HP 54501A NVRAM Replacement

Introduction: HP 54501A NVRAM Replacement

About: I'm an engineering student at a local community college and will be transferring to a 4 year school in 1-3 years. I'm a ham radio enthusiast and enjoy building things and disassembling them.

The problem: you have a great scope but it doesn't hold calibration when you turn it off. The source of the problem is the Dallas NVRAM chip from the late 80's has finally run it's battery down. The solution: replace that dead chip with a new Maxim chip.

If you see the errors from the photo here when you turn on and run selftest, but they all pass if you rerun after the self calibration, this should fix your problem.

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Step 1: Parts and Tools

There are very few necessary tools and fewer parts.
  • Torx 10 bit
  • 5/8inch wrench
  • soldering iron
  • solder sucker
  • optional: desoldering iron (cheap RadioShack one does the job)

  • Maxim Integrated DS1230Y-120+ (approx $30, but available as a sample from Maxim)
  • 28 pin IC socket, mouser #575-199628 works great (not necessary but highly recommended)

Step 2: Remove the Cover

Safety first: Unplug the unit.
Remove the cover, there are 6 screws on top and 2 on each side.

You will be greeted by a nice warning against touching things and a nice view of the beautiful through-hole construction.

Remove the power supply: Do not remove any of the power supply screws, they're not necessary to this step. Remove the two high tech power supply retention pins (two white things that look like styluses), disconnect the ribbon cable and mains connector, the power supply then just slides out.

Step 3: Remove the Back Cover

Remove the 8 screws from the back panel and pull it off carefully, it will stick in the bottom left corner until the pin inside unseats from its socket.

Disconnect the three connectors to the main board, feed the mains connector through it's hole in the chassis and set the back cover aside.

Step 4: Remove the Main Board

Flip the scope on its side and remove the 8 screws on the bottom. Then remove the nuts on the 4 input connectors on the front.

Next disconnect the fan's power connector, the ribbon cable that connects to the side board, and the power extension ribbon cable.

The main board should now slide out the back smoothly.

Step 5: Replace the Chip

The chip in question is marked U37, it's a Dallas DS1235YW.

Flip the board over and locate the opposite side of the chip. Remove the chip by desoldering each of the 14 pins, there are many guides on doing so, just be patient and don't force it. If you pry at the chip you'll scratch the board.

I added a socket when I replaced my chips, to make any future replacements easier and to prevent possible damage to the chip. Pay attention to which direction pin 1 goes.

Put the new chip in the socket, paying attention to pin 1 orientation.

Step 6: Put the Whole Thing Back Together

Slide the board back into the chassis.
Put the nuts back on the input connectors and put the 8 screws back into the bottom of the unit to hold the main board in. Reattach the fan power connector and the side board connector. Connect one end of the power supply cable.
Reattach the back panel, reconnect the three cables and then the pin on the bottom left seats into a connector on the main board.
Slide the power supply back in and reconnect the ribbon cable and mains connector.
Put the top cover and its 10 screws back in place.

Step 7: Power Up and Calibrate

Set the calibration switch on the back to unprotected then power on the unit while holding a key in on the front panel. The scope should alert you that the default calibration has been loaded.
Now you can go through the self calibration process, which you may be familiar with, if not follow along.

Press the Util button, go to the Self Cal menu. Select cal 0, channel 1 (both are the default selections), then press Start cal. The scope will remind you to connect a cable from the DC port on the back to the channel 1 input, then press continue. After several minutes the calibration will succeed, change to cal select 1, swap the cable from DC to the AC port and press continue. The delay cal goes much quicker.

Repeat this process for each channel and you're done.

The calibration will now be saved through powering off and back on, no more recalibration.
Move the switch on the back to the protected position to prevent overwriting the calibration.

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8 Discussions


3 years ago

Is there a difference between "original" ds1230y-120, and the chinese ones i see on ebay. It would save me good 20 bucks.


Reply 3 years ago

I do not know if you are still looking but I found one for about $11 in US. see eBay item number 361787213071. Otherwise watch the clock speed. IE: -120 and not the 150.


3 years ago

Hello. I have replaced the chip. ALL system tests pass. Set the switch to UNPROTECT and do a self cal. All cals are successful. Then switch back to PROTECT and power off. When it boots it gives a "CAL RAM checksum error, RE-CAL Unit"

Does anyone have any ideas what I am doing wrong? Thanks!

Thanks for the write-up. Great job!


4 years ago on Introduction

Hi, thank you for the explanation. What does it happen if I use ds1230y-150 instead of ds1230y-120?


Reply 4 years ago on Introduction

I'm sorry, I don't have a definite answer for you.

According to the specifications the only difference is the access time, the 120 being faster than the 150. The original chip did not have a marking for the access time, but in my research the 120ns was mentioned as a replacement.

Maxim recommends replacing with either the same or faster speed here. So I would stick with the 120 or better if at all possible. 

However, I don't know if it will work or not. So, if you're going the socket route and have a 150 on hand, it might be worth a try. If ordering though, I'd stick with 120.


Thank you so much! I'm not sure if I would have had the courage to go through with it without this tutorial. My oscilloscope works perfectly now!