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  #16  
Old 07-25-2017, 12:53 PM
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Try a different modulator, and try reducing signal to the set as well as increasing it (IIRC contrast is IF gain+video gain so try changing contrast first*).
* best to do this with a DVD or some repeatable content that does it bad so you can see how the signal level affects it....Also don't worry about watchable brightness or contrast during this test, as long as you have enough to tell if it is rolling that is all that matters.
If signal is too strong or too weak then it will fall outside the designed range of the sync clipper resulting in weak sync or sync with noise (which can disrupt lock).

If it looses it under the same conditions regardless of signal then the sync sep and osc stages should be suspected.
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  #17  
Old 07-25-2017, 01:08 PM
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Well I have been running it with the cable wire I was not using an RF modulator so this should be good enough to figure what is what with the signal or sync. I wonder if the 2 .002 ceramic disc caps I have in the horiz may be affecting the sync.
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  #18  
Old 07-25-2017, 01:43 PM
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I just checked v10 clipper tube pin 6 should have b+ at least 125 Vdc on it but all I'm getting is 82 Vdc it don't make sense to me I checked the cap that feeds pin 6 even tried another cap but only 82 Vdc so that should indicate weak sync . This is one of the power supply caps and recapping I did one at a time and never messed up with power supply caps so now I'm stumped. I did replace the seleniums with 1n4007 diodes as I done with the other sets I have with no dropping resistors.
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  #19  
Old 07-25-2017, 03:41 PM
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Assuming the whole B+ rail is low and not just the sync tap:
B+ is created from B++ at the audio output tube which acts as a B+ dropping resistor.
Check the bias on the audio output tube, the tube it's self and associated parts....If those are good look for excessive loads on the B+ rail (by disconnecting things hooked to B+ one by one and noting voltage change or by resistance testing).

[edit] Scratch the audio output tube being the B+ supply divider resistor, the circuit does not look right for that...Odd thing is I'm not seeing any clear B+ supply/divider...Might be time for some coffee.
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Last edited by Electronic M; 07-25-2017 at 03:54 PM.
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  #20  
Old 07-25-2017, 04:21 PM
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You're right, it's not the usual stacked design using the audio output tube. Instead B+ is created by the drop from several tubes. Sorry for the poor scan quality.

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  #21  
Old 07-25-2017, 04:25 PM
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timmy if you have not done so already, down load the Motorola manual on the VT-71 from the ETF it gives detailed description of the circuits and bite size schematic of each section of circuitry. Hopefully will make it a little bit easier to follow. All the best,Tom.J
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  #22  
Old 07-25-2017, 05:42 PM
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Ok yes I have the schematic for the ts4j ill look at it closer. So if I'm correct this could be the vert problem not staying locked.
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  #23  
Old 07-26-2017, 08:52 AM
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If your picture is crisp, and, clean, I would say it's more likely in your sync circuit somewhere. Perhaps the sink clipper..
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  #24  
Old 07-26-2017, 09:02 AM
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Quote:
Originally Posted by Bill Cahill View Post
If your picture is crisp, and, clean, I would say it's more likely in your sync circuit somewhere. Perhaps the sink clipper..
Well it's not really crisp it's close but not crispy which is nice to be coming out of a 70 or so year old tv. I'm going to get into it today to see if the whole b+ rail is low. Maybe a defective diode I put in.
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  #25  
Old 07-26-2017, 09:46 AM
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Given the explanation of the B+ source that bob posted, if B++ is right then PS diodes should not affect the B+ .

It looks like B+ is obtained by a balance of net currents and voltage drops through V-2,4,6, 10a 11A, and 12 (if I'm reading correctly). Basically B+ is created by a voltage divider created by the impedance of each tube connected to it....It is like the audio output B+ divider/dropper concept I thought it was, on crack (ie they took it too far).

To figure out the low B+ issue you are pretty much going to want to examine the biases of all the tube sections it feeds look for bad tubes, and bad passive components. You will want to look for bias voltage changes in tubes that are not immediately explained by changes to the B+ rail.

Tubes shown in Bobs diagram between B++ and B+ will be conducting too little and or tubes shown between B+ and the Common B return/b- will be conducting too much.

SS sets that use this type of multi-stacked parallel B+ (I hear 2 way radios do this a lot) can be a b!tch to troubleshoot.
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  #26  
Old 07-26-2017, 12:05 PM
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So the low value resistors between the heaters I should check. Could this be a tube drawing to much current ?
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  #27  
Old 07-26-2017, 01:00 PM
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He doesn't mean the tube heater (filament) is drawing too much current. He means that if the tube grid bias is off, it could be drawing too much or too little current between the cathode and plate.
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  #28  
Old 07-26-2017, 01:02 PM
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Quote:
Originally Posted by timmy View Post
So the low value resistors between the heaters I should check. Could this be a tube drawing to much current ?

I don't see any interaction between the heaters and the B+, so I don't get why you are asking about the heaters (they should not matter).

As I said in my last post your B+ issue is the lower tubes in Bobs diagram conducting too much current or the upper conducting too little...

The B+ voltage is created from the B++ through use an interesting idea: If a tube draws a certain amount of average B current and drops a certain amount of average B voltage, then (if you filter the spikes out) you can model a tube stage as a power resistor loading the B supply....They modeled several lower B+ voltage stages as resistors then connected those stages to B++ in a series parallel voltage divider network the center of which is B+.

If your not an electrical engineer, and don't understand: Ohm's law, series and parallel resistor networks, and the voltage divider rule then it will be extremely difficult to understand the way in which the B+ is obtained in this design*, and how to best troubleshoot it.

*It is probably the most complex unregulated supply I've seen in tube era electronic gear.
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  #29  
Old 07-26-2017, 02:13 PM
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Well if there are bias issues somewhere I have not found any high or low value resistors that were bad . I have a Dvom that I can check capacitance on capacitors so I am wondering if I got a possible bad cap because I know when you get small waves in the picture that usually means a bad cap , not filtering the ac out and that's really the only noticeable thing other then the weak vert lock that I see going on with this set because of probably the low b+ . So now I should look into the caps in the p s that I put in as possibly bad. And I do know it uses half to get the 125 Vdc for the b+ . So now I will have to doubt my Dvom for cap checks if I find a bad one.
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  #30  
Old 07-26-2017, 04:01 PM
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I don't know if it's feasible to remove each b+ wire at the tie point one by one as I think there are like 4 or 5 places b+ Is tied to, maybe narrow down where the drop is originating from. The b++ is where it should be on the chassis at 248 Vdc and there are different voltages on each p s cap and the next to b+ voltage is 132 Vdc . I don't have any voltage chart that is with some schematics as it's just the download from early television and it's kind of difficult to judge what's right or wrong if I don't have any references to go by.
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