Zenith 25EC58 Restoration Thread

[vol.2]

Originally Posted by TinCanAlley

I tested them and the 68pf tested as 56pf with an ESR of 21kohms and the .0047 tested as .0038 with an ESR of 411ohms. I tested them for value at 100Hz and ESR at 100kHz. That is correct? When the new ones arrive, I'll check them the same way.

Yeah, that ESR sounds way too high at 100kHz.

It's worth noting that you would have to have a fairly expensive and fancy tester to get a really accurate reading on something as low as 68pf, but I think it's safe to say that those are bad ~kohms, or 411ohms.

You should test the new ones before you put them in to see if your meter might be misleading you for some reason. Cheaper testers might not be able handle really small value caps. If the new caps also test crazy high ESR, then you know something's up

[TinCanAlley]

Originally Posted by vol.2

Yeah, that ESR sounds way too high at 100kHz.

It's worth noting that you would have to have a fairly expensive and fancy tester to get a really accurate reading on something as low as 68pf, but I think it's safe to say that those are bad ~kohms, or 411ohms.

You should test the new ones before you put them in to see if your meter might be misleading you for some reason. Cheaper testers might not be able handle really small value caps. If the new caps also test crazy high ESR, then you know something's up

It was kohms. I did read that lower values and higher voltage ceramics don't measure will with basic ESR testers. I will definitely test the new ones and compare the reads to see if my tester isn't up to the task.

I do have a question you might be able to answer. The chassis is hand built, so when they are building them is there instructions on lead length for certain components, or is the length determined by fit and clearance? The reason I ask is the 68pf was pretty much bottomed out on the pad, so I had to clip it as high up toward the body as possible, leaving short stubs sticking out of the buckets. I plan on sitting the new leads into the bucket and using a small tip to reach the inside of the bucket and then flowing more solder into it. Since this cap had very short leads, do I need to do the same or can they be longer? I wasn't sure if they were short based on design or just that the cap fit better lower down. I know, odd question.

[vol.2]

Originally Posted by TinCanAlley

The chassis is hand built, so when they are building them is there instructions on lead length for certain components, or is the length determined by fit and clearance?

That would be determined by best practices at the time. Lead length could absolutely make a difference in specific situations, but I'm not familiar enough with the make of your TV in general to know all of the spots that would be sensitive. I would assume that anything involved in a trimmer circuit would be important as they could pick up interference.

The older guys in the Solid State forum will be able to tell you a lot better as they know the 70s US make stuff really well and I don't.

[old_tv_nut]

Manufacturing practice was to make a prototype agreed on by the engineering and production departments, and install it on the assembly line for physical comparison. The prototypes could range from individual modules to complete chassis in a plastic housing to illustrate it completely finished including things like lead dress of the wiring. No deviation was allowed whether it was actually important or not.

So in some cases, lead length was just a matter of neatness, and in others it had an effect on performance. Wiring harness lead dress could be important if it affected issues like jail bars, or it could just be an issue of neatness or easy order of assembly. And of course there were also consideration of tie-downs for long leads and heavy components to prevent breakage during vibration and shipping tests.

You can see these kind of considerations in the old Heathkit assembly manuals, where you are told to connect this or that but not solder it yet because something else will be added in the net step.

[TinCanAlley]

Good information. I will try to keep the leads of the 2 replacement caps as close to original lead length as possible. They are in the Horizontal Blanking circuit tied to the diode and also to the setup side of the setup switch. As you may remember, I can't get the setup mode to produce anything but a faint blue line at full and nothing from the other guns at full. I have to adjust with a greyscale ramp pattern and with the setup in normal mode, same max rotation of the G2s brings up a super bright raster, so I know the controls work.

[BeamT]

+1 old_tv_nut, duplicate what was there as much as possible.

Stay on the track you are working, finish, and then move forward.

and I remember. Alternative Ways of Adjusting G2, is only 4 threads away. IMO you never fully explored that issue. ElectronicM gave you excellent advice and recommended looking at the voltages associated with the CRT. You looked at a lot of things but it appears you never returned to that. The CRT and Video Outputs are supplied by the 3 boost sources that are Horiz derived. The TV needs to produce 3 pictures, a red one, a blue one, and a green one, of equal intensity dark to light (gray scale -tracking) on top of each other (convergence). Thats why in the immortal words of Zeno, start with a good black and white picture.

I concede that a really tired/weak CRT might not be capable of producing one or more RGB setup lines but from the brief pictures you have posted, your CRT actually looks really good. Did you ever test the CRT?

ElectronicM has also chimed in with soldering advice, I think in more than one of your threads. Do you think you have enough wattage in your soldering devices? You should be able to pull component leads from the solder cones (pots), and re-insert leads to the depth of the cone. I use a Weller gun. If I did this more often, I would use the higher wattage iron that he uses.

[TinCanAlley]

Originally Posted by BeamT

+1 old_tv_nut, duplicate what was there as much as possible.

Stay on the track you are working, finish, and then move forward.

and I remember. Alternative Ways of Adjusting G2, is only 4 threads away. IMO you never fully explored that issue. ElectronicM gave you excellent advice and recommended looking at the voltages associated with the CRT. You looked at a lot of things but it appears you never returned to that. The CRT and Video Outputs are supplied by the 3 boost sources that are Horiz derived. The TV needs to produce 3 pictures, a red one, a blue one, and a green one, of equal intensity dark to light (gray scale -tracking) on top of each other (convergence). Thats why in the immortal words of Zeno, start with a good black and white picture.

I concede that a really tired/weak CRT might not be capable of producing one or more RGB setup lines but from the brief pictures you have posted, your CRT actually looks really good. Did you ever test the CRT?

ElectronicM has also chimed in with soldering advice, I think in more than one of your threads. Do you think you have enough wattage in your soldering devices? You should be able to pull component leads from the solder cones (pots), and re-insert leads to the depth of the cone. I use a Weller gun. If I did this more often, I would use the higher wattage iron that he uses.

I figured that since I'm replacing the C202 and 204, and they are connected to the setup side of the setup switch, it might help with that issue.

In the past, I tested all those voltages and they all were good. Since I haven't had any new issues that might point there, I haven't checked them again, but I can once it's back together.

The CRT is good. I've tested it a couple of times and each time it tests good and strong on all 3 guns.

My iron is 200W, but I might need to put the largest chisel tip on it for maximum heat transfer. Should I be concentrating the heat on the tip of the bucket, or the side? I think the side give more contact surface, now that I'm thinking about it.

I think for the setup switch issue, I might bypass it for testing. I've cleaned it a number of times, but it might be extremely dirty, or damaged. What I want to do is put it in setup and jumper the connections on the back side of the switch. That should determine if the switch is good or bad. The other area I need to look into is are the Chroma Gain Taps. I accidentally found out that while in setup I couldn't get a red line adjusting the G2 control, but I could by pulling the lead off the tap. I realized I had the tap on low when it should have been on Hi, so I pulled it to move it and a red line appeared. When I moved to Hi, the line went away. I tested the resistors in that network, but nothing stood out. I tried deciphering the schematic for that section, but I'm not sure what I was seeing.

[TinCanAlley]

Originally Posted by BeamT

+1 old_tv_nut, duplicate what was there as much as possible.

Stay on the track you are working, finish, and then move forward.

and I remember. Alternative Ways of Adjusting G2, is only 4 threads away. IMO you never fully explored that issue. ElectronicM gave you excellent advice and recommended looking at the voltages associated with the CRT. You looked at a lot of things but it appears you never returned to that. The CRT and Video Outputs are supplied by the 3 boost sources that are Horiz derived. The TV needs to produce 3 pictures, a red one, a blue one, and a green one, of equal intensity dark to light (gray scale -tracking) on top of each other (convergence). Thats why in the immortal words of Zeno, start with a good black and white picture.

I concede that a really tired/weak CRT might not be capable of producing one or more RGB setup lines but from the brief pictures you have posted, your CRT actually looks really good. Did you ever test the CRT?

ElectronicM has also chimed in with soldering advice, I think in more than one of your threads. Do you think you have enough wattage in your soldering devices? You should be able to pull component leads from the solder cones (pots), and re-insert leads to the depth of the cone. I use a Weller gun. If I did this more often, I would use the higher wattage iron that he uses.

I figured that since I'm replacing the C202 and 204, and they are connected to the setup side of the setup switch, it might help with that issue.

In the past, I tested all those voltages and they all were good. Since I haven't had any new issues that might point there, I haven't checked them again, but I can once it's back together.

The CRT is good. I've tested it a couple of times and each time it tests good and strong on all 3 guns.

My iron is 200W, but I might need to put the largest chisel tip on it for maximum heat transfer. Should I be concentrating the heat on the tip of the bucket, or the side? I think the side give more contact surface, now that I'm thinking about it.

I think for the setup switch issue, I might bypass it for testing. I've cleaned it a number of times, but it might be extremely dirty, or damaged. What I want to do is put it in setup and jumper the connections on the back side of the switch. That should determine if the switch is good or bad. The other area I need to look into is are the Chroma Gain Taps. I accidentally found out that while in setup I couldn't get a red line adjusting the G2 control, but I could by pulling the lead off the tap. I realized I had the tap on low when it should have been on Hi, so I pulled it to move it and a red line appeared. When I moved to Hi, the line went away. I tested the resistors in that network, but nothing stood out. I tried deciphering the schematic for that section, but I'm not sure what I was seeing.

[TinCanAlley]

I have tested all the resistors in the setup circuit from the switch to the convergence board. So don't know enough about reading schematics to understand why setup connects the video outputs to the inductor coils on the convergence board. I'm pretty sure when you flip the switch it's main function is to disconnect all the video amps and collapse the vertical.

I've also tested all the resistors in the gain taps and they are withing tolerances. So unless I'm missing something, my inability to bring up lines in setup looks to be either in the switch itself, or the connection of the outputs to the convergence board.

[Electronic M]

The side of the solder pot terminals usually works best. I recommend choosing the biggest tip for best thermal mass and conduction to the terminal.

[TinCanAlley]

Originally Posted by Electronic M

The side of the solder pot terminals usually works best. I recommend choosing the biggest tip for best thermal mass and conduction to the terminal.

Got the biggest chisel tip ready to go. Replacement caps will be here tomorrow, so I should be able to get them installed, check over the entire chassis for anything I might have disturbed in my work and all new transistors for proper orientation. Then it's time to reinstall the chassis and fire it up. Fingers crossed.

[TinCanAlley]

I managed to get the other 3 leads from the cut capacitors out. It seems a bigger tip and holding in on the side of the solder bucket helped, but I think extra pulling also helped. I'm attaching a pic of one of the leads I got out. I'm not sure what that stuff is, but it is hard and was on the three leads I couldn't get out, but not on the one I did get out. So maybe that's why only one lead came out.

Also note the crud on the lead. Is that old flux? I'm assuming the discolored lead is only from the removal and it wasn't that way in the solder.

So when I work to install the new capacitor leads, should I put a drop of flux in the bucket first? I figure I'll let it heat up for about 20 to 30 seconds before trying to insert the lead.

Oh, and with the amount of time it takes to head up the bucket to melt the solder, is the heat transfer to other components like resistors bad? They get pretty warm/hot during the process. Should I clip a heatsink to the leads of the other components sharing the bucket?

[TinCanAlley]

Question about transistors. I've noticed that several transistors noted in the SM, each with different Zenith part numbers, have PNs for other manufactures, but those part numbers are all identical. Does this mean when you buy from another manufacturer you're getting replacements that aren't the exact specs, but close enough to work? For instance, transistors 695, 888, 931 and 447 replacements are all ECG123A or NTE123AP. The same happens will all other manufacturer's replacements (RCA RT102).

[vol.2]

Originally Posted by TinCanAlley

Question about transistors.

So the thing about transistors is that they have a lot of characteristics, but which ones make a difference entirely depends on your application.

When a manufacturer is deciding which transistor to use in which spot, they have an entirely different set of considerations than a general repair-person does, or in fact you or I have many years later.

Usually, they are working with a bunch of boxes of parts they buy in bulk, and that is all down to contracts and maybe just what's around at the moment. They try to make it as cheap as possible while still doing the job, but that might actually mean using a way over-spec'd part if that's what they have a whole lot of in the back room right now.

The repair manuals are looking at the whole field of parts available and giving you choices because different repair shops have their own stock and their own prices based on the volume of parts they can afford to buy at the same time.

Sometimes (a lot of times) the exact transistor you chose doesn't make a heck of a lot of difference. As long as it sort of fits the voltage and current requirements as the junctions, it's going to do the job.

Other times, the gain characteristics and/or the transition time of the transistor is going to make a big difference, and you have to pay attention to it in the circuit. In these cases, even a single transistor replacement can require reverse engineering the whole portion of the circuit that it is found in.

If you are seeing a single transistor type shown as a replacement for many different transistors in BOM, then it's likely that is a common replacement part used for a variety of transistors from different manufacturers. And so is more of a general replacement than an exact fit. They are usually pretty much okay, but I have seen situations in which Sam's replacements didn't work in a circuit. Not often, but I've seen it.

[TinCanAlley]

Originally Posted by vol.2

So the thing about transistors is that they have a lot of characteristics, but which ones make a difference entirely depends on your application.

When a manufacturer is deciding which transistor to use in which spot, they have an entirely different set of considerations than a general repair-person does, or in fact you or I have many years later.

Usually, they are working with a bunch of boxes of parts they buy in bulk, and that is all down to contracts and maybe just what's around at the moment. They try to make it as cheap as possible while still doing the job, but that might actually mean using a way over-spec'd part if that's what they have a whole lot of in the back room right now.

The repair manuals are looking at the whole field of parts available and giving you choices because different repair shops have their own stock and their own prices based on the volume of parts they can afford to buy at the same time.

Sometimes (a lot of times) the exact transistor you chose doesn't make a heck of a lot of difference. As long as it sort of fits the voltage and current requirements as the junctions, it's going to do the job.

Other times, the gain characteristics and/or the transition time of the transistor is going to make a big difference, and you have to pay attention to it in the circuit. In these cases, even a single transistor replacement can require reverse engineering the whole portion of the circuit that it is found in.

If you are seeing a single transistor type shown as a replacement for many different transistors in BOM, then it's likely that is a common replacement part used for a variety of transistors from different manufacturers. And so is more of a general replacement than an exact fit. They are usually pretty much okay, but I have seen situations in which Sam's replacements didn't work in a circuit. Not often, but I've seen it.

Got it. Was just wonder as I needed two 695s and found a seller with a lot of 10 for next to nothing. So I bought them and wondered what I could do with the rest. If the ECG123A is a basic replacement for a number of transistors, and one of those is the 695, then if need be, I can use the 695 as a replacement for those other transistors?