No Horiz Oscillator on Zenith Porthole- Solved!

[Eric H]

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.

[old_coot88]

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).

[Eric H]

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

[Username1]

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...?

[kramden66]

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

[Eric H]

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.

[jr_tech]

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.
jr

[Eric H]

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

[Username1]

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.

[DavGoodlin]

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.

[Eric H]

Originally Posted by DavGoodlin

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.

[Username1]

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)

.

[Eric H]

Originally Posted by Username1

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.

[Eric H]

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.

[Eric H]

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.