[cgl18]

Hello, good afternoon, I would like to ask what the possible problem could be if I don't have an image on the screen. I changed the capacitors, resistors, and the networks. I did it myself because the factory doesn't send them to Argentina. I tried with another tuner from another one that I have and it didn't change.

[GarySP]

Welcome to the Phorum, cgl18! I am not your resource for television repair, but lots of friendly help is available here. You should get some responses soon. Take care, - Gary

[TV MAN]

Bob Anderson (Bandersontv) on Youtube would be the go-to guy on these. Lots of good videos. I don't know what model you have, but it's obvious there's no video. The good is that you have sweep and light on the screen. You swapped tuners with no change. So your problem has to be between the 1st IF stage and the video output (unless you are missing a voltage to your tuner or have a bad tube (between the two??). Couple questions: Do you have sound? If you have good sound, no video then start looking around the video amp/out stage. Check voltages etc. If no sound, then could bring you back up the IF chain. Double check your work, make sure the caps you replaced are the correct value, wires connected correctly, none broken, no solder blobs stuck somewhere. Beyond that, I hesitate to go further ... do you have schematic? Is it a transformer set or live chassis? What test equipment do you have? Isolation transformer?

[RadioSvit]

If you have a television signal generator (a must-have for TV repair) and an oscilloscope (the same), then this problem is solved very simply. Just connect the generator to the TV input, and then observe the signal passing through the cascades with an oscilloscope.

[MrFixr55]

Hi CGL18,

First off, welcome to the Philco Phorum. I have not worked on a TV for a while but here are some givens:

First off, safety is of extreme importance. There are many high voltage, high current points in any TV that employs vacuum tubes and a CRT (Cathode Ray Tube / Picture Tube). These voltages vary from line voltage to somewhere between 15KV and 30 KV. Even if an electric shock is not strong enough to kill you, the body's reaction to the shock could cause you to lose balance or slam into something, casing injuries, possibly severe injuries.

In addition, be extremely careful around the CRT itself. Wear eye face and body protection when handling the CRT. I had an accident with an old 21" DuMont TV when I was a kid. The face plate was off the set for cleaning and for some reason the CRT imploded. I was standing at the side of the set when it happened. If I was standing in front or back of the set, I would have been blinded, disfigured and disabled by life-changing injuries.

Having said all of this, if you do not have a signal generator or oscilloscope, repair is difficult but still possible.

Did the set work or work better before recapping it? If so, check your work. If not, follow TV Man's advice. Almost all sets of this vintage are "intercarrier" stye, where the sound comes off the Video IF stage. Therefore, if you have sound, then signal is getting from the antenna through the tuner and video IF stages.

What model and chassis is this set? Do you have a schematic?

Start out by adjusting brightness and contrast controls. Anty changes? If not, troubleshoot the CRT and associated circuits. Is this the model with the long "umbilical cord" that allows the set to be at your chairside but the CRT to be on a separate table? Check this cable for shorts or open circuits.

Next, make voltage and resistance measurements, starting with the Video Output and CRT, and working backwards. Abnormal voltages indicate trouble with that stage. abnormal voltages for several stages indicate a power supply issue. If you don't have a signal generator, then inject (very low voltage) AC hum, using a transformer, potentiometer and capacitor, again, working backwards from the video output.

I did n0t ever try this, but I remember some of the old electronics magazines discuss modifying a TV set to provide various signals, Video, IF, sync, etc. outputs. It is vitally important that if either the signal generating set or the set under test are "hot chassis" types, then the chassis must both be at neutral potential.

Hope this still helps.


There seem to be 2 different chassis styles for these sets, one is transformer powered, the other is a "series heater string "hot chassis" type. It is also important to note that some CRTs for these sets have only a 2.5 V or near to that) heater.

Here are some links:
Sams PhotoFact Schematic for Philco Predicta Princess from Early Television Foundation:
https://www.earlytelevision.org/pdf/phil..._466-1.pdf

Repair Hints from Early Television Foundation:
https://www.earlytelevision.org/pdf/pred..._final.pdf

[cgl18]

The model is the holiday without UHF It has no sound and I don't have an oscilloscope or signal generator.

[MrFixr55]

Hi CGL18,

Here is a link for the schematic from the Early television Foundation:
https://earlytelevision.org/pdf/philco_g..._439-1.pdf

WARNING!! One side of the Chassis is directly connected to the AC Line! Fortunately, it is not the switched side. Please ensure that when connecting to house power that the Chassis is at neutral potential by measuring between the chassis and earth ground!

Avoid all temptation to adjust any slugs in the video or sound IF.

Slowly turn the tuner and "rock" the station selector back and forth. A lot of noise in picture and sound indicates possible poor contacts in the tuner, enough that no signal is provided.

Analyzing the schematic, there are several levels of B+. The 140V B+ is responsible for Video and Sound IF, and the Sound (quadrature) Detector. The 140V supply also supplies the screen of the Video Output tube, and the cathode of the Audio Output tube. However, the 140V supply is not used for the CRT, the Horizontal or Vertical sweep circuits, or the damper and no circuit fed by 140V is responsible for raster on the CRT. Therefore, this is the first place to check because if this supply is dead, it will cause exactly the problem that you describe. If this supply is good, the next step is to confirm B+ to the tuner and working forward, to the plate and screen of every tube in the Video and Audio IF sections.

The signal for the Sound IF comes from the Video Detector, so a bad diode section of the 5AM8 will cause the issue.

If the plate and screen voltages are good, check the capacitors that couple each stage and check all coils. the primaries are likely good or you will not have B+ at the plate. If a secondary of an IF or video coil is open, the signal will be blocked. Leaky caps will cause saturation of a tube (and possible red-plating).

Hope this helps.

[TV MAN]

As Mr. Fix says, this is a live chassis so without an isolation transformer, it can be dangerous to work on. There's good and bad to this circuit and problem. From your description, the problem most likely is in the video IF chain, low/missing voltage, bad tube. Do you have any tubes to substitute? That 5AM8 detector could be dead, which might have retired the set. You could be missing or have a low voltage to the tuner or a bad tube. They do go bad in tuners. Did you change any caps or resistors in the video IF section? I really can't recommend doing more due to some safety issues. The isolation transformer mentioned simply separates your set from the ac power line which would help eliminate electrical shocks (or worse) & fried test equipment.

[cgl18]

I found that where I have the 140v B+ there is 50v and the tuner receives 235v through the orange cable

[MrFixr55]

Hi CGL,

Yes, troubleshoot the 140V Power supply, something is definitely loading it down, and since this supply exclusively powers the IF section, some parts of the Video section and is involved in the audio output, your problem is there. It is likely that one stage has a shorted components your tuner power is somewhat but not greatly loaded down, not by the tuner but by the fact that the 140V supply is overloaded, pulling down the 280V Source of all power. You state that you receive 235V from the 250V tuner supply. Measure the 280V to determine if that is correct.

One can safely work on this set by ensuring that the power is connected in such a way that the chassis is connected to the neutral leg of the power source and not the "hot" leg. If the "interlock" is not polarized by one leg being larger than the other, then consider "tacking" a polarized power cord to the input in such a way that you confirm that the chassis is connected to the larger prong on the polarized power cord. Then confirm the correct wiring of your power source.

Other than the hot chassis, one issue in troubleshooting this set is the "Christmas Tree" arrangement of the tube heaters. My first troubleshooting technique for this power supply would be to pull all the tubes associated with it and while monitoring the 140V PS, insert each tube, one at a time, and troubleshoot the stage that pulls the PS down. Unfortunately, one can't use this technique because if you pull one tube, the heaters on all tubs go out.

First off, check the C3100 uF cap to verify the polarity and working voltage. Replace C51, the 470 pF cap or cut it out of the circuit. Next, verify or replace R75, the 18KOhm 2W resistor that connects this leg of the supply to the 280V supply. Next, disconnect B+ and cathode from the audio output tube. I have no idea why they sed the arrangement that they did on this stage, where the whole stage is 140V above ground, but a heater-cathode leakage could definitely mess up the supply.

Otherwise, the only way to troubleshoot this supply is to cut wires to the various loads to see which one pulls the supply down.

Hope this helps.