HP 4145A Error A01 - Page 1
Forum section: electronics / repair

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: November 15, 2024, 12:01:28 am
content: So, the first time I turned on the device, the error was not present. But next time the A01 is displaying constantly. I checked everything (voltages, oscillograms) that is possible. Went through all Test Sequence Flow Diagrams from OSM for various boards. All tests and LEDs are showing that there should be no errors. Packets of data SRD SCD and SCK are flowing nicely between A2 and A3 boards after self test button is pressed. Still, error A01 means basically that there is a problem in communications between A2 and A3 boards (optocouplers are ok). Cant figure out yet. Any help or ideas are much appreciated.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: November 25, 2024, 08:25:07 pm
content: I narrowed the issue so now it is obvious how to reproduce it. When device is cold it shows the A01 error but once it is turned on and heats up for 30 seconds, after you quickly turn off and on again the error will gone and device passes self test.Can someone with device post oscillogram of how RESET signal looks like? This is A12 DC power supply board TP8, reference to chassis ground.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 26, 2024, 12:08:01 am
content: I'll dig mine out in the next few days and hook up the scope.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: November 27, 2024, 10:44:16 pm
content: Got better understanding with time. The exact reason of the error is A3 board initialization delay when cold. The 4 didoes on board are blinking all the time, means that board is not doing any internal tests. A2 boards expect immediate response but A3 is not initialized thus error appears. Once it heats up for 30 seconds, the initialization is immediate. I tried to heat up device without faulty board inserted but it is not helping, this means that A3 board need to be hot itself.Also tried to heat with hot air gun ROM, CPU and ASIC ICs, but it is not helping. Still, the board needs to be inserted and heated up in the 4145A itself.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 28, 2024, 02:08:20 pm
content: Did you still need any measurements taken?Have you checked the voltage rails for voltage and ripple?

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: November 28, 2024, 02:30:14 pm
content: Quote from: TERRA Operative on November 28, 2024, 02:08:20 pmYes, I need to make sure that my RESET is ok.Voltage rails have 1V ptp ripple but it is just a high frequency from switching power supply and digital logic itself. Anyway all caps were replaced, so there is nothing to improve.TERRA Operativecan you also please post how ripple looks in your 4145 on +5VF and -+15VF lines?

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 29, 2024, 02:50:53 am
content: I'll finish excavating my pile of gear in storage and take the measurements this weekend.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 01, 2024, 06:12:04 am
content: Here are the ripple values I get on my voltage rails.Scope set to AC coupling, 10x probe, 20mV/Div Vert, 10ms/div Horiz, 20MHz bandwidth.Images in order of Table 5-3 on page 5-10 of the Service manual.For the reset line on TP8 on PSU board A12, it just goes straight to +5VDC as soon as I turn the power on and stays there.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: December 03, 2024, 04:52:44 pm
content: Quote from: TERRA Operative on December 01, 2024, 06:12:04 amWith oscilloscope settings like yours, my peak to peak values look identical.I moved another step closed in solving this puzzle. I tried heating IC one by one and stating the device immediately. Heating U33 turned to be fixing the error A01. The case 5 out 5 reproducible, I tried ICs around U33 as well. However replacing U33 with new one did not solved the issue, new IC behaves in the same way, on cold A3 is not initializing, when heated up to 60C A3 board initializes immediately after power on. I checked all traces from it they are all ok. So I suspect there is a timing issue somewhere, TTL logic tends to decrease threshold voltages with temperature.

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 03, 2024, 04:57:30 pm
content: It seems something is marginal, a TTL chip hanging on for dear life?

author: picburner
author_rank: Frequent Contributor
timestamp: December 04, 2024, 05:05:54 am
content: It seems that the "A01" error when cold is endemic to this instrument, my 4145B does the same thing but once warmed up it works perfectly without any problems.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: December 04, 2024, 08:16:00 am
content: Quote from: picburner on December 04, 2024, 05:05:54 amFrom the manual it looks like I should wait 40 min for the instrument to be functional and that is normal behaviour? Does not seems like HP thing.The second notice states that A01 will be shown if device is in test mode because of DIP switch settings. S1 switch is located in IO bus and sets some bits on it.TERRA Operativedoes your show the same behaviour when cold?

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 05, 2024, 12:29:10 am
content: I have never seen that error on my 4145A or 4145B that I remember.I'll boot the 4145A up again and see. (And the 4145B too if I can actually find my HxC floppy emulator, if that would help)Maybe I'll stick it outside in the cold for a bit and see if that does anything.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: December 19, 2024, 10:49:36 pm
content: So, one step closer. After replacing U33 74LS245 with much slower 74HC245 board starts initialization immediately and passes all self test steps but never finishes it. It starts it over and over again. So it is 9ns vs 15ns, I guess, I need to find something like 11-12 ns delay.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: January 02, 2025, 07:31:36 pm
content: Can someone give me a hint what is going on ? When I remove U33 the board initialize normally and stops at point of AD circuit communication, which is normal because U33 is just a buffer for AD circuit and nothing else.When I install new U33 the board is not initializing at all, not even self test! Something from AD circuit is spoiling the communication?

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: March 21, 2025, 07:59:18 am
content: So the reason for this Error A01 was degradation of Test SW Buffer U36 DM81LS95N, after replacing it device initialized immediately even when cold.Figured out by replacing all ICs in data bus. Thanks to all who tried to help here.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: April 20, 2025, 07:41:48 pm
content: If anyone is interested, I’ve made a few videos showing the full restoration and repair process of my HP 4145A