Hallicrafters T-506 Late Production Set
 

Greetings, Gentlemen:

I recently acquired the Hallicrafters, and I'm replacing the selenium rectifier with a 1N4007 diode. It's been a long time since I've been on the bench, and I forgot how to tell the " + " end of the diode.

Also, I hear you may need to install a resistor, and don't know what value to start with and where it goes.

Thanks;

Lee:smoke:

The end of the resistor body with the stripe is the + end. Start with about 50 ohms, at least 5 watts, and adjust from there. Sets with voltage doublers for the main B+ need a smaller resistor (typically 10-20 ohms, 10 watts) but I believe in this set that rectifier carries a smaller load. The added resistor simply goes in series with the diode, either before or after.

There are, obviously, two leads connected to the selenium rectifier. One to the positive marked end, and one to the negative. I take it the diode connects in similar fashion.

Does the 50 ohm resistor go between the negative lead to the diode or the positive?

Thanks. :smoke:

PS: It's been awhile since I've done this.

Dumb as it seems, the '+' end of the selenium corresponds to the banded (cathode) end of the Si diode. The + sign simply means "the B plus comes out of here".

The resistor would connect "upstream" of the diode.

I should try proofreading some time. :tears:

The end of the diode with the stripe is the + end.

The added resistor goes on either side -- it doesn't matter which you choose.

Which side doesn't matter, but conventionally it went on the input side of the diode.

Gentlemen ?!? Who come in ? (Grin) Lotsa luck fixin' the Halli... They are ALWAYS neat old sets...Me, I got an SX-28, S-36, SX-62B, SX-73 & a couple of others whose monikers escape me currently...Great mementos from the era when American radios & related stuff were second to NONE in the world...

"You don't understand. The missing king is King Rootin Tootin the Third.":smoke:

Yup. Neat set. I've sent out the mahogany cabinet to be refinished. The front plate and bezels, etc I'll restore myself.

I want this one to come out primo.

Waiting for the high voltage caps to come in...:smoke:

Like a dummy, I only ordered two 470pF 15kV axial caps. I need four.

Can I substitute ceramic disc caps? Two are in the HV Cage, two underneath near the bleeder resistors. The parts list calls for 500pF 6kV. :smoke:

Another question: I hear the HV RF Coil is a soft spot on these TVs.

Should I:

1. Clean and bake it at 180 degrees?

2. Space the top lid by .25"

3. Add a muffin fan?

Love to see a pic of the HV Cage with the fan installed.

Thanks.:smoke:

You can give the ceramic caps a try, but they might not work well. I did a study of this a few years ago and was able to show how the applied DC voltage across high voltage ceramic caps actually changes their capacitance. This causes severe distortion of the scanned raster.

That problem, however, is mainly with vertical deflection. In your set, those use much larger value capacitors like .01 uF (or I believe even .05 uF in the Hallicrafters). Your 470 pF caps are probably for horizontal deflection or HV filtering, which have less of a problem with this.

What I found with ceramic capacitors is that if you simply use a significantly higher capacitance value than the original tubular capacitor, ceramic will work. So for example, if you see problems using 470 pF ceramic capacitors, then using something like 2000 pF (.002 uF) or more will probably work just fine. Maybe even .001 uF would do the trick.

For some of my previous work on this subject, see: http://www.videokarma.org/showthread...=260749&page=2

I have a more complete write up of it somewhere else, but I can't find a link to it right now.

My experience with a Hallicrafters set that had a problematic coil was as follows:

1. Clean, bake, and coat with fresh varnish: Temporarily fixes the problem, but not long term. (although others have reported better results)

2. Lift the lid by 1/4 inch: not sufficient to help.

3. Add muffin fan and lift the lid: works well. I had a very small muffin fan that fit inside the cage. About 30 mm square, 10 mm or so thick. Didn't like the noise it made.

4. Best fix: replace the coil with a different one (I used a donor from a Teletone junker)

Not all Hallicrafters sets have this problem. You might be lucky and have one of the good ones.

I have this to look forward to! I have one in queue. I also like the stainless steel paint that was used on the metal cabinet, but have not found it locally.

[QUOTE=Tom Albrecht;3212247]My experience with a Hallicrafters set that had a problematic coil was as follows:

1. Clean, bake, and coat with fresh varnish: Temporarily fixes the problem, but not long term. (although others have reported better results)

2. Lift the lid by 1/4 inch: not sufficient to help.

3. Add muffin fan and lift the lid: works well. I had a very small muffin fan that fit inside the cage. About 30 mm square, 10 mm or so thick. Didn't like the noise it made.

4. Best fix: replace the coil with a different one (I used a donor from a Teletone junker)

Not all Hallicrafters sets have this problem. You might be lucky and have one of the good ones.[Quote]

Would doing the 1st 2 suggestions help? I mean before I power it up.:smoke:

I ordered up more of the proper caps.:smoke:

Maybe, but probably not. I did the steps I reported in the sequence listed. I started to have success with #3.

Over in the ARF the subject of the H.V. transformer has recently come up.
https://antiqueradios.com/forums/vie...p?f=3&t=359879

The bottom of my HV Coil assembly has a resistor on Pins 5 and 6. It doesn't show up on the schematics that I'm looking at (late production 6AU6 T-54) and I can barely read the bands. I THINK it's Green-Black-Yellow. 500 kohms. Half watt.

It tests OL on my Fluke. The 6C4 oscillator is such a critical tube so I want to get it right. My HV Coil board has two .25uF 600V and one .001 6kV.

Any ideas, Gents?:smoke:

It's a cap. Measures 2.7 pF. Specs at 4.7pF. Duh.:smoke:

The way I always think about it is the stripe on the diode is the | from the schematic symbol ->|-

Which would also be the opposite of what Tom said, I believe he meant the + comes out of the stripe side. But the stripe side is the cathode (-) and the arrow side is the anode (+). So remember my mnemonic device so you don't get confused: in the symbol ->|- the arrow points in the conventional direction of electricity flow from + to - (not the actual direction in physics, which is backwards O_o [but don't think about that]) and the stripe on the diode is the vertical line |.

https://cdn.sparkfun.com/assets/lear...wardsDiode.png
https://learn.sparkfun.com/tutorials...d-led-polarity

Thanks for jumping in. Soooo, the TAIL of the arrow attaches to the lead that was attached to the " + " lead of the Selenium Rectifier?

Huh? Oh, Boy.

NEGATORY. Please see post#4. The banded end (stripe) on the Si diode is the cathode. The "+" marking on the selenium is the cathode, dumb as this seems. The only sensible interpretation is "the B plus voltage comes from here."

Nope. Look at a tube rectifier the B+ line comes from the cathode....He is right in that there does need to be a voltage potential across the diode as marked in his drawing to conduct. Think of the old selenium power supply and the load of the TV as 2 batteries in series plus on one connects to minus of the other. They marked the selenium not with it's own polarity but with the polarity of the other battery it connects to.

The stripe on a new diode is the B + end of the selenium.

The old selenium polarity notation is bassackwards from modern convention.

Oooookay.

I removed the diode and reattached the leads to the rectifier when I attempted to first power it up. Bringing it up on the Variac yielded nothing. No tubes lit.

I checked the switch continuity. It's fine. Checked the leads coming into the switch. They're ok.

Did a resistance check on 6X5GT. Strange readings:

Pin 1: Inf. Test Inf.

Pin 2: 2 ohms Test 1.6

Pin 3: 60k Test soars from 350k

Pin 4: Inf. Test Inf.

Pin 5: 60k Test soars from 460k

Pin 6: Inf. Test Inf.

Pin 7: 0 ohm Test 0.2

Pin 8: 70k Test soars past 1 meg

What's up with this?

Thanks:smoke:

25Z6 Readings. Also way off:

Pin 1: 60k Test Inf

Pin 2: 25 ohms Test 144k

Pin 3: 160k Test 1.3 meg and climbing

Pin 4: 64 ohms Test 144k

Pin 5: 10k Test 11.43k

Pin 6: 46 ohms Test Inf

Pin 7 26 ohms Test 144k

Pin 8: 60k Test 1.1 meg and climbing

Weird. And the chassis is recapped, and I've checked everything twice, at least. :smoke:

Series string set...did you check all of the heaters and assure that tube pins are clean and making connection? 18 ohm resistor ok? All bets are off on the resistance readings until you have heater strings intact.

Also, did you notice that the Sams shows c4a & b connected backwards? that cap actually has common plus ! Riders has it correct.

jr

I already cleaned all the tube sockets with contact cleaner and a pipe cleaner.

I replaced all the 'Lytics as they were wired ex-factory, same grounding points, replaced all wiring with new. Double checked it. Tested all components before install.

Tested the tubes before fire up.

Would a dead selenium rectifier prevent start up? When I was waiting for an answer on the diode polarity I hooked up the rectifier and attempted to fire it up that way.:smoke:

Both 18 ohm resistors near the rectifier test fine.

I had checked all resistors on the chassis before fire up. Replaced only the four 4.7 meg resistors in the sweep section as they were beyond 20%.:smoke:

I have C4A and C4B common plussed to the ground on the 25Z6, just like the factory.

Still checking...:smoke:

Any chance of posting a schematic?

There must be an open connection somewhere in the heater string if the tubes are not lighting... a bad selenium rectifier will not be a cause of the tubes not lighting.
Perhaps continuity check the heater strings from ground, working back to the power cord end... there must be an open circuit in the heater strings.
CRT heater ok? Fuse ok?

jr

How do I check the CRT heater? Fuse is ok.:smoke:

An Ohm meter on pins 1 and 12 (the pins on either side of the ridge on the locating pin on the CRT's connector) should read very low ohms , high ohms (or infinity) mean the heater is open .

CRT heater is pins 1 and 14 (the pins next to the key-way, on either side). It should ohm out pretty low, around 2 to 3 ohms.

jr

CRT Heater reads 1.9 ohms. :smoke:

Okay. did a test of the AC string. Everything checks out except the following pins measured no continuity to ground. Per Sam's T-54:

V4 6AU6 Pin 4

V5 6AU6 Pins 3 & 4

V6 6AU6 Pins 3 & 4

I'm hoping this is an ah-ha! moment. All the wiring in this area looks fine. I'm thinking (hoping?) one of you guys can take a look at a schematic armed with this info and say "check ceramic cap on V4" or words to that effect.

I say this because I don't want to go into this area half-cocked and mess something up. Especially after this real pretty recap.

Thanks.:smoke:

The isolation of the pins from ground described suggests two open circuits:

1. between pin 3 & 4 of V4, perhaps open heater, dirty pins, broken or poorly tensioned socket pins.

2. between pin 3 of V8 and pin 4 of V6, perhaps broken wire, cold solder joints.

It is also possible that your probe was not making good connection to these pins due to flux left after changing capacitors. :scratch2:

jr

I was very careful with the probe. No caps were replaced in that area (V4 - V6) because they're all ceramics.

I'll check the suggested areas later today.

Thanks.:smoke:

Resoldered the AC Pins on V1 through V7. Plugged it in, brought it up gradually with the Variac and all the tubes except the 1B3 lit. No raster with brightness all the way up. I assume full clockwise is full brightness.

Well. I'll be back at it after dinner.

Thanks!:smoke: