[tryppyr]

Yesterday I picked up an HP 1741A 100Mhz dual trace oscilloscope in non-working condition. The fact that I work for HP explains my affinity for old HP hardware, but it doesn't afford me any inside access to knowledge of our old products.

For that I turned to the web and found what is supposed to be the operations and service manual for this unit on the Agilent web site. That's a great start. Only problem is the manual is older than the unit, and some of the parts are different from the ones described in the manual.

The very first thing I noticed when I did a power on test was that the high voltage power supply board was making a clicking sound.

HVPS video

HPVS close-up photo

You can see in the photo there are some stand off posts and mount points for something to be positioned above the HVPS, but it didn't come with the unit, so I have no idea what it might be yet.

I also noticed there was a metal bracket loose inside the case. I have not yet figured out what it is supposed to support.

 

[tryppyr]

By studying the diagrams I found what the bracket is supposed to do, but I still don't see the right side mount point for it. I mounted it on the left, though.

 

[tryppyr]

Today I replaced a resistor that the manual said should be a 68K resistor, but was instead a pair of 68 Ohm resistors tied together. I used at 68K resistor and suddenly the clicking sound and flashing lights on the HVPS board ceased. No other improvement in functionality.

 

[KrisBlueNZ]

Does the service manual include a schematic diagram of the high-voltage power supply section? Even if it's older than the unit you have, it should be relevant. Can you post it?

Also, can you mark it up to indicate the resistor that was the wrong value, and the neon(s) that are flashing in time with the clicking sound?

 

[tryppyr]

The documentation does include a schematic, but that schematic does not reference any of the neons, which is one of the reasons I felt it was from an older model.

The HVPS is module A15 in these diagrams. On the parts list the resistor in question is A15R14, which is described as "Resistor 68K 10% .25W FC TC=-400/+800".

 

[KrisBlueNZ]

It looks like there are three neons connected in series at the top right of the schematic, named OS1~3. They connect between the first grid and the cathode of the CRT.

And the resistor that had been replaced connects between the cathode and one side of the heater. I wonder why it was replaced.

Is it possible that there's a short inside the CRT? Check for shorts between heater, cathode, and the first five grids (up to but not including the deflection plates).

You can also try running the power supply with the CRT base unplugged. If the neons stop blinking, that suggests the problem is inside the CRT. Also, with the CRT plugged in, in a dark room with the neons covered up, check for sparking inside the neck of the CRT.

If the neons flash with the CRT unplugged there might be a regulation problem in the high-voltage generator, I suppose.

 

[tryppyr]

Thanks for the tips.

I checked for shorts in the CRT but found none between any of the terminals except the obvious (1 and 14, opposite ends of the heater). By the way the neons that were flashing before are not among the three in series (DS1 - DS3). They are the other two (DS4 and DS5). Note, they no longer light up at all. If you remember from the video clip there was also a neon flashing on the daughter card containing the Focus and Intensity pots as well as the Line and Beam Find switches. The neon on that card also doesn't light up now.

In the dark there were no signs of any activity in the neck of the CRT.

One good thing about this unit is that the PCB is marked with numbers indicating which wire from the CRT connects to which post on the board. Makes it easy to insure you hooked things up correctly. The only exception is number 14, which us connected to the STD pin on the board (right beside the connection for #1, labeled COM 1.

 

[KrisBlueNZ]

Hmm, OK. Are you saying that the neons aren't flashing any more? Are you still getting the clicking sound? If so, is any sparking visible anywhere?

Re the wire from pin 14 of the CRT, does it look like it was originally connected to that point or is there fresh solder?

I've run out of suggestions I'm afraid. I'm not familiar with the model - nor any HP scopes for that matter :-(

 

[tryppyr]

That's correct, the neons are all dark and quiet (as DS1 - DS3 have been all along). There are other signs of life in the unit, such as the LEDs in the front still functioning normally, but nothing on the HVPS is lit.

The wires connecting the CRT are all on pin posts, not soldered. They slide on and off. I tested them and the electrical connections are all sound.

I appreciate your assistance. It's given me a few ideas to pursue.

 

[tryppyr]

I see I forgot to tell part of the story. Before I replaced the suspect resistor I was testing voltages at various test points on the LVPS. All the while the neons were clicking on the other side of the system. At one point I heard a little pop sound and everything went quiet. When I turned the unit over I saw that the two 68 ohm resistors had magically popped off. That's what triggered my decision to replace the resistor.

I checked the fuses and looked for any signs that something had blown, but so far haven't seen anything.

 

[KrisBlueNZ]

So if you replace the 68k resistor with two 68 ohm resistors in series, would you expect the clicking and flashing to reappear?

I don't think the actual value of the resistor should make much difference... unless there's a short in the high-voltage transformer. I should have suggested this before; these are a common failure item. You can check for direct shorts between windings that are supposed to be isolated, but often you'll get a shorted turn, which can only be detected by testing the Q of the transformer, for example by "ringing" it. That requires it to be taken out of circuit.

Would you be interested in tracing out the part of the circuit that relates to the two neons that aren't on the original circuit? Could they be the only difference between the board you have and the schematic?

 

[tryppyr]

So if you replace the 68k resistor with two 68 ohm resistors in series, would you expect the clicking and flashing to reappear?

I don't think the actual value of the resistor should make much difference... unless there's a short in the high-voltage transformer. I should have suggested this before; these are a common failure item. You can check for direct shorts between windings that are supposed to be isolated, but often you'll get a shorted turn, which can only be detected by testing the Q of the transformer, for example by "ringing" it. That requires it to be taken out of circuit.

Would you be interested in tracing out the part of the circuit that relates to the two neons that aren't on the original circuit? Could they be the only difference between the board you have and the schematic?

I would not expect the replacement of the resistors to cause the old behavior to return. That said, I can't readily explain why the resistors fell off in the first place. That suggests to me they were poorly installed to begin with.

I don't really have the means to check the transformer at this time, apart from checking the outputs. Taking it out of the circuit is a bit of a job, and with no means of testing the Q, it wouldn't be worthwhile.

What I have been doing is making sure all the wiring agrees with the schematic. I initially assumed it was all correct, and have been referring to my photos instead of the schematic, but now I feel it's best to verify.

 

[tryppyr]

Now that I'm done messing around with other projects I have begun to turn my attention more fully to this one. Today I started digging into the manual in some depth to get a better sense of how to approach this project.

One chapter in particular is interesting and informative. The Service chapter has a circuit theory section for each of the circuits in the system. Reading through that offers some insights about the logical progression of steps. Frankly, I am amazed by the amount of information it contains. Here is an excerpt:

HIGH-VOLTAGE POWER SUPPLY-CIRCUIT THEORY

HIGH-VOLTAGE OSCILLATOR. The high-voltage power supply consists of an oscillator (Q1) and a rectifying circuit. When the instrument is turned on +15 volts unregulated is applied to Q1, turning it on. As Ql conducts through the primary winding of A15TI (pins 3 and 4), positive feedback to the base of Q1 occurs through another winding of the transformer (pins 1 and 2). When conduction through Ql reaches saturation, the magnetic field developed in primary winding (pins 3 and 4) starts to collapse. This induces reverse feed back in the other winding, causing reduced conduction through Q1. With varying conduction through QI, the circuit oscillates at a rate determined by the inherent distributed inductance and capacitance of the oscillator circuit. The magnitude of the .oscillations, and consequently the output of the power supply, is controlled by the voltage at the output of differential amplifier Al5Ul.

HIGH VOLTAGE REGULATOR. A reference voltage from the +15-volt supply is established at the junction of A15Rl3 and A15R37_ The voltage is applied to a differential input on amplifier A15U1 through AI6R23. A sample of the cathode voltage for the CRT is also applied at the junction of A15R23 and A15R37 through A15Rl31 A15C8. Any change in the cathode voltage is amplified by A15U1 and applied through the primary winding on A15Tl to the base of high voltage oscillator Ql, changing the amplitude of its oscillation. The change is in such a direction as to correct the original change in the rectified cathode voltage. Diodes A15CR1 / A15CR2 protect the oscillator transistor from excess reverse voltage.

HV DISABLE. Transistor A15Q2 monitors the +120 V supply through resistor network A15R41 and A15R31. If the output of the +120 V supply drops below approximately +100 volts, A15Q2 conducts, turning on A15Q1. With A15Q1 conducting, a ground is applied to the base of high-voltage oscillator QI, cutting it off. In addition, if the +120 V supply surges above approximately +138 volts, zener diode A15VR5 conducts, turning on A15Ql. Again the high-voltage oscillator turns off. This protects the CRT from high- intensity bums. DEEP ERASE switch A15S1, when engaged, disables the high-voltage oscillator and gate circuitry to the CRT. The switch is engaged when deep erasure is required to remove deeply written images from the storage mesh. In the event that the -15 V supply is shorted to ground, A15Q2 turns on, turning off A15Ql, protecting the CRT from high intensity bums. Shorting the +15 V supply to ground causes the output of A15U1 to go low, turning off A15Ql, again protecting the CRT. A protection circuit consisting of A15Q4 / A15U21 / A15Q5 is incorporated in the cathode circuit to prevent accidental burning of the CRT during deep erase operations, while turning the instrument on and off, and during quick power dropouts.

When the instrument is turned off or the DEEP ERASE switch is engaged, A15C18 discharges, turning off A15Q4. When the instrument is turned on or the DEEP ERASE switch is returned to its NORMAL position, A15CIB starts charging towards the -15 volt supply through A15R44. Until A15CIB becomes fully charged, A15Q4 is cut off and there is no current flow through A15R45. A15U2. is an opto-electrical device consisting of an LED and a light-sensitive transistor. With no current flowing through A15R45, the LED is non-conducting and the light sensitive transistor is turned off. This turns off A15Q5 and the CRT cathode is now 100 volts positive with respect to the grid by AI5VR3. This action disables the write gun in the CRT.

The time that it takes A15C18 to charge to -15 volts allows the high-voltage regulator circuit to stabilize the output from the high-voltage oscillator. When A15C18 becomes fully charged, A15Q4 conducts causing the LED in A15U2 to turn on. This turns on the light-sensitive transistor, turning on A15Q5, effectively bypassing A15VR3. This action enables the write gun of the CRT.

HIGH VOLTAGE RECTIFIER. The CRT cathode and grid 'voltages are developed in the secondary of high voltage transformer A15T1. The cathode voltage is rectified by A15CR3 and filtered by network A15C6 / A15R11 / A15C7 before application to the CRT. The rectified cathode voltage is also used as. a reference level for the CRT filament voltage (developed on a separate secondary winding on A15T1), the grid bias voltage, and focus voltage. The cathode voltage is adjusted to -2295 V by HV ADJ potentiometer A15R38.

The CRT grid voltage is picked off the secondary winding of transformer A15Tl at pin 5. The voltage is applied through an RC network (A15C21 A15R3) to diodes that clamp the voltage swing between that established by intensity limit control A15R2 and the gate de levels. The peak-to-peak voltage swing is rectified and applied to the CRT grid which is voltage- referenced to the cathode. This controls the brightness of the CRT trace.

The HV Multiplier voltage is picked off the secondary winding of transformer A15T1 at pin 7. The voltage is applied to HV Multiplier Assembly A6 where it is multiplied (X3) and then applied to the post accelerator of the CRT. The post accelerator voltage is approximately +5 kV.

 

[KrisBlueNZ]

That's a pretty thorough circuit description. This used to be typical for really responsible, professional companies who respect their customers. Nowadays, providing technical information on products is often considered a legal risk (because it could be seen to encourage people to poke around in the thing and electrocute themselves) and an avoidable effort. Hence the "no user-serviceable parts inside" claim (which is often wrong, if the user is me .

But are you sure that's the right schematic? It's hard to be sure because many of the references and values aren't quite readable, but I think there are some differences between that diagram and the circuit description.

The last sentence of the paragraph that starts with HV DISABLE references Q4 and Q5 but I can only see four transistors on that schematic. Also an earlier reference to CR5 doesn't seem to fit with the schematic as shown.

 

[tryppyr]

You are correct in observing that my unit doesn't exactly match the manual. But there are enough similarities that I believe everything (or very nearly everything) about the circuit theory description applies. Some of the specifics I am curious about aren't there (such as the location and purpose of some of the lamps), but the basics are there.

When assessing the quality and accuracy of the text, bear in mind the journey that text has undertaken. It was originally printed into a paper manual. That manual was then scanned into a computer (at low resolution) and spit out as a PDF. That PDF was then printed out on paper again. That paper was then scanned into a computer again, and OCR software was used to convert it back to text. And then I spent some time correcting most of the errors I saw, some of which were of the type you describe (part names slightly scrambled). I noticed after posting that I missed quite a few such errors, but I thought the text was close enough to give you an idea what was on the original (which is slightly harder to read but less :scrambled").

- Greg

 

[KrisBlueNZ]

I don't know about the unit you have. I was saying that the circuit description does not seem to match the schematic. Not fully, anyway. I suppose that could be due to OCR errors but I'm not convinced. Can you check the references to Q4 and Q5 in the explanation are correctly OCRed, and see if you can find them on the schematic?

 

[tryppyr]

The reference to A15Q4 and A15Q5 are correct. Both are shown on the schematic on page 8-21, which immediately follows the section of text I excerpted. Q4 is almost dead center of the schematic, and Q5 is a bit to the right and higher.

Now that I stop to study that particular schematic, I also found the additional lamps I've been looking for.

 

[KrisBlueNZ]

Perhaps you should post that schematic! The one you posted in post #5, which I have been working from, doesn't seem to have a Q5.

 

[tryppyr]

Here you go.

 

[KrisBlueNZ]

OK, good. Now what's the current state of the unit? What are the symptoms exactly?