Horizontal Drive waveforms on my 1950 Zenith porthole

[Mike Stamper]

I'm confused as to why the high voltage fades out as I increase the horizontal drive setting. I took pictures of the waveforms at three different settings: minimum, halfway, and maximum rotation of the horizontal drive control. Here is the picture at minimum:

http://img02.picoodle.com/img/img02/...dm_600e144.jpg

Here it is at midpoint:

http://img01.picoodle.com/img/img01/...dm_ba3d48d.jpg

At at maximum:

http://img03.picoodle.com/img/img03/...dm_26c47b0.jpg

Notice that the sawtooth wave form becomes more and more distorted as the control is advanced. The voltage (at pin 5 of the 6BG6G where these pictures were taken) changes from 13VAC to 21VAC as the control is advanced to maximum. I don't think this is right. Can anyone tell me what SHOULD happen as the horizontal drive is advanced? Should the voltage go up? Should the wave form change? And finally, if this indicates a problem, where should I look? Thanks!

[Old1625]

Have you recapped that set? Checked the tubes?

It would seem that you might have a leaky capacitor, or perhaps a resistor or two that have drifted in value.

Does the HOT plate get red?

I laid my hands on a Sams for a slightly later model Zenith using a 6SN7 for horiz osc/disch and a 6BQ6 for HOT. THe waveform they show is a reasonably pure sawtooth, and not one where the top is clipped the way yours is.

Checking the operating voltages against design figures may uncover the culprit. The screen voltage at the HOT could be low. Also the plate resistor in the preceding stage could have changed in value. Also check the amount of developed negative voltage there is on the HOT control grid. The Sams I have shows -15 volts for control and 150V for screen. I feel fairly certain that your 6BG6 should operate at voltages not far from these....

[Mike Stamper]

Every cap was replaced, as well as every out-of-tolerance resistor. I'll take some voltage measurements and post them tomorrow. Thanks for commenting on my post.

[Old1625]

Does the raster show adequate width before the HV starts going down? Bear in mind that as the accelerating voltage goes down the raster size will lose brightness, focus--and become larger.

If your voltages all jibe then my gut is telling me that the HOT is played out, or that perhaps the flyback has issues....

[Mike Stamper]

Flyback is a brand new old stock FLY-51. 6BG6GA is NOS. Here are some voltage readings:

Horizontal discharge 6SN7: Pin 2 anode: expected +65v, actual +37v; Pin 1 grid: expected -33v, actual -30v

Horizontal output NOS 6BG6GA (tube plugged in, anode NOT connected to flyback): Pin 8 screen grid: expected 255v actual 267v; pin 5 control grid: -7v expected -27v actual; pin 3 cathode: +14v expected +2v actual

All caps replaced, and I have triple-checked accuracy. All resistors in that section replaced, again, have rechecked accuracy. The biggest variations seem to be the anode of the horizontal discharge, and the control grid of the horizontal output. Does all of this suggest anything? Thanks!

[Mike Stamper]

Re: "Bear in mind that as the accelerating voltage goes down the raster size will lose brightness, focus--and become larger." Yes, that is what happens as I turn the horizontal drive clockwise from start.

[Mike Stamper]

C77 and C84 are both new. I pulled them and subbed the identical parts. No change. The "phonevision socket was corroded and measured 27 ohms across each connection. I bridge the connections and remeasured (down to 0.2 ohms). No change. Still running about 7500-8000 volts - not enough to drive my NOS 12LP4 to more than medium brightness. I subbed the 6BG6, 1b3gt, and 6w4 (all of which were NOS and tested great). No change in HV or brightness. I'm fast running out of ideas and patience. Too bad; otherwise, it's a great set.

[Old1625]

"I'm fast running out of ideas and patience."

Patience unfortunately is one of the most important tools in dealing with antique electronics. I've chased myself around in circles on some of them, and then I'd shelve the project for a while and walk away from it. Later I'd attack it again with a fresh frame of mind. Giving up altogether for me was never an option; I always managed to find the gremlin in the end. Sometimes it turned out to be so simple that I felt awfully stupid.... That set of yours worked before, so it can work again; it's simply that we're missing something here that will eventually show itself.

That being said....

Not sure if this is going to solve your problem, but you need to be doing it anyway: If that corroded "phonevision" link socket is of bakelite wafer sandwich construction (Zenith loved using such construction for connector sockets, tube sockets, etc.... ) then you need to be sure that it is entirely excluded--i.e. not just bridgeing the connections, but removing the leads entirely from that socket and making mid-air splices if you have to, so that the socket is not involved in any way. Those can become leaky when the corrosive salts of the metal contacts leach into the bakelite. (FWIW I never use bakelite sockets in either new construction or renewal of existing parts; they've broken my heart too many times.)

I've now got this thread copied to Word, and on the desktop alongside icons of the two files you sent me. I'll look at them from time to time and get these rusty gears of mine turning.

Hang in there! We'll make this set happen yet!

[Mike Stamper]

OK. Thanks. I've been advised to try to measure the cathode current on the 6BG6. That will be my project for tomorrow.

[Old1625]

Originally Posted by Mike Stamper

OK. Thanks. I've been advised to try to measure the cathode current on the 6BG6. That will be my project for tomorrow.

No project at all: Use Ohm's law equation, and the value of resistance as shown, and your voltage reading on the cathode pin as you've already posted. Calculate. Done. No milliammeter jigged into the ckt--just a calculator.

[Mike Stamper]

It's occurred to me that I am overdriving the 6BG6. The screen voltage is 290v - the expected voltage is 255v. I think that the purpose of the 6BG6 is to amplify the sawtooth pulse coming from the horizontal discharge. The rising and falling voltage associated with the amplified sawtooth wave passes back and forth through the primary coil of the flyback transformer, creating a magnetic field which passes through the secondary windings. If the 6BG6 is being overdriven, wouldn't the sawtooth wave clip (at the maximum gain of the tube)? And if it clipped, would the nominal difference between the top and the bottom of the wave be shorter, hence less voltage change, hence smaller magnetic field generated by the flyback, hence less voltage generated in the secondary winding? Well, that's the theory. Tomorrow, I'll test it by increasing the value of the 10k resistor in series with the 6BG6 screen. I'll drop the screen voltage to the recommended 255v, and see what happens.

[Mike Stamper]

Increasing the 6bg6 screen resistor did bring down the screen voltage, and did bring the control grid voltage down to -5v. However, it had no effect on HV, which remained at about 7300v. Disconnecting the width control (and leaving the B+ connected to pin 4 of the FLY-53) produced an immediate HV boost to 10K, which went up even further as the horizontal drive control was advanced clockwise. However, trying to run the set this way with the CRT installed didn't work (raster was completely messed up, and whining noises ensued). By the way, does it matter whether I have the B+ side of the width coil connected to pin 3 or 4 of the flyback? The fly-53 directions seem to indicate putting it on pin 3 in the wiring chart, and pin 4 on the wiring diagram (pin 5 is unused). Does it matter what pin the B+ is assigned to? I'm a little unclear as to what the width control is supposed to do. How does it affect HV? Why would the HV jump up with it disconnected? If it were defective, would it contribute to drawing too much current from the 6BG6? Sorry for all the questions. I'm at my wit's end.

[Mike Stamper]

Using your suggestion (re: Ohm's Law to measure the 6BG6 cathode current). I measured the resistance to ground (149 ohms) and the DC voltage to ground (12.92v). I found an ohms law calculator, which yielded the result of 0.08658 amps, or 85 milliamps.

[Old1625]

I've felt right along that the HOT is being overdriven for whatever reason. And your voltage readings and calculated cathode current seem to jibe with this. As well as your clipped sawtooth waveforms, which indicate the control grid being driven well into conduction, which would result in an increase of developed negative bias. IIRC a normal cathode current would be more on the order of 100-110mA....

Assuming the replacement flyback has the same numbering as original as shown in the schematic the width coil goes to pin 3 and returns to the B+ line, while pin 4 of the flyback is connected to boosted B+ through a fuse.

The reactive effect of the width coil loads the circuit so as to change the current through the deflection coils to control width. That removal of the width coil should so profoundly change the generation of HV in a separate winding makes me wonder if the coil has an internal short, or that perhaps there is some confusion in the numbered callout of terminals on the flyback itself.

[Mike Stamper]

OK. I give up. It was the fly-53. There was nothing wrong anywhere else. I rescued the OEM flyback from the parts bin, repaired it, and reinstalled it. I got 10k immediately, and more when I advanced the horizontal drive. One of the wires (to pin 4) on the original flyback broke. I had lost all hope of fixing it. Today, I thought I would give it one more try. Using an x-acto knife, I scrapped the insulation from the remaining wire. I then found a small piece of stranded wire, wound it around the remains of wire #4, got my tiniest solder tip, and flowed a little solder onto the connection. I verified that it conducted, then soldered the stranded wire to the post. I pulled the fly53 and reinstalled the original. I turned the set on and it worked perfectly. A little tweaking of the width and drive controls (which now work correctly) and the picture was full and bright. I am now done with this set, at least until it breaks again. Either:

1) The fly53 doesn't work very well in this set
2) The fly53 I bought was damaged
3) The instructions on connecting it were faulty

I'll post a picture soon. Thanks for your help.