Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 65655 times)

SAUL BRITTO · « **Reply #125 on:** February 16, 2016, 01:43:10 am »

courageously I disconnected the cable from the anode and the earth and it continues measuring -1832V.

rf+tech · « **Reply #126 on:** February 16, 2016, 02:06:34 am »

This is interesting. My best guess is someone tried to "fix" the original problem shown in your video by adjusting the HV.

Always, always mark any control before turning, so it can be put back in the original place.

Now it is right to try HV ADJ R38, to see if the HV can be increased to -2295 Volts. If it only goes less, tell me what the measurement is.

Of course, reconnect the tripler first.

SAUL BRITTO · « **Reply #127 on:** February 16, 2016, 02:16:48 am »

Online rf+tech really,all the electrolityc capacitors was replaced in all the PS. Power oscillator is with strange behavior on DMM.
I will back with more.

SAUL BRITTO · « **Reply #128 on:** February 16, 2016, 02:37:30 am »

On the HV ADJ I can -1922 maximum on the cathode test point.

rf+tech · « **Reply #129 on:** February 16, 2016, 02:47:31 am »

Okay, I would put this back to -1832 Volts.

How dirty is the pc board around CR3, C6, R11, C7, R12 and all components that connect to the cathode circuit? Could there be leakage around these parts?

Look closely at R13, 30 MOhm resistor. If there is electrical leakage across R13, the HV will be low and not adjust. Better to remove R13 and measure it.

SAUL BRITTO · « **Reply #130 on:** February 16, 2016, 02:51:16 am »

I found a resistor of 560 Ohms measuring 598 Ohms,just need to know what is it's connection. I don't know ifit is R18 or R19 on the pin 6 of IC U1.Or that one connected to CR3 it seems R11.

SAUL BRITTO · « **Reply #131 on:** February 16, 2016, 03:06:44 am »

Quote from: rf+tech on February 16, 2016, 02:47:31 am

Okay, I would put this back to -1832 Volts.

How dirty is the pc board around CR3, C6, R11, C7, R12 and all components that connect to the cathode circuit? Could there be leakage around these parts?

Look closely at R13, 30 MOhm resistor. If there is electrical leakage across R13, the HV will be low and not adjust. Better to remove R13 and measure it.

The board is very clean in all areas.

SAUL BRITTO · « **Reply #132 on:** February 16, 2016, 03:15:16 am »

R13 is 29.99M

rf+tech · « **Reply #133 on:** February 16, 2016, 03:36:46 am »

Good information on R13 and very clean pc board.

The voltage on U1 pin 6 may tell something.

SAUL BRITTO · « **Reply #134 on:** February 16, 2016, 03:56:56 am »

Quote from: rf+tech on February 16, 2016, 03:36:46 am

Good information on R13 and very clean pc board.

The voltage on U1 pin 6 may tell something.

R21 1M on pin 6 is more than 20% altered,it measures 1.260M

SAUL BRITTO · « **Reply #135 on:** February 16, 2016, 04:24:23 am »

rf+tech at the pin 6 of U1 i have -2.6V.

rf+tech · « **Reply #136 on:** February 16, 2016, 02:46:59 pm »

Quote

rf+tech at the pin 6 of U1 i have -2.6V.

This is good and shows that the feedback loop is regulating properly.

But, this tells me the RCA HV probe calibration may be not be good.

Since U1 pin 2 voltage is zero (returned to ground via R24), the voltage at U1 pin 3 must also be zero.

Therefore, the two currents through the resistor dividers must also sum to zero.

-1832 Volts / 29.3 MOhms = -62.5 uA, derived from the -HV

If the curent through R13 is correct, then the current through R36, R38 and R37 must also be the same magnitude, but opposite polarity. So we solve to find the total resistance required for +62.5 uA from +15 Volts:

15 Volts / 62.5 uA = 240 kOhms

The total resistance of R36, R38 and R37:

182 kOhms + 10 kOhms + 9.09 kOhms = 201 kOhms, with R38 at maximum
201 kOhms - 5 kOhms = 196 kOhms R38 mid control

Now we calculate the current from +15 Volts:

+15 Volts / 196 kOhms = +76.5 uA

This must be balanced by the same current of the opposite polarity from -HV:

-76.5 uA * 29.3 MOhms = -2242 Volts

This shows that the -HV must really be -2242 Volts and shows that the RCA HV probe cannot be trusted.

SAUL BRITTO · « **Reply #137 on:** February 16, 2016, 04:37:46 pm »

rf+tech Ok. That probe is really bad then.Despite I've measured some other voltages with it and was ok,voltages of 220V or below. In this case what do you think that can be the defect?

SAUL BRITTO · « **Reply #138 on:** February 16, 2016, 04:49:16 pm »

I was thinking about an overload caused by any things as tube,can it be possible?

SAUL BRITTO · « **Reply #139 on:** February 16, 2016, 04:51:52 pm »

But with this news I am a bit confuse.If the cathode V is right maybe the anode V is right too.

SAUL BRITTO · « **Reply #140 on:** February 16, 2016, 04:59:19 pm »

rf+tech I do have here some VTVM in good condictions it can measure 1500V and it's input impedance is 11M in DC skill. Can I double or duplicate the impedance with another resistor of 11M in series? I think there is a way to resolve this.Of coarse: My hand the probe will not see

SAUL BRITTO · « **Reply #141 on:** February 16, 2016, 05:42:56 pm »

I did this question before I don't know if you remember: It's normal the 15V Unreg be 24V? It's 24V on the PS board and HV board.

rf+tech · « **Reply #142 on:** February 16, 2016, 11:20:33 pm »

Saul,

I'll try to answer some things.

Quote

It's normal the 15V Unreg be 24V? It's 24V on the PS board and HV board.

Depending on your mains voltage, if it is higher than normal, then yes, the unregulated 15 will be high. This really does not affect the HV output, U1 compensates for any deviation on the HV output, or the input, by adjusting the DC voltage to the base of the power oscillator transistor. Since U1 is the regulator, the 15 volts to the power oscillator does not need to be regulated.

Quote

But with this news I am a bit confuse.If the cathode V is right maybe the anode V is right too.

More math: The HV probe error is 2242 - 1832 = 410 Volts low.

410 / 2242 = 18.3%

2800 * 18.3% = 512 Volts

2800 + 512 = 3312 Volts from the tripler

3312 / 2 = 1656 Volts into the tripler

1656 * 3 = 4968 Volts out of the tripler.

4956 Volts is "close enough" to 5 kV, yes?

Looks like the tripler is only a doubler, it is defective.

Quote

That probe is really bad then.Despite I've measured some other voltages with it and was ok,voltages of 220V or below. In this case what do you think that can be the defect?

Defective may be a bit strong. 18.3% error is excessive. The probe reading ok at 220 V may indicate the high value resistor inside has some voltage-dependent non-linearity. If this is true then even the 18.3% figure may in fact need to be increased with higher voltages, such as at 2.8 kV. Post a photo of the probe and how it is connected to your meter. I am guessing this probe is a color TV HV probe and it is being used with a DVM? If this is true, the probe is intended to work into an 11 MOhm VTVM, Using this probe with a 10 MOhm DVM introduces an error immediately. But I am guessing because I do not know any of the important details to make a good conclusion.

I have experience working with feedback resistances from 2 GOhms (2*10^9) to 2 TOhms (2*10^12). This is an art all of its own, just as with RF and microwaves. There may be some hope for your probe, but I need to see it first and know the details about it.

RF+ Tech

SAUL BRITTO · « **Reply #143 on:** February 17, 2016, 01:28:54 am »

Hi rf+tech Good night. Thank you. Yes looks like the probe was not so good seems it was a bad contact and really I WAS USING A DMM! wow! But I has some VTVM like the heathkit IM-10 RCA-Voltohmyst etc. The probe is an old RCA WG-297

SAUL BRITTO · « **Reply #144 on:** February 17, 2016, 02:11:46 am »

rf+tech can I measure with a VTVM directly with the HV probe,without that 11M one?

SAUL BRITTO · « **Reply #145 on:** February 17, 2016, 02:40:31 am »

rf+tech Using a VTVM on the post tripler I can measure 5kV now

SAUL BRITTO · « **Reply #146 on:** February 17, 2016, 02:46:38 am »

Including I can hear a noise of HV coming from tripler when I touch with the HV probe inside the connector even when the scope is off.

SAUL BRITTO · « **Reply #147 on:** February 17, 2016, 03:12:53 am »

Where is marked I can measure exactly 5kV,can this be a gridd not polarized or tha HV post-tripler cable with some kind of problem? Did you see any thing like this? Times ago I remember not exactly if in same way a problem on HV connection of anode of a tv tube, the contact was oxidized.

rf+tech · « **Reply #148 on:** February 17, 2016, 03:30:34 am »

Saul,

Quote

rf+tech

My memory is fuzzy, the Heathkit and RCA VTVMs were 10 MOhm at the front panel connector and have an additional 1 MOhm resistor in the probe body. The over-all input resistance is 11 Megohms.

The RCA WV-98A manual states:

Quote

DC voltages as high as 50,000 Volts can be measured directly by the WV-98A when the WG-289 High-Voltage Probe is used. The probe uses the WG-206 Multiplier Resistor, having a value of 1090 megohms, to present an overall voltmeter input resistance of 1100 megohms.

Quote

Using a VTVM on the post tripler I can measure 5kV now

Well, this is good news, indeed.

And you have learned that improper results and incorrect conclusions occur when using HV probes with the wrong meters.
You may ask yourself "how did I know your mistake?"

Quote

I can hear a noise of HV coming from tripler when I touch with the HV probe inside the connector even when the scope is off.

There may not be a bleeder resistor present in the tripler to discharge the output filter capacitor. When the probe tip approaches the connector, the air will ionize and a small sizzle noise will be heard. Leaving the probe in contact should show the voltage falling, as the probe discharges the output capacitor. Perfectly normal.

Quote

can this be a gridd not polarized

I think what you are saying is that the grid may be in cutoff, so there is no beam from the cathode. This is what I believe to be the *next* problem. I do not think oxidation of the HV anode is the problem. Really, how can a little oxidation stop 5000 Volts?

Next, the cathode and grid voltages need to be measured, to see if the CRT is cutoff. This should be done with the HV probe and VTVM because both are at high voltages.

RF+ Tech

SAUL BRITTO · « **Reply #149 on:** February 17, 2016, 03:36:53 am »

rf+tech I can read -2450V on the TP5 of gridd,is it ok? I do not see at the schematic the indication of voltage for that.


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Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 65655 times)

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