HP1740A Oscilloscope

[jwax]

Had this oldie but goodie 2-channel analog scope for a few years, with never a problem.
Yesterday, I couldn't get the trace on the screen. The Beam Finder puts it right back in the middle, with both Focus and Intensity working OK. But the Vertical Position Control, of either channel, will not bring the trace back on the screen.
Went through the usual triggering scenarios, but still no trace.
Anybody familiar with this old scope know where I begin to look for the problem? The interior visual looks as clean as the day it left the factory.
TIA!
John

[Externet]

Hi John.
Repairing oscilloscopes has been my favorite fun for ~a decade. I enjoy bringing them back to life !

Check power supply feeding the vertical deflection circuit to start. Do you have a second oscilloscope to diagnose the ill one ?

Is there +43V at pin 16 0f the vertical output IC ?
Is there +12.6V at emitters of Q1 and Q3 ?
Is there -12V at the junction of R10 and R12 ?
Are the vertical deflection plates contacts tight and clean ?

Miguel

[jwax]

Negative on the spare scope.
Power Supply voltages at that board check with test point values.

Locating and identifying parts on the vert output board is troublesome. I believe I've identified the board (the one with two wires going to the vert deflection plates on the CRT!), but none of those components have identifiers. There's a large ceramic 18 pin IC on a large heat sink, but can't tell where pin 1, or pin 16 is.
R10/R12? No idea which those are, or Q1/Q2. Can you send me to a picture of the board?

Appreciate the help Miguel!
John

[jwax]

OK, found a schematic. +43 on pin 16 is OK, but +12 on pin 18 does not change with position control (should be 5 to 30 volts, varies with position control)
Also, have 3.3 volts on pin 13, which should be 4.8 by the zener. Bad zener?
More later.....

[Externet]

OK.
The zener could be bad, confirm by disconnecting the ceramic integrated pin 13 and measure the cathode. If recovers to +4.8V, zener is good; ceramic chip is in trouble, loading pin 13 too much. Make sure the position control is centered. Probes in GND.
a-Check pin 2 and 8 have the same voltage.
b-Check pin 3 and 7 have the same voltage.
c-Check pins 10 and 18 have the same voltage.
d-Confirm continuity from pins 10 and 18 to the deflection plates.
Check R2, R4 for identical values and heat.
If a,b,c,d are yes, highly suspect of the ceramic output module. Typical and not cheap failure.
----> http://www.ebay.com/itm/HP-Agilent-1740 ... 1533652430
Miguel

[jwax]

I'm going to replace the Vert Out hybrid module. Too bad it will take 2-4 weeks to get here! Having worked with hybrid microelectronics before, I am confident that it is the defective component.
Been there, done that!
Thanks Miguel, I'll post the result!
John

[Externet]

Really ?
At least tell results of R2, R4 and a,b,c,d ! We are not done diagnosing !
Miguel

[jwax]

OK Miguel, here we are:
Ends of R2 and R4 are 14.3 V, and 12.7 V respectively.
Pin 2: 2.1 V
Pin 8: 1.65 V
Pin 3: 0 V
Pin 7: 1.6 V
Pin 10: 12 V
Pin 18: 35 V
The bases of Q1 and Q3 are at 11.9 V

What do you think?

[Externet]

Hi John.
The different voltages in the plates voltage branches are what causes the sweep not being centered when the position knobs are centered.
Two possibilities:
-The position potentiometers are not providing same voltage to the vertical output stage branches after the delay line, or
-The circuits after the delay line are not balanced themselves.

I would isolate both areas by disconnecting pins (4) and (5) entering that output board and measure again a,b,c,d

If voltages still differ, the position knobs circuits are assumed behaving well. If they equalize, the position knobs circuits are to blame

If the voltages are still different, Check Q1 is not dead open. If fine, I would momentarily short emitter of Q1 to emitter of Q3 and check if when doing that, trace appears centered.

If trace gets centered, first suspects are R2 and R4 not being equal value.
With scope off, measure resistance between pins 11-12 and 16-17 of the ceramic module.
Let's pause here and come back with results.
>You have not tweaked R24, right ?
>Was the zener diode healthy ?

Miguel

[jwax]

OMG! Anybody need a Vertical Output Module for an HP1740A oscilloscope? I seem to have a spare now!

Miguel- I removed the two leads from the delay line going to the vert output board. The trace centered!
I measured the voltage on the two leads from the delay line: 2.0 VDC each. Balanced!
Reconnected the leads to the board, and voila! The dang trace is back where it belongs!

Appaently those PCB pins, and dangling wire sockets are not the most reliable joint!

Thanks very much Miguel, you kept me from wandering down the wrong road!

Let me know what I owe you!

John

[Externet]

That is not fair ! I was just starting to have fun and you took it away...

Well, as payment (compensation for taking the fun away, not for repairing anything); would you have a spare LM3900 new or used but good behind your spider webs ? (Am cheap)

Am happy if you are happy with the HP1740. Next time just hit it with a hammer and will work.
Miguel

[jwax]

So sorry Miguel, I checked around, and we're fresh out of LM3900's.

[Edd]

.
Sr Miguel . . . . .



Vell . . . . .I tell you vats . . . .I'm dun bin checkink und findink dat I'm dun bin receivink und fresh shipmink of der LM3900 Eye Sees frum dose dere National peoples.
They are havinks und dately code uf der 12th weeks of 1984 . . . . fresh enuffs fer ye ?


I have three chip version options for you.
Dash 1 . . . . with the internal current mirrors being cheap silver mylar.
Dash 2 . . . . . incorporates use of optical grade glass and surface silvering for absolute minimal diffraction .
Dash 3 . . . . .the individual internal current mirrors have handles . . . . just in case "you're so vain" . . . . . . . so says Carly Simon.


Sooooooooooooo let me step right up to the plate and fill in the debt now due from jwax . . . . .aka "The Waxster", such that he will not be in arrears to you.

Homer . . . .the pet carrier pigeon . . . . . is currently crashed out in his regal coop, after just having made sucessive winged treks to Beaverton( in Tek country) with a dog leg down to Californias Malibu Beach.

(I fully feel that his "bushed" out condition might be fully attributable to that delayed overnighter that he spent with the "pigeonettes" at the beach.)

So, with that type of delivery to you, currently not an opton, let me just slip it down in the deepest fathoms of the re purpopsed /and/ relabeled bubble wrap bag that my i-Watch just came in.

Fill me in on your mail addee via our private messaging option.

BTW . . . . . on that Hooooolet Plastered, EARLY in my testing procedure would be the going over of all of its underneath and its PCB's with a small plastic handled screwdriver
and a methodic tappa-tappa to seek out and find any connectivity breaches, whilst observing the scopes trace.



Thassssitttt.. . . . . . . . . .





73's de Edd


[email protected]. . . . . . . . . (Interstellar~~~~~Warp~~~~Speed)
[email protected]. . . . . . . . . (Firewalled*Spam*Cookies*Crumbs)



I took my car in for inspection, but was worried that the mechanic might try to rip me off.
However, I was tremendously relieved when he told me all that I needed was some turn signal fluid.

.

[jwax]

Miguel, I'll be happy to have Edd send you the LM3900!
Thank you very much Edd!

However, the plot must get revisited- The day after the scope was "fixed", I powered it up, watched the trace go "noisy", then disappeared once again!

Upon expert advise from Edd, I wacked, tapped, whistled and wapped various connectors, boards and fittings to no avail! Even tried freeze spray. No trace.

Having just received the Vert Output Module, I replaced it. No trace still.

So now, without the module in place (to simplify things), I see these readings, indicating something is still amiss among Q1, Q2, Q3, and Q4:

Q1 emitter: 14.33 V
Q1 collector: -1.1 V

Q3 emitter: 12.7 V
Q3 collector: 0.8 V

Q2 emitter: -1.1 V
Q2 base: -.7 V

Q4 emitter: 0.0 V
Q4 base: -.7 V

So, let's have some fun guys!

The delay line wires are connected, the module removed, and the trace is centered on the screen, for these readings.

[Externet]

Well, typing all again as my response dissapeared...

Thanks, Edd, will send a message when/if I deserve the prize. And right, a cold solder joint will do it.

John, you broke it again. Hammering the unit is with a feather, not a sledge !

OK. Trace is centered with uneven voltages because the deflection plates are not connected without the module in place.
I would disconnect the delay line from entering the board, and confirm your voltage readings reported above.
If branches differ; measure R2,R4 for identical values lifting one pin.
If same value for R2,R4, do the same for R8,R11 ; R9,R13 ; R15,R16 and come back with findings.

Something is unequal in the symmetrical branches.