11A-series plug-in NVRAM replacement

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The following detailed explanation of how to replace the NVRAM of an 11A16, 11A32, 11A33, 11A34, 11A52, 11A72 plugin is courtesy of John Addis.

Some portions of John’s procedure do not appear in any Tek manual which makes it impossible to permanently recalibrate the plugin after the NVRAM has been replaced without the information John compiled into this procedure.

An expired battery inside the NVRAM in 11A16, 11A32, 11A33, 11A34, 11A52, 11A72 plugins results in three problems:

  1. “Serial Sum” diagnostic failure at turn-on (loss of the plugin’s serial number).  
  2. Running calibration routine at every turn on even though the plugin has not been removed from the mainframe.

  3. Failure to retain V/Div, Offset, Probe calibration, etc. after turn off.

There may be other failures directly caused by the NVRAM, so multiple failures should not discourage you.

Note: the 11A52 is a special case in that the high frequency response (a single pot per channel in the 11A32 and 11A34) is instead encoded in 12 calibration constants (six per channel). These are lost in the 11A52 along with the serial number, instrument settings, and probe settings at last turn off.

These are the result of a failed battery inside the NVRAM (U801). "Serial Sum" error code is a check on the integrity of the NVRAM part. The information stored there is the plugin’s calibration settings, the last control settings, and the serial number, which the mainframe interrogates to determine if that plugin was the last one residing in that plugin slot (LEFT, CENTER, or RIGHT).

The NVRAM is identified as a Dallas Semiconductor 1220Y and is the tallest, thickest chip on the board, 24 pins. The internal battery is not 
rechargeable even if you could get to it. One of mine is still working 29 years after manufacture without failing. A replacement part is available on the internet (DS1220Y-200 works). The date code (format: YYWW) indicates when the part was manufactured and was originally stated to last at least 10 years after that date. I have found DS1220Y-200s on the internet with a 1503 date code (third week in 2015).

The part is soldered into the board. After removing the DS1220Y, I suggest soldering a socket into the board and installing the replacement part in the socket. The 24 pin socket is Mill-Max 110-41-624-41-001000.

I have successfully programmed the serial number into my 11A32 using an NVRAM purchased for $7.50 from China. Reprogramming the serial number is necessary to prevent the plugin from going through a calibration cycle at every turn on. Any legitimate serial number will work,but I suggest you program the serial number printed on the top rail of the plugin. That is what Tektronix did.

The plugin service manual tells you how to reprogram the serial number, BUT THAT DOES NOT WORK BECAUSE IT IS INCOMPLETE.

The well-kept secret is that a mainframe J450 (Cal Lock) Jumper on the time base board on the bottom of the mainframe needs to be installed in order to program the plugin. It is not like they tell you where to find a spare jumper to do this, no, they will, however, sell you one. Or, you can borrow one from an unused plugin if you happen to know that the jumper is a short. You can also jump the pins with alligator clips.  Be careful!

So both the plugin and mainframe need jumpers installed to program the plugin serial number. The mainframe SN can be programmed without a plugin, if you have a spare jumper someplace.

NEITHER the plugin manual NOR the mainframe manual makes any statement (that I have found) to the effect that BOTH INSTRUMENTS have to have jumpers installed to program the plugin serial number.

Here is what you do:

Remove the bottom panel of the mainframe. On the board closest to the front panel (Time Base board), locate J450 labeled “CAL-LOCK”. This is hard to find. It is near the center of the board, about 1” toward the rear of center and 1.5” away from the CRT side. See Fig 3-15 if you have a manual. Short these two pins together with a jumper of some sort. J450 is probably the name on all 11K mainframes.

In the plugin, remove the left side panel, remove and rotate the J611 jumper on the plugin from vertical (normal) position to horizontal position, and re-install. Install the plugin in the mainframe. (I used the center hole, and the commands below assume you used the center hole. If not, substitute RIGHT or LEFT for CENTER in the following instructions.)

Connect either a GPIB or an RS232 cable between the mainframe real panel and a computer. In the case of an RS232, you will have to have a cable capable of mating to both ends (RS232, or computer COM port connectors varied over time, but they all work).

I recommend setting the 11K mainframe RS-232-C settings as follows:

Baud Rate: 19,200
Parity: EVEN
EOL String: CR/LF
Echo: ON Flagging: HARD
Verbose: ON
Stop bits: 1
Delay: 0
Debug: ON

Some COM ports will not support 19,200 speed.   Just use a much slower speed on both the computer and the mainframe.

If you are using GPIB, you are on your own.

To connect the computer to the mainframe, I used HyperTerminal, a free program on any Windows XP computer (START > All Programs > Accessories > Communications > HyperTerminal). I set the HyperTerminal options (Baud rate, Parity, etc.) to match that on the oscilloscope mainframe (Utilities button > Page to Utility 2 Menu > RS-232-C). HyperTerm is not free on Windows versions after XP but any serial communications program will work. See http://helpdeskgeek.com/windows-7/windows-7-hyperterminal for some alternative suggestions.

You can check that this is working by typing in any legitimate (probably case sensitive) command:

CHC2 SENS: 0.1

CH means CHannel. The letter following CH specifies the plugin hole you are using L, C, or R.  LEF, RIG, and CEN are also acceptable. The number following L, C, or R is the channel number. Once this is all working (answering in English), type in the following command to program the serial number:

UID CENTER:″B010200″ where you substitute the serial number on the top rail of your plugin for B010200 and LEFT or RIGHT for CENTER as appropriate. No space between the colon and the double quotation mark.

You can use all the same commands using GPIB.

The response on the computer should be OK. Check that the scope identifies the plugin serial number.

Mission accomplished, unless you have an 11A52, and that is the subject of another article.

Remove the CAL-LOCK jumper from the mainframe. Reinstall the plugin jumper in the vertical position.

And you are done. You can confirm the serial number and the firmware version on the mainframe (Utility button > Page 1 > Ident).

Turn the mainframe off and restart. If there are probe error codes, they can be cleared by installing a probe. 11A52s will still have an "HFAdj Cksum" or similar error code. The transient response on the 11A52 will not be correct until new constants are installed. See elsewhere.

NOTE: John did this from memory so he hopes it is all correct.

Dennis Tillman W7PF