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 Post subject: Motorola 17T21 RF or IF Overloading LARGE IMAGES
PostPosted: Mar Thu 23, 2017 10:23 pm 
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Hello,
I have been working on this Motorola 17" set from 1955 for the past month. I have not been able to solve this problem for weeks.
When the set is turned on the horizontal takes anywhere from 1 to 5 minutes to sync up. When locked, there is a another problem
that I believe is the underlying cause of the sync issue. There is several points on the screen that shift horizontally and fluctuate
with the video on the screen. When the screen is black it goes away. This is affected by the contrast control, when you turn the
contrast up it gets worse. I have probed the sync section with a oscilloscope and it shows the sync waveforms being modulated
by some sort of video signal or noise. There is a resistor and capacitor in parallel with the input to the sync separator tube. I replaced both
with no effect. Where is the noise coming from? Is it upstream of the video output stage or is it from something else.

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Last edited by MrE12AX7 on Nov Tue 21, 2017 7:34 pm, edited 2 times in total.

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 Post subject: Re: Motorola 17T21 Video noise on sync signal
PostPosted: Mar Fri 24, 2017 4:22 am 
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What kinds of things have you replaced? Maybe the video is getting to other sections through the supply voltage. Did you look at the supply voltage with a scope to see if the modulation is there?

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"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


Last edited by Notimetolooz on Mar Fri 24, 2017 3:43 pm, edited 1 time in total.

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 Post subject: Re: Motorola 17T21 Video noise on sync signal
PostPosted: Mar Fri 24, 2017 5:00 am 
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Location: Wyoming, Michigan
It's difficult to assist without a model number and schematic to look at. Start by pulling out a sync tube and then carefully adjust the horizontal and vertical hold controls until you obtain a momentarily synced frame. See if the offending horizontal shifting goes away. If so, the problem is isolated to just a few parts. If not it's isolated to a few other parts.

Post a schematic if you can.


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 Post subject: Re: Motorola 17T21 Video noise on sync signal
PostPosted: Mar Fri 24, 2017 2:34 pm 
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This is a series string set so I cannot pull out any tubes.
The noise is not coming from the power supply as it disappears when the screen is black.
I have replaced all the paper and electrolytic caps as well as the selenium rectifiers. I also replaced a 220pf cap and a 470k resistor in
parallel on the input to the sync separator. The vertical is unaffected as far as I can tell.
Should I try a different sync separator tube?
Image
Image
Image

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Fri 24, 2017 4:03 pm 
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Location: Dallas, TX
I re-read your first post. I think you should look at the signals around the Horiz AFC V11 next.
Then Sync Phase Inverter of V13.
There would normally be some video at the input to the sync separator.

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"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Fri 24, 2017 4:13 pm 
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I second the above. Sounds like the phasing is out to lunch. In older sets, that's the other half of the oscillator coil. Here they used caps and resistors...smaller and cheaper. Craig


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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Fri 24, 2017 4:31 pm 
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analog.tv wrote:
I second the above. Sounds like the phasing is out to lunch. In older sets, that's the other half of the oscillator coil. Here they used caps and resistors...smaller and cheaper. Craig

Should I check that resistor across the osc coil?

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Fri 24, 2017 4:56 pm 
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I measured the voltages on V11 and V13
V11
pin 4 -3.2v
Sams -.9v
pin 5 0.360v
Sams -.1v
pin 6 1.384v
Sams .5v
V13
pin 4 6.81v
Sams 5.2v
pin 5 112v
Sams 110v
pin 6 10v
Sams 8v

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 Post subject: Re: Motorola 17T21 Video noise on sync signal
PostPosted: Mar Sat 25, 2017 12:32 am 
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MrE12AX7 wrote:
This is a series string set so I cannot pull out any tubes...


Funny the schematic tells you to remove a tube (V14) from its socket as well. Of course that wouldn't work on this series-strung set. But I think any TV repairman of the day would have known what was meant.

I would be suspicious of C72 or C73. It's always possible to get a bad one in a new order. Or sometimes accidentally replace one with the wrong value.

Or C70.


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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 1:30 am 
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I cannot check the capacitors as I don't have a cap checker (although I am trying to get one)
They are wired correctly and are the correct values. They are high quality Panasonic film caps.

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 1:32 am 
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If you haven't already tested or tried replacing the tubes in the sync and horizontal area,
it would be a good idea.
The Phase Inverter voltages look OK. The Horizontal AFC are off. Did you take these reading with the
picture looking good or bad? I was thinking that you should compare the appearance of scope displays
with what SAMS shows. You said that the contrast control effects the problem, this made me suspect
that the video output signal was getting back into the sync circuits, probably through the common power
supply. However if you have recapped this set that seems unlikely.

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 1:43 am 
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Note 5 says to take all readings with the controls set for normal operation and no signal applied. I could imagine the AFC values being off with video and sync signal. Make sure you were just viewing snow.

Couldn't hurt to swap V13 in case the problem is in the tube.


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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 2:08 am 
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You can vary the contrast control while you monitor signals with a scope and see if it effects the
signal at various spots.

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"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 3:27 am 
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I have looked at various signals with the scope and turning up the contrast makes it much worse.
Replacing v13 made no difference. I then swapped v11 and v15 (they are the only 6sn7s I have)
When this was done I did not leave it on enough see a picture as there was some high pitched squealing noise
coming from the flyback. I guess it does not like the tubes being swapped.
I forgot to mention that the sync locks only when the horizontal hold control is at one extreme.

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 3:33 am 
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Well that sort of tells you V11 is bad. Putting it in place of V15 made your horizontal osc flip out, hence the high pitched squeal. Do you have a tube tester? What kind? The ones that only test emissions can sometimes mislead you. If both tubes were good, swapping them would have no effect.


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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 3:59 am 
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I have a basic emissions tester and V11 tests just barely in the green on that tester, no shorts.
I found another 6sn7 and tried it. No change. EDIT that 6sn7 tests about the same as the other one.

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 5:38 pm 
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What no one has mentioned so far is that the sync separator may not be separating the sync out correctly. To do it's job of
amplifying only part of the signal fed into it, it has to be run with somewhat funny voltages on the tubes. And those voltages
are quite critical. You should check all the resistors in the sync separator circuit as those control the voltages on the tubes.
Also, how does that 12SN7 test?

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 6:21 pm 
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Tom Schulz wrote:
What no one has mentioned so far is that the sync separator may not be separating the sync out correctly.


Because that's been fairly obvious but thanks for being explicit. It's why I originally doubted C72 and C73 and said it wouldn't hurt to sub V13.

He has a cold-set problem that corrects to some degree after a 5 minute warmup. Suggests a weak tube (or tubes). Confirmed by his tester to boot. Replacing it may correct the poor warmup and lock only occurring at extreme range of hold control. The jittery left edge still seems like bad coupling cap or possibly flaky tube in the sync separation or possibly phase inversion. Both functions are within one tube, so a test swap of it would rule tube flakiness in or out. He says he tried that, but wondering if he just has a stash of weak used tubes.

A photo of the left edge of the screen (when locked but ragged) might help.


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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Sat 25, 2017 6:56 pm 
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I think I tried 2-3 12SN7s including one NOS one over the course of trying to fix this thing.
I will try and get new 6SN7s soon.

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 Post subject: Re: Motorola 17T21 Video noise on sync signal LARGE IMAGES
PostPosted: Mar Mon 27, 2017 2:06 am 
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That might be an integrator issue...both sweeps' front end have decoupling for the wrong pulses. Scope's a good friend here. Craig


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