HP 3440a digital voltmeter repair/troubleshooting: moving on to PSU ripple

[valley001]

Hi all,

I am new here, been working on repairing this old HP volt meter.  I also have a thread going on antique radio forum. 

Short story on this meter, it is a parts meter I bought to repair my once (and now again) working HP3440A, that I damaged while adjusting the -8.000v reference (screwdriver slipped and touched the back of A2  ).  The meter then stopped functioning.  I used A2 from this parts unit and installed it in my good meter and it is now functional, so I know the problem is in A2, but there are other problems with the parts unit.  I would like to get it going if possible. 

I have verified the voltages out of the PSU per 5-17 in the manual.

I have verified 400hz out of the oscillator per 5-18.

I do not get a reset pulse per 5-32.

I do not get any waveform out of A2TP1.

I do not get a "narrow positive pulse" out of A2CR5B on my scope (I am also not certain what a "narrow positive pulse" is but I get nothing on the scope.

Voltage readings per table 5-6:

A2Q1: -.672
A2Q2: -1.33
A2Q3: -.519
A2Q4: -.321
A2Q5: -.734

A2Q11: -.378
A2Q12: -.832
A2Q13: -20.5

This is the extent of my testing so far, and has revealed some problems.  A2Q1-5, all voltages are low.  I removed Q4 and 5 and tested them out of circuit with the diode check function on my DMM and they seem fine.  I wonder if some other issue might cause all the voltages to be low on these transistors? A2Q11-13 seem to read mostly in spec. 

I am interested in ideas on where to go next. Am I at the point of just subbing in transistors and see if things change?  Is there a more thoughtful way to approach these problems? 

[rhb]

I suggest you ask here:

HP-Agilent-Keysight-equipment@groups.io

the guy who designed the meter way well reply.  I've seen that happen in several discussions on the list.  It was on yahoo groups, but finally people got fed up and David Kirkby moved it to groups.io.

[Armadillo]

Yoo!, I think we are sufficiently able here. 

The reset pulse is a train of synchronized reset pulses.

[5-32]
I would advise you to;

Clean ALL the contacts with contact cleaner and eraser if you can reach the surface, particular wiper of R6
Change C5.

If there are still no pulses then;

Then with respect to Power Ground, measure DC Voltage of;

Q5 Emitter =
     Base =
     Collector =

Q6

Q2

Q3

[valley001]

OK, took some measurements.  Have not yet changed C5.  The neon responds to turning to the sample pot. 

Q5 Emitter = 1.6 to 2.2v
     Base = Sawtooth -2.08 to -4.4
     Collector = Square wave +13.6 to -14.4v

Q6 Emitter = -1.2 to -1.6
     Base = Square wave -1.72 to -.64
     Collector = Squareish wave -24.8v to -.4v

Q2 Emitter = Squareish wave -24v to -13.6v
     Base = Squareish wave -24.8v to 0v
     Collector = Sawtooth -35v to -13.6v

Q3 Emitter = -21.6 no wave pattern
     Base = -10.8 no wave pattern
     Collector = -10.4 no wave pattern

[rhb]

Yoo!, I think we are sufficiently able here. 

I certainly agree, but there is a lot to be said for getting advice from the engineer who designed the instrument.  And very few of them hang out here.

[valley001]

Darnit now the count gate neon is no longer flashing.  I may have grounded pin 9 or 10 while testing 

[Armadillo]

Change Q3. Its defective.

[Armadillo]

Now? Why is Q2 collector a sawtooth? Could be affected by shorted Q3. But as a caution;

Can you check what capacitor is C1? After you take out Q3 [empty] and before you replace with new one, measure and check C1. It should be very very high ohms "OL". [With POWER OFF].

[valley001]

OK, Q3 is HP 1851-0024 or 2N383 I will need to place an order. I will remove and test the cap in the mean time. 

Now that the sample rate neon is dead, I should look at A3Q10 and 11 correct?

[Armadillo]

Please carefully probe them for pulses and read the Base, Collector, and Emitter voltages.

Also probe A3Q1.

[150v on Q1 Note!] Safety for you and equipment.

[valley001]

As someone who is not trained in electronics but is very interested (20 years ago I could have pursued this option) what am I looking for when testing transistor voltages?  What does a good transistor look like compared to a bad transistor?  I have used the diode check function on my DMM in the past but perhaps that does not tell the whole story? 

[Armadillo]

Hi;

Buy the component tester, you will love it.

Diode Mode, for NPN, red on base, black on emitter and black on collector should reads like diode. Then red to collector, black to emitter, should be open. Then red to emitter and black to collector should be open.

Do Reverse polarity for PNP.

But please see Youtube for how to test.


https://www.ebay.com/itm/All-in-1-LCR-Component-Tester-Transistor-Diode-Capacitance-ESR-Meter-Inductance/322762949895?hash=item4b262bc507:rk:1:pf:0

[valley001]

Well, good news. I only blew the .75a -35v fuse, so the count gate is intact.

[valley001]

Back to the A2 board (the one that received the screwdriver slip). A2Q1-5, should I see a wave form out of the base and collector on these transistors?  Probing a few I get a voltage but no wave pattern on the scope.  Shall I post these base, collector, emitter readings or is this premature pending  repair on A3?

[Armadillo]

Hi, we should get A3 working first.

is A3Q1, Q10, Q11, Q12, Q16, Q17 producing the pulse? If not we need the voltages.

Why don't you extract the component from the spare board first and start procurement when 1 set is working?

[valley001]

Results:

Q1-

Emitter = .4 to .2v no wave
Base = Squareish wave 3.2 to -.8v
Collector = Square wave 58 to -55v (sometimes a spike to 100v happens at the edge of the square)

Q10

Emitter = 40mv to -80mv no wave
Base = Square wave 6.6 to -.6v
Collector = Square wave .6 to -12.6v

Q11

Emitter = Square wave 14.4 to -13.6
Base = no wave -12v
Collector = squareish wave 18.4 to -13.6v

Q12

Emitter = .8 to -1.6v no wave
Base = squareish wave 1v to -13.2
Collector = square wave 36 to -1.6v

Q16

Emitter = sawtooth 12 to -12v
Base = Sawtooth same as Emitter 12 to -12v
Collector = no wave -11.6 to -13.2v

Q17

Emitter = no wave 22.23v
Base = no wave 21-22v
Collector = sawtooth 18 to -6v

[Armadillo]

Are you able to get A3Q3 replaced? This Q3 reset the plug in unit, the voltage comparator and the decade counter. Man this chip is important.
Without this Q3, best to wait for now.

Oh, I glance your measurement, looks fine to me. I will take a closer look waiting for your Q3.

By the way, don't you have the spare board to temporary borrow the Q3?

[Armadillo]

After you replaced the Q3, measure the voltages, capture the "short" pulse. "short"!!
If you can get reset pulse;
temporary assemble up and try it out the unit and observe what's up?. If still not working then we go A2. 

[valley001]

I do have the other 3440A functional, but I would rather not remove parts from it now that it is working and calibrated. 

I checked through my stock of used transistors and do not have a suitable replacement so I will have to wait until the replacement 2N383 arrives this week. 

[Armadillo]

Oops! my mistake. I thought that was a spare not working unit.
You're right, wait, then.   

[valley001]

I probably did not word it well.  The unit we are troublshooting is the parts unit with the damaged A2 board from the other unit.  So I know A2 is damaged but the parts unit was not functional when received so there are problems with it, I am hoping the Q3 will fix the A3 board on the one we are troubleshooting and then we can move on to A2. 

I am getting better with my scope and have been able to verify some of the wave forms per 5-11.  5-11 A and C are correct.  5-11 B, D, E and not correct for the A3 board.

[valley001]

I should probably also mention we are troubleshooting with the 3441A range selector plugin installed.

[Armadillo]

5-11 B - Waiting for your Q3

5-11 D - In your previous reply, you said you have got 12 to -12 sawtooth at the emitter of Q16, if it is not the same now, please picture the curve over so that I can understand it what you saying. Otherwise do a remeasurement of BCE of Q16 again.

5-11 E - Is dependent on 5-11D above, please picture a curve over.

Regarding the A2, in your original 1st message, it appears there are no power coming into the board or I have no idea where you measured from as per your writing.
So you can start making sure the board edge connector is making good connection, clean those contacts and measure all the power are coming into the board correctly at the mean time.

[Armadillo]

Also for the A2 board, with the power supplies incoming checked OK, then you can test all the TPs and Wave Points, for any that is wrong, then the upstreams Transistors BCE needs to be measured.

[valley001]

5-11 D - In your previous reply, you said you have got 12 to -12 sawtooth at the emitter of Q16, if it is not the same now, please picture the curve over so that I can understand it what you saying. Otherwise do a remeasurement of BCE of Q16 again.

Interesting. Emitter of Q16 shows the ramp per 5-11D, but I do not get the same wave pattern at the indicated test point A3 (6) (pin 6 correct?)

5-11 E - Is dependent on 5-11D above, please picture a curve over.

OK, I am going to confess that I am very new to this.  I had the scope on AC coupling...I switched to DC coupling and now I see a mostly correct 5-11E.  Same for 5-11D

Regarding the A2, in your original 1st message, it appears there are no power coming into the board or I have no idea where you measured from as per your writing.
So you can start making sure the board edge connector is making good connection, clean those contacts and measure all the power are coming into the board correctly at the mean time.

Will do.