Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 65695 times)

Alex Eisenhut · « **Reply #25 on:** February 12, 2016, 12:58:30 am »

Quote from: SAUL BRITTO on February 11, 2016, 11:52:54 pm

Hi friends. When I worked with tube tv, seen many triplers causing a similar defect. As the anode lost the ability to attract the electron beam causing, this type of screen. Tomorrow the courier will deliver the scope in my home.

Good luck! But before tearing everything apart, remember it's tricky to get working in storage mode; don't assume it's broken right away!

SAUL BRITTO · « **Reply #26 on:** February 12, 2016, 02:40:54 am »

Quote from: Alex Eisenhut on February 12, 2016, 12:58:30 am

Quote from: SAUL BRITTO on February 11, 2016, 11:52:54 pm
Hi friends. When I worked with tube tv, seen many triplers causing a similar defect. As the anode lost the ability to attract the electron beam causing, this type of screen. Tomorrow the courier will deliver the scope in my home.

Good luck! But before tearing everything apart, remember it's tricky to get working in storage mode; don't assume it's broken right away!

Hi Alex,how are you guy? Thank you for advise.

I'm not interested storage. I just want it running smoothly

SAUL BRITTO · « **Reply #27 on:** February 12, 2016, 02:43:28 am »

I do have a tektronix 2220 that have storage mode but ,I never used,my use of oscilloscope is very simple

SAUL BRITTO · « **Reply #28 on:** February 12, 2016, 10:20:32 pm »

Hi All. The scope finally is in my bench. I did a small video,look:
https://youtu.be/SpsUYWx4-_0

rf+tech · « **Reply #29 on:** February 13, 2016, 03:44:55 am »

Hello Saul,

First thing to try is turn OFF storage mode.

Turn the scope around, on the back panel is a pushbutton marked "Conv". This to the right of the black CRT cover.
Press this button to place the scope in Conventional mode.
The "Conv" light on the front panel should now be ON and the scope should display a trace that can be properly focused.

Here's the HP1741 manual: http://cp.literature.agilent.com/litweb/pdf/01741-90911.pdf.

Section 3 covers the operation of this analog storage scope, and Section 5-59 describes the storage adjustments procedures.

Without the knowledge necessary to use analog storage mode - and most importantly - how to properly adjust these "unconventional" scope controls, it is possible to end up looking for something that does not need to be repaired. The blooming present in the video appears "normal" to me for an old scope that has not had the storage adjustment procedure performed in many years.

I once owned an HP1727 275 MHz storage scope, getting the collimator and flood guns properly adjusted on these old HP analog storage scopes can be a challenge. Once adjusted, the illumination may still not be uniform across the screen. It all becomes a compromise.

RF+ Tech

SAUL BRITTO · « **Reply #30 on:** February 13, 2016, 04:20:59 am »

Wow! Thank you so mutch rf+tech.It was of a big help to me! I will do this and report what happened.

SAUL BRITTO · « **Reply #31 on:** February 13, 2016, 04:30:34 am »

rf+tech when I use conventional mode I get a black screen. No trace,no bright, nothing but,I found a transistor
Q17 on the vertical board that seems in short thre is a diode near to it that looks in short. There is a -12 Volts with -14,2V. You see a connection between these things?

SAUL BRITTO · « **Reply #32 on:** February 13, 2016, 04:57:39 am »

SAUL BRITTO · « **Reply #33 on:** February 13, 2016, 05:02:34 am »

At this place has a 2n4035

rf+tech · « **Reply #34 on:** February 13, 2016, 01:21:32 pm »

Saul,

Where is this diode that may be shorted? I see no diodes in the attached schematic.

Please provide the voltages measured on each lead of Q17. One measurement by itself is meaningless.
Since Q17 and Q18 comprise a differential amplifier, it is better to evaluate both sides together.
Let's see all six measurements.

And have all power supply voltages been checked, before jumping into the vertical amplifier?

RF+ Tech

SAUL BRITTO · « **Reply #35 on:** February 13, 2016, 03:54:48 pm »

Quote from: rf+tech on February 13, 2016, 01:21:32 pm

Saul,

Where is this diode that may be shorted? I see no diodes in the attached schematic.

Please provide the voltages measured on each lead of Q17. One measurement by itself is meaningless.
Since Q17 and Q18 comprise a differential amplifier, it is better to evaluate both sides together.
Let's see all six measurements.

And have all power supply voltages been checked, before jumping into the vertical amplifier?

RF+ Tech

Q17 is: base -12.17V, collector and emmiter -14.12V. Q18 is working very very hot! I will to see all the PS, despite I did a preview yesterday night and, just found the 120V rail with 124V and, on the vertical board a -12V with -14,2V,That diode is fisicly near to this transistor but on schematic no, it isn't in this image.

rf+tech · « **Reply #36 on:** February 13, 2016, 04:54:38 pm »

Saul,

Something to make note of: the A17 schematic shows +/- 12 Volts coming from A3. The A3 schematic does not show where +/- 12 Volts comes from.

Looking at the interconnecting diagrams, sheet 1 of 3 shows +/- 12 Volts at A3 block but *nowhere* can I find the source for +/- 12 Volts.

Since you measure 14.2 Volts where the schematic indicates 12 Volts, and there is no 12 Volt supply to be found, the conclusion is an error in the manual. HP was very good about releasing errata/change notices in paper form. Without access to these for the 1741, we have to conclude from the evidence of your observations that 12 Volts is really 15 Volts. 14.2 Volts is only 5.3% low from 15 Volts and does not present a problem.

Please report what the -12 Volt point measures to see if this theory is further supported.

This illustrates the importance of conveying as much information as possible. With minimal information, we can only waste time guessing at things. Even if you think the power supply voltages are good, post them for our benefit. Everyone sees things a little differently.

As for Q18 running hot, consider that the A17 is DC coupled from the A3 - specifically from A3A1. And the output balance of A3A1 depends on many more inputs. It is possible that Q18 is hot because the real problem is in the A3.

By providing the DC voltage measurements on both Q17 and Q18 emitter, base and collector, we can better evaluate and advise on the problem.

The 120 Volt supply measuring 124 Volts is of little concern as this is only 3.2% high.

RF+ Tech

SAUL BRITTO · « **Reply #37 on:** February 13, 2016, 05:07:21 pm »

That diode is connected to a -15V rail,but I can not find in the schematic,for now.

SAUL BRITTO · « **Reply #38 on:** February 13, 2016, 05:21:43 pm »

RF+ Tech >> I do have two more boards like this one (Two scopes) and those boards has different measures with the DMM in diode scale, the junction colletor emmiter of Q17 is 0.600v on the other boards and on the sick scope is full short 0.0V on diode scale. That one diode measures 0.2V and on the other board is 0.500V on diode scale.
Q17 and Q18 has each one a equal diode near it,they are connected to a -15V rail,but on the A3 vertical board I just found a transistor in this rail,at the eschematic. If you prefer I can make an image of exactly place on the board.

SAUL BRITTO · « **Reply #39 on:** February 13, 2016, 05:30:42 pm »

Here is the image.Sorry for this webcam.

SAUL BRITTO · « **Reply #40 on:** February 13, 2016, 05:41:59 pm »

The pins 5 and 6 of this connector are -12 and +12V rails.Near to Q17 has a test point connected to -12V that's measuring -14,2V.

SAUL BRITTO · « **Reply #41 on:** February 13, 2016, 06:20:25 pm »

If you see on the schematic Q17 is polarized with 12.6V on emmiter and 0.6 on collector but,here is measuring -14,2V base and emmiter and Q18 on the schema is PNP on the board is NPN wow!!

rf+tech · « **Reply #42 on:** February 13, 2016, 06:47:44 pm »

Very good information Saul.

The diodes are not likely to be VR2 and VR3, on the right side of the A17 schematic, based on their connection to -15 Volts. Do the other two boards look identical to this defective board, in this area? The fact that HP revised this differential amplifier and added these diodes, together with +/-14.2 Volts on the "+/-12" Volt rails adds confirmation that these are indeed +/- 15 Volt rails.

The diode test on Q17 certainly is conclusive. The diode test on this unknown diode may be influenced by the shorted Q17. Can you see if this diode connects in some way to Q17, perhaps through a low value resistor? It would be good to unsolder one leg of the suspect diode and try the diode test again.

I don't see the mystery Q13 (on the A3 parts layout page) on the A3 schematic, either. So far, this manual has not been very helpful. More searching has not turned up different scans of the manual.

Just now saw your new post - it would appear that someone has "repaired" this board. I "think" some people do this just to help us sharpen out troubleshooting skills.

For a quick fix, a 2N3906 should work. The bandwidth of the amplifier may suffer, but at least we will know more.

RF+ Tech

SAUL BRITTO · « **Reply #43 on:** February 13, 2016, 06:53:10 pm »

I removed the Q17 it's in full short and the short on the diode was your fault,diode is good.

SAUL BRITTO · « **Reply #44 on:** February 13, 2016, 06:59:11 pm »

That -12V seems to have origin in any way in Q17 because when I removed Q17 did a measure on that -12V and it is 0.2V now. On the others boards are absolutly equal all you find in one find on the other and,Q18 is a NPN transistor on all that.

SAUL BRITTO · « **Reply #45 on:** February 13, 2016, 07:14:24 pm »

I have an original one Q17 but, I think: what cold be te cause of this short in Q17? What do you think about?

SAUL BRITTO · « **Reply #46 on:** February 13, 2016, 08:08:28 pm »

Replaced the Q17 and some things changed your behavior,that big IC on the vertical board seems working normally,
Beause it was working cold and now is in a temperature as the other scopes. Just don't have a trace,maybe I have a tube polarization problem or in lacking of HV.

SAUL BRITTO · « **Reply #47 on:** February 13, 2016, 08:42:17 pm »

I found a tantalum cap in short on the vertical board,I will replace and see what happens

rf+tech · « **Reply #48 on:** February 13, 2016, 09:59:50 pm »

That is too strange to find NPN transistors in place of PNP, especially in a differential amplifier where both sides mirror each other. Have you compared the E-B and B-C diode drops to a known PNP or NPN to confirm this?

On another forum, I found a marked up A5 schematic for the HP1740 vertical amplifier. It is very similar to the A17 and someone has recorded many voltage measurements that may be of help:

HP 1740A Vertical Mod -0001: http://www.electro-tech-online.com/attachments/hp-1740a-vertical-mod-0001-pdf.44405/

Measuring the voltage on the vertical deflection plates will show if the amplifier can be balanced when turning the vertical position control, or if there is still another problem to locate. Knowing that the trace is not off the top or bottom is more important than jumping to the high voltage power supply.

You are making good progress.

RF+ Tech

SAUL BRITTO · « **Reply #49 on:** February 13, 2016, 10:06:28 pm »

rf+tech>> I am almost sure that the tripler is bad,Has a 58kv probe here and the way is to measure this.

I did measure the HV on the output of tripler and got about 2.8kV I think it'snt right...


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Author Topic: Fixed HP1741A in a big shape but... I bet in a focus problem, and you? (Read 65695 times)

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