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  #16  
Old 06-03-2017, 01:56 AM
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Eric H Eric H is offline
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And a few more caps, a little more progress. None of the Electrolytics have been replaced at this point.
The vertical was cutting in and out when I tapped on the 12SN7 tube so I replaced it, it seems solid now, I'll see what happens when it cools back down.
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  #17  
Old 06-03-2017, 05:23 AM
kvflyer kvflyer is offline
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Quote:
Originally Posted by Eric H View Post
And a few more caps...
The vertical was cutting in and out when I tapped on the 12SN7 tube so I replaced it, it seems solid now, ...
Could that cutting out and tapping be a dirty tube socket, or possibly the tube that you took out having dirty pins?

I recently brought back to life a factory wired Dynaco PAS pre-amp. One channel was fine; the other was dead. I had not cleaned the sockets yet and wiggling one of the 12AX7 tubes brought it back temporarily.

Long story short, DeOxIt on a toothpick into each socket and all is well now.

Just a thought, could be still a good tube!
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  #18  
Old 06-03-2017, 03:30 PM
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It could be a dirty socket, the tube tested good so I'll try it again, but for now its solid and the vertical is taller.
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  #19  
Old 06-03-2017, 08:47 PM
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Quote:
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It could be a dirty socket, the tube tested good so I'll try it again, but for now its solid and the vertical is taller.
"I hear ya..."
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  #20  
Old 06-04-2017, 04:45 AM
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I tried the tube again and no vertical, until I whacked it with a screwdriver.

Tested the tube again, very good on both sections and it's a fairly new looking Sylvania.

I tried it in the Horizontal Oscillator circuit and it works fine, in fact I was able to adjust the horizontal down to a single image.

It's an odd looking 12SN7, taller than most and the plates are different than the usual thin, flat ones usually seen, it actually looks more like a 12SL7.

The chassis still has a lot of old caps in it, none of the Electrolytics have been changed yet so it could be the circuit is just being fussy at this point.
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  #21  
Old 06-06-2017, 01:04 PM
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Update/Comments on my set:

1. Received the 75 ohm 10 watt ceramic resistors for the heater circuits... they seem to be about the right value. At 125 volts line input, I get about 6.5 volts on the CRT heater, and pretty close to 6.3 volts at 117 volts line. Turn on surge at CRT heater is slightly greater than 8 volts, so I may add a TVS diode later, when set is no longer used with the variac. The resistors run quite warm, so I positioned them against the back of the HV cage and used a bit of heat sink grease between them and the aluminum surface. They now run under 200F.

2. Pulled off the front (indeed, I protected the speaker, as recommended) repaired the speaker (previously damaged by careless fingers) and replaced the rectifiers and power resistor (corroded wire fell off).

3. Finished up the caps (29 in my set) and replaced the electrolytics. (see pix). Picture has improved only slightly from previous photo, but is now stable.

4. Getting about 10.3 kV HV, other voltages are now at spec or slightly higher.

5. Bright spot on center of CRT lingers for nearly a minute after turn off... anybody have a mod to eliminate it ?

6. Next, check the tubes, upgrade if necessary, fine adjust horiz and vert size linearity, polish cabinet.

fun project!
jr
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  #22  
Old 06-06-2017, 08:09 PM
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I hooked this thing directly to the DVD player instead of using over the air with the Agile Modulator, after a bit of fiddling I got a really decent picture, the sound was weak and had static like a control was arcing, I started tapping around on the controls and the field coil of the speaker and it started getting better and better, now it has reasonably good sound, not sure if something was shorted or what but it cleared it up.

This is the slowest warming set I've ever seen, it takes at least a full minute before I hear the oscillator kick in, did I read somewhere it has some voltage dependent resistors in the heater string? Possibly those black round things in the HV cage?

The pictures a little cockeyed and I had to remove the focus magnet so it would center on the 5AXP4 test CRT but it's bright, has great contrast and no retrace lines.
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  #23  
Old 06-06-2017, 08:38 PM
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Quote:
Originally Posted by Eric H View Post
This is the slowest warming set I've ever seen, it takes at least a full minute before I hear the oscillator kick in, did I read somewhere it has some voltage dependent resistors in the heater string? Possibly those black round things in the HV cage?
Yep! I think that those glowbar (sp?) resistors exhibit an increase cold resistance as they age. With the 75 ohm resistors installed, my set produces a full, stable picture in about 20 seconds after turn on. The 6Al5s light up pretty bright at initial turn on, but seem to be holding up ok.

jr

Last edited by jr_tech; 06-06-2017 at 11:02 PM.
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  #24  
Old 06-07-2017, 11:57 PM
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My parts chassis had some 75 ohm resistors installed instead of the Globars so I swapped them into this chassis, it warms up quickly now, some of the small tubes come on a bit brightly but the CRT warms up normally so I can live with that, I might experiment with a Thermistor in the future but I wonder if it can handle the load.

Theses resistors look pretty hefty, maybe 10 watt and they get really hot.
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  #25  
Old 06-08-2017, 12:21 AM
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The best of luck on it. Information much appreciated.
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  #26  
Old 06-09-2017, 03:58 PM
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I don't have one of these but this thread makes me want one. They respond so well to your attention.

Between the RCA 630, Motorola VT-71, Admiral and GE 800/805 sets all so different yet rugged post-war designs.
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  #27  
Old 06-09-2017, 07:39 PM
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Update on mine:

1. The other night during prolonged operation, I experienced a failure... oddly it was a tall Sylvania 12SN7 with unusual looking plates (vert osc/amp), strange coincidence, for sure. Vertical went flat line, and I quickly reduced brightness, to avoid phosphor damage. Replaced the tube quickly, and caught the end of BBT, just in time for the cameo appearance of "the great and powerful Woz".

2. After prolonged operation, the 75 ohm/10 watt resistors are actually closer to 275 degrees F. Much of the heat is transferred to the HV cage through heat sink grease (the back side of the cage reaches about 150 degrees directly behind the resistors). I am working on a "cooler" solution involvind diodes and higher value 200-250 ohm resistors shunting them.

3. For grins, I pulled the chassis out of the wood cased 10T4 and repaired a few problems. It still performs fairly well with *mostly* original parts.

jr
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File Type: jpg IMG_1117.jpg (71.0 KB, 31 views)
File Type: jpg IMG_1178.jpg (103.3 KB, 35 views)

Last edited by jr_tech; 06-09-2017 at 08:40 PM. Reason: pix orientation
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