I’ve finished checking about half of the resistors on my Zenith H2445R porthole. Most of the resistors are out of tolerance. What is strange is that I’ve found 3 resistors so far that don’t match what the Photofact says should be there. There are a couple others but then in the notes section it states the correction for some models. Here are the 3 in question:

R24 is supposed to be 1,000 ohms (+/- 10%) but a 1,200 ohm resistor is installed. Measures 1,180 ohms.

R43 is supposed to be a 1M resistor (+/- 20%) but a 1.5M resistor is installed. Measures 1.42M.

R49 is supposed to be a 33K ohm resistor (+/- 10%) but a 30K resistor is installed. It’s in parallel with coil L17 so I can’t measure it without unsoldering one end.

For the value installed in R24 and R43 both are in tolerance with the resistor color code and appear to be original. Should I leave these as is or install the value listed in the Photofact?

Here is a high resolution scan of the service information (600 dpi, 54MB):

https://drive.google.com/file/d/0BzQx4cDSWtozdDZ1Mmp0WkFvYzQ/view?usp=sharing

I've explained this to many on here before.

Most schematics do not reflect all production changes a chassis under goes in the months or years that it is in production. It is not always easy to ascertain whether the schematic or chassis reflects the most recent/best revision.

I recommend you stick with the values you find in your chassis unless you have reason to suspect they are not original/hack work. If after caps, resistors, tubes and adjustments the circuit one of the different parts is in refuses to work well, then try the schematic value.

Thanks for the feedback Tom. I'll stick with what was originally installed.

Photofacts are only a snapshot of one production run - usually the base model set, or the most popular in sales. Sams published production change bulletins (PCB) in later Sams Photofacts, but only when received. Many manufacturers, Zenith included, kept production changes from Sams - and the multitude would never be published by Sams anyhow. RCA made 36 (that I know of) production changes to their CTC76 chassis - Sams reflects only a couple.

The online Sams site lists zero PCBs - you gotta use a paper index to find them, if Sams ever published them. Same for a lot of the Sams Servicer info - only the (older) paper indexes list them. Get an index prior to the 2003 index - after that, a lot of older Sams aren't listed - they started their Photocopy Service...

I have the original Zenith schematics for the 24H20 (16") and 24H21 (19") chassis, and when compared to the Sams, Sams seems to put an average value
in to cover both chassis... "Close enough..."

This is a good topic as to determine when to or not to change a resistor. The old carbon resistors are notorious not only for out of tolerance values but also drift with temperature.

An absolute match is essential in a discriminator circuit. However most tube circuits will run without problem with a tolerance of +/-100%. Resistors in bias dropping circuits are perhaps more critical but for example the grid return resistor on an audio amplifier can vary by 1000% and you would be hard to notice any difference in performance.

For example I find that sets that rely in the vertical sweep circuit on capacitive-resistive feedback instead of a blocking transformer for the vertical oscillator will generally benefit from a "shot gun" change of resistors in the entire sweep circuit to offset any value drift which will cause the vertical height/linearity to change.

It is important to determine where the resistors are in a circuit and their application to determine if it may cause a problem. With a 70 year old TV set, I will do a general visual inspection for charred or stressed resistors. I will also do random spot checks to ensure my chassis run in production did not get a bad batch of resistors.

I consider a resistor's stability to temperature generally more important than its absolute value in many cases.

Terry