Thursday, April 7, 2016

HP 54503A NVRAM Replacement - DS1235YW to DS1230Y

A few weeks ago I turned on my HP 54503A to find it behaving strangely. While probing a circuit under test, I could not get any waveforms at all. Was very unusual, so ran a quick self test and was greeted with the following:

HP 54503A Selftest Failure - Protected Non-Volatile RAM

This didn't look good.  An important key to the cause of the failures was the failure of the Non-Volatile ram. Most older equipment like this used a Dallas Semiconductor real time clock / NVRAM IC to store the calibration data along with waveform storage. In this case it is not a real-time clock, just a battery backed DS1235YW RAM ic. This classic chip has an internal battery to backup the contents inside, technology that existed way before inexpensive flash memory that has a limited lifetime. Based on the manufacturing date on the late 1980s, this ic was almost 30 years old, way beyond its expected lifetime but about average for how long I see these devices last.

The HP 5450NA family of scopes 54501A, 54502A, and 54503A are really nice scopes readily available for cheap. My favorite part about them is the interface, it is intuitive and lightning fast. Response from button inputs is instantaneous, unlike many modern scopes which makes it a pleasure to use. This was definitely getting repaired.

A quick way to check to see if the NVRAM is indeed the issue is to flip the write protect switch on the back of the unit off and attempt calibration. Once you calibrate this scope, you flip this switch to protect the contents from accidental writing, but in this case I want to re-calibrate it and to do so you must enable writing. Once this switch was changed I ran the calibration procedure for channel 1. It calibrated successfully and brought the scope back to a working state. So this was the only problem and the NVRAM ic will need to be replaced, unless you feel like going through a full calibration every time you power the scope on. Since a full calibration of all channels takes around 25 minutes, this is not ideal.

HP 54503A Main Board

This scope comes apart very easily, the main board slides out of the back with little effort. Once removed the Dallas NVRAM was easy to spot:

HP 54503A DS1235YW

Removing this ic is pretty easy, use a nice fat hot iron and some fine solder wick and the ic falls right out with no damage to traces.

HP 54503A NVRAM Removed

As far as a replacement, the original DS1235YW is not easily available, but the pin compatible DS1230Y is available. It is still made and Digikey sells it for $30. Then there is the eBay source of them direct from China for only $5. While the Chinese version is guaranteed to be a knock off, it does work as I have used them before. I went ahead and used one for this replacement since I have a few laying around.

There are also available pin compatible flash based alternative ics that you can use in this scope, but I have never personally tried them. One last note is as long as you are going through the trouble of this, go ahead and place a socket on the board so in the event your new NVRAM doesn't work, you won't have to stress the board with another de-solder. Here is the replaced ic:

HP 54503A DS1230Y NVRAM

Once replaced and everything is back together, calibration is the last step. Set the write protect switch to off and run through all the calibration procedures. The calibration for this scope requires a 50ohm bnc cable to be fed from two outputs on the back of the unit to each channel is sequence, the scope really walks you through everything. The manual will explain all of this as well, but is really a simple procedure and a really nice feature.

HP 54503A Calibration
When done, set the switch back to write protect, test it out and power cycle it to verify all calibration was held. Good as new.

1 comment:

  1. I'm a bit of a newb. I wasn't careful when I ordered the new NVSRAM DS1230Y IC and it reads DS1230Y-150. I understand this to be the read/write time. I have the correct DS1230Y-120 coming, but I just wondered if the other would work as well.