Packard Bell Color TV

[zeno]

The trouble is this set has something thats just off. Odds are
its not a wide open resistor or dead shorted cap. If you have
a scope look at the drive. Should be one ( or 2) BIG spike
& otherwise clean.
One thing that gets missed is the hoz blanking tube. That can cause some
strange problems. Change it & check around that area. It may be transtorised
also.

73 Zeno
LFOD !

[bhegges]

More measuring and also fired up my new Tek scope. The best part of a project is getting to acquire new tools:

• AC line voltage 123 volts
• 390v source measured 405 volts
• 275v source measured 294 volts
• Horizontal output grid 2,6 spec -46v measured -43v volts
• Horizontal output grid 2,6 spec less than 225ma. The best I could adjust for was 240ma current at 24k high voltage
• Horizontal output grid 1,7 spec 130v measured 130 volts
• Horizontal output grid 8 spec 37v measured 42 volts
• Horizontal output grid 2,6 drive. Scope setting 50v per division, 10us per division
  
  

Zeno, thanks again for your advice, the horizontal blanking amp is a 6GH8A. Other than replacing all of the electrolytic caps on this set I have not checked any parts in this circuit or other remaining circuits.

[bhegges]

Been working on a home improvement project so not much progress but I did pick up a Sencore CR70 and was able to test the 2 CRTs I have. Both are Packard Bell labeled 25xp22 which makes me think they are original to the TVs.

Naturally the tube in my project TV has no cataract or discoloration but measured almost no cutoff on red or green and barely any for blue. Emission is very poor for red, marginal on green, and fair on blue.

Now the CRT out of the large parts console I picked up measures good all around but it is quite discolored under the safety glass. I have watched shango066 pull off the safety glass on several CRTs so I see it is possible. What does everyone think for this one? Also let me know if anyone has a CRT they want to sell and ship to me (I will post a separate thread in the classified section).

[dieseljeep]

Originally Posted by bhegges

Been working on a home improvement project so not much progress but I did pick up a Sencore CR70 and was able to test the 2 CRTs I have. Both are Packard Bell labeled 25xp22 which makes me think they are original to the TVs.

Naturally the tube in my project TV has no cataract or discoloration but measured almost no cutoff on red or green and barely any for blue. Emission is very poor for red, marginal on green, and fair on blue.

Now the CRT out of the large parts console I picked up measures good all around but it is quite discolored under the safety glass. I have watched shango066 pull off the safety glass on several CRTs so I see it is possible. What does everyone think for this one? Also let me know if anyone has a CRT they want to sell and ship to me (I will post a separate thread in the classified section).

That CRT shown is a Zenith/Rauland sourced CRT, with the famous halo.
There's a few entries on this forum dealing with this. It's an easy fix.

[Electronic M]

The Zenith style cataracts (green halo) are easy to fix. With the tube removed from the cabinet laying face up peel the tape off the edge, then take a number 18 steel guitar string* and securely affix each end to it's own wood handle. Hold the wire tight with a handle on each hand and press it firmly into the glue while sliding the wire along the glue to saw through. I typically start at a corner, and get the wire in along one side of the rectangle and cut towards the other side (instead of going top of screen to bottom). The glue sawing usually takes less than 10min once you get a feel for it. Once the wire is running down a side it typically has a LOT of resistance so I pull one handle with both hands till the other handle has an 1-3" of wire separating it from the glass then, I switch to pulling that handle and repeat. The tube doesn't have to be warmer than room temperature for this...I've done this on tubes in 45 degree weather at night and had it go well.
Once the glass is off peel the glue by hand (use an old credit card if it's stubborn) and clean residue with Goof Off then glass cleaner, then use 1/4" foam double stick tape spacers in the middle of the 4 sides of the screen and caulk the safety glass back on. Don't caulk more than 1/4" in from the edge of it may be visible in the cabinet, and let it dry the full 24 hours before reinstalling or putting weight on the safety glass.
Before you caulk make sure the CRT face and safety glass are spotless cause you don't want to have to open it to clean it once it's caulked.

*I've been using smooth guitar string but some folks have suggested using the bumpy stuff might improve the sawing action...I worry the bumpy wire may scratch the glass and or make the handles too hard to pull (they are enough of a work out with smooth wire). I haven't tried the bumpy wire or heard of anyone using it so I can't say if it changes anything in practice.

If you come over here when the weather gets above 50 degrees I'd be happy to do the cataract removal for you.

[bhegges]

New update - I got some time to check components in the color circuits. Film capacitors and out of spec resistors all got replaced. There were a few resistors over 30+% but I suspect the issue was an open resistor that connected +275v to part of the 3.58 osc circuit. Other work included replacing the circuit breaker with a fuse and wiring in a bucking transformer to bring the line voltage down.

I picked up a RF modulator so I could connect a DVD player. With some basic adjustment I was able to get the picture reasonably centered, not much for color as I previously noted the CRT is weak. I continued to monitor the horizontal output tube current, I was able to get it down to a steady 210ma.

Next up:

• Swap out CRT, and make applicable adjustments
• Clean tuner, it seemed like I could maybe get a bit better picture and sound as I adjusted the fine tuning
• Figure out what is causing the visible retrace lines

[LukeSimon]

That TV looks amazing! A fully vacuum tube high voltage power supply is cool to see in a color TV. Those shunt regulator tubes emit soft x-rays. The high voltage cage doesn’t just block accidental arcing. It also blocks the x-rays. Even without the cage, the x-rays probably can’t travel more than a few inches thru the air.

I currently have a dozen CRT TVs, but only one non-solid state TV. It uses semiconductor diodes in the flyback power supply. I am looking to see expand my non-solid state collection. The circuit designs are more beautiful when they are only tubes.

[Electronic M]

I'm not familiar with the rectangular Packard Bell sets, but most sets tend to take the retrace blanking off the plate of the vertical output tube. The components that couple that into the video chain need to be good, and the screens, drives, bias, brightness all need to be adjusted properly. Even a perfectly working set can produce retrace lines if adjustments are wrong.

[bhegges]

After some further repairs I am questioning how well the RF/IF sections are working. My thought was to splice in composite signals to see what the set can look like.

I found a few posts with various ideas along with a few points of caution such as transformer isolation, inverted phase signal, and signal strength mismatch. It is not clear to me where I would splice in composite video. My first thought might be at the video detector diode cathode (X9). I am confused around if there is signal that is needed ahead of the pulse gate diode (X11) as this feeds the horizontal output tx and deflection circuit.

Next post has the relevant portion of the schematic.

[bhegges]

[Electronic M]

If your set has a power transformer (and is not hot chassis) you don't need an isolation transformer.

It looks like sync is peeled off the emitter of the video amp so the only place you CAN inject without loosing sync is between the detector diode and the base of the video amp.

If polarity or amplitude is wrong you'll need to build an external video amp to fix that.

[old_tv_nut]

X11 does not feed the horizontal deflection circuit. The horizontal deflection circuit feeds X11 and Q6 to develop the AGC voltage. (X11 protects Q6 from the negative part of the pulse waveform). So, no modification is needed here.

You need to connect inverted video to point B (base of Q4). The average DC of the inverted video should match the 7.5 volts shown on the schematic. The part of schematic you posted doesn't show what the peak to peak video should be. Does your service info show a waveform for point B?

[bhegges]

old_tv_nut and Electronic M, thank you for the replies. The set is a power tx set so no need for isolation and it sounds like I will need an inverted signal.

I found an old post from you with a circuit design. I understand I can adjust the collector resistor to change the output, also can I skip the .22uf on the collector as I am feeding directly into Q4? Additionally read that a clean 12v is important. I have 19v on the board to work with so I could add a voltage regulator to drop down to 12v. Some quick searching suggests this transistor should work: https://www.digikey.com/en/products/...4401TA/3478153

As for details on the waveform I tried to clean up and shrink the schematic to better fit. See below from Sams which includes the waveform, also posted is the inverter/amp circuit.

[old_tv_nut]

My first try would be:
Disconnect L8.
Connect the collector of the inverting amplifier directly to base of Q4 (don't use the 0.22 uF.)
Add 330k across the 33k in the inverting amplifier to bring the collector voltage down slightly, closer to the schematic voltage for point B.

[bhegges]

I will give this a try and report back. Would there be any noise coming from the AGC keying or Noise Inv circuits (Q6 & Q7). Would it be helpful to disconnect these from where they tie into the sync amp?