Carbon composite in place of carbon film

[TinCanAlley]

As many of you know, I'm trying to fix jail bars on my Zenith Avanti (yes, still). With the help of a few here, I'm about to replace the resistors, diode and transistor in the blanking circuit. I managed to get an NOS Zenith diode and transistor, but the resistors weren't available. I ordered the replacements from Mouser and they arrived on Saturday. When I went through them, I noticed for the 100K 1/4w 5% carbon film I ordered the 100K 1/4w 5% Carbon Composite. I was ordering the 33K and 82 composites and I must have zoned out and kept in that section when ordering the 100Ks.

Anyway, I've searched and read lots on the internet and found the composites are not as stable as film. Now, granted, most sites were in regards to music amps and such, and talked about noise as well. So what I need to know is if I should use them or run out to Radio Shack and get the metal. Only issue with RS is they don't list a manufacturer, so I don't know the quality.

Anyway, thanks for any input.

[marty59]

Since you have them handy I'd use 'em. While true that carbon comp's can drift, become noisey, etc. you do have some new ones at your disposal. Not all carbon comp's go bad "just because" and I'm sure there are still lots out there operating just fine including my sets and equipment too.

[TinCanAlley]

Originally Posted by marty59

Since you have them handy I'd use 'em. While true that carbon comp's can drift, become noisey, etc. you do have some new ones at your disposal. Not all carbon comp's go bad "just because" and I'm sure there are still lots out there operating just fine including my sets and equipment too.

Cool. Just wasn't sure if they would be a fit in that circuit. I know they're all over the bottom of the chassis, but there's also lots of carbon films as well. Can't figure out the logic behind using two different types unless the characteristics of each type fit the intended use.

Thanks for the input.

[DaveWM]

good luck with it I hope it fixes the problem. Try the transistor 1st as its just a plug in IIRC.

[DaveWM]

oh and if it was me I would prob just j hook in those parts to avoid stress on other stuff. I am pretty sure the diode is silicon but I would put a heat sink on it any way.

Do yourself a favor, check the NOS parts before installing thats the diode and transistor.

[TinCanAlley]

Originally Posted by DaveWM

good luck with it I hope it fixes the problem. Try the transistor 1st as its just a plug in IIRC.

You read my mind. I have the transistor in front of me and am about to test it. If it tests okay, it goes in and I fire her up. If that doesn't help, I flip her over and start the resistors and diodes.

[DaveWM]

BtW, does your schematic give you voltage readings for that transistor? if so it would not hurt to check them. Make sure you check the notes regarding without signal, line voltage etc...

Be VERY careful reading voltage with SS parts, a slip of a prob can instantly destroy a diode or transistor.

[TinCanAlley]

Originally Posted by DaveWM

BtW, does your schematic give you voltage readings for that transistor? if so it would not hurt to check them. Make sure you check the notes regarding without signal, line voltage etc...

Be VERY careful reading voltage with SS parts, a slip of a prob can instantly destroy a diode or transistor.

Yep, it has the voltages. Is it better to probe the bottom lead of the socket, or try to read directly from the exposed part of the legs of the transistor itself?

[TinCanAlley]

I realize I put this thread in the wrong forum. How do I move it?

[TinCanAlley]

Originally Posted by DaveWM

BtW, does your schematic give you voltage readings for that transistor? if so it would not hurt to check them. Make sure you check the notes regarding without signal, line voltage etc...

Be VERY careful reading voltage with SS parts, a slip of a prob can instantly destroy a diode or transistor.

Well I replaced the transistor and it didn't help. Figured since I was in there I change the other since I had the replacements. I managed to change all of them except for the 3rd Video Amp. When I installed it, there was no picture. I reinstalled the original and it worked fine. The legs on the replacement are not in a row (they in an arc). I know the center back is the base and I had assumed that since the outer two were forward of the base, it would install in the socket that way. Am I wrong? Do I need to bend the legs backwards to rotate and install it?

[DaveWM]

the sams should id the pins of the existing trans or you can look a the schematic to figure it out. then you need to look at the replacement and check the package for what pins are what.

[DaveWM]

Originally Posted by TinCanAlley

Yep, it has the voltages. Is it better to probe the bottom lead of the socket, or try to read directly from the exposed part of the legs of the transistor itself?

I find it best to go from top of chassis, get some test prob clips from rat shack, they fit over the tips of reg test probs and have little j hooks that extend and retract, hook up everything with the set off and make sure there are no shorts and that the DMM is in the correct setting, then power up. take readings, power off and remove the clips.

BTW no need to replace anything but the Blanking transistor the others would not account for the jail bars (at least should not).

Don't make too many changes or substitutes. You want to try and diagnose the problem and not shot gun parts at it. If the diode/ blanking transistor/cap/resistors to not resolve it then you are into tough dog territory, and you need someone that has a scope and some real talent to fix it.

I only hope that the target parts are it, again its really just an educated guess.

[TinCanAlley]

Originally Posted by DaveWM

I find it best to go from top of chassis, get some test prob clips from rat shack, they fit over the tips of reg test probs and have little j hooks that extend and retract, hook up everything with the set off and make sure there are no shorts and that the DMM is in the correct setting, then power up. take readings, power off and remove the clips.

BTW no need to replace anything but the Blanking transistor the others would not account for the jail bars (at least should not).

Don't make too many changes or substitutes. You want to try and diagnose the problem and not shot gun parts at it. If the diode/ blanking transistor/cap/resistors to not resolve it then you are into tough dog territory, and you need someone that has a scope and some real talent to fix it.

I only hope that the target parts are it, again its really just an educated guess.

I have those little suckers with the hooks.

I'm going to do the testing.

[TinCanAlley]

Okay, after a total brain fart and leaving the positive lead in the OHM connection, I finally have the numbers.

For the blanking transistor I got 6.8V for the Emitter (SM has 7.19V), Base was 3.9V (SM has 4.43V) and the Collector as 23.4V (SM has 23.5V).

[DaveWM]

those all look pretty good, You can still try the diode (I would just unsolder and check the a DMM compare to the new one) then the resistors. This may be very late question, but do you have the brightness cranked up? or the contrast real low?