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  #16  
Old 05-28-2017, 04:04 PM
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VintagePC VintagePC is offline
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That's something I considered, but they're both the correct 1/4 amp value. I guess it's a free spare for me then... that said this is a replacement flyback, so perhaps it was part of the service kit to put it in as it draws more current.

Spent the day checking various things. The important bits like transformers seem OK, though the flyback does have a high resistance on the primary... schematic says ~180 ohms, but it's reading about 460.

Most of the controls were pretty crusty. Apart from the already found busted focus, they came good with disassembly and some deoxit.

Also spent some time panel-beating the front brass decoration. It was pretty beat up and bent. Managed to get it quite decent again... only downside is the little metal spring that is supposed to hold it closed was bent and broke when I tried to bend it back. We'll see whether there's enough tension in the hinge to keep it closed or if I'll have to figure out something else.

Also got my cap list together, will be ordering that shortly. Looking forward to first power-up once that's done!
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  #17  
Old 06-02-2017, 12:41 PM
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DavGoodlin DavGoodlin is offline
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I've been helping a noob work on a Hallicrafters 602 floor model, not unlike yours. After a complete recapping, we have a very dim but full raster. The 12RP4 is pretty weak though.

I think yours may be a bit newer but the 602 uses 12AU7 tubes in many places including as the video amp. cranking up the contrast control causes this video amp to red-plate, because it alters the cathode bias resistance which seems archaic considering most video amp circuits only vary the resistance in series with capacitance in the cathode circuit, leaving bias unchanged. Very strange circuit, may have to change it for it to work right.

Once you replace the paper and electrolytic caps throughout your set, you'll have some real clues to what it still needs.
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  #18  
Old 06-03-2017, 08:35 PM
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I've finished nearly all of the recapping, only a 4 section can left. I stopped because I can't make sense of what I'm seeing vs the schematic:

What is *supposed* to be happening is a 190v line from the LV rectifier feeds one of the sections (40 mfd), which then bridges thru a 10W resistor, to another 40mfd section, for a ~120v DC line.

The bits after the 750/10W resistor all make sense. What doesn't is that the first 40mfd section (the 190v line) is bridged to the 40mfd section that resides in the audio circuit just off the output transformer. That's already a WTF, since that section sees 260v and the bridged section is only rated for 250. (the section for that area is a 450v one)

... I wonder if that's what made the focus control burn up in the first place?

... and should it be corrected? I've found only a few minor differences between my schematic and the set otherwise (39k vs 47k resistors in some spots, and swapped component order in one spot) otherwise.
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  #19  
Old 06-04-2017, 12:29 AM
EdKozk2 EdKozk2 is offline
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Pin 8 of the secondary B+ (150V) supply rectifier only connects to the junction of C135c and R164. R164 the 750 ohm 10w resistor is connected between C135c and C135d, which forms a standard PI filter. C135c and C135d are the 40-40mfd 250 volt sections. They share the the same ground with C135a a 20mfd 25vdc and C135b the 40mfd 450vdc sections. The focus control may have cooked if C135b had shorted. The primary B+ (260V) is filtered at the audio output transformer. The 260v supply voltage comes from the primary rectifier a 5U4.
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  #20  
Old 06-04-2017, 07:45 AM
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Yes, that's what I gathered it should have been. Looks like the person that replaced the focus control shunt resistor made a pretty severe mistake, as its lead is the one that appears to bridge the two sections.
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  #21  
Old 06-04-2017, 04:19 PM
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Well well... I think we found the reason that bridge existed. Just went for first powerup with only the 6AX5 installed... no HV. 390VAC into the tube, filament is lit, no output.

This also explains the double-ganged fuse on the primary B+... naturally it's going to draw more current if it's got more stuff on it.

Don't have any spares on hand... I was planning on getting some spare tubes for this set next time there's a local swap meet. Guess it's on the shelf 'till then.
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  #22  
Old 06-04-2017, 06:26 PM
mrjukebox160 mrjukebox160 is offline
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http://www.ebay.com/itm/Vintage-6AX5...sAAOSwrklVKGoI
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  #23  
Old 06-04-2017, 06:34 PM
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Why would I spend the better part of $10 on a single tube + shipping when I can wait a few weeks and probably get most of my tubes for $1 or $2 each?

That said, I am being a little silly... I have a parts box full of things and I just realized i can almost certainly find some 1N4007s and a resistor to make a temporary replacement....
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  #24  
Old 06-04-2017, 08:56 PM
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At the risk of sounding cliché...

WELL THERE'S YOUR PROBLEM:





Pretty sure 6AX5s do NOT have inbuilt cathode fuses! Yowza! (Yes, it's melted, not just snapped)
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  #25  
Old 06-05-2017, 08:08 PM
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First signs are promising... I pulled the horizontal oscillator and tried a power-up with the 5U4 and my improvised diode rectifier. (Ended up adding a switch to it as well as a resistor since the DC voltage is too high for the caps until the tubes warm up, but too low if I have the correct resistor value to keep it in range... so I just give them a bit to heat up and then switch the AC to the diodes.

First time I heard a rather ugly crackling noise and killed the power... fortunately it was just the speaker and nothing emitting magic smoke.

Anyway... I was rewarded with audio from the speaker!

Everything seemed fine with both B+ voltages up, so I plugged in the oscillator and powered it up again. Heard the HV come up, and was treated to a pretty light show in the rectifier.


I'm going to go ahead and guess my rectifier is just a teeny bit gassy... but I'd say that probably means my flyback is good and producing plenty of HV. Awesome!
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  #26  
Old 06-05-2017, 08:31 PM
EdKozk2 EdKozk2 is offline
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Did you try another HV rectifier, to see if the blue glow goes away ? That's really bright.

Last edited by EdKozk2; 06-09-2017 at 09:51 PM.
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  #27  
Old 06-05-2017, 08:50 PM
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Don't have one at the moment, I'm tube-poor. I'll be picking up a full compliment of spares in a week or two.
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  #28  
Old 06-05-2017, 09:28 PM
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Electronic M Electronic M is offline
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Future TV variac advice: DO NOT pull the H oscillator tube(!), pull the H output tube instead (if series string pull the top cap or better yet clip only the heater leads in with clip leads).

If the output gets no signal from the osc, then the output will red-plate when that happens for more than ~10 seconds you will ruin the (often pricey) output tube, or weaken it greatly, and if you run it longer you put other sometimes unobtainium parts like the flyback at risk of destruction.
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  #29  
Old 06-06-2017, 06:54 AM
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My bad, though the output tube is right up top and I always watch for those kinds of things when doing a powerup if the set is not known working.

I could have sworn I saw a comment somewhere in the service literature about pulling the H. oscillator for one reason or another (which is why I did it) but can't for the life of me find it again by skimming the pages.

Did not see any signs of it red-plating, and it was indeed only for a few seconds.
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  #30  
Old 06-06-2017, 06:58 PM
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Found an ebay seller with a large variety of NOS tubes and a free shipping promo for over 20 items. I stocked up and got everything I need for about a dollar each except a spare 5u4 and some 6sn7s.

Looked closer at my 1B3, getter is completely gone, so it's totally gone to air. No obvious cracks.

Electronic M - In retrospect, I'm intrigued by your comment... One would think sets would be designed so that typical failure of any arbitrary tube would result in minimal or no damage. What is it about the circuit design that makes the HO tube red-plate if there is no oscillator? Are there other parts of the circuit that are similarly affected if another single tube fails?

Granted, in a typical case I would assume tubes wear out rather than failing abruptly and a weak oscillator would get replaced before it reached that state, but I can imagine it certainly wouldn't be uncommon for a tube to sometimes go poof from thermal stresses or filament burnout.

Last edited by VintagePC; 06-06-2017 at 09:21 PM.
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