1967 Admiral Horizontal Section Troubles
 

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

Success?
 

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

Update
 

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

Update?
 

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

Could you post photos of this set?
 

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/attachm...7&d=1157431059

I agree - pictures do make it more interesting!

Jim