Flyback Failure

[7"estatdef]

What causes the flyback to overheat? How can I check it short of measuring the resistances of the windings? Is it common for it to devolop a HV short to ground after it been overheated? Other than checking grid drive and for shorts in the sweep tube what else should I take a look at? This set is a 1951 Zenith with two 6BQ6's in the H output. BTW does anyone rewind them?

Full of questions
Terry

[Don Lindsly]

Terry:

Is the TV working now or are you just concerned with flyback overheating? What is the part number? Does it mount underneath the chassis?

Don

[7"estatdef]

No I don't think it's working. I just got it and was giving it a cleaning when I opened the HV cage and saw the flyback with about half of it's wax on the chaiss. Sam's doesn't list a replacement part for it. The Zenith p/n:S-17233. It mounts on top of the chaiss
Terry

[Don Lindsly]

Lots of wax is a bad sign. Many older flybacks woould drip wax over the years and still operate. I would check for operation as usual. If there is no high voltage, then make sure tubes are good and the tube voltages are close to normal. Remove the damper tube and check for high voltage and a narrow picture.

If you get high voltage with the damper removed, then there is a shorted capacitor from boost to B+. That's easy. If not, make sure there is grid drive and look close to see if the horizontal output tube plate is running slightly red. If so, remove the horizontal yoke wires and see if the boost increases. If so, suspect the yoke.

If not, remove power and feel the flyback down close to the core. If it's hot, 95% chance it's bad. That's a quick troubleshooting technique if you don't have a scope or flyback checker.

There is also a ringing test using a cheap scope and a small cap that has never been wrong for me.

Don

[polaraman]

The flyback is crossed to Fly-52 in Thordarson. Pm me because I think can get it for you.


polaraman

If the flyback "tire" or tertiary winding is overheating,that can be caused
by the plate cap not having a good,sound connection on the hv
rectifier tube.I had this happen on my Conrac color monitor and just
recently with a rca KCS-153b chassis.After the set was on for about a half hour,the picture began to shrink and get real dim.I could smell the flyback
burning.I was really bummed out about having a bad flyback when I decided
to pull off the rectifier plate cap and clean the cap contacts and the
2bj2 plate cap itself.Well,I was rewarded with sucess.The flyback no longer
overheats and I can run the set all day without any problems.By the way,
having a good connection there is extremely importaint for color sets,
especially ones that use shunt regulators.A bad contact with the hv rect.
tube in those sets could cause the set to burst into flames.Flyback fires
were a common cause of tv fires back in the old days.
Regards,
Swanson

[blue_lateral]

There is also a ringing test using a cheap scope and a small cap that has never been wrong for me.

Don

Don, could you please elaborate on how to do this?

Thanks,

John

[Don Lindsly]

John:

This is intended for tube type TVs. The concept can be made work on solid state flybacks, but that's not the topic.

For this application, a cheap scope works out better than a good one.

Connect a small cap, about 33uuf to 47uuf at 500 volts or more to the plate of the scope's horizontal sweep oscillator plate, or buffer plate if it has one.

Connect the other end to an unused jack on the front of the scope or install a new jack for the purpose.

To test a flyback, run a wire from the new pulse output to the scope vertical input probe. Set the scope horizontal freq to the 30 khz range. You should see a drooping line from upper left to lower right.

WITH THE TV POWER OFF, remove 1B3 and damper and disconnect the yoke. Connect the scope ground to the chassis. Touch the probe to the horiz plate connection on the flyback.

On a good flyback, you will see a distinct ringing pattern with large sine waves on the left diminishing to the right. If it decays too quickly or does not ring at all, the flyback is bad.

Try it out on a good flyback to get the feel of the scope settings. Once you have marked the scope settings for a good transformer, a bad one will be obvious. If you have a known good flyback, you can simulate a bad one by shorting any two terminals together with a clip lead. You will instantly see the difference. Even shorting the agc winding will produce the result.

It will work on yokes as well, but the indications are not as consistent. It works on flybacks out of the circuit so you can test ones that are laying around the shop. Use the low end (B+ connection) of the primary for the scope ground and touch the probe to the HO tube plate connection.

That is essentially how the low cost flyback testers work. They just read the ringing voltage on a meter. I prefer a scope. Try it out and let me know if it works for you.

Don

[nasadowsk]

I've got Sparton I'm trying to get rolling now - I can't get HV or vertical sweep. No vert is related - this set's vertical section is 'slaved' off the horizontal's boost B+. The horizontal also steals some boost too.

I can't get the 1B3 to light up (I have Bendix that sure can...) and at best, the HV at the CRT suction cup is a rip roaring 4kv

There's at least some drive and the set sounds like it's trying HARD - the horizontal's got a LOUD whine to it. Real loud.

The flyback's not open on the HV section. At the AGC pulse winding, I'm seeing 200v pulses. The yoke's not shorted as best as I can tell.

I'm thinking It could be a shorted damper - was the 6W4 known to be flakey? Otherwise, barring a major problem in the vert section that's loading the boost heavily , I can't seem to find anything that's blindingly obvious.

Naturally, the schematic doesn't show a boost voltage - the main power supply's best is 340 - and it's surely there. So, I'm guessing I should see a boost far higher, but no idea how much.

I'm stuck - the flyback looks fine, but I can tell SOMETHING major happened in this set - the 5U4's filliment was broken when I got it, and looked burnt in a few spots. There's very little besides the sweep that the 5U4 powers (a 5Y3 runs everything else).

I'm stumped here guys I want to think it's not the flyback or yoke, but barring a damper or vert section issue, I'm out of ideas

[polaraman]

What is the model and chassis number of the set. Do you have the part number of the flyback?

polaraman

I think the only problems the 6W4 had was heater-cathode leakage.I
would imagine that the Sparton has a seperate heater winding dedicated
to the 6W4 so the leakage would not be a problem.You could temporarily
sub a 6AX4 or even a 6AU4 in it's place just to see if it is the problem.
The pinout is the same on those tubes.Very loud whine usually means
the horiz. osc is running way off frequency.I would start troubleshooting
there first.Don't forget the flyback depends on resonance with the
yoke to work properly.The entire stage needs to be at resonance
at the proper frequency to get maximum efficency and output
power.Just like the output tubes in a rf linear amplifier.Check for the
usual leaky caps and way off value resistors in the horiz. osc. circuit.
Do you have a scope to look at the horiz. waveform with?If you do not
have one and since you are seriously into fixing old tv's you should get one.
It will make the troubleshooting a whole lot easier.
Regards,
Swanson

[nasadowsk]

Ok - I got scope It's a cow-orker's Tek he's got on extended loan to me. I think that 15,750hz is 63 microseconds and I can get it in that area - my rough look on the scope was in that range. Being a Tek, I can believe it.

The set is a Sparton 5020something, chassis 26SD160. Folder 13, set 128 Photofact FWIW.

They were crafty on this set, not only does the heater float, it's tied to the cathode on one side anyway. It's a dedicted winding - no surprise there.

I can push it on frequency (or close), or way off either way - no dice. The range of the oscilator is quite something, and I can run it down to around 14khz and up to like 18, at least off the DMM's built in counter. I can look closer with the scope though.

I'm going to see if I can disconnect the dead as a doornail vert section just as a check to see if that's killing things - it looks like a simple wire unhook.

I've got a 6W4 comming but I've got a few 6AX4s laying around I can try now.

There's no sub listed for the flyback, only Sparton part PC70010

I'll try to get my image scanner here shortly so I can scan in schematic clips if needed, too.

I'm not ready to condem the flyback - it doesn't get hot much at all.

Thanks for the advice!

I forgot to ask,Is the boost voltage completely gone
or is there something there?Normally it should be
around 500 to 600 volts.The vert. sweep section
draws very little current from the boost supply.
Only the vert. osc. uses it.It should also go to
the focus grid on the crt.I think that set is too
old to have it going to the sound detector.The
yoke should have a mica damping cap across
at least one of the horiz. windings.The cap
will probably look like a domino.When this cap
shorts out it will cause the symptoms that you
are seeing.Sometimes you will have enough hv
to see an image on the screen that looks like
a Dorito chip(keystoning).If you really want
to get mental,you can disconnect one side
of the 1B3 heater and carefully hook up a
D battery to the 1B3 heater.Make sure you
float that battery real good above ground!
This is not for the faint hearted.This should get
you more hv to the crt so maybe you can see
some form of raster.
Regards,
Swanson

[polaraman]

PC70010 is a Thordarson FLY7A. Let me know if you need one. I know where to get my paws on one.

polaraman

[nasadowsk]

Ok, I disconnected the vert section from the boost - this set hangs the vert osc AND output off the boost, amazingly.

There was a 20mfd cap that was holding things down, but even still, my boost is only at 320V or so. Enough to light the 1B3 and get a dim fuzzy line. I temp wound a few extra turns (ok, 1 extra) on the flyback with some 25kv wire for the 1b3's filiment. Seems safer than a D cell

I'm getting 9kv now. It seems the horz section's weak, but it's on frequency best I can tell - I ran it up/down until I got max HV. I see the mica on the yoke but no value. I'm gonna pull the tube to clean it in a while anyway, so I'll look at it then.

Am I right in that I can't check for the dreaded keystone without vert sweep?

The flyback *still* isn't getting warm, in fact it feels ice cold after a few min of operation and then powering off. I see a .25 that runs between the 340 supply and the damper's cathode - that looks like a good suspect.

I'm not sure where the boost should be though. I'm guessing above 340 (else no reason to have it!) but the .25 is rated 600V and the 20mfd I think is a 450 unit (but has a 10k resistor feeding it). My out of ass number is in the 400's...

I think I see what's going on here, almost. I'm gonna replace all the caps soon anyway, but I wanna see if I can tackle this boost issue now cause it's driving me NUTS.

I'm not even close to sticking a fork in the flyback, but I'll keep in mind you can probbably find one, polaraman... I think the yoke's a bigger suspect though I'm not convinced it's bad either yet...