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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Nov Sun 26, 2017 11:41 pm 
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thomas13202 wrote:
i think a b&k television analyst would help you locate the trouble in short order. you can inject a video signal into the video output tube and work your way back till you find the stage with the problem. then voltage and resistance measurements will locate the bad component. that is if it is a problem in the video circuit. the same can be done with the sync circuits. this is the way i learned to troubleshoot a tv. personally i wouldnt attempt to troubleshoot a tv without one.

when making a probe for your oscope i would try to stick to the component values of the original probe as close as possible.

this tv is giving you a baptism by fire. once you fix this you should be able to fix anything. just remember just throwing new components in with no particular reason never was and never will be a substitute for proper troubleshooting procedure. the tv books you mentioned should explain the logical step by step procedure for troubleshooting.

i have never tried to use an oscope to troubleshoot so i can not advise about that. good luck and keep us posted as to progress.

does the tearing start right away or does the set have to warm up first? if the set has to warm up awhile before the tearing appears it could be some kind of thermal problem. spraying the resistors and condensers one at a time with freeze spray in the video and sync sections might help locate the problem.


went to edit my origional post and some how i posted this twice. please remove this copy of my post moderator as i dont know how


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Jul Tue 31, 2018 12:10 am 
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Well I have came back to this finally with some new tools. I now have the precious B&K 1077 Tv analyst and I have started using it to figure out why this strange problem is occurring. Removing the 3rd IF tube and jumpering the filament, I injected video into the input of the video amplifier tube directly. Doing so resulted in a image with no tearing to the side. Using the contrast control did not make it appear again. I now plan to go backwards through the IF stage to find the section causing this mess. There is still a vertical foldover on the bottom of the screen. This is most likely a separate issue. I am glad that I am finally making progress on this thing.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Jul Tue 31, 2018 2:36 pm 
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Here are the results of the latest round of tests. Injecting IF into the input of the 3rd IF tube produced a weak signal but no pulling or tearing. Injecting into the input of the 2nd IF tube produced a much stronger picture but also brought the issue back.
In between these 2 points are 3 1000pf caps, a 47 ohm resistor, a 6.8k resistor, a IF transformer with a 8.2k resistor across the secondary and another 8.2k resistor. I plan to check all of these parts next.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Jul Tue 31, 2018 4:55 pm 
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Well nothing in that section tests bad so that leaves only a video IF alignment as the issue. I will take a break until I have the scope is working then I will see about aligning it.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Wed 01, 2018 5:22 am 
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Before you go aligning it check to see if the agc is aplied to that stage and any others before it. If so the agc line may be dirty or at the wrong level for the signal coming in causing the tubes to clip the sync as the sync is the highest part of the video modulation and therefore gets clipped first. Gassy i.f. tubes or those with grid emmision can cause this as well as far back as the r.f. amp.These other tubes up stream can upset the agc by leaking voltage onto or from the line. also look for very large resistors in the agc line that have drifted. As I do not have your schematic in front of me I can give you general info only.


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Wed 01, 2018 8:59 am 
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Whatever you do, DON'T TOUCH the VISION IF ALIGNMENT unless you have all the right gear (sweep and marker generator) and the full alignment instructions.
And you know what you are doing (very important).
And there is clear evidence that someone has been fiddling with it (look at the slots in the core adjustments).
Vision IF alignment does not go out of whack by itself.

Let's measure some voltages first to get a clear picture of what is going on.
Start with the cathode resistor values (measure them) and the voltages across them for each IF stage.
Get back to me and I can tell you where to go next.

Measure, don't guess......


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Wed 01, 2018 1:14 pm 
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The only sort of AGC in this set is a local suburban distant switch on the back. When I was injecting the tuner was disconnected from the IF stage. I could certainly check all of the resistors downstream of the 2nd if tube input. I also would like to get my scope working and check out the waveform. I have a demod probe. I want to get the scope working first and then I am going to come back to this and figure out whats making the IF overload the rest of the set.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Wed 01, 2018 5:46 pm 
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I agree with the above posts to do voltage checks on the IF tubes and check for defective tubes. I often find IF tubes that have shorts or grid leakage.

I'm sure your set uses AGC but there isn't a control for it. The will be an AGC line going from the detector or video amp stage back to the IF and maybe tuner/RF amp. The alignment instructions will typically tell you to set that line to a fixed voltage like -3 volts to disable it.


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Wed 01, 2018 6:45 pm 
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If I remember I had swapped all of the if tubes as part of the troubleshooting. I will check voltages and resistors in the IF but I would like to finish the scope project first. I have that all figured out. Just need to order some tantalum caps.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Thu 02, 2018 3:45 am 
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It's not just to check the tubes. It's quite possible a mistake has been made when shotgunning the parts. Cathode bias on each tube should show this up.
Will also show if the AGC is wrong.


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Thu 02, 2018 5:45 pm 
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injecting a signal at the grid of the 3rd if tube gives a clear signal and at the 2nd does not. easy way to clear the parts between the 2nd and third if tube is to unplug the 2nd if tube and using a blocking condenser between the b@k probe and the plate terminal inject a signal at the plate of the 2nd video if tube. if you get a picture that is good that clears parts downstream in my mind.

assuming you get a good picture at the plate of the 2nd if tube next step is to test all voltages on the 2nd if tube. do you have tube test socket? this will make voltage measurements easier. if not u can make the measurements at the tube socket pins underneath the chassis. any that are off by more than 20 percent test all parts connected to that socket terminal. this was standard proceedure when i worked on these tvs.

never ever touch alignment till all other possible causes of a problem has been eliminated and then only if you have proper test gear and the knowledge to use it correctly. follow the instructions in the service notes exactly.


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Fri 03, 2018 12:16 am 
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thomas13202 wrote:
injecting a signal at the grid of the 3rd if tube gives a clear signal and at the 2nd does not. easy way to clear the parts between the 2nd and third if tube is to unplug the 2nd if tube and using a blocking condenser between the b@k probe and the plate terminal inject a signal at the plate of the 2nd video if tube. if you get a picture that is good that clears parts downstream in my mind.

assuming you get a good picture at the plate of the 2nd if tube next step is to test all voltages on the 2nd if tube. do you have tube test socket? this will make voltage measurements easier. if not u can make the measurements at the tube socket pins underneath the chassis. any that are off by more than 20 percent test all parts connected to that socket terminal. this was standard proceedure when i worked on these tvs.

never ever touch alignment till all other possible causes of a problem has been eliminated and then only if you have proper test gear and the knowledge to use it correctly. follow the instructions in the service notes exactly.
What value of cap would you use for this? would a .01 work?

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Fri 03, 2018 12:44 am 
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Here are the voltages on v6 and v7.
V6 plate spec 200v i got 160v
V6 cathode spec .3v i got .5v
V6 screen spec 130 I got 116ish
V6 control spec -1.7v i got 0.382v
V7 plate spec 130v i got 123v
V7 cathode spec 1.7v i got 1.5v
V7 screen spec 130v i got 110
V7 control spec 0v i got 0v

This is with the if signal being injected at pin 1 of V6. I think the next step is to check the resistors feeding b+ to the tubes. Also the vertical is doing a weird foldover thing on the bottom edge. This issue has some effect on it but its not entirely related. I want to get a clean sync signal through the IF before going after the height.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Fri 03, 2018 4:07 am 
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OK that is telling me all DC conditions look OK, forget about the plate and screen resistors, they, and the tubes, are all OK.
Variations around V6 are consistent with the fact that there is no AGC being developed, were you injecting into G1 of V6 via a coupling cap?
If not, your signal source will be shorting out the AGC voltage, that explains the discrepancy.

Can you vary the output level of your signal source? Is it even roughly calibrated? In mV?
What carrier frequency is it set to? Does it match the IF frequency of that set?
We need to know how much signal you are injecting into G1 of each of the IF amp tubes to see something on the screen.


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Fri 03, 2018 12:42 pm 
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Yes I ca vary the output in 2 ways the video or the RF attenuator. They are not calibrated in any way just marked 0-10. I have the video at max and the attenuator at min attenuation. I can turn them back a bit but too far and it loses the signal. No cap is being used. Im just following the 1077 instructions. The AGC used in this set is little more then a 3 position switch.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Sun 05, 2018 10:07 pm 
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If you inject a strong signal into an AGC- controlled stage and you are effectively shunting the AGC to ground (the 0v tells me you are) you will overdrive the stages and that might explain what is happening.

Use a .01uF cap in series.


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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Mon 06, 2018 12:53 pm 
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irob2345 wrote:
If you inject a strong signal into an AGC- controlled stage and you are effectively shunting the AGC to ground (the 0v tells me you are) you will overdrive the stages and that might explain what is happening.

Use a .01uF cap in series.
Ok I will try a .01 cap.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Tue 07, 2018 9:43 pm 
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Ok. I tried injecting a signal into the set with a .01 capacitor in series with the output. I trued all 3 if stages. Stage one produced the worst results with the picture severely distorting when the video control on the 1077 was turned up to the max output. I kept the video output level around the 6-8 mark. Bringing this up to 10 will distort the image at the input of stage one. turning down the contrast control brings it back into reason but the picture is quite washed out at that point. Stage 2 only has a slight hint of instability of the horizontal at the extreme end of the contrast control. Stage three produces a very very faint picture with no distortion as stated before.
Edit. Just thought I would put this in here as well

There is a local/suburban/fringe agc switch on the back of the set. Has no effect whatsoever. To me it seems that the primitive AGC does not work.

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 Post subject: Re: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Aug Wed 08, 2018 1:00 am 
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Placing a -3v bias on the AGC line results in the issue going away. Looks like a AGC issue.

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 Post subject: Re: Motorola 17T21 IF Overloading/No AGC LARGE IMAGES
PostPosted: Aug Thu 09, 2018 5:57 pm 
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keep the signal level of the 1077 as low as possible to get the desired result. you can overload the stage if u put in to hot a signal. each stage you move back toward the front end you might want to lower the signal a bit to compensate for added gain of the aditional stage. you might want to do the test again injecting a lower level signal. if you are still having problems check components in the agc line. i cant remember what signal level i would use but 8 seems a bit high for injecting into the front end.

i am not sure about the 1077 as i never saw one but if my memory is correct for the 1075 i used they had a seperate probe to use for rf if signal injection and if memory serves a seperate control to control the rf signal leval. i havnt used one of these in about 20 years.

one other thing remember a tube can test good in a tube tester but not function in a particular circuit. as i recall the horizontal section this could especially happen. this is why if the set had two of the same type tubes it was common practice to switch them and see if that helped the problem. sometimes this might work. if the origional problem was now cleared but a new problem showed up in the stage the tube was swapped into you knew you had a bad tube. replace the tube and you are done.

when testing tubes dont forget to do a nose test and on amplifier tubes dont forget to do a gas test if your tube tester has provision made for those tests. in a push pull audio output stage the tube should be matched as close as possible also. if one tube is weak and one strong this can cause distortion.


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