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/img02/4/2/2/f_horizontaldm_600e144.jpg

Here it is at midpoint:

http://img01.picoodle.com/img/img01/4/2/2/f_horizontaldm_ba3d48d.jpg

At at maximum:

http://img03.picoodle.com/img/img03/4/2/2/f_horizontaldm_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!

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....

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.

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....

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!

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.

The voltage readings on the HOT tend to point the finger at excessive drive. :scratch2: This would jibe with the waveforms you show the more I think about it. The clipping at the top suggests diode action of the control grid caused by its being driven well into conduction. There is some of this happening to a degree in normal operation, as that is how the HOT develops the negative bias that keeps the cathode current in check. In cases of horizontal oscillator failure the HOT would melt down completely. But it would seem that the reverse is happening here, as your cathode voltage reading and the control grid negative reading reflects little current flow, as if the tube is conducting only during a very small part of the waveform cycle. If that be the case then one would think that the width of the raster would be affected, and that one side of the raster would actually show a brighter edge, reflecting the beam lingering there during the HOT's cutoff. Just the same such condition would cause lower output from the HOT, as it is only conducting for short bursts. Attempts to further increase drive will only make matters worse.

It seems to me that you need to be going in the other direction, if possible. :dunno:

Do you actually have a video program source or a test pattern in your testing? Or are you just working with a white raster or snow?

Tonight I will input a test pattern and re-do the voltage measurements. Is the low voltage on pin 2 of the 6sn7 a problem? Should I reduce the value of the 560k plate resistor to increase the voltage? Would it be helpful if I sent the schematic? If so, can you send me a personal message with your email address? Thanks!

PM'd you.

That 560K does seem kinda high for plate resistor. The schematic I have is for a Zenith J2027, which has a different HOT but uses a 6SN7 that employs triode on pins 4,5 &6 as a Hartley osc and pins 1,2 &3 triode driving the HOT. It receives a B+ of 310V through a plate resistor of 150K. The plate op voltage is 50. There is a little difference, so seeing your schematic will be helpful. Lowering the value of the plate resistor may bring your gain down, but will also help with loading issues involving the network at the HOT control grid. This may bring things to where they belong, restoring your HT. That yours calls for a 560K is a head scratcher. :dunno: :o

Mike,

Here are photos of the waveforms I get on my set. These were taken at the grid of the 6BG6:

Drive full CCW (http://www.fidnet.com/~seanbart/HOT_grid_drive_ccw.jpg)

Drive midpoint (http://www.fidnet.com/~seanbart/HOT_grid_drive_mid.jpg)

Drive full CW (http://www.fidnet.com/~seanbart/HOT_grid_drive_cw.jpg)

Zoom in of the waveform (http://www.fidnet.com/~seanbart/HOT_grid_drive_zoom.jpg)

The vertical scale is 20V/div. With the drive pot in full CCW I also measured -5Vdc on the 6BG6 grid and 34Vdc on the Hor. discharge plate, full CW I got -7.4Vdc on the 6BG6 grid and 66Vdc on the discharge plate.

And here is a schematic of this set: Zenith 23G24 (http://www.angelfire.com/electronic2/radiosean/tv/Zenith_G2350/G2350Z_Schem.jpg)

Many thanks for the input, Sean! :thmbsp: The clipping on top I'm guessing is where the HOT ctrl grid is being driven into conduction. There's something here we're not seeing. :scratch2: :dunno:

I've got the scams from him, and I'm giving it some thought. Lately I've been out straight, but let's see what we can do.

Thanks again for your input! :thmbsp:

Many thanks for the input, Sean! :thmbsp: The clipping on top I'm guessing is where the HOT ctrl grid is being driven into conduction. There's something here we're not seeing. :scratch2: :dunno: If the HOT gets insufficient drive there is no HT. And the HOT goes to pieces with all that B+ and no developed grid bias. At the other extreme excessive drive can cause the output tube to be biased stone-cold, and thus not deliver the necessary output to the flyback in the doghouse--again: Insufficient HV. Just my humble theory... :o

This is my current thought on this matter: He's got a resistance value for plate res on his disch stage than I see called for in my scam of a similar model that shares a lot of the same stuff. Are we dealing p'raps with a typo here...? I'm still puzzling over the call for a 560K resistor where I am not used to seeing such plate res so high. And the scam I've been working from for a similar model shows a value of res of 150K. If he changes to that value I think his problems will be over. IMHO....

I've got the scams from him, and I'm giving it some thought. Lately I've been out straight, but let's see what we can do. Man! It has been so long since I've enjoyed the glory of troubleshooting an old set! :D

Thanks again for your input! :thmbsp:

The 560k value for the plate resistor is correct. That is what was originally in my set. I also had problems with the HV being low like Mike. This was due to low HOT drive caused by the 560k resistor having doubled in value.

Here are some fresh voltage readings. All tubes connected, including the HOT plate:

Pin 2 V14/6sn7 (plate of horizontal discharge; 65v expected):
-Drive at minimum: 34v
-Drive at midpoint: 48v
-Drive at maximum: 62v

Pin 3 of 6bg6 (14v expected):
-Drive at minimum: 13v
-Drive at midpoint: 14v
-Drive at maximum: 15.4v

Pin 5 of 6bg6 (-7v expected):
-Drive at minimum: -0.9v
-Drive at midpoint: -0.7v
-Drive at maximum: -0.5v

Pin 8 of 6bg6 (255v expected):
-Drive at minimum: 289v
-Drive at midpoint: 282v
-Drive at maximum: 275v

The above readings were made with no signal. Introducing a test pattern made little difference. Changing the brightness made no difference. HV output (as measured at the HT lead) was about 7,500 volts (10,000 expected). I replaced the 560k resistor with another 560K. No change. All caps replaced. All out of tolerance resistors replaced. Flyback is a new FLY-51. 6bg6g is new. Does any of this help? Thanks.

Did your recapping schedule include the one across the HT?

Something about the voltage dropping on pin 2 of the 6SN7 makes me want to look at 680mmf C77 more closely. With the HOT anode disconnected try pulling the 6SN7 and study pin 2 voltage while rotating the drive pot....

Also what are you getting for boosted B+ at the cathode of the damper?

As for suspecting new parts as culprit I learned the hard way never to assume a replaced part is good. This goes for glass, caps, flybacks, etc. You can measure a new resistor and verify that it is within tolerance, but leaky capacitors and somehow-defective transformers can be insidious. I don't trust 'em.

If you mean C79 (the .001/1000v cap from pin 2/plate of the 6sn7 to pin 5 of the 6bg6) then yes, it's new. On the chance it was defective, I substituted a .0015/1600v cap (the closest I had). Voltages went up a little, although the HV was unchanged. I guess what I am trying to figure out is: do I have a problem before the flyback, or are my new HOT and flyback failing to deliver high (10k) voltage? That's why I got into the subject of the waveforms from the horizontal discharge. They just didn't look right.

Looking at your supplied schematic, C77 runs from the resistor on one end of the drive pot to a special "phonevision" socket where it eventually connects to pin 2 of the 6SN7. If that capacitor turns out to be defective and leaky, it could cause that drop in plate voltage you noted as the drive control is turned to min. Before one condemns the HOT or flyback I'd like to see if there is something going on with C77.

Also would still like to know about the 500mmf capacitor C84, as well as the state of the damper tube.

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.

"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.... :o 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.... :rolleyes:) 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. :o

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

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

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.

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.

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.

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.

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. :eek::saywhat::headscrat

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.

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.

Glad to hear you got it going! Good show! :thmbsp:

Of the 3 probable causes you listed regarding FLY-53 it could've been any one of them.

A pic of this triumph will certainly be well received and enjoyed here.