Found a Philco

[andy]

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[andy]

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[Chad Hauris]

You could supply a signal from another tuner into the IF stages to rule out any problems in the tuner, provided the Philco uses the modern IF frequency...also have you checked all screen voltages on the IF tubes, maybe a shorted cap burned out a resistor, also could be a bad video detector diode? What is the negative AGC voltage...you may need to bias the if stages with adjustable negative voltage from a power supply to check how they respond.

I'm hoping to get that far as to get any kind of picture, but your story gives me hope. At least I know the history of my DuMont, that it worked until around 1980 or so and hasn't been moved or plugged in since.

I'm just waiting for the caps to arrive from JustRadios in Canada.

[andy]

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[andy]

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[andy]

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[bgadow]

Looks great! Its still cool to watch an old set "paint a picture" like that. That could not have been a real common crt, it doesn't look familiar to me.

[andy]

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[andy]

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[Dave S]

Originally Posted by andy

I also added a DC restoration diode. It was really easy to do

Andy,

As a self-taught "shade tree" engineer, there are so many things I am really, really good at being ignorant about. I've been pretty good at diagnosing and repairing many of the common problems since I was a kid, but I've never had the luxury of a comprehensive education on the subject, so certain things still leave me scratching my head or other parts of my anatomy. I have NEVER been able to fully grok the mechanics of DC restoration. Can you (or anyone else) point me to a reference that serves as "DC Restoration for Dummies", or maybe be kind enough to try to explain it either here or privately in a way that my aging and deteriorating brain cells can understand?

I'm actually not TOO stupid, I mean I know why we use DC Rest., but I have some sets in which it's not working and I can't seem to get my head into the circuit enough to home in on the problem. Also, the concept of simply "adding a diode" to improve an existing circuit as you said you did is something that is definitely outside my ability to cook up.

Thanks in advance if you're up to making me less ignorant on the subject!

--Dave

[old_tv_nut]

Originally Posted by andy

I'm having a problem getting it to focus after I disassembled the focus magnet assembly to clean out the old sticky grease and lubricate it. I'm sure I put it back together exactly the same way, but now the focus knob has very little effect and the picture is always very blurry. It's so blurry that 1" high text is completely illegible. I've tried adjusting all the CRT neck adjustments with no improvement. Has anyone run into this before with a permanent magnet focus assembly?

Here's a close up picture of the focus assembly before I dissasembled it. It uses 3 magnets located 120 deg. apart around the CRT neck. The focus is adjusted by moving a metal sleeve in, or out.

Random thought:
Is it possible you have one or more of the 3 magnets installed in reverse?

[old_tv_nut]

A DC restorer is a "clamp" circuit, and has five parts:
A low impedance signal source (not shown)
A coupling capacitor.
A switch.
A voltage supply (low impedance, could be ground).
A high impedance device (like a vacuum tube grid input) that senses/uses the output voltage (not shown).

The function of the input source is to force the input side of the capacitor to whatever voltage is being sent, even when the switch is closed.
The function of the switch is to connect the output side of the capacitor to the fixed voltage supply at the desired time.
The function of the capacitor is to charge or discharge until the voltage across the capacitor is equal to the difference between the input voltage and the desired voltage (the voltage source) when the switch is closed, then to hold that difference voltage when the switch is open (which it can do if the load is high impedance and therefore doesn't bleed off the charge.)

So..
We start with the wrong voltage across the capacitor. As the signal voltage goes up and down on the input, it goes up and down by the same amount on the output. However, the difference in voltage between the input and output is not necessarily what is desired; that is, the output has a DC offset.

At the proper time (sync or blanking), the switch is closed, forcing the output voltage to equal the desired voltage. However, the low impedance input forces the input side of the capacitor to remain the same - therefore the voltage across the capacitor is changed so that the output is correct. When the switch opens, the input voltage goes up and down with video again, and the output follows exactly, but witht the correct offset voltage.

The parts described above may be hard to see at first in some circuits, but they will be present.

Now to some nitty gritties:
The switch (diode or triode) may be gated by an external pulse, or it may be self operating (a diode). In the case of the self-operating diode, it is the sync pulse itself that causes the diode to conduct. This situation gets a little tricky - a perfect high impedance load will not work exactly right, since amplitude variations in the input may cause the negative peaks of the waveform to drift upward, and then the sync pulses will not make the diode conduct. Such self-gating clamps must have some discharge path so that the voltage on the output droops enough between sync pulses so that conduction through the diode always occurs. Gated or pulsed DC restorers avoid this problem, and can have a longer discharge time constant, so there is less droop in the waveform across one line time.

[andy]

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[old_tv_nut]

Loss of magnet strength is puzzling to me, unless the focus was bad BEFORE you disassembled it. I know generator armatures have to be poled if they have been dropped, but that is just restoring a weak remnant magnetism - the focus magnets should be strong and I would think shouldn't lose their magnetism suddenly. (I could be corrected by someone who knows!)

Well, at least you have it fixed, so the mystery can be contemplated at your ease.