Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 65784 times)

SAUL BRITTO · « **Reply #75 on:** February 14, 2016, 08:30:12 pm »

Is about this your comment?When you say: Does the vertical imbalance exist at the output of A3A1?
If so, have you disconnected the A3 preamp output from the A17 input and checked the A3 output to see if the imbalance is still present?
If so, have you checked to see if some input to the A3A1 is forcing the output out of balance?
If the A3A1 is defective and a replacement is not economical, why bother fixing the HV?
What good is a scope with working HV if the vertical amp is beyond repair and cannot bring the beam on screen?

SAUL BRITTO · « **Reply #76 on:** February 14, 2016, 08:33:46 pm »

In CR29 and CR30 I found a low resistence,where is marked 12V I get a small variation changing Position.In one channel is around 0.3 V on the other almost nothing it's stoped in 12,1V

SAUL BRITTO · « **Reply #77 on:** February 14, 2016, 09:08:31 pm »

Yes on the anode of diode CR27 I get 14.97V and on the collector of Q19 14.2V. 14.97-0.7 of diode= 14.2 ok?
On the base 12.7 and emmiter 12.01. 12.7-0.6 (base emmiter)= 12.01. Correct?
This is that I get here.

SAUL BRITTO · « **Reply #78 on:** February 14, 2016, 09:10:25 pm »

If you are not so experienced I think it would here turn a Babylonian tower

SAUL BRITTO · « **Reply #79 on:** February 14, 2016, 09:19:22 pm »

You remember,that one in short? Was Q17, +12 -12 are ok.I replaced Q17 with an original one.

rf+tech · « **Reply #80 on:** February 14, 2016, 09:47:13 pm »

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In CR29 and CR30 I found a low resistence,where is marked 12V I get a small variation changing Position.In one channel is around 0.3 V on the other almost nothing it's stoped in 12,1V

I don't understand about these voltage measurements - from where to where?

The back to back diodes prevent the *differential* voltage from exceeding about 1.2 Volts - as measured directly across the two outputs. When this preamp is working properly, with a meter connected across the two outputs, the vertical position control should swing this voltage from +0.6 Volts, through zero Volts and to -0.6 Volts. By adjusting the vertical position control for zero volts - balanced output - the beam will be almost exactly centered.

If the series combination of R64, R65 and R66 in parallel with the diodes is close only several hundred Ohms, the diode test reading will not be reliable.

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You remeber,that one in short? Was Q17, +12 -12 are ok.I replaced Q17 for an original one.

This is where we were Babel-ing on about Q17 in two different places. I was looking at the A17 and you were looking at the A3.

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If you are not so experienced I think it would here turn a Babylonian tower

I can Babel with the best, I do this professionally in my own business - and repair things too.

RF+ Tech

SAUL BRITTO · « **Reply #81 on:** February 14, 2016, 10:00:59 pm »

Measuring in CR 29 and CR30 Vertical output, I get a variation of -403mV to +500mV,in both channels.

rf+tech · « **Reply #82 on:** February 14, 2016, 11:47:30 pm »

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Measuring in CR 29 and CR30 Vertical output, I get a variation of -403mV to +500mV,in both channels.

That is good to see. Set this to close to zero Volts, reconnect the A3 output to the A17 input and let's measure the voltage from the collector of Q17 to the collector of Q18 on the A17. It should be close to zero Volts here and move positive/negative with the vertical position control. Set the vertical position back to zero Volts.

Then proceed to measure from the cathode of VR2 to the cathode of VR3. The same positive/negative swing should be measured here, but with much greater voltage range. Again, set the vertical position control for zero Volts.

Each point where the voltage can be set to zero, the two halves of the differential amplifier will be balanced. When the output across the deflection plates can be set to zero Volts, we will know the beam will be very close to the center of the screen.

Let's see how this goes.

SAUL BRITTO · « **Reply #83 on:** February 15, 2016, 12:25:57 am »

rf+tech,when I put a probe in X1 to channel B Using CAL 1V at .1V scale I get around 0.6 Volts pp, in other scope, after delay line on the input of that power vertical circuit,when I put position full left it shows 0.2V at the other scope and when I put full right it goes to 0.2V too.

rf+tech · « **Reply #84 on:** February 15, 2016, 12:45:55 am »

Saul,

For now, it is not important about the difference between the two scopes. Our concern is only with no signal, will the difference adjust to zero?.

Set the input coupling to GND and continue with the procedure above.

Once we know that the differential output can be balanced to zero Volts all the way to the deflection plates, then we can evaluate this difference between the two scopes.

RF+ Tech

SAUL BRITTO · « **Reply #85 on:** February 15, 2016, 12:52:40 am »

Are that diodes a kind of 1n4148? it shows 13.8 to -36.4. I did all that tests and very near to 0V, i can but, it's very critical I just can 0.03V,ok?

SAUL BRITTO · « **Reply #86 on:** February 15, 2016, 12:56:44 am »

rf+tech> Ok, it was the same way with GND connected,I get 0V and the other voltages above.

SAUL BRITTO · « **Reply #87 on:** February 15, 2016, 01:10:57 am »

Guys please sorry for my crazy english!

rf+tech · « **Reply #88 on:** February 15, 2016, 01:45:40 am »

Saul, your English is not a problem. My problem is guessing about missing details to be certain we are talking about the same thing, the same place and the same type of measurement.

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If that diodes are as 1n4148 it shows 13.8 to -36.4.

Guessing that 13.8 to -36.4 is from one diode to ground?

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I did all that tests and very near to 0V,

So the difference across the deflection plates can be set to zero, then the entire vertical amplifier is working okay, except maybe the gain may be wrong.

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it's very critical it was 0.03,ok?

0.03 Volts is close enough to zero and that is very good.

So you are ready for the next problem, yes?

SAUL BRITTO · « **Reply #89 on:** February 15, 2016, 01:50:13 am »

OH sorry it was to measure cathode to ground? I did it cathode to cathode,but I can do it to ground.Please confirm that I did it wrong.

SAUL BRITTO · « **Reply #90 on:** February 15, 2016, 01:56:36 am »

Measuring to ground it is 43.9 to 70 Volts in one diode. The other was 32.7 to 50.0 Volts

SAUL BRITTO · « **Reply #91 on:** February 15, 2016, 02:03:32 am »

If I made it wrong I interpreted you text in bad way,when you said: That is good to see. Set this to close to zero Volts, reconnect the A3 output to the A17 input and let's measure the voltage from the collector of Q17 to the collector of Q18 on the A17. It should be close to zero Volts here and move positive/negative with the vertical position control. Set the vertical position back to zero Volts.

Then proceed to measure from the cathode of VR2 to the cathode of VR3. The same positive/negative swing should be measured here, but with much greater voltage range. Again, set the vertical position control for zero Volts.

Each point where the voltage can be set to zero, the two halves of the differential amplifier will be balanced. When the output across the deflection plates can be set to zero Volts, we will know the beam will be very close to the center of the screen.

Let's see how this goes.

"let's measure the voltage from the collector of Q17 "to the collector of Q18 on the A17."

It was to measure the difference between the two collectors,the two probes on the collectors?

rf+tech · « **Reply #92 on:** February 15, 2016, 02:13:24 am »

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OH sorry it was to measure cathode to ground? I did it cathode to cathode,but I can do it to ground.Please confirm that I did it wrong.

Thank you for clarifying. Both types of measurement are valid.

Maybe my use of the word differential is part of the problem. Push-pull may be easier understand. The deflection plates push and pull the beam, depending on the voltage across the deflection plates and the polarity of one plate to the other plate. The actual voltage from one plate to ground is of lesser importance.

Here is a thought experiment: with only one meter, measure the voltage from one deflection plate to ground. Next measure the other plate to ground. Then adjust the vertical center control, measure one plate voltage to ground, then the other, back and forth until both plates are at the same voltage. Now measure the voltage across the two plates and it should be close to zero Volts. This is the condition required for the beam to be in the middle of the screen.

See how much easier it is to simply place the meter across both sides of a push-pull amplifier and adjust the vertical center control for zero Volts? This is why I am saying the voltage to ground is not important. What is most important is the difference between the two sides.

--

Okay, I am very slow to write my thoughts.

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It was to measure the difference between the two collectors,the two probes on the collectors?

Yes, this is the correct way. Please, if there is anything you do not understand, ask. I do not want you to feel frustrated.

SAUL BRITTO · « **Reply #93 on:** February 15, 2016, 02:23:45 am »

Thank you rf+tech! When you says: adjust de vertical center,you want to say that adjust ,on the board, not the position,right?

SAUL BRITTO · « **Reply #94 on:** February 15, 2016, 02:31:37 am »

I see on the board polarity balance... This is on the shield cover with drawings.

rf+tech · « **Reply #95 on:** February 15, 2016, 02:34:30 am »

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When you say adjust de vertical center,you want to say that adjust on the board, not the position,right?

No, adjust the "vertical position" control on the front panel. Think of how the vertical position control works on a good scope. The beam moves up or down with the vertical position control.

The adjustments on the board should not be altered yet. This may add more confusion.

Please allow me more time to write.

SAUL BRITTO · « **Reply #96 on:** February 15, 2016, 02:36:29 am »

Oh my friend you have all the time of the word! Relax. Maybe I am the nervous here !

SAUL BRITTO · « **Reply #97 on:** February 15, 2016, 02:50:43 am »

rf+tech > near to zero I can, around 0.053mV.

SAUL BRITTO · « **Reply #98 on:** February 15, 2016, 02:53:27 am »

I am measuring with a keysight DMM but has a fluke 179 here...

SAUL BRITTO · « **Reply #99 on:** February 15, 2016, 02:58:30 am »

Ok I did the work. I can measure 0.0V on the two channels but with de position a bit to left, around 20%.


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Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 65784 times)

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