Hi all -

Looking for some help on my 1967 Admiral - the one with the arcing damper tube. The tube snapped once or twice and the picture returned to normal. As soon as I saw that, I turned the power off and unplugged the set. I've since ordered a tube and have finally gotten back around to messing with the set.

I replaced the damper, fired it up, and although I didn't get any arcing, I also didn't get any high voltage - but I did get a nice red glow from the plate of the 6KD6! Pulled the plug immediately and have been doing some troubleshooting with the horizontal output tube removed.

My determination so far is that the horizontal oscillator isn't running; I'm seeing .3 to .4VDC at pin 5 of the 6KD6 rather than the -60VDC the service manual mentions. I've attached a scan from the service manual detailing the horizontal section - maybe this will help.

I replaced a few of the resistors in the oscillator section; most were at the high end of +10% if not a little over; a couple were 1.5x their rated value. I only replaced 1/2 watt resistors as that was all I had on hand.

I haven't replaced any capacitors yet (except for the 20uF electrolytic) and I'm trying to learn from this rather than just shotgunning the whole thing.

Measuring voltages at the pins of the horizontal oscillator gave me the following values:

Pin 1- control plate +240 VDC
Pin 2- oscillator grid -28VDC
Pin 3- oscillator screen +233VDC
Pin 4- heater +6.3VDC
pin 5- heater 0VDC
pin 6- oscillator plate +174VDC
pin 7- oscillator cathode 0VDC
pin 8- control cathode +8.2VDC
pin 9- control grid -10mVDC

I'd like to puzzle this out and ensure that the circuit is functioning correctly before popping another HO tube in and trying it out.

Now for the questions:

1) I'm assuming that the two plate voltages are elevated over what's on the schematic since the tube isn't conducting (oscillator grid is at -28V and the control cathode is 8V above the control grid) - correct?

2) Can somebody shed some light on how this circuit works? I understand so far that it's a free-running oscillator that locks to the pulses from the sync separator, but what acts as the feedback path - is it the horizontal hold control?

2a) In light of question 1, is there something that would 'kick' this circuit to get it started?

3) Are there any areas that I should focus on especially - and if so, what would I be looking for?

Thanks in advance,

Jim

Looks like the horiz control side may be soaking down the osc side because the triode cathode is at 28 volts. Look to see if R454 has shifted high and running the triode into cutoff. One thing I have done in the past at least with my Sylvania and a few others is sub in a 6GH8A for in my case 6BL8 and had good results. 6GH8 is a medium mu triode/sharp cutoff pentode and pins all line up internally with the 6JW8 that you are running now and the tubes are similar electrically but not exactly, this may only help you isolate if its a bad tube or other problem.
One thing is do you have a replacement for the 6KD6 by chance? Ive had dampers arc out in the past and the resulting current swings have also taken out my horiz out tubes. A 6JS6, 6LR6 would be good subs as they pin the same and have similar ratings within 10% of nominal voltages. The 6KD6 is a bit more robust than the 6JS6 however the JS6 will survive as I have used this tube in several sets that have called for a 6KD6 and 6LR6 such as my Sylvania.
Last thing to check is supply voltages themselves however they will be elevated with the horiz tube out of its socket but all the voltages should be present.

Im not an Admiral expert by any means but Ive burned my fingers a bit over time with glowing horiz tubes.

109

109 -

Thanks for the reply!

R454 has been changed; if I remember right it was reading a little high; possibly at the upper end of 10% but nothing to cause alarm. Mainly changed it since I had one end free already to check it.

I did try to sub in the 6GH8A (there's one in there for color killer/3.58 mHz oscillator) but still the same results.

Don't have a replacement for the 6KD6 as of yet; was planning on getting the oscillator running first. Don't think I want to reuse the other tube; looks like it got cooked pretty well and has an internal rattle. I'll have to order one of those guys up but they seem to be pretty plentiful on Ebay.

Jim

I think the oscilloscope would be put to good use to see what kind of an AC signal the oscillator tube is producing.

It does sound like there is no horizontal drive, but be aware that the DC voltage on the HOT grid won't show any negative bias without the tube in the socekt. The diode action of the grid to cathode of the HOT is needed to create the negative bias from the drive signal. A scope would be the best way to check it.

Check the small silver colored Mial capacitors in the oscillator. There are usually 2 or three low value caps in the horizontal osclliator and control circuit. They are notoriously unreliable. Both shorts and opens are common. Silver micas make good replacements.

Thanks to all for the help!

Chad and Andy - I will be digging my scope out this evening and will see if I can get any waveforms that make sense. The scope is probably 10 years older than the set :) but worked well the last time I tried it.

Andy - Interesting information on the bias for the HO tube. I wasn't aware that such was the case. Can you explain a little more how it works (I'm taking notes!)? I'm still a little fuzzy on tube theory and am used to seeing tubes with cathode resistors rather than grounded cathodes.

Don - Happy Birthday, and I will be looking for the caps you mentioned. IIRC, the 820pF on pin 2 of the oscillator is one of those plastic coated caps, and I think there may be another one in there as well. Hadn't suspected them since I thought they were reliable.

I'll post more information when I get a chance to dig into the set (hopefully this evening). I really appreciate all the help and information - I'm trying to learn how this thing works and how to fix it rather than just changing all the parts and hoping something happens. :)

Jim

Dug the scope out and looked at the waveform at the grid of the HO tube. Much to my surprise, it looks exactly like it's supposed to as per the service manual. They mention 220V p-p, I'm maybe seeing 175-180 (if I'm reading my weird scope correctly).

Checked the HO tube with an ohmmeter and didn't see any shorts between elements, but that internal rattle is still there. It's a strange sound, with more "springiness" to it than just something rattling around in the tube. Is it possible for an overheated tube to lose connection from the pins to the elements?

Looks like it's time to order a tube and give it another shot.

Thanks again!

Jim

Basically, what happens is that when the grid of the HOT goes positive, current flows from the grid to the cathode. If the HOT grid was DC coupled to the osc circuit, it would just clip off the positive peaks of the drive signal. Since it's AC coupled through C448, this cap gets charged which results in a DC bias on the grid.

I would check the screen grid supply and also the suppresor grid circuit to the HO tube...have had some problems in the screen circuit on RCA's. Make sure the screen supply is good before connecting the tube...lack of bias can permanently destroy the tube quickly. You might try a 6.3 volt light bulb wired into the cathode circuit to act as a fuse and indicator. The bulb should glow with a normal full brightness on a properly functioning circuit. If the bulb burns out or is super-bright there is likely still trouble.

Andy - thanks for the explanation; it makes a lot more sense now.It takes a while for some of this stuff to sink in for someone who deals mostly with digital electronics. I think that's the main reason I fool around with this stuff - analog has a certain element of "magic" to it.

Chad - Checked the screen voltage and I have B+ (although high) at pin 3 of the 6KD6 (with tube out of socket). Will verify that parts in that circuit are okay tomorrow; it's getting late here. Light bulb is a good idea - there is a RCA jack on top of the chassis with a shorting jumper in it for checking cathode current, seems like as good a place as any to put it.

Hopefully will order a new tube tomorrow, then once it's working properly, it's on to cataract surgery.

Thanks!

Jim

Finally got back to the set after a week or so of other responsibilities and commitments. Have been looking at the various waveforms in the service manual and comparing them to what I get on the scope. All of the waveforms are almost dead-on in terms of shape, but a couple are out of whack voltage-wise. For example, the plate of the oscillator control should have a ~75V sine wave, but it's only about 5 or 6 volts. I've got some capacitors on order and will change them out when they get here. Based on what I'm seeing and what other forum members have said, that's probably my best course of action. Will keep everyone posted.

Jim

Good evening all,

Sorry for the delay in getting back to this topic; it's been a busy month for family and work responsibilities.

I received the caps I ordered and recapped the horizontal section, save for ceramic discs. This resulted in some improvement; the waveform at the grid of the HO tube is now in spec with the service manual (220V P-P). I still have to check the frequency against a known source (scope doesn't have a calibrated timebase)

I'm still getting excessive heating of the 6KD6 plate, though. I've checked the screen voltage with the tube out, and I have B+ at that point. Are there any other things I should be checking?

Is it possible the flyback or some other component in the high voltage section has gone south? How to tell?

Jim

It is nice to see the model we are working on!

Thanks

Bobby -

This is the set in question. It was also part of a thread on cataract removal along with a 1965 Admiral (which I'm watching at the moment).

http://audiokarma.org/forums/attachment.php?attachmentid=25057&d=1157431059

I agree - pictures do make it more interesting!

Jim

Well... I dont know much about Admiral, but.... These old Color sets are for the most part pretty generic...

Do you have high voltage now? There is a jumper on your schematic to check the H.O. current, (which will of course be too high if the plate is too hot, but I would see how much it is). I would check that. These Horiz. sections are tuned much like transmitters. If they're out of tune, the HO tube draws too much current. There should be a "horizontal effeciency coil" somewhere. On the other hand, this was a sudden failure?

Got a bit of the schematic showing the HO, damper, HV rect., 6bk4, etc?

John

John -

No high voltage whatsoever at the moment. This was indeed a sudden failure - the damper tube arced a couple of times, but the set was still working when I turned it off 15-20 seconds later. I didn't power it up again until after I replaced the damper tube (about a week or so), and it was dead at that point.

Re: tuning the frequency: I'm going to bring my function generator home from work tomorrow to set up my scope; that way I can make sure that the oscillator is indeed on frequency.

There is a horizontal efficiency coil in the plate circuit of the damper tube.

I'll scan in the high-voltage section of the schematic tomorrow and post it.

Thanks!

Jim

The oscillator should set the frequency, since it is locked by the horiz AFC. The HO tunes like a PA tube, using the efficiency coil, in most if not all cases. Minimum current, or minimum current and 1/4 turn to eliminate foldover or something like that.

Thats a really cool looking set. I used to have a Zenith in a similar looking cabinet. It wasnt as pretty as your Admiral, though. :)

Your new damper tube could easily be the problem. Been there done that.

Here is a scan of the schematic from the HV section. There is one change - the previous owner replaced the flyback some 20-odd years ago and the instructions for the new flyback state that pins 4 and 10 of the 6KD6 are to be grounded.

Holmesuser01 - thanks for the suggestion about the damper tube. Is there a way to test that theory or is it just a replace-it-and-see type of thing? There's a guy fairly local to me who sells tubes; that's where I got the replacement horizontal tube. I'll have to give him a try. If nothing else, I'll have enough spares to last me for quite a while :)

To all - Thanks again for all your help on this set and with cataract removal. I'm still getting my feet wet, and at times these problems can be quite daunting. Hopefully someday I will be able to be one of the ones providing the help!

Jim

Hmmm....

Your idea of checking the exact frequency of the Horiz. oscillator is probably a good one. It would also be a good idea to check again the DC voltage, and the AC waveform on the grid of the HO tube. Were you checking it at the tube socket? Could r729 be open?

How about c448? The equivalent capacitor in one of my sets had gone up(!) in capacitance, and it threw the horiz. frequency off. It didn't throw it off far enough to cause this kind of trouble (the AFC pulled it back in), but it was off nonetheless. I have heard of the capacitance of this cap going down in RCA sets, causing insufficient drive, and constant HO tube failures, due to excessive current.

Ok, if we have good drive and the correct amount of DC on the grid of the HO, and the HO is still getting hot, probably I think HV system is overloaded. The HO screen voltage would be a good clue here. Is it too high or too low? Do I understand that the HO is getting hot and the damper is not?

I also like the idea of trying another damper.

If the CRT guns are biased full on, it would probably pull enough current to kill the HV. You might try it with the CRT socket disconnected, and see if the HV comes back.

If it isn't that, I guess we'll have to go looking in the focus rect. or HV regulator circuits. :scratch2:

Regarding the grounding of the suppressor grid, some sets (other brands) came this way and others had the option of connecting it either to a voltage divider that supplies about 80v (like the one on your schematic), or ground. You would change it if you had a parasitic oscillation (snivet) in the HO tube. You knew you had one, because it would modulate something, causing a dark vertical line or area on the screen. If you don't have a snivet, I would leave it the way the flyback manufacturer wanted it.

Good luck,

John

John -

Yes, I wanted to verify that the oscillator was on frequency after recapping and changing resistors in that section. Plus I wanted to make sure that the horizontal hold control hadn't been disturbed (the chassis has only been in and out about eight or ten times so far.. :) )

The waveform I am seeing is at the grid pin of the HO tube socket. I've checked R729 and it is within spec. I have the HO tube out for testing, but am going to put it in and see what I get at that pin as well. I also plan on measuring the screen voltage then as well - with the tube out the screen voltage is simply B+ - and a bit high at that.

I haven't looked at C448 as it is a ceramic according to the parts list, but could measure it and change it if necessary.

As far as I know, the damper isn't getting hot. I haven't looked at it closely though; I focused on the HO tube since that was the first one I saw with a glowing plate.

Will keep everyone updated.... :)

Jim

Will remove the socket from the CRT and see what happens when I get home tonight.

John -

Just finished messing with the set for this evening. Here's what I've found out:

1. The oscillator is on frequency (at least as close as I can get with my scope).

2. Screen voltage starts out at about 450 at power-on, and drops to a low of about 75 and then begins to rise again. Haven't made it past this point since the HO tube starts to glow. The two schematics I have for this set show 90VDC and 180 VDC for the screen voltage.

3. Removing the picture tube socket had no effect on the high voltage (or lack thereof). Same with removing the focus rectifier.

4. The cathode of the damper tube is also glowing. I hadn't noticed until you mentioned to look for it. I'm going to pull it out in a second and check for shorts between the elements.

Jim

You might want to check the caps and windings of the yoke. The yoke coils are in parallel with the damper usually. I have had bad new damper tubes.

Bill R.

Here is a silly idea but how about unplugging the convergence board...just to make sure something in that circuit isn't causing a problem.
Also I would take the yoke off the CRT and check carefully for burns on the underside. Have some RCA sets where somehow the yokes shorted out and burned the enamel off the wiring.

Well, unplugged the convergence board, and still no go. Also removed all of the tubes from the high voltage section and looked for inter-element shorts. Didn't find anything of note.

Can the set be operated without any of these tubes in place (HV rectifier, etc?)? I would assume probably not the HV regulator though.

Have an order in for a new damper tube but it won't ship until after Thanksgiving.

Going to pull the yoke off hopefully this weekend and look for burnt spots and see if there is anything I can tell through further examination.

Any way to see if the flyback is the problem? This one seems to be a lot tougher than the one in my other Admiral - it's encased in silicone rubber rather than wax.

Thanks again to everyone for all the help. I'm learning slowly but surely - and I will get this set going eventually.

Jim

Well, unplugged the convergence board, and still no go. Also removed all of the tubes from the high voltage section and looked for inter-element shorts. Didn't find anything of note.

Can the set be operated without any of these tubes in place (HV rectifier, etc?)? I would assume probably not the HV regulator though.

Have an order in for a new damper tube but it won't ship until after Thanksgiving.

Going to pull the yoke off hopefully this weekend and look for burnt spots and see if there is anything I can tell through further examination.

Any way to see if the flyback is the problem? This one seems to be a lot tougher than the one in my other Admiral - it's encased in silicone rubber rather than wax.

Thanks again to everyone for all the help. I'm learning slowly but surely - and I will get this set going eventually.

Jim

You can safely remove all the HV tubes, including the regulator as long as the HV rectifier is also removed. See if you can get any kind of an arc between the HV regulator plate wire and a screwdriver with an insulated handle (there's no need to ground the screwdriver blade).

There is a test you can do if you have a scope and a square wave generator. You have to disconnect all the wires from it and apply a square wave to one of the windings through a resistor. Monitor this signal on the winding with a scope. A good flyback will ring with each pulse that hits it. Any shorted turns will cause the oscillations to stop very qucikly.

A google search should find a detailed description of this test.

Also you might check the 6bk4 current. There is a 1k resistor in the cathode circuit. Make sure it's still close to 1k, then hook a meter across it and read the current as voltage. Sometimes if a 6bk4 arcs or otherwise fails, it will toast some of the resistors in the grid circuit. Also the grid capacitor can short. I see from the schematic that the grid capacitor has a spark gap across it, probably to prevent damage, but I would be a bit suspicious of it nonetheless. If the bias on that tube was all screwed up, it might be drawing all the current the system can produce.

John

I have had the 6BK4 problem too. Burned resistors in the grid/cathode circuit and the 6BK4 was drawing about 2000 microamps (2 milliamps) and overloading the flyback causing it to overheat and no HV going to the CRT.
The recommended current is about 800-1000 microamps (.8 to 1 volt on the meter across the 1000 ohm resistor) Both sides of the resistor are "Hot" B+ voltage.

Get a neon test lamp and tape it to a non-conductive stick and hold it carefully against the horiz output plate lead insulation (no direct connection) and this will tell you if there is AC high voltge being developed at the flyback primary if it lights.

Well, it was the damper tube after all. Been busy here with work and all and finally got another damper tube. Put it in and the set came right up - horizontal on frequency and all! :banana:

Probably should have subbed it out when all of this started, but figured it couldn't be something as simple as a tube, now could it.. :)

Well, it's now got a new horizontal output tube and a rebuilt horizontal section, so I shouldn't have to worry about that for a while :)

Have to get a spot cleared in my basement so I can get this monster down there and work on it instead of in the garage (It's cold in Joliet this time of year!).

Going to go fire it up again, check the horizontal current, and do a quick setup of focus, screen, etc

A very heartfelt thanks to everyone who offered assistance. I'm sure this stuff seems like a cakewalk to you guys most of the time, but to a newbie staring at a set that just <bzzzzzt> crapped out, it can be quite daunting. Your information was very helpful and hopefully stimulated the thought process for the next time I've got a set to work on. I only hope that one of these days I can 'pull my weight' on these forums.

Jim

Glad you got it sorted out! :thmbsp:

John

I always replaced the HO, damper, HV rectifier and regulator at the same time if there was a problem with one. Cost a little more, but saved a lot of call backs and troubleshooting.

Bill R

I may still do that for this set - I'm halfway there already. :)

Spent the past couple nights tracking down an intermittent connection at a tube socket resulting in loss of color. Seems to be working well now - have been watching Laugh In for the past hour or so while working on other stuff.

Next up - cataract removal as soon as I get some space cleared in the basement.

Jim