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  #1  
Old 10-27-2015, 07:07 AM
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Kamakiri Kamakiri is offline
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Need some help on an 8TS30....

I've got this 8TS30 on the bench, and I should start by saying that this is the 2nd 8TS30 I've done, and about the 8th 630 chassis set....so I'm very familiar with them.....but I've run into an issue that I can't seem to get a handle on.

Started with a recap, and replaced the wirewound resistor and bad sections of the candohm in the metal box behind the transformer. Powered it up, got perfect sound but no HV. The only thing that I didn't check was the large wirewound in the HV cage. Sure enough, it was open. Now here's where things get interesting:

http://www.earlytelevision.org/pdf/r...ts-sams-54.pdf

Third page, illustrated about 10:00 to the 5V4 in the diagram, is R123....which was the open resistor. I replaced it, and variac'd it up. About 5 seconds after I hit the power at about 78 volts or so, R127B started arcing and smoking (shown on page 2, lower right). I replaced R127B with a Dale chassis mounted resistor of the correct resistance and wattage.

The first thing I thought of was that I wired the resistors incorrectly. I then stuck an untouched 8TS30 on the bench next to it, and traced out the wiring. Everything looks correct. Next, I pulled the chassis out of a restored 8TS30 to triple check my work. Again, everything I did is correct from what I can see.

Kind of at a loss as to how to proceed. Ideas/thoughts? I'm thinking there's another issue that this set has, as there's a lot of circuitry between R127B and R123....but since R123 comes right off the flyback, being open it wasn't engaging any of the HV.
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Old 10-27-2015, 11:56 AM
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If R123 is now a chassis mount dale, then what are the odds that the yoke voltage on it is enough to short thru the dale to it's chassis mounting and load the heck out of the fly?
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  #3  
Old 10-27-2015, 12:02 PM
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Yeah, but he said he replaced 127B with a Dale chassis mount,which should have no connection to the yoke?

I can't see that R127B supplies any source voltages to the horizontal or vertical circuitry. Looks like it supplies negative bias to the video and sync circuits. R127B is in series with the negative voltage on the center tap of the 5U4 transformer winding - that feeds the focus coil and Ion trap, which is also in series with R128B-C and continues to ground. So basically you have a negative voltage source being divided across several resistors all in series to ground, negative bias voltages picked off to bias the video and sync circuits. Have you slowly powered back up since you replaced R127B?
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Old 10-27-2015, 12:15 PM
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I just noticed that R127B also feeds a negative bias voltage to the grid of the 6K6 audio output, but that should be isolated through a 270k resistor in series with the 6K6 grid. However the 270K(R76) may be worth investigating.
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Old 10-27-2015, 12:20 PM
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Quote:
Originally Posted by Kevin Kuehn View Post
Yeah, but he said he replaced 127B with a Dale chassis mount,which should have no connection to the yoke?
I never mentioned 127B. I was talking about R123. I think you misread my post....Though Kamakiri did not say what he replaced R123 with so perhaps I misread his post.
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Old 10-27-2015, 12:56 PM
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Sorry. I lumped R127B and chassis mount Dale together. And I took it he was now concerned with why R127B smoked. We may be missing information.
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Old 10-27-2015, 02:20 PM
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I replaced R123 with a wirewound of the same spec. That's why the set had no HV, it was open. I replaced R127B with a chassis mounted Dale resistor, but now that I think about it, I dressed the lead at a 90 degree angle, and there was *maybe* 1/4" clearance between the chassis and the resistor. Is it possible that all I did was create a gap just large enough for spark to jump to chassis ground?

I haven't replaced the resistor and powered it up since. Figured I'd verify all my work before I did.

When that thing sparked, it was like a welding arc......
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Old 10-27-2015, 04:33 PM
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The source of voltage across that group of series resistors that forms the focus coil, Ion trap, and bias network( includes R127B) , comes from the center tap of the 5U4 low voltage rectifier winding. Measuring across C1C to ground, you should have a negative voltage of less than 150.

Last edited by Kevin Kuehn; 10-27-2015 at 04:42 PM.
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Old 10-27-2015, 10:16 PM
old_coot88 old_coot88 is offline
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FWIW, I would first suspect a hard B+ short-to-ground (possibly from the H.centering control or V.centering control to ground).
A hard short on the B+ line would throw a huge current draw on the 5U4 and its centertapped HV winding, and cause a large overcurrent through the resistor chain off the centertap, which includes R127B (it's the highest resistance in the chain, so it will run the hottest).
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Old 10-27-2015, 11:44 PM
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I think you probably nailed it there old-coot88. But I was sort of hoping we could at least fire up the dim bulb tester, maybe get a few voltage measurements. At least for me, most of the fun is learning how to walk through the circuit and do some basic DC troubleshooting. But I also understand that time is money.
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Old 10-28-2015, 02:01 AM
old_coot88 old_coot88 is offline
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Well, it's also an opportunity for a noob to see how the back-bias system works to supply negative voltages. The centertap of the HV winding, instead of going directly to ground as it normally would, goes through a resistance that's tapped for the different (-) voltages.

Troubleshooting without understanding the back-bias thing would be confusing, as evidenced in the thread.
(Of course in this case, there's also a focus circuit and ion trap whose summed resistance forms part of the resistor chain.)
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Old 10-28-2015, 05:04 AM
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Well, you've given me a place to start, appreciate that!

The set that I restored that was working fine (not this one obviously), all I did was remove and reinstall the resistor cover and look at my wiring that I did. That's IT.

Decided to watch that set for a little while, turned it on, and guess what? No raster. Removed the cover, and everything looked as it was. I didn't even pull the chassis. So now I have to figure that one out too.

#&%@ televisions.
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  #13  
Old 10-28-2015, 05:16 AM
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Quote:
Originally Posted by Kevin Kuehn View Post
I think you probably nailed it there old-coot88. But I was sort of hoping we could at least fire up the dim bulb tester, maybe get a few voltage measurements. At least for me, most of the fun is learning how to walk through the circuit and do some basic DC troubleshooting. But I also understand that time is money.
For me, it's difficult because I've never had any formal electronics training whatsoever. Heck, I've never even had the benefit of someone in person walking me through a set step by step. It was just something that I wanted to do ever since I was a boy, so I pick it up as I go and have been doing that for the last decade or so.

I've restored close to 100 sets over the years, but every one of them was done by either recapping and getting lucky, or working through electronic principles and developing a rudimentary understanding of how things work along with problems associated with different circuits and how to resolve them.

I can recap a 630 chassis in 2-3 weeks, and my work is very precise. Hands, I've got. Diagnostic skills, not so much. Doesn't stop me from trying and learning though.
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  #14  
Old 10-28-2015, 08:01 AM
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Quote:
Originally Posted by Kamakiri View Post
Well, you've given me a place to start, appreciate that!

The set that I restored that was working fine (not this one obviously), all I did was remove and reinstall the resistor cover and look at my wiring that I did. That's IT.

Decided to watch that set for a little while, turned it on, and guess what? No raster. Removed the cover, and everything looked as it was. I didn't even pull the chassis. So now I have to figure that one out too.

#&%@ televisions.
Everything with my restored 8TS30 (mentioned above) is now okay.

I disassembled and tested each resistor in the network, the wirewound in the HV cage, the resistor at the bottom of the 1B3, the doorknob cap, all of the tubes in the HV section, even the picture tube. Found not a damn thing wrong. Put it back together, and for some reason now it works.

Now back on to the problem child at hand
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  #15  
Old 10-28-2015, 11:48 AM
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Quote:
Originally Posted by Kamakiri View Post
For me, it's difficult because I've never had any formal electronics training whatsoever. Heck, I've never even had the benefit of someone in person walking me through a set step by step. It was just something that I wanted to do ever since I was a boy, so I pick it up as I go and have been doing that for the last decade or so.

I've restored close to 100 sets over the years, but every one of them was done by either recapping and getting lucky, or working through electronic principles and developing a rudimentary understanding of how things work along with problems associated with different circuits and how to resolve them.

I can recap a 630 chassis in 2-3 weeks, and my work is very precise. Hands, I've got. Diagnostic skills, not so much. Doesn't stop me from trying and learning though.
Well I hope my remarks didn't come off as sounding harsh or criticizing, that surely is never my intention here. I suppose I'm guilty of being a little too sarcastic some days.

I went to electronic servicing school way to late in the consumer electronics game to make any career out of it. So I ended up working many years for an industrial supplier to the weighing industry. Therefore most of my professional experience has been with DC circuits and analog to digital conversion. But in my opinion, understanding the very basics of DC and AC circuits is the key to efficiently troubleshooting these TV's, or for that matter any AC/DC electronic circuit - being able to apply Ohms law, and understanding how current flows through a circuit branch, and how that current flow creates voltage drops across the various components. In my case, seeing is believing, and so very early on I got hooked on using a scope, even when looking at simple things like power supply voltages. Where I have a major weakness is in understanding RF flow through capacitors,coils, and tuned circuits, and it's because I never really took a liking to the math involved in AC circuits. I can still look at the signals, but the results often don't make sense to me. Major stumbling block for me, that I didn't focus more on that math in my younger days.
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