Philco 50-T1403 with focus problem

[Don Lindsly]

Roger:

The focus pot and a 150 ohm resisitor are across the focus coil. To route more current through the focus coil, make sure the voltages on the 6V6 audio output are correct. Check the 5K B+ stacking shunt and grid divider resistors.

Since the high voltage, width and vertical are OK, we can assume they are drawing the proper current, or close enough for focus.

The focus control circuitry is across the focus coil so routing more current through the focus coil requires raising the value of the shunt resistors. If the 6V6 voltages are OK, then raise the 150 ohm resistor on the focus control to about 330-500 ohms, 5 watt.

Don

[andy]

...

[cbenham]

[QUOTE=dreyfoos;1697038] "I reversed the focus coil leads. Huge difference – went from 0 to maybe 90% perfect."

Just to confirm, I looked at my 50-T1403 and the focus unit wires [PM with a coil] are connected green to red and red to brown.

Try to find a copy of the 1950 Philco Yearbook which contains full information for all the 50-T sets. There are several pages of factory updates which will greatly improve the performance and reliability of these sets.

[dreyfoos]

Here's where I am. First to the Kolordapter Kid (cbenham): Yours works - mine didn't. I put a Color-dapter together some fifty years ago but never got it to work. I don't know if I would do any better today. If I still had it, I would sure give it another try.

Yes those focus coil wire connections do make the difference between no affect by the focus control and, for me, an "almost there" situation.

Don, my version (50T1403 code 125) uses the 6AQ5 for audio output and the only voltage that seems significantly off is the 132VDC on the grid where Sams calls for -38VDC. Remember, my audio does seem a little anemic. Could this be an error in Sams or, perhaps more likely, this is a problem to track down. I've got a lot to learn (or remember) before I get this straightened out. Now I have found a couple of B+ resistors that were 40-50% high and have replaced them and am looking for more. There might be some slight improvement in the focus range as I can now see lines on the 12KB4 where I couldn't previously. The range is still not right though, as best focus is still with it cranked all the way clockwise for maximum current. Still wondering whether the PM is carrying its share of the focusing task.

I subbed a higher 300-500 ohm resistance for that 100 ohm shunt and noticed no change except that at 500 ohms - no raster.

In some of the old Philco production run changes quoted in Riders I found an earlier model with this "R66 changed from 180 to 100 to center range of focus control. So, that's where I'm at.

Maybe I just need new glasses.

Roger

[dreyfoos]

Whoops. That should be a -3.8 VDC from the Sams.

Roger

[Don Lindsly]

I will look up another print.

Don

[dreyfoos]

Update: I just subbed a focus assembly from a slightly earlier 10" Philco model - with no improvement - best focus setting still max CW. I had restored that chassis and then put it on the shelf. I recall no focus problem when I was working on that one. I only remember an excellent picture. It's not convenient to fire it up right now but I did check the position of the focus control and it was very low - not quite fully CCW.

Roger

[Don Lindsly]

I found another print showing the 6AQ5 audio. What are the grid, plate and cathode voltages? Earlier posts state the B+ voltages are slightly high. That may be due to slightly higher line voltages.

Is the 5K stacking resistor OK? When you remove the 6AQ5, the sound and picture should go out and the screen should go blank. If not, something is bleeding through to the 135 volt source.

There is too little current through the focus coil, (no kidding). Do you detect any circuit revisions? Is something left out? You posted the current through the coil. What is the total current drain?

What is the exact focus coil resistance when measured with an analog VOM? Digital meters will give false readings on high inductances.

[andy]

...

[dreyfoos]

Okay Don, I've got fresh numbers for voltages on the 6AQ5 audio output - all with a VTVM. Sam's calls for -3.8 VDC on the control grid, pin 1/7, and I get 135 whether to ground or to the cathode, pin 2. For the screen grid Sam's calls for 165VDC measured to pin 2 and I measure 150 (Rider's calls for 290 to ground and I get 285). Sam's calls for 145 on the plate to cathode and I get 140 (Rider's 280 to ground and I get 275).

Specified line voltage is 117 and that's what I measure.

B+ voltages measured against Sams are 330/330, 325/312 and 310/270 - with the focus control at mid position.

The focus coil should be 170 and I read 180 ohms on the original and 184 on the "sub" from another Philco. That sub had just a hair better focus - very subjective (and still full CW).

The focus pot ranges from 0 to 500 ohms - on the money.

You asked about the stacking resistor. I presume that would be the Sam's R78 sand type listed as a "decoupling-wirewound." I subbed a new 5K sand and there was no change in focus.

As to measuring the total current drain - I don't have an ammeter that will handle AC. Perhaps I could measure somewhere after rectification?

I mention again - since we're "focusing" on the audio amp - that my audio is just barely enough - with the volume control fully advanced there is no "extra" volume available.

Also, I notice that according to my tube manual, the 6V6 does not use pin 6. Yet, Rider's shows the pin 6 connected to the 135V supply. Sam's shows two possible arrangements - one connects pin 6 to the 6AQ6 cathode and the other connects pin 6 through a resistor to the 310V supply. My 6V6 socket has a pin 6 that's clean as a whistle - looks like it's never been used. Could there have been a later factory mod?

When I pulled the 6AQ5 it killed the sound but a somewhat "faded" pix remained.

Roger

[Don Lindsly]

Roger:

The Sams is wrong on the control grid voltage. You are right. It should be about 4-5 volts less than the cathode and about 135 volts. I am using the Philco yearbook.

Sorry I wasn't clear on total current drain. I meant the total DC B+ drain at the 5U4 output, pin 8 where it feeds the filter choke and caps.

I like Andy's suggestion to set the focus pot to minimum resistance and drive the focus coil with an external power supply. Then measure the current and voltage it takes for proper focus. The 180 ohms is high, but not too high. It is within 3%.

The audio can be affected from a few things. That TV is no boom box. It has a small speaker in front. Remember people sat lots closer to a 12" TV than a 25" so high volume was not necessary. It should be about like a 5 tube radio. Make sure the 2 electrolytics are OK and the .0068 across the transformer is not leaky. There should be about 2 volts audio at the volume control high side and 6-8 driving the 6AQ5 at high volume setting. Check the audio output transformer resistance, particularly the secondary. Some develop high resistance joints internally at the connecting leads. It should be around an ohm or less.

Also check the 2uf electrolytic in the FM sound detector. When that cap opens, it will cause a serious buzz in an intercarrier TV, but that has split sound so it will only cause weak audio.

Don

[dreyfoos]

Goal: Still trying to get the best focus moved to the center of the focus control range rather than just beyond fully CW. (Audio volume is marginally adequate.)

Correction: In my last post I thought there was something out of place on the 6V6 Vid Output pin 6 situation. Never mind. As some of you well know, Philco made it pretty “narrow and deep” in that area of the chassis and I somehow got disoriented on the tube sockets. All wiring is according to both Rider’s and Sam’s schematics.

Update: Going back to what Don said: “When you remove the 6AQ5 (audio output), the sound and picture should go out and the screen should go blank. If not, something is bleeding through to the 135 volt source.” I repeated that trick and I would describe the raster and video that remain as degraded but watchable – perhaps half the contrast as with the tube in. So where would I start to track down that possible short or low resistance? BTW, the 6AQ5 tests fine on my RCA WT-10A, but I don’t have any spares so I’ve ordered some new ones.

As to using another power source to run a higher current through the focus coil... I’m going slowly on this to make sure that I know what I’m doing. I don’t want to have to go shopping to replace an open coil. To make sure that I don’t burn out the coil, should I use a sort of dim bulb setup? (Not talking about me!) I previously measured 52-165 mA through the coil connected to the set’s B+ of 330VDC. So, do we not care what the voltage is so long as we keep the mA current, say, within 200 mA? What source would you use? Would a 13.8VDC source be okay and what would be best to limit the current? Would a 69 ohm resistor plus a pot be enough?

When I put the digital mA meter between the 5U4 output, pin 8 and the filter choke and caps, I read 0.11 A.

As to the audio: the electrolytics are new as is the .0068 and the output xmfr checks okay. The 2uf in the detector is new. I find 6V driving the 6AQ5 - but only about 1V on the volume control.

I continue to try to get schooled up on all fronts – including what it would take to make sure the permanent magnet is “saturated.” Just reading the Rider’s TV volumes 5 and 6 covering all brands is giving me a better feel for this. It's “Heavy” reading. Sorry, couldn’t resist.

Roger

[Don Lindsly]

Roger:

You're doing fine.

It is possible that enough current passes through the 5K stacking shunt to power the signal circuits. I would start by removing the 6AQ5 and one end of the 5K. If the picture and sound dissappear, then let's get away from there for a while.

At 110 ma, it seems the TV is not drawing enough current. That is in conflict with the earlier measurement of 52-180 ma through the coil. We're missing something.

Disconnect the focus coil from the TV and put a 150-200 ohm, 5-10 watt, resistor in its place to protect the focus pot.

Apply somewhere around 30-40 volts DC to the focus coil. At 180 ohms, you don't need any protection if you keep the voltage low. Two hundred ma works out to 36 volts. Adjust voltage for best focus and note the current. I suspect it will be somewhat more than 180 ma. If so, we can go from there.

I thought we eliminated the focus coil earlier by substitution. Did we?

Don

[dreyfoos]

Don

Picture and sound do disappear with the 5K and 6AQ5 out and one end of the 5K lifted.

We did eliminate the focus coil as the culprit with a sub. There is still a remote but possible problem with the PM. It could have lost some of its pull (or push) and that would make the coil have to take up the slack.

With the focus pot protected I connected the focus coil to the 25VDC bias supply on my old B&K 415 Sweep-Marker generator. I could run the voltage up from zero to 27 and get about the same result as from the Philco B+, which is from fuzzy to almost sharp before I run out of range and the current is 147mA. The B&K also has a 50VDC bias supply measuring a smooth run-up from 0 to 50. With it connected to the coil I don’t get any focus – even at halfway – about 25VDC – nor at 30-40VDC. For some reason that 50VDC supply wimps out.

So I decided to lash something up to my variac. Got a RS bridge rectifier and tacked on a 10 MFD 450V for “smoothing.” I was reading a higher voltage than my input so I decided I had better find a formula for the cap value needed. I did – and it said I needed .01MFD. That didn’t work any better so I figured it needed some resistance across the output. That made the output read right and I dialed in about 30VDC.

So then I hooked the lash up to the focus coil. At 160 mA and 33VDC I got PERFECT focus - in alternating horizontal bands of the pix. Looks like way too much AC ripple in my lash-up. I really did take “electricity” in H.S.

Now it was time to reconnect the old chassis supply to the focus coil. Fully clockwise I get 160mA @ 28V, max current. Focus not perfect – kind of like when we take our reading glasses off. By the way, with the focus pot at the halfway point the voltage reads 19VDC.

I sure hope this is making sense to you. Any idea why I’m getting the ripple? Does it need something on the order of a 10, 20, 30MFD across that output and then take new readings?

I have no idea why the current was 147mA with the chassis hook up earlier this afternoon.

I haven’t worked on that “no make sense” lower current on the 5U4 than on the focus coil. Hope it doesn’t have anything to do with the meter I’m using – a $3 item from Harbor Freight. Thought I’d give you the other stuff to chew on first and take a fresh look at the discrepancy with the current tomorrow.

Roger

[Don Lindsly]

Very ingenious. You have eliminated the CRT and some other stuff.

Don't expect 180 ma from a bias supply. It may deliver 5-25 ma, surely not 200.

The bands are coming from AC ripple in your power source. It could use a low resistance choke and second electrolytic. A single pole filter won't do it for high current drain. Try a 5-10 henry and a couple of 10-30 uf caps. Anything that looks like it will work probably will.

Digital meters can be fooled by high AC ripple riding on the DC. That is why some measurements don't make sense. A Simpson 260 or Triplett 630 will give readings that more closely match the schematics on older TVs. They were the standard at the time.

I am still puzzled as to why there is not enough current going through the focus coil. Try a 10-20 uf electrolytic on the 5U4 side of the B+ source when measuring total TV current drain. That way both sides of the meter see (relatively) clean DC. Recheck electrolytic cap wiring, if any were replaced. Some high current circuit on the wrong side of the focus coil will cause poor focus.

Don