Electronics > Repair

HP 3478A: How to read/write cal SRAM

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radar_macgyver:
I replaced the battery on my 3478A prototype without losing the existing cal. After that, I figured I will try to cal the DCI ranges per the instructions in the service manual, but I found that I was unable to switch the instrument into cal mode even though I moved the front panel cal enable switch to the vertical position. I wanted to test if the signal was making it to the main board (in case there was a bad wiper), and found that the board is somewhat different from the one shown in the service manual. I don't have a U515 anywhere on the board, this is the dual 4-input NAND gate used to gate the WR signal to the CMOS RAM.

It almost looks like the switch is simply not connected to anything on the main board - probing the wiper while turning the switch between its two positions does not change the voltages on either terminal of the switch. Pin 9 Maybe they hadn't implemented the self-calibration routine in the prototype.

How does the firmware know what position the cal enable switch is in (in the non-prototype version)? It seems like S550 enables the microprocessor's WR signal to get connected to U512's R/W pin. Without any direct connection to the switch input, how does the micro know that U512 is not write-enabled and put up the "ENABLE CAL" message?

Edit: removed an incomplete sentence

pqass:

--- Quote from: radar_macgyver on April 15, 2024, 04:48:50 am ---How does the firmware know what position the cal enable switch is in (in the non-prototype version)? It seems like S550 enables the microprocessor's WR signal to get connected to U512's R/W pin. Without any direct connection to the switch input, how does the micro know that U512 is not write-enabled and put up the "ENABLE CAL" message?

--- End quote ---

U515 should be just behind the LCD and in front of the red relays.  See attached photo.

I think it was mentioned earlier in this thread that the meter tries to periodically write alternating values into location 0 of the cal ram. If a new value is written and it sticks, then it's in cal enable mode.  Note, you must be aligned perfectly vertical in the detent; a couple of degrees either way won't close the switch.

m k:
TP6 is write, active low, cal switch is letting it through.
You can scope RAM pin 20, it goes low for write.

Middle of manual Sep-87 ed. 2 E0987 has another set of schematics.
There figure 6-5 cal uses C and D parts of regular two input NOR U515.
Table 6-10 replaces 7420 to 7402.

radar_macgyver:
See below for a pic of my meter's front end - U515 is not at the location indicated by the assembly drawing in the service guide. Another thing that I had started to type up but forgot to finish is that the schematic indicates pin 9 from the front panel board carries the Cal Enable signal. On my board, this pin is grounded. In the attached picture, you can see the trace from the front panel connection is a thicker one, that connects to the ground plane.

I think I just have the older rev board, referenced by the schematic figure 6-5 that m k mentioned above. The difference is that while that schematic shows the two-input NOR gates as U515, on my board this is labeled U508 (part number is 74LS02). Interestingly, the theory of operation section 7-F-63 specifies U508 as the device that drives the CMOS RAM's R/W pin. They must not have updated the text after the schematic change.

It also turns out the plastic tab that holds the cal enable switch wiper was very brittle and a piece of it broke off while I was probing the wiper so it probably wasn't making good contact. I soldered a small jumper between pin 10 of the front panel cable. When grounded, the display shows an extra 'C' indicating cal mode.

I attempted a calibration of the DCI 300 mA range and while the zero cal worked, the 1/3 scale cal gave a 'value error'. I measured the shunt resistor R107 using the 4-wire mode on my DMM6500 and it showed 0.098 ohm, so about a 2% error. This should not result in the nearly 25% error observed in the readings. The fuse installed is rated 2A.

Just for fun I tried running the cal procedure again and it worked. Not clear why it failed the first time around - maybe because the initial error was so large? Repeated this in DCI 3A and all seems well. I verified the results with my DMMcheck and readings were within 0.001 mA.

The service manual seems to indicate that a separate cal for the AC current is not required since the cal values for AC current are derived from DC current, though mine still showed the same ~25% error it used to. Will attempt an ACI cal, but I need to borrow some equipment from work. Thanks all for the help with this!

m k:
Figure 7-G-5 (03478-66501) is the older schematics.
Figure 6-5 (C3478-66501) is the newer one, with 7402.

So your machine has a newer configuration.

What kind of U502 you have?
Section 6-25 says that revision D changes it to 28 pin chip.

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