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#16
10-05-2011, 11:30 AM
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I wonder if both 6BG6's are working.. You might try (very briefly) running the set with one, than the other. Possibly something goofy, bad solder or contact on one..
All the symptoms point to low power into the flyback, rather than an overload. Weak damper, or poor boost filtering could be a problem.. May be worth scoping the boost supply. Also worth scoping the B+ that feeds the damper, although I'd expect other problems if the filtering here was poor.. 
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#17
10-05-2011, 12:20 PM
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Both 6BG6s are working. Removing either one results in much lower width. Filtering is fine. Very little hum on the B+, plus I've tried paralleling another 40 mfd cap on that line. Replaced the .082 boost cap, no change. I still suspect the flyback.
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#18
10-05-2011, 06:58 PM
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Before tearing into the HOT, I am still curious about the Horiz Output tube current. You say that with the screen grd feed resistor shorted, is the cathode voltage, hence current correct? With the screen resistor shorted, was the screen voltage correct? (eg high enough?)
Could be the HOT has a shorted winding. But then again there may be a problem in the boost circuit. What makes me think it may not be the HOT is that the horizontal output tubes are running cool.
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#19
10-05-2011, 07:06 PM
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Shorting the screen grid resistor brings the screen up to the supply voltage, well above what the schematic shows as proper. Cathode voltage comes close to what it should be (17 vs 18 v relative to the the -135v buss).
Every part in the damper and boost sections has been changed. As you can see, there is very little connected to the boost (hor. osc and CRT screen grid pots). All those components have been checked too. I see no other possibility other than the flyback.
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#20
10-05-2011, 07:37 PM
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#21
10-05-2011, 07:38 PM
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Hmmm. Did the set work correctly at one point, or has it always been thus? (Wondering if it's a failure, or some other influence) I'd take a hard look at C131, the 1uF in the cathode circuit. You're dealing with a pretty hefty pulse current here, and an aluminum electrolytic may have a high enough ESR to not work very well.. The sch seems to indicate it's not an electrolytic. Also R20 and R156 100 Ohm and 180 Ohm basically in parallel, (hard to keep jumping from one page to another) Scoping the -130V line may tell something too.
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#22
10-05-2011, 08:56 PM
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The waveform on the sine wave coil is correct. Turning the hold control to either extreme makes no difference in the width. c124, r147 and c127 have been checked.
I've owned the set for a couple of years but am just getting around to restoring it. There is evidence that the previous owner had the same problem, since many parts in the horizontal circuit have been changed and a few were partially removed. C131 is not an electrolytic, and I've changed it. The cathode resistors and the pot have been checked. The -130 v line is clean, with little hum. I've also tried adding additional filtering on all supply lines with no effect. I don't have a spare flyback, and they are impossible to find, so I'll probably end up with no screen resistor and live with it. There is a problem with the RF HV power supplies in some 7 inch black and white sets that seems to be related to accumulated moisture in the RF transformer. I've never heard of such a thing with flybacks, but could that be a possibility? I mention it because this set has probably not been operated since the mid 50s. It will be a couple of weeks before I get back to this.
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#23
10-05-2011, 10:44 PM
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Steve,
It seems to me there used to be a width boosting product on the market which plugged into the damper tube socket and the damper tube plugged in turn into it. The result was boosted width. If I recollect correctly, this gizmo had a capacitor inside that connected between the damper tube's plate and cathode. I don't believe that I ever knew the value or voltage rating of the device's internal capacitor. Maybe this memory of mine will jog the memory of someone who can recollect some more details. If I had a set with such a rare flyback and stubbornly persistent width problem, I wouldn't be afraid to experiment with an added capacitor's capacitance using temporary capacitors that have a relatively high breakdown voltage. Incidentally, I think your Sparton was once offered for sale to me as a source for a 15GP22 for a CT-100 that I once had. I couldn't see destroying one historically important TV to fix another historic set, so I passed on it. I just found my 20 year old note where I thought it might be - it says "15GP22 Dan (847) 823-2379." It's a very cold trail I'm sure but white pages.com reverse lookup say it belongs to Daniel L Gustafson of Chicago, Illinois. I now see your set came from California, so the one once offered to me is probably a different Sparton. Someone might try calling Dan to inquire about the set. Not much to lose. Last edited by Mal Fuller; 10-05-2011 at 11:00 PM. Reason: to improve layout
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#24
10-05-2011, 11:45 PM
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Quote:
 Dan G has been dead for a while, sorry to be the one. I won his 21CT55 at the ETF auction this year, no one will be getting in contact with him anytime soon lest they join him. Let's pray that's not the case.
__________________
Evolution... 
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#25
10-06-2011, 09:36 AM
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Quote:
I'm a little confused by your response. Are you saying that Steve's Sparton is the same set that Dan and I discussed years ago?
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#26
10-06-2011, 10:09 AM
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Quote:
Now that you've got near-normal width, I was gonna suggest the old damper shunt cap trick, but see another poster has already mentioned it. The cap should be disc ceramic, rated at 6KV minimum. Value can be anywhere from 68 to 150 mmf (pf). We used to keep several values on hand for this very purpose (higher value gives more width). The cap goes directly between cathode and plate of the damper tube. But there's a trade-off as in the old saying "there is no free lunch". The HV drops a bit with each increase in width, as does the HO tube current. So the mod is only useful as a last resort in cases of intransigent marginal width such as this set appears to have. The same trick can be used on B&W sets with the cap rated at 4KV minimum. Last edited by old_coot88; 10-06-2011 at 10:12 AM.
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#27
10-06-2011, 03:25 PM
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Thank you old coot88 for providing "the rest of the story." Does the Sparton use a shunt regulator for HV regulation? If it does, would the "damper cap" have any real effect on 2nd anode voltage?
Dropping the two horizontal output tubes' current can't be a bad thing. Last edited by Mal Fuller; 10-06-2011 at 03:32 PM.
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#28
10-06-2011, 03:56 PM
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The Sparton I have didn't come from Danny.
Yes, it uses a shunt regulator. I now have just about 21 kv max. The question is whether the capacitor trick reduces it to below 20.
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#29
10-06-2011, 07:25 PM
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Steve, I would try adjusting the value of the cathode resistor in the horizontal output tube(s), to see if this could result in a bit more width. Check the amplitude and shape of the parabolic waveform before you change the resistor value so you know what waveform is needed to drive the convergence circuit. But I would think there is some margin available for the convergence circuit.
__________________
John Folsom
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#30
10-06-2011, 08:03 PM
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Quote:
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Linear Mode
