No Horiz Oscillator on Zenith Porthole- Solved!
 

I've been working on this one for a while now off and on. Model 28T964 and I'm stumped.

I've done a partial recap, mostly around the Horizontal section trying to get the oscillator running but no luck.

The problem seems to be at the oscillator tube itself, I have some voltages that are off, particularly the -18 at pin 5, it's at zero.

The resistance on the Horizontal Osc coils seem to be okay as far as resistance but the board they are on is dipped in Beeswax making it most unpleasant to work on, not much there other than the coils except a couple resistors and some caps, I replaced the one paper cap, the other two appear to be Mica.

Voltages at V22 are
Pin
3 = 80
4 = 142
5 = 0
6 = 448
8 = .6

The Horizontal Discharge tube (V-18) is also missing it's grid voltage at pin 4, it's .5 where it should be -20, I suppose that's because the oscillator isn't running?

the Power supply hasn't been recapped but seems to be working OK, I subbed the one out of my Mayflower set and it made no difference either.

The only Can Cap I've re-stuffed is C-4, that was because it was getting hot, the others are running cool, but of course I could be wasting my time by trying to get it working without changing the rest, I would like to get at least the HV running though to confirm the flyback is good.


Link to Sams here Sams is for a 28T963 but seems to be identical.

Check the "phone vision" socket, pins 7 and 8 (counting clockwise). There's a jumper in the plug that if missing, will kill the drive signal going to the discharge tube (via C82).

Checked the socket, all the jumpers appear to be intact.

With pin 5=0 and 3=80, I would say the tube is ON and in saturation.
Is there most of your supply voltage across the dropping resistor....?

What is the supply voltage to that plate... 480?

I hate these broken up schematics....

Is the cathose pin 8 or 6.

is r120 that 5k good...?

be aware of that little board if this one uses it with the horz coils ( refered to as a transformer ? ) , my table model i traced it to that and a coil was open.
mike

I'm wondering if it isn't a bad Horiz Frequency coil (B2).
It's grounded at one end, the center tap goes to the cathode on pin 8, and the other end is connected to pin 5 through the .0047 cap, then to a 27k resistor then to ground through the 50k hold pot.

The plate (pin 8) has over 400 volts on it, pin 3 starts out over 300 but drops to 140 as the tube warms up and starts to conduct.

Just did a few more checks.

The Horizontal Phase coil is equal on either side of the center tap, about 36 ohms, however the horizontal frequency coil is about 50 ohms on one side and 20 on the other, not sure how that could happen exactly, overall it measures in the ballpark.

6K6:

1-NC
2&7- Heater
3-Plate
4-G2
5-G1
6-NC
7&2-Heater
8-Cathode
Now *which* pin has the 400+ volts on it? I'm confused. :scratch2:
jr

You're right, I screwed up on that post, my first post has the correct voltages I'm reading on V22 6K6.

Voltages at V22 are
Pin
3 = 80 (should be 160)
4 = 142 (should be 180)
5 = 0 (should be -18)
6 = 448 (no connection to the tube but Sams calls out the voltage anyway at 400v.)
8 = .6

Pins 3 and 4 start out high (200+ & 300+) but drop to the value listed as it warms up.
Pin 5 seems to be where the problem lies.

Re: resistance measurements on Horiz Osc coil, total resistance is 58 ohms with center tap at 15 ohms so the 20 is probably close enough.

Probably I should finish changing the paper caps, (quite a few left in areas outside of the Horizontal) and see what happens.

The vertical seems to be working though, the transformer rattles fairly loud when I mess with the Vert Height control

Plate voltage is too low, either a resistor is open, or the tube is in saturation.
Since the G1 is too high, and the plate voltage starts out high, then
goes low.... the tube is in saturation.

Can you check resistance on V22 6K6 pin 5, then rotate the hold control to see if the range is within tolerance?
The 6AC7 could be pulled to see if the AFC stage is loading and stopping the oscillator, or what else is connected to V22's cathode.....

No experience on early Zenith, just wondering.

I've checked the hold control and it seems fine, I've also disconnected every other connection to the grid on pin 5 (one at a time) to see if something was killing it.

The Cathode is on pin 8 and the .6 is very close to what Sams calls for .4 I think?

I suppose it might be a good idea to check the resistances against what Sams says and see if anything stands out.

What voltages you getting on the horiz output tube....?
it's not hot is it...? All those low ohm resistors good...?
Are you getting a plate voltage...? (yes you can check it with no HV, no Osc running)

.

The HO tube has voltage on the plate, it's missing the voltage from the Osc tube as I recall. It's not getting hot though.

I'll check it when I get home tonight and report back.

Lets try a different approach here, I'm not understanding how this circuit works.

I'm trying to understand where the -18 volts that's supposed to be on pin 5 of the Horiz Osc comes from?

The horiz phase and horiz osc coils are two separate coils that are physically strapped together, they appear to be inductively coupled on the schematic (No iron between them so not coupled?) does the -18v depend on the horiz phase coil working or is it independent of that?

There is 200+ volts coming to C72, the 150mmf cap that goes to the phase coil, that's the only place I can see any voltage at all entering this circuit but maybe I'm missing something obvious?

The voltages to the 6AL5 are not horribly far off, somewhat low on pin 7 I think, 3.something where it should be 9.0, but I' going from memory here.

I know it's a hassle to have to download the schematic to see the whole circuit, sorry about that, perhaps I can scan it and post it in a single image later.

http://www.videokarma.org/attachment...1&d=1411339796

Got a little help over at A.R.F. that led me in the right direction and found I had miswired a cap!

http://www.antiqueradios.com/forums/...72016#p2172016

Got HV and a raster. :yes:

Ooops, hate it when that happens....

SR

Yeah, I've wasted a lot of time over the years from dumb mistakes like this. :drool:

Eric,
Seeing only positive supply voltages to these stages, I used to wonder how there could be a negative DC voltage on the grid G1 of the HOT or in your case, the oscillator. Normally when you measure an AC sinewave (linear with no DC offset component) with a DC meter, you will get zero because there is an equal amount of voltage above and below zero reference.

The negative voltage is the average DC voltage of the non-linear sawtooth. A majority of this voltage is below the zero crossing, a clue that the oscillator is running if you don't have an oscilloscope.

With all the parts we replace, its no wonder this happens.

Congratulations!

Not to be a fuddyduddy:o, but you're overlooking the cathode/G1 diode action which also generates negative grid voltage (as in grid leak detection).
A sine wave on the grid will generate it (provided the grid's at high impedance and the signal is capacity-coupled. A superhet local oscillator will make negative voltage on the grid.

Hi Eric,

The grid voltage on the oscillator comes about by grid rectification. It is known as grid leak bias. The feedback is via the cathode to the grid and when the oscillator is running, grid current with the positive signal excursion forces grid makes the grid negative. When there is no oscillation, there is no negative voltage! This is the same bias on the subsequent stage and because there is no signal, there is no bias.

If the capacitor is the correct value and the grid resistances are okay, it will be either the 6K6 or the oscillator coil. Are you sure the coil connections are correct?

Terry

Hi Terry, I finally figured out I had installed a cap wrong.

The explanation is here: http://www.antiqueradios.com/forums/...72016#p2172016

Excellent news. Yep, the cap misconnected would effectively kill the oscillator!

Replaced nearly all the paper caps in this thing, but almost none of the cans yet, still it's starting to work fairly well.
It has a bad Height control that prevents me from getting the optimal vertical adjustment but it's not too far off.

I put the 16AP4 back in it to see how it looked, it was really weak originally so I rejuved it with the Beltron, the emissions came up really good but the Cutoff reading is non existent.

I wonder if that's why it has the retrace lines, or maybe the vertical misadjustment could be causing them?
They are there even with the brightness at a minimum.

The High Voltage is good, the speaker wires got to close to the front of the metal tube and it was quite a show.

retrace lines can show up on portholes easy especialy if it is copyguard but it too could be that the crt is weak and rejuved with no cut off or a combo of the two , are there retrace lines on anything you feed into the set ? you try tapes and dvds?

The signal is a burned copy (so no Macrovision) of a Japanese version of one of the X-Men films I think, it was in the DVD player when I got it from the Thrift, it's being broadcast with the B-T Agile Modulator.

does it have a retrace blanking circuit? I have added those to sets that lack them if I have a weak CRT. also how well does the contrast control work.

The contrast works well, one thing though, only one of the electrolytic cans has been rebuilt, all the others are original.

We'll have to see how it looks after everything is done, there may be some bad tubes as well, I was more focused on getting the basics working first, now I'll have to do the rest. Works pretty well considering.

I could also stick a better CRT in and see how it looks, I have a 16GP4 but it won't slide in without loosening the yoke assembly, I think a 12LP4 would be okay for a test, have to see if it can handle the HV but I don't think it's all that much higher than a 12" set.

Making progress
 

I replaced the last three paper caps tonight.

I was having a problem with the brightness, the range was from too way bright and blurry, to burn the phosphors off the screen bright!
Turned out to be a leaky Cathode bypass cap (C46), it was a Mica-Mold domino style plastic case cap, replaced it and now it goes from complete black, to very bright.

I'm even getting a little bit of sound, it's very low though, possibly the 6V6 is near dead from running it without a speaker but more likely it's somewhat out of alignment.

the sync is very twitchy, it jumps vertically at scene changes and flops out of horizontal now and then.

It still has the retrace lines (except with the brightness way down) but the CRT is sharp and bright.

the jumping when scene changes occur could be agc is too high ?
as a test you could put a brightener on it and if you get less retrace lines then you will know it is the crt , those don't remind me of retrace lines from a broadcast but more of the type you see with a weak crt , so add a brightener , if you get enough brighteness when turning the control for less retrace then you will have the answer.
mike

Quote:
I'm even getting a little bit of sound, it's very low though, possibly the 6V6 is near dead from running it without a speaker but more likely it's somewhat out of alignment.

Could be the dreaded silver mica disease causing that, buried inside the IF cans.

to verify i turned on a set that has a weak crt , it plays ok but if you turn the brightness high enough i get those same retrace lines like in your pictures , they are spaced far apart , thin just like you show , these aren't the kind you get from broadcasts or copyguard , so if you can turn the brightness down to where they go away and the image is still bright or bright enough it should be ok , if not try a brightener and see if you an get enough brightness without the lines appearing , it is unfortunate that these things happen , no idea if adding a retrace circuit to the set will get rid of it since these aren't the broadcast retrace lines , broadcast / copy guard retrace lines they are brighter , closer together toward the top of the screen and pretty sure they are thicker lines and there are more lines then what is showing in your pictures.
mike

I tried a different CRT but it still had lines. I still need to replace the electrolytics in this set so I won't get too worried about it just yet.

I think it might have something to do with the vertical being a bit messed up, the blanking comes from the vertical circuit doesn't it? So if it's not right...