Repair Log: HP 5216A 7-Digit Counter

 

This blog post chronicles my efforts to repair an older HP 5216A Nixie tube counter.

The HP 5216A is a 7-digit counter specified to count frequencies between 3 Hz and 12.5 MHz.  In addition to frequency measurements, it can also perform period measurements, period-average measurements, ratio measurements, totalizing measurements, and time-interval measurements.

Documentation:

Although an Operating-Service Manual for the HP 5216A can be downloaded from the Keysight website, this version is of very poor quality with the schematics essentially unusable.  I would instead recommend purchasing a manual from Artek Manuals.  The one I purchased (and obtained via electronic download) is of excellent quality and at the time of this writing only $12.50.

Issues with my 5216A:

• The counter, when it counts, only counts in "Check" mode (left-hand front panel rotary switch in the "Check" position), but it doesn't count when a signal is applied to the front-panel input BNC.
• Sometimes the counter doesn't count at all, irrespective of mode, and the gate lamp never illuminates.  And sometimes the power doesn't seem to come on.
• The most-significant (MS) digit does not illuminate reliably -- it will briefly flicker on, but remains off most of the time.

Repair Efforts:

1.  Power Supply Recapping:

The serial number of my 5216A has a three-digit prefix of 748, implying it was manufactured in 1967 (i.e. take the first two digits of a four-digit prefix (in this case "07") and add them to "1960").

Given that this counter was almost 60 years old, I decided to recap all of the electrolytic capacitors on the power supply board, using caps from my collection of miscellaneous electrolytic caps.

My replacement caps were often the wrong form factor, having radial leads rather than axial, but I was able to attach them without too much effort.  And if I did not have the exact capacitance or working-voltage specified in the manual for a capacitor, I would choose one with a higher capacitance and/or higher working-voltage rating.

With these caps replaced, the issues listed above still existed.

2.  Only counts in "Check" mode, but not when a signal is attached to the front panel input BNC.

The counter would only count when the left-hand rotary switch was placed in "Check" mode, letting it count the counter's internal frequency reference (or a divided-down product thereof).

To trouble shoot, I applied an external signal to the counter's front panel BNC input and followed it through the circuitry.  It appeared to not make it through IC11 on the Main Board (A4).  

As shown in the 'red' annotation on the schematic, above, the input signal appears at IC11 pin 4, but not at its output, pin 6.

Yet pin 5 was high (i.e. TTL input-floating level of around 2 V), which should have gated the signal on pin 4 through.  And also pin 3 was 0, so the other AND gate was cut off and should not interfere with the propagation of the signal from pin 4 to pin 6.

So most likely IC11 (HP part number 1820-0072) was bad.

P/N 1820-0072 is equivalent to an SN7450.  I was able to obtain one via eBay.  (Note, if you cannot locate an SN7450, you can use an SN7451 in a pinch.  It should support all counter functions except those requiring that the 'Time Interval' switch on the counter's back panel be set to "TIME INT" in lieu of "FREQ-PER").

Replacing IC11 fixed the problem of the counter not counting when a signal was applied to the counter's input.

3.  Intermittent Problems:

Sometimes the counter would not count at all in either "Check" mode or in Normal mode.  Sometimes it did not seem to even turn-on.

All of these problems were traced to wires that had broken at their solder joints.  I'm theorizing a previous owner  (one or more), when trying to repair the counter (there were several repairs that had been made previously to the main board, for example), overstressed the wiring harnesses while disassembling the unit, and these wires eventually broke from over flexing.

Here's an example:

Repairing the various broken wires (there were three or four) fixed the intermittent problems.

4.  Left-most Nixie digit not working.

The Nixies are driven by decoder-drivers ICs, HP part number 1820-0092.  I verified that the input to IC15, the decoder-driver for the MS (Most-Significant) digit was correct (a four-bit BCD-encoded low-true value), but the correct Nixie digit representing that BCD code was not being illuminated.

It looked to me as though the Nixie tube was soldered to the PCB (important note: IT IS NOT!), and so I decided that trying to replace the 1820-0092 would be a better path to take.

Unfortunately, a replacement 1820-0092 is almost impossible to find, so I designed an equivalent circuit using available parts, in this case an SN74141 decoder-driver (or its equivalent, a Russian K155ID1 IC, available on Amazon and eBay), and a 74LS04 hex-inverter to convert the 4-bit low-true BCD code input to high-true for the 74141 (or its equivalent) input.

This circuit is described in this blog post:  https://k6jca.blogspot.com/2024/07/hp-1820-0092-nixie-decoderdriver.html

I removed the suspect 1820-0092 and replaced it with an adapter board on which I'd built the new replacement circuit.  The Nixie still did not work.  Oh oh -- was the problem with my replacement design, or with the Nixie tube, itself?

(Here is an image of the Main PCB with my 1820-0092 replacement circuit installed).

Poking around a bit more, I discovered that the Nixie tubes are NOT soldered onto the PCB, but instead they are inserted into pin-sockets, and so they are easy to remove.  I swapped the bad tube with a working tube.  The problem followed the bad Nixie tube, and I was able to show that my 1820-0092 replacement circuit worked properly for decoding digits 0-9 and blanking the tube for non-valid input BCD codes.

These Nixie tubes (HP part number 1970-0025) display their digits "upside down" and are very difficult to find.  Luckily, I have an old HP 5221B counter that uses the same Nixie tubes, but it is only a 5 digit counter, not 7, and it is spec'd to 10 MHz instead of the 5216A's higher frequency of 12.5 MHz.

So I decided it would be the sacrificial lamb, giving one of its Nixie tubes to replace the bad Nixie in the 5216A.  (Now I'm trying to find a replacement Nixie for the HP 5221B).

Other Notes:

1.  Creating a 1 MHz Reference Frequency for the 5216A from 10 MHz:

The HP 5216A's External Frequency Reference Input Specification is:

• 1 MHz Sine Wave (note: square-wave should be adequate, too).
• 1 V rms into 1000 Ohms (10V rms maximum).

I have a "house" 10 MHz frequency reference (based on GPS) that I use to maintain the frequency accuracy of various pieces of test gear in my lab.  But I would need to divide this down to 1 MHz for the 5216A counter.

Here's the design I came up with.

Schematic Notes:

• C2 AC-couples the 10 MHz signal at the input BNC connector to the input of the inverter (NC7S14M5X).
• Resistors R1 and R2 set  the inverter's input DC level to midway between VCC and GND.
• Diodes D1 and D2 are clamping diodes to clamp overshoots and undershoots on the input signal, with R3 acting as a current limiter.  (The inverter also has integrated internal clamp diodes, but they are limited to 20 mA).  
• U2 is a CMOS inverter with a Schmitt-Trigger Input (to prevent false counting of the counter U1 from noise on the input signal).  I used an NC7S14M5 device because I had it in my junkbox.  A 74AHC1G14, or equivalent, should work just fine.
• U1 is a dual divide-by-10 counter, each counter consisting of two stages:  first a divide-by-5 stage that produces an asymmetrical 2 MHz intermediary signal, and then a divide-by-2 stage that also "squares up" the asymmetric signal; the result being a 1 MHz square wave.  Only one of the two divide-by-10 counters is used.
• C1 AC couples U1's output to the output BNC connector.
• R4 both limits output current and also provides a one-pole low-pass filter in conjunction with C4, removing ringing on the output signal, and its value is not large enough to significantly affect signal level, given the 5216A's input impedance of 1K ohms for the reference signal.  Note, too, that given the low-frequency of operation and the short lengths of coax that might be used to connect the 1 MHz output to a downstream input, R4's value does not need to equal the characteristic impedance of the coax.

Implementation:

I built the circuit on a small perf board that could be inserted into a three-BNC Pomona Box, as shown below.  Note that the NC7S14M5 inverter is in a SOT23-5 package and thus mounted on a small adapter board of its own to make signal connection easier.

The three-BNC Pomona Box housing the circuit is shown below.  Note that the 5.1 VDC power comes from a BNC on the front panel of the HP 5216A counter (a modification added to the counter's front panel by a previous owner), which connects to the counter's internal 5.1 VDC rail.

The 10 MHz input:

And the 1 MHz output:

Below is the counter using the 1 MHz External Reference.  The gray coax cable provides 5.1 VDC from the (added) BNC on the counter's front panel to the divide-by-10 circuitry attached to the External Frequency Reference Input BNC on the counter's rear panel.

2.  Removing the Main PCB (A4) for repair/troubleshooting.

Unless you have an Operating-Service Manual, it isn't clear how to remove Main PCB to get access to all of the IC pins and components for trouble-shooting or repair.  When mounted within the chassis, significant parts of the PCB are difficult to access with, for example, a scope probe.

But removal of this PCB is actually straightforward.  Per the manual's section 5-14:

First, remove both side panels and the top cover.

Then, remove the transparent colored plastic front-panel window by sliding it out either side of the unit.

Next, reach through the sides of the chassis and gently lift the sides of the main PCB.  Pull the board forward with your fingers.

And after the board is started, remove connector XA4 (the connector on the left-side of the rear of the PCB, when viewed from the front of the counter).

(This works better with two hands -- I was holding my camera with my other hand).

Push or pull the board out of the counter, being careful to keep the board moving in a straight line.

With the PCB now outside of the counter, you can reattach it to connector XA4 for trouble-shooting with power applied.  But there are two important caveats!

First, be sure to place something under the PCB (and behind connector XA4) to prevent either the board or the connector from shorting to the case.

Second, there is high voltage on the board, and it is easy to touch.  (Been there, done that!)  I now wear  gloves when making measurements on the board,

In the image above, several sheets of paper from a yellow legal-pad serve to temporarily isolate the board and connector XA4 from the chassis.

Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.