with weak distorted or no sound. As you continue to rotate the knob, the sound gets louder then dips to nothing and then as you continue rotating, a point will be reached where the sound is clear and loud. Further rotation will hit another dip or null and the sound will reappear quieter and a bit distorted. As you continue to rotate the knob the distorted sound will diminish.

Where the sound is clear and loud between the two dips is the point where the picture is the sharpest. Note where the picture is sharpest is not the brightest as the video if carrier will be at the 50% point on the response slope. This is how all split sound set tune and operate. The later intercarrier sets tuning was for best picture because the sound was always present with the fixed intercarrier 4.5MHz sound if.

Last edited by Penthode; 03-07-2019 at 10:40 PM.
Reply With Quote
Audiokarma
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 03:26 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
ęCopyright 2012 VideoKarma.org, All rights reserved.