Rca ctc-5

[DaveWM]

ah thanks Steve, One of them was missing the little metal clip, but I am pretty sure I can just pull it with some needle nose, the other one moved easy.

[DaveWM]

well the one missing the little clip is really in there tight, maybe that is why the clip is missing....

I don't want to damage the wood by a half ass approach with the needle nose so I will have to see if I can machine a tool to fit under the head of the pin and allow me to pull down. Dang so close.

[DaveWM]

got it with some hemostats, bezel out, glass out, magnets fully CCW so tomorrow I can do a degauss, thanks again to Steve for the hints. I hit the little mech with some lube so it should go easier next time.

[Penthode]

Originally Posted by Penthode

My thought is the added reactance to the cathode circuit (5ufd cap and wiring) may hinder the ability for th HO tube to be turned off at the end of forward scan. This would reduce the pulse height and inrease the effective series impedance of the HV supply. This is only my thought and I'd next like to explore.

I tried simply grounding the cathode. The was little difference. Perhaps a minor improvement but this was not the crux of the problem.

The CTC5 chassis seems to have HV issues. As Nick pointed out, it may have to do with the B supply. I noticed that as the brightness is advanced to blooming, the vertical scan increases (as would be expected with drooping HV) but the horizontal scan decreases. This suggests the horizontal sweep is has problems.

However, the HO Tube cathode current is low after I minimized it with the adjustment of the efficiency coil. I read elsewhere the that on another chassis the instructions called for a minimum adjustment and then the core was adjusted further into the coil to raise the HO Tube current to the correct value.

Any thoughts on this?

[DaveWM]

I will have to check the cathode current again. I did not check the line voltage, but the fly is getting pretty hot, and I do not seem to have any HV blooming (but I do have the video out contrast mod in). My concern is the heat in the fly HV tire. what currents are you seeing? are you checking the AC in and setting at 117v?

I bought several HO tubes to compare with.

[miniman82]

My opinion at this point is that the flyback is wound in such a way that it is somehow current limited. In other words once you get to a certain point, perhaps the core begins to saturate. Can't do anything about that, because messing with the core (or its gaps) changes the magnetic properties of the entire transformer in possibly undesireable ways. Lets remember the whole transformer is a precision engineered part, especially with regard to frequency. Mess up one thing in any of the windings, and it goes haywire.

Or maybe the primary simply has too much resistance, since the amount of current supplied is influenced by the resistance of the primary. More primary resistance means less current when HO tube conducts, which means a weaker magnetic field and corresponding flyback pulse when the stage suddenly cuts off. Obvisouly, there is nothing short of rewinding the flyback that can cure it if that's the problem either. Folsom is the only one I know of that's tried (re?)winding one, and the results were less than satisfactory.

The secondary might not have enough turns in it, but that seems unlikely since it does in fact produce the required voltage at low brightness. That tells me the problem has more to do with the supply side of things, since the problem doesn't manifest till more demand is placed on it. It's not that there's a lack of HV per se, 21-22kv would be just fine for a 21" CRT. The problem is lack of adequite current delivery, which is why HV sags when brightness approaches normal viewing intensity- the beam current required by the picture tube outstrips the flybacks ability to produce it, so the picture blooms and lacks brightness. The situation is analagous to power grid sagging during the hot summer months, when everyone is running their AC units. Too much demand, and line voltage sags. The fact that the horizontal scan decreases in width is another clue, pointing to a lack in magnetic flux to power the scan coils. Again, I can only think of too much primary resistance being the cause.

If I'm right, about the only thing that's likely to make things better is an increase in B+ voltage. Since you can't get a load with less resistance, you can always try driving a larger waveform into it in an attempt to extract more power on the other side. This would likely require vast changes to the LV power supply, including a switch to solid state rectifiers and juggling the dropping resistors to all circuits besides horizontal scanning. Careful attention needs to be paid to cathode current in this case, since a small change in input voltage can push current way overboard in a hurry.

A good test would be to put the set on a variac, with the brightness set so that the picture just starts to bloom. At this point, you should be able to read a slight drop in HV. Try increasing the AC input voltage to say 125 from 117, and see if HV tries to come up at all. If it does, there may be something to investigate.

[Penthode]

Originally Posted by DaveWM

I will have to check the cathode current again. I did not check the line voltage, but the fly is getting pretty hot, and I do not seem to have any HV blooming (but I do have the video out contrast mod in). My concern is the heat in the fly HV tire. what currents are you seeing? are you checking the AC in and setting at 117v?

I bought several HO tubes to compare with.

I was measuring about 210mA with the efficiency coil adjusted to minimize current. I believe the current should be a bit higher than this. The schematic inicates there should be 7 volts on the cathode with respect to ground. I measure 5.5 volts.

My thought is that adjusting the efficiency coil for slightly higher current will result in and increase to the unregulated HV. However, as Nick points out, the problem may relate to a limitation of the HV transformer.

I found the 6BK4 regulator is not really regulating the HV. This appears because the HV unregulated (with the 6BK4 cap off) is only about 23kV. When increasing the brightness hence the beam current in the CRT, the HV sags below 23kV.

Here is what I'd like to test: if the focus voltage does not track with the HV eg the focus voltage remain constant with the HV dropping, this means it is an unregulated HV supply issue. That is, the HV transformer is okay but the pulse to the HV rectifier is not of sufficient amplitude. If however the focus voltage also sags with the HV, then the HV transformer is likely at fault, as Nick suggests.

As for raising the B supply with a variac on the 117vac side, if the HV regulates better, that would suggest the former and not the latter.

Do you agree?

[DaveWM]

I was at 195ma, tweeked it just a bit to get to 190ma (simpson 260 VOM), solid 21kv with no noticeable flux on the HV meter (simpson 248 hv probe).

I am going to try some tube swaps just to see the effect with the current setup.


I don't have my good VTVM handy to get a control grid voltage reading on the HO tube, but will use my DMM and see what I get. Then will replace the 82k with a 120k in that voltage divider on the osc plate just to see if that will effect the drive/bias at that control grid (and perhaps further reduce the current thru the HOT). As long as I get good regulated HV I will see how low I can go.

I suspect some of the variations we are seeing is maybe do to the calibration of the test equipment?

[DaveWM]

matix balance pot (deluxe chassis) I get that is seems like a way to balance the gain of the X and Y demodulator tubes, but I could not find anything on how to set this up.

[DaveWM]

wow, new tube, gets me to 180ma and still good 21kv

yes pics coming soon

[old_tv_nut]

Originally Posted by Penthode

I found the 6BK4 regulator is not really regulating the HV. This appears because the HV unregulated (with the 6BK4 cap off) is only about 23kV.

The 6BK4 regulator does not work by regulating the voltage. It works by regulating the total current load on the high voltage. It measures this current at the bottom of the flyback. That is, when the CRT draws less current (dark picture), the 6BK4 draws more current from the high voltage anode lead, keeping the load constant. Therefore, the load (and the voltage) should be constant, until the picture is so bright that the 6BK4 has reduced to zero current. If the CRT draw is raised higher, then the draw on the high voltage output does increase and the voltage drops.

So: if operation is proper, the high voltage should remain constant as brightness is increased up to a certain point, and then the voltage should start to droop as you try to get more beam current. If the voltage is drooping gradually as you increase brightness from black, the regulator is not drawing compensating current like it is supposed to.

[DaveWM]

hmmm while getting set the pic starts slowly bloom check the current 230ma and climbing, yikes, tried another NOS tube (elmenco branded) and back to 180ma, going to have to watch this.

[old_tv_nut]

Originally Posted by DaveWM

matix balance pot (deluxe chassis) I get that is seems like a way to balance the gain of the X and Y demodulator tubes, but I could not find anything on how to set this up.

This appears to set the relative gain of X and Z. The effect should be most visible on saturated colors, like color bars. This will affect all the color bars in some way. For example, one direction should make magenta more reddish and the other direction should make magenta more bluish.

[miniman82]

Originally Posted by Penthode

I found the 6BK4 regulator is not really regulating the HV. This appears because the HV unregulated (with the 6BK4 cap off) is only about 23kV. When increasing the brightness hence the beam current in the CRT, the HV sags below 23kV.

As Wayne pointed out, I think you're misunderstanding how the 6BK4 does what it does. It doesn't tell the flyback to 'make' more power, which is what I understand when I read your post, it's only there to provide a constant load on the flyback. The flyback always operates wide open, only thing that can drag it down is the CRT or the 6BK4. When beam current at the CRT is low (dark images), the 6BK4 picks up the slack so to speak. When beam current at the CRT is high (bright images), the 6BK4 will be cut off. In this way the load is always constant on the anode, but if you ask me it's a pretty crude and wasteful way to do things.

Here is what I'd like to test: if the focus voltage does not track with the HV eg the focus voltage remain constant with the HV dropping, this means it is an unregulated HV supply issue.

Regulation isn't the issue here, since the 6BK4 only absorbs 'excess' HV when the CRT doesn't need it. The issue is lack of sufficient headroom in the HV supply to begin with, which explains why the chassis works the same wether the regulator tube is installed or not. It made little to no difference to the images on screen for me, there's just not enough HV to make a difference. Other sets will overshoot by a lot without the regulator, my CTC-9 for example would hit nearly 32kv unregulated. In that situation there would be a difference in images on screen, but not the anemic CTC-5.


Do the variac test, see what happens.

[DaveWM]

I disconnected my shunt and the HV shot up to 25kv with a MUCH brighter screen, still holding at 180ma... So I will mess with the HV setup and regulation tomorrow.

I just realized there is no HV pot, so I will start with some tube swapping on the 6BK4