Carbon composite in place of carbon film
 

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.

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.

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

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.

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.

:thmbsp::yes:

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?

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

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?

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.

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.

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).

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?

Funny you should ask. I have the contrast at a comfortable level and there's plenty of travel left on the control. You only need to turn less than a 1/4 to get the right level.

The brightness, on the other hand, is almost full. There's not headroom in the control. The full rotation gets you just over where it should be. I back it down just a bit to get to the right level. I could have sworn it had much more range before I started my repair work.

Why are you asking? Is there something in the numbers?

Also, if the numbers look okay, does that mean the resistors are probably okay as well? I will do the diode on Wed. I'm running out of steam and don't feel like flipping her over at the moment.

well if it checks out (the blanking) then I was thinking something may be a miss in the overall bias of the CRT.

You should get that working (brightness control).


Check all the voltages on the CRT (G1 G2 and Cathodes).

The CRT checks out okay on the B&K tester and the G2 voltages were good as well. Could it be possible I didn't set the G2s high enough when I did the grayscale adjustment? I disconnected the IF, turned all G2s down and then turned red up until dim raster and backed off until it disappeared ( don't like the setup line). I did the same for the blue and Green. I didn't see a need to move the taps, so they're on medium.

well how do the voltage look at the G2? based on your setup, warning they will be pretty high so make sure your meter can handle it.

If the brightness control is turned way high, and its part of the same circuit that has the blanking maybe there could be a problem..

Each G2 measured around 752 with pot turned up and around 420 with it turned down.

where was it when you did your setup?

When did what setup? If you mean the grayscale, they were all the way CCW which would make them about 420V. I don't know what they ended up at as I didn't think to measure.

when you did the orig CRT setup you said something about not liking to use the setup switch bur rather seting up the red raster then the other colors.

Now you have the set working but you say the brightness control must be set to max.

I presume the jail bars are happening with this setup.

So

now measure the crt pin voltages. don't move the pots I just want to know what the voltage are after you did the setup and had the picture that you say is low in brighness.

Oh, I get it. That setup was the grayscale. The setup switch puts a single horizontal line across the screen and you're supposed to turn up the G2s so you get a white line. It's to hard to do this, so I used another's suggestion about pulling the IF cable and going with a full screen raster. The only difference is, and this could be the issue, is that with the IF disconnect way, you turn up till visible and then down til just off. With the setup line, you want each visible. So maybe turning back til just off is making it too dim for good brightness.

Oh, and I'm not maxed out, but just about. There's about 1/8" movement left in the control. I know on my other CCII, there's about 3/16 or more left and can get pretty bright.

When I flip her over Wednesday, I'll measure the G2 voltages as set.

That seem odd, with a good CRT and the G2's turned up 7/8 the pic should be way bright. Something is not right. The thing with CRTs is if you the correct HV and the votages are right and the CRT has good emissons it pretty much has to work right (brighness).

should be interesting to get the readings. something is off on the bias maybe in the cathode circuit after all (back to the video amp).

if all the cathodes are too high it would cutoff the CRT, the driver transistors get there signal for the luma from the emitter circuit, the base is the chorma.

for grins check the emitter voltages of those 3 power transistors (should all be the same about 16v no signal)again be careful on slip and you will ruin the transistors.

Okay, the G2s are: Red=488V Green=453V Blue=456V

How are those numbers? And it's not the G2s that are turned up that far, it's the brightness control on the front of the set that up.

Okay, unsoldered the leg of the diode. It measured 584 or so one way, infinity the other. So I'm assuming it's good, right? That's really the only test you can do on a diode, right?

what does the sams say the G2s should be? that seems low to me.

before doing anymore work on the jail bars you need to get the brightness thing worked out.

do the setup per the sams, (with the service switch) then check the brightness control and the G2 voltages.

and yes the diode is fine.

Okay, the SM shows: Red=590 Green=640 and Blue=540

So I need to bring the G2s up to roughly those values? I know they go that high and higher from testing, but didn't know I was aiming for those values when adjusting.

I'll have to do it tomorrow. Got a busy day today.

its not that you are aiming for those that is just what sams got, my concern is if you deviated from the correct G2 setup procedure, then the bias of the tube may be off, making other things (maybe the jail bars) become an issue.

just do the setup as defined in sams, check the voltages and post back the results if brightness is still an issue then need to look at the other voltages and try to figure out why, brightness should not be an issue with a good tube with correct HV and pin voltages.

I'll readjust them first thing on Wednesday per SM instructions.

Okay, I could not go the setup switch approach. The only G2 that put a line on the screen was the blue. The red and green wouldn't show. So I brought up a grayscale pattern and adjusted it that way. After doing that and adjusting the taps, the red voltage was 590V, blue was 550V and Green was 540V. The SM has the Green at 640V, Blue at 540V and Red at 590V. So other than the Green, the other two are pretty close.

Now I do have more brightness range, so that is good. As for the jail bars, they're still there. I changed the diode since I had to unsolder the existing to test. I didn't bother with the 100K resistors as I have the scope now and figured I'd try that before more part replacement.

you need to check the other voltage at the pins of the CRT the cathodes/G1 and G2.

It does not follow that you can not get the setup to work with the switch. If the crt test good it should work, if it does not work there is a problem.

...

Okay, here's where I get lost. Finding the G2 lines was easy. I just connected to them at the pots. As for the G1 lines, I don't know which ones they are. The G2s are all white with red, blue and green stripes. Then I have yellow wires with red, green and blue stripes and solid color red, green and blue wires. That makes three sets of wires and no indication in the SM as to which sets are which. I do know the black is focus, though.

During testing, the range for each G2 control was roughly 450 to 729. The reading was taken directly from the output tap on the G2 pot.

the best way is to simply pull the crt socket back just enough to expose the pins, then using the rat shack spring loaded prob you clip on to one pin at a time (neg to chassis) and get a reading. you want the socket to be on enough to still work the CRT. As far as what pin is what, you should look up the CRT (google) and it will be listed along with pin out. stay away from the focus pin it will be the one that has space between it and the other pins on both sides.

dont try and just use the prob directly to the expose pin, it can be done but you risk slipping and shorting out on pin to another. far better to get it all hooked up and make sure nothing is shorted then power up, get a reading, power down and move to the next pin.

oh one last thing, if you do this a lot you use a socket adapter that goes between the CRT pins and the socket, it has test point that are clearly labeled that make checking pin voltage easy.

Well I used the hook probe and got all the voltages. The only ones I know for sure are the G2 leads. I'm assuming the G1s are the yellow wires since their voltages are around 140V which I believe is the cutoff voltage? I was unable to find anything on the internet searching for my CRT model and pinout.

I'll put the values in the SM first and then actual.

SM Values Actual Values

2 = 143 153
6 = 142 144
11 = 142 147

3 = 36.4 33.4
7 = 36.4 33.4
12 = 36.4 33.4

4 = 590 600
5 = 640 548
13 = 540 540

All the numbers seem good and within range.

Pin

hmm, well thats odd that you can't get the setup to work then.

my service manual shows 180v at the collectors of output transistors when in the service mode. check that. again extreme care with the probes or you can smoke a SS device is a fraction of a second.

Kind of late, but my skills at reading the schematics must be getting better. I just found that there are test points for the G1s at the chroma gain taps. Seems T, S and R are the G1s and you can measure the voltage from there. I just did that and got the same voltages there as the pins (well within a volt). :banana: