Repair Log: HP 3438A Digital Multimeter

This blog post is a record of my notes made while repairing an HP 3438A Digital Multimeter I had picked up last year at a local electronics swap meet.

The 3438A is a 3.5 digit HP-IB controllable multimeter.  It has five selectable functions:  DC Volts, AC Volts, DC Amps, AC Amps, and Ohms.  Of these five functions, three can be auto-ranged:  DC Volts, AC Volts, and Ohms.

The serial number on my meter had a "1717" prefix, making its vintage 1977 (coincidentally the same year I graduated from college  as a new wet-behind-the-ears engineer).

The price was inexpensive, and so I anticipated that it might have some problems.  When I got it home I discovered the following:

• Intermittent On/Off Pushbutton (sometimes it would turn on, and sometimes it would not).
• Ohms improperly reading "OL" on the 2000 Kohm and 20 Megohm scales for a load of 1.1 Megohms.
• The "kΩ" LED annunciator would illuminate in lieu of either the "uA" or the "mA" LED annunciators when the AC mA switch was depressed.
• Incorrect readings. 

Preliminary DC voltage checks:

Over time, power supply electrolytic capacitors can become faulty.  One of the first things I like to do before I dive into a repair job is to verify DC voltages with both a DVM and with a scope.

Table 5-2 in section 5-13 of the Operating and Service Manual lists the Power Supply voltages to check and their acceptable tolerances.  This table does not include all voltages (e.g. V_BG, the Back Gate Bias voltage), so I like to measure voltages across all electrolytics that look like they might be involved in power-supply filtering.

All DC voltages looked fine except for one -- it was the voltage across C609 .  Rather than being DC, the voltage across it looked unfiltered, as you can see, below:

Here's the location of the capacitor on the A3 PCB -- note:  it is a 20uF, 50V axial-leaded aluminium electrolytic capacitor.

And here it is in the schematic:

I clipped out the original capacitor and measured its capacitance on my GR 1657 RLC Digibridge.  Rather than measuring around 20 uF, its measured capacitance was 1.3 nF  (yes, nanofarads!).  Clearly it was bad and needed to be replaced.  So I dug around in my junkbox and, not finding an exact replacement, I used a cap with better specs: a 47 uF, 100V radial leaded electrolytic cap (as shown, below):

Here's the DC voltage measurement across the new C609 capacitor:

Much better!


Preliminary Heat Tests:

I also like to check if any components are over-heating (using my fingertip).  Everything seemed fine except for Q402, which was very warm to the touch.  Was this to be expected, or could it indicate a problem somewhere?

Here's the location of Q2 (on the A1 PCB).

And here it is in the schematic, with its Collector and Emitter voltages annotated from my measurements:

To measure Q402's collector current I wired a 10 ohm resistor in series with CR408 (see schematic, above).  The voltage drop across this resistor was 1.1 volts, thus Q402's collector current was 110 mA.  And therefore, for a measured Vce of 5.5 volts, the transistor was dissipating 0.6 watts.

Per the manual's parts list, the power dissipation specification of Q402 is described to be 1 Watt.  So Q402 was operating within spec, and I decided not to look any further.


Repairing the Push-buttons:

I suspected that much of the flaky operation (including LED annunciators incorrectly displaying the measured function) was due to the front panel push-buttons having dirty contacts.

This problem had been recognized earlier by HP, and they acknowledged that "occasionally switch contacts become dirty and introduce numerous problems" with Service Note 3438A-5A.

(Note that Service Note 3438A-5A was included as a PDF when I purchased the 3438A's Operating and Service Manual PDF from Artek Manuals).

HP's solution to intermittent switches?  Clean the switch contacts.

The switches are latching push-button switches.  And you must remove their plungers to clean their contacts

But to remove the plungers, you must have access to the tops of the switches.  To gain this access you must first remove the top PCB assembly and the meter's front panel.  Here are the steps I followed:

1. Disconnect the AC power cable.
2. Remove the top and bottom covers (four screws).
3. Remove the five screws on the back panel that hold the shield of the top A3 PCB assembly to the back panel.
4. Disconnect the three cables of colored wires that connect to the top A3 PCB Assembly.
5. Remove the top A3 PCB assembly by sliding the assembly forward (toward the front panel) and upward.
6. Remove the four screws at the corners of the front panel that attach the front panel to the side panels.
7. Disconnect the multi-conductor cable consisting of white wires from the A1 (bottom) PCB.
8. Pull the front panel forward and tilt it so that you can remove the screw on the inside of the front-panel's right side -- this screw attaches the green-yellow Earth Ground wire to the inside of the front panel.
9. Remove the two-pin cable (consisting of two gray wires) from P602 on the Front Panel (PCB A2). 

With these steps competed, you should be able to move the front panel out of the way, giving you access to the tops of the front-panel's "Range" push-buttons, as shown below:

A final step:  If you are going to clean the On/Off and the Function-select push-buttons, also remove the aluminium shield covering the circuitry on the left-hand side of A1 (see picture, above).  This shield is held on with one screw, through its top.

The Service Note mentions that there could be three different types of latching push-buttons, per the figures, below:

In my 3438A only the Power On/Off switch has an "External Metal Latch" (figure 1).  All the other switches are the "no latch" (figure 3) type.

Here are the Service Note's steps to remove a switch's plunger so that its contacts can be cleaned (my annotations are in italics):

1.  Remove the PC Board with the specific switch on it from the instrument case (note: I left the board in the case).  The PC Board contains static sensitive components, therefore handle the PC Board at a static-free work station.

2.  Observe the switch to be cleaned, if it has an external metal latch, then go to step 3.  If it has an external plastic latch, go to step 4.  If it has no latch, go to step 5.

3.  Refer to Figure 1.  Remove the metal latch by pushing the spring away from the metal latch, then pull the metal latch up and out and proceed to step 5.

4.  Refer to Figure 2.  (Make sure the switch whose plunger is to be removed is not in its depressed state -- otherwise it might propel itself out of the housing with the next step, and you could lose a contact or its associated spring).  Remove the latch by pushing the switch spring away from the latch and gently pull the latch out.  It may stick to the switch case so gently wiggle the switch plunger out.  Pull the switch plunger out.  Take care in removing the plunger as the contacts and springs have a tendency to fall out.  Proceed to step 6.

5.  Refer to Figure 3.  (Make sure the switch whose plunger is to be removed is not in its depressed state -- otherwise it might propel itself out of the housing with the next step, and you could lose a contact or its associated spring).  Take a small thin probe (a jewelers' screwdriver is ideal) and gently insert it as indicated.  Carefully pry up on the plastic retainer until the switch plunger slightly pops out.  Then push in an adjacent switch and the switch plunger will easily slide out.  Take care in removing the plunger as the contacts have a tendency to fall out.

6.  Clean the switch contacts with a contact cleaner.  Use a cotton swab to clean the contacts inside the switch case.  (Note:  I instead sprayed the inside of each switch housing with compressed air (after first removing its plunger) to blow out any debris, and then I put drops of DeoxIT on all of the plunger's contacts before re-inserting the plunger back into its housing.  And if you do use a cotton swab to clean out the inside of the switch housing, I would recommend that it be lint-free so that no fibers remain within the housing after swabbing.)

7.  Return the switch plunger to the main switch case by inserting it and pushing it completely down.

8.  Re-insert the metal or plastic latch, if removed.

9.  Return the PCB to the instrument case (not necessary in my case).

For reference, here's a plunger properly oriented for insertion into its housing (note that the contacts are along the lower sides of the plunger, not the upper sides).

And here's a plunger, upside down.  Note the springs!  Be careful when touching the contacts -- if a contact falls out, the spring will leap away!  (Been there, done that.  And luckily found the spring in the carpet).

(By the way -- now might be a good time to also add a drop or two of DeoxIT to the contacts of the Input Selection switch.)


With the switches cleaned and my repairs finished, it was time to do a "mini" calibration of the 3438A.


Calibration:

Per the Operating and Service Manual (section 5), calibration requires an HP 740B DC Calibrator and an HP 745A AC Calibrator.  I have neither of these.  Fortunately, one can perform a DC calibration without a 740B, as long as you have a second DVM whose accuracy you trust.

AC Calibration, however, can be an issue if you don't have an AC Calibrator (such as an HP 745A), because three of the adjustment procedures require 19 VRMS AC signals at frequencies of 200 Hz, 20 KHz, and 100 KHz., which I cannot currently provide.  Thus, for the moment, I've skipped these three adjustments.

So consider this my abbreviated, "mini," calibration.

The manual's calibration steps are:

1A: +7V Power Supply Adjustment.   (Procedure is self-explanatory in manual).

1B:  U725 Back Gate Bias Adjustment.  (Procedure is self-explanatory in manual.  Note that the JMVB measurement "loop" is just to the right of C611, at the upper left-hand corner of the R603 potentiometer).

2:  Clock Frequency Adjustment.  (Procedure is self-explanatory in manual).

3:  AC Zero Adjustment.  (Procedure is self-explanatory in manual).

4.  20 Ohms Zero Adjustment.  (Procedure is self-explanatory in manual).

5.  DC Gain Adjustment.  To make this measurement I use an external variable DC supply and connect its output to both the HP 3438A that I am calibrating and to my more accurate HP 34401A DVM.  I then adjust the power supply for about 19 VDC and adjust R403 so that the reading on the 3438A is the same as  the reading on my 34401A.

6.  Ohms Gain Adjustment.  I first verify that a resistor's value is close to 19 Kohms on the HP 34401A. I note its measured value and then I remove this resistor from the 34401A and connect it to the 3438A.  Then, following the steps in the manual, I adjust R119 so that the value displayed on the 3438A equals the value measured on the 34401A.

7.  AC Gain Adjustment.  (This step requires a 19 VRMS, 200 Hz AC source, which I do not have.  So I have skipped this step).

8.  20V AC Range, 20 KHz Adjustment.  (This step requires a 19 VRMS, 20 KHz AC source, which I do not have.  So I have skipped this step).

9.  2V AC  Range, 20 KHz Adjustment.  To make this measurement I use an HP 8904A Function Generator and connect its output to both the HP 3438A that I am calibrating and to my more accurate HP 34401A DVM.  I then set the 8904A to give me a 1.9 VRMS sine wave signal at a frequency of 20KHz.  I then adjust R110 so that the reading on the 3438A is the same as the one measured on the 34401A.

10.  20V AC Range, 100 KHz Adjustment.  (This step requires a 19 VRMS, 100 KHz AC source, which I do not have.  So I have skipped this step).

A note regarding the skipped AC adjustments -- if you have an audio power amplifier that can provide a low-distortion 19 VRMS signal into a high-impedance load, over the frequency range of 200 Hz to 100 KHz, and an appropriate sine-wave generator with which to drive the power amplifier, you can perform the three AC adjustments, above, that I skipped.  Simply drive the 3438A in parallel with a more accurate AC voltmeter (e.g. HP 34401A), and adjust the 3438A until its reading matches the 34401A's reading.

Other Notes:  Noise Pickup:

I noticed while I was photographing my 3438A (for the photo at the top of this post in which the 3438A displays +17.89 VDC), that the voltage reading was continually bouncing around between 17.86 to 17.91 volts, rather than being a steady 17.89 volts.

I initially thought there was a noise issue with the circuitry inside the meter, but when I moved the 3438A to my desktop from its original position on top of the HP E3616A DC Power Supply (while keeping it attached to the E3616A generating 17.89 volts), the display settled down.

My conclusion is that the meter is somehow sensitive to stray magnetic-field pickup, such as the B-field from the E3616A's power transformer.  Moving the meter away from the E3616A power-supply fixed the problem.


Resources:

Although Keysight provides a free 3438A Operating and Service PDF on their website, its schematics are unreadable and thus useless.

Fortunately, one can purchase a readable manual from Artek Manuals:  http://artekmanuals.com/
(Note:  When I purchased Artek's 3438A manual as a PDF download, it also included a PDF of the 3438A-5A Service Note).


Standard Caveat:

I might have made a mistake in my designs, schematics, equations, models, etc.  If anything looks confusing or wrong to you, please feel free to leave a comment or send me an email.

Also, I will note:

This design and any associated information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without an implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.