Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 68756 times)

SAUL BRITTO · « **Reply #175 on:** February 18, 2016, 01:53:30 am »

Wow measuring the R45 I see 6.2 mV one lead to the other.Is this you want?

SAUL BRITTO · « **Reply #176 on:** February 18, 2016, 02:01:17 am »

6.2mV/320 Ohms (270) will this led work with this current?

SAUL BRITTO · « **Reply #177 on:** February 18, 2016, 02:05:59 am »

Is it 19uA or, am I confused?

rf+tech · « **Reply #178 on:** February 18, 2016, 02:09:13 am »

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Seems the resistor 2700 ohms on the normal position do not works, right? It is altered to 3.200 ohms too.

Is this really 3200 Ohms (3.2 kOhms)? Maybe it is time to check your meter with some 1% precision resistors?

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C18 ... seems a mica cap.

22 uF in mica will be about as large as the entire HV board. The parts list has the letters "TA" which indicates this is a Tantalum capacitor.

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Across R45 I read around 4.2 mV

Okay, there is no current flowing through the LED in U2.
.0042 Volts / 270 Ohms = 15.5 uA. The LED should be off.
.0062 Volts / 320 Ohms = 19.3 uA.
The LED would need maybe 10 mA to turn on U2.

Move the Normal/Deep Erase to Deep Erase. This will turn on Q4 and the LED in U2. Now measure the voltage across R45.

With the scope turned off, some tests on the HV side of U2. Check zener diode VR3 (zener diodes like to fail shorted), diode-test Q5 for shorted and diode test U2 pins 5 to 6 for shorted.

Oh, yes, verify sweep at the deflection plates. Using another scope, check both horizontal deflection plates for the sweep ramp.

SAUL BRITTO · « **Reply #179 on:** February 18, 2016, 02:18:52 am »

The voltage over C18 is -11.2 V

SAUL BRITTO · « **Reply #180 on:** February 18, 2016, 02:20:47 am »

Is this really 3200 Ohms (3.2 kOhms)? Maybe it is time to check your meter with some 1% precision resistors?

no I said 320 Ohms (270).

SAUL BRITTO · « **Reply #181 on:** February 18, 2016, 02:28:48 am »

With deep erase on I read 0.2mV.

rf+tech · « **Reply #182 on:** February 18, 2016, 03:06:09 am »

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The voltage over C18 is -11.2 V

This looks okay. With very low current, VR4 will be less than 11 Volts. 11.2 Volts minus 0.6 Volts for Q4 base-emitter = 10.6 Volts across VR4. I did not look closely at this.

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no I said 320 Ohms (270).

Yes, but read again what you wrote earlier:

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Seems the resistor 2700 ohms on the normal position do not works, right? It is altered to 3.200 ohms too.

2700 Ohms implies R43
270 Ohms implies R45.
Who is

So please explain "3.200 Ohms."
3.2 Ohms is 2696.8 Ohms less than 2700 Ohms.
I have to guess is this really Ohms or kOhms? I think this is 3200 Ohms (3.2 kOhms).

If your meter measures 270 Ohms as 320 Ohms, 15.6% higher, and measures 2700 Ohms as 3200 Ohms, also 15.6% higher --- then your meter may not be accurate. The meter needs to be checked with known good 1% resistors. It would be very unusual for all resistors in this HP scope to be 15.6% high.

Quote

With deep erase on I read 0.2mV.

Hmm. This is only 625 uA, I don't think U2 can turn on with this low current.
What part number appears on U2? The parts list shows 1990-0607.
What are the emitter, base and collector voltages on Q4, in normal and deep erase?

SAUL BRITTO · « **Reply #183 on:** February 18, 2016, 03:19:07 am »

I measured with three meters and was all near basicly the same thing.The resitor of 2700 Omhs is 3.000 Right now
Maybe because I removed U2 and replaced it the pins was very dirty,I don't know...
I did look if was a short in collector and emmiter of U2 but, no.

SAUL BRITTO · « **Reply #184 on:** February 18, 2016, 03:27:18 am »

The part number is 199006077812

SAUL BRITTO · « **Reply #185 on:** February 18, 2016, 03:34:45 am »

Normal

Base= -11.23V
Emmiter= -10.63V
Collector= -10.64V

Deep erase

Base= -0.6
Collector= -13.37
Emitter= -3.7

SAUL BRITTO · « **Reply #186 on:** February 18, 2016, 03:52:34 am »

Man! I think this resistor of 68K don't smell good...

SAUL BRITTO · « **Reply #187 on:** February 18, 2016, 04:07:24 am »

The 68k is 76.6k Omhs

rf+tech · « **Reply #188 on:** February 18, 2016, 04:49:19 am »

The voltages on Q4 show that my interpretation of Q4 on/off is reversed.

These old HP schematics where transistors are drawn both "right side up" and "up side down" always make my head hurt.

The most common convention is to place the transistor terminal, emitter of PNP and collector of NPN at the top where positive voltage is also at the top. Negative voltages and transistor terminals connected to negative voltages are drawn lower. Here is an example where PNP transistors are drawn two different ways in the same schematic. Crazy. This just makes it harder to understand the circuit.

After drawing this circuit on paper the "proper" way, it is obvious at a glance how Q4 works. When the scope is off, the -15 Volt supply is at zero. C18 discharges through R44. When switch S1 is in "normal" and the scope is first turned on, C18 looks like a short and holds Q4 off. R44 charges C18 with current toward -15 Volts. When the voltage on C18 exceeds VR4 plus the base-emitter drop of Q4, about -11.2 Volts, Q4 turns on.

When "deep erase" connects R45 to ground through switch S1, C18 is discharged and Q4 is turned off.

In "normal" with Q4 turned on, current through R45 turns on the LED in U2 and the output transistor of U2 also turns on. This action turns on Q5 and shorts VR3.

VR3 causes the CRT cathode to be 100 Volts more positive than the grid, the CRT is more deeply cutoff. This protects the CRT from burning when the scope is turned on and turned off, or when electric power is lost momentarily and comes back in very quickly.

After a short delay by C18 and R44, the protection is released.

Based on all the voltage measurements, I think the problem is the LED inside U2 has failed open circuit. The output transistor inside U2 cannot turn on and the extra 100 Volts keeps the CRT deeply cutoff.

The single most valuable measurement is the voltage drop across R45 (270 Ohms). Using a 4N25 datasheet as a reference, 10 mA of LED forward current is used to describe the transfer characteristics. Data found for U2 part number 1990-0607 is too minimal to be 100% conclusive, but 625 uA seems not sufficient to turn on U2.

4N25 is not suitable for replacing U2. A good parametric search is necessary to ensure a replacement capable of withstanding 120 volts across the emitter-collector of the output and 4 kV across the input side to the output side.

You indicated that U2 is in a socket, do you know how to test U2 out of the circuit?

SAUL BRITTO · « **Reply #189 on:** February 18, 2016, 04:51:07 am »

rf+tech I measured some resistors of 1% 1M,100K,10K and the fluke is ok.

SAUL BRITTO · « **Reply #190 on:** February 18, 2016, 05:01:40 am »

This troubleshooting is better than fix the scope

. Yes I know how to test this LED.
Despite I have another CI of a HP1740A just don't know if it's the same.

SAUL BRITTO · « **Reply #191 on:** February 18, 2016, 05:10:59 am »

OOps the 1740A do not have this CI. Fail.

SAUL BRITTO · « **Reply #192 on:** February 18, 2016, 05:15:15 am »

I have a PS of a tektronix TDS320 tha has two CI Like this one,just don't know the electric feature.

SAUL BRITTO · « **Reply #193 on:** February 18, 2016, 06:11:30 am »

rf+tech how many volts I can use in this IC?i see -15V on the cathode,can I use +15 on the anode, on inverted situation? i jusT did put 0.6 V.Looks like it is with inttermitent work but,I am not sure.What is the right way to test that IC?

rf+tech · « **Reply #194 on:** February 18, 2016, 06:36:33 am »

Saul,

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I have a PS of a tektronix TDS320 tha has two CI Like this one,just don't know the electric feature.

Use the IC part number and search for a datasheet and post the link here.

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how many volts I can use in this IC?i see -15V on the cathode,can I use +15 on the anode, on inverted situation? i jusT did put 0.6 V.Looks like it is with inttermitent work but,I am not sure.What is the right way to test that IC?

Yes, inverted is okay, but the LED does not see 15 Volts in the circuit. The voltage is 15 - 11 = 4 Volts, with 270 Ohms in series. Measure the voltage drop across 270 Ohms, exactly as in the circuit.

A good test to show that Q4 is working correctly would be to carefully push the leads of red 20 mA LED into IC socket pins 1 and 2, with the proper polarity. The LED should turn on about 1.5 seconds after the scope is turned on. The actual current through R45 (270 Ohms) and the LED will be a function of Q4 gain and Q4 base current supplied by R44 (68 kOhms). Measure the voltage drop across R45 with the red LED for reference.

Continue tomorrow,

RF+ Tech

RF+ Tech

SAUL BRITTO · « **Reply #195 on:** February 18, 2016, 06:40:56 am »

I used breadboard

SAUL BRITTO · « **Reply #196 on:** February 18, 2016, 07:24:07 am »

re+tec here is the datasheet http://www.vishay.com/docs/83660/moc8101.pdf

SAUL BRITTO · « **Reply #197 on:** February 18, 2016, 07:54:48 am »

Well, I tested this IC of datasheet and now I have 2.45V across R45 and maybe a current of 9mA.

rf+tech · « **Reply #198 on:** February 18, 2016, 03:03:28 pm »

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I tested this IC of datasheet and now I have 2.45V across R45 and maybe a current of 9mA.

Very good Saul, this is the kind data needed.

The MOC8101 minimum breakdown voltage (BVceo) is only 30 Volts. U2 is expected to withstand 100 Volts, the MOC8101 output may have already failed.

Please put original U2 on a breadboard with 270 Ohms in series and supply 4 Volts.
What is the voltage across 270 Ohms?

Good engineering decisions depend on good data, not guesses.

SAUL BRITTO · « **Reply #199 on:** February 18, 2016, 06:15:59 pm »

Look: contrary to logic I put MOC81 on the oscilloscope and got 2.7V acros R45 and -14 on pin 1 of U1 but the picture remains the same (Just for quick test).


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Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 68756 times)

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