Data Precision 2020 Function Generator repair - Page 1
Forum section: electronics / testgear

author: ioprev
author_rank: Contributor
timestamp: March 18, 2020, 05:52:56 pm
content: Hello,New member here in the forum (not that new in the field)I got a Data Precision 2020 function generator that doesn't boot up. With power on, only fan works everything is off in front panel. First thing I checked was the PSU, which seems to work as it gives stable +5V, -5V, +15V, -15V and 3.8V(?) outputs.However since I only have the User’s manual, I cannot say if this is the correct voltages and not really sure what to check next. Is there anyone with experience repairing this instrument?Service manual would help if someone has a copy of it.Thank you very much!John

author: fcb
author_rank: Super Contributor
timestamp: March 18, 2020, 09:31:11 pm
content: 3.8V sounds obscure.1. See if you can reverse engineer the PSU to work out what it should produce?2. Look at what it drives, does 3.8V seem right?Can you measure the current running through each supply? This might give you an idea. Alot of this old gear had tantalum caps..

author: duak
author_rank: Super Contributor
timestamp: March 19, 2020, 12:34:27 am
content: Ioprev,As fcb says look at where the power supply voltages go. Look for leaking or damaged capacitors and for loose connectors.Since the display is not working I would see what voltages it needs. There should be a low voltage AC signal for the filament and a higher DC voltage for the anodes.Is the display board from a different company?The 3.8 V is unusual. Since this unit is likely from the 80's or 90's it probably has some ECL in it for the high speed waveform generator. ECL usually used 5.2 V so perhaps the 3.8 V shows there is a fault on that line.Let us know what you find.Good Luck,

author: Testtech
author_rank: Regular Contributor
timestamp: March 19, 2020, 10:39:00 pm
content: Could the 3.8V be the VFD filament supply?

author: duak
author_rank: Super Contributor
timestamp: March 20, 2020, 12:52:20 am
content: The 3.8 V could be for the filament - is there an AC signal in addition to the DC?.VFDs usually use AC because DC affects the brightness from side-to-side. see:https://www.noritake-elec.com/technology/general-technical-information/vfd-operation

author: ioprev
author_rank: Contributor
timestamp: March 21, 2020, 08:07:08 pm
content: Thanks for your lightning fast response! Let me share some more findings as of now.Some things I forgot to mention in my first post:- This machine indeed has hundreds of tantalum caps inside. To give you an idea, apart from the PSU, I found only two electrolytics on the other boards. (One in front-panel, rated 350V, that seem to be part of a dc-dc converter for powering VFD anode, and another one on a side board)- As of now, I have visually inspected every board, looking for physical damage on components. Inspected every single cap (looking for burn marks or any leakage) but found nothing as of now.- No AC voltage is leaving the PSU. I didnt had the chance to measure current on PSU output lines (will do so), but comparing between load attached and without load, there is very little variation on voltage (e.g. 5.16V goes to 5.08V with load, etc.). This also made me believe that there is not any short on the lines.Bellow is a photo of the PSU board (please ignore the sharpie marks that I made on the output header), and what I measured on the output lines with load disconnected (Pins 1-2-3 are connected into the same line, so does 4-5-6)Output (bottom to top)Pin 1 GNDPin 2 GNDPin 3 GNDPin 4 5.16VPin 5 5.16VPin 6 5.16VPin 7 GNDPin 8 5.12VPin 9 -5.42VPin 10 GNDPin 11 GNDPin 12 14.99VPin 13 -14.84VPin 14 GNDPin 15 3.8VNow, 3.8V is really obscure, so I tried to follow the path of this signal. Starting from the PSU, this signal goes into the main board assembly. From there I found only one connection of this signal that goes into the main logic-board (which is the fourth plugin card; photo bellow)Into the main logic-board, the first branch of this signalgoes to the ~CS input of an MC68230P8 parallel interface driver(red arrow) and a second branch goes into a jumper (red arrow). I didnt traced further than that.As noticed from datasheet 3.8V should be logic HIGH for this IC, setting it into a disabled state. My main suspicion for this signal now, is whether it could be some kind of "PSU ready" signal, preventing output on the bus until it gets low (?). That brings me back into further investigating the PSU...Will follow up with any more findings, meanwhile any thoughts are more than welcome!

author: ioprev
author_rank: Contributor
timestamp: March 22, 2020, 12:42:12 pm
content: Measured currents:+5V line: 7.3A-5V line: 2.1A+15V line: 0.14A-15V line: 0.15A

author: srb1954
author_rank: Super Contributor
timestamp: April 11, 2020, 10:59:29 am
content: I also have a Data Precision 2020 for repair. It has a bad PSU and I haven't made much progress on it yet.The PSU in mine (Todd MAX354-9920) differs from yours but it has a separate header with a connector pin labelled PF, which is presumably stands for Power Fail.This would allow the CPU to shut down in an orderly fashion if there is a power failure. A wire connects from this pin to pin 5 on the power connector on main backplane. Presumably this is a logic level signal, which would explain the 3.8V you are reading. It might pay to check to see if this signal changes state as the unit is powered on and off.I can't verify the state of this signal on my unit due to the PSU not working properly.Good luck with your repair efforts. If you manage to find any schematics for this unit please share them with the community. This device is a very interesting piece of test gear but there seems to be practically no documentation available for it.

author: Rerouter
author_rank: Super Contributor
timestamp: April 11, 2020, 11:09:33 am
content: Worked on one of there oscilloscopes,check all the dipped tantalum's for shorts (some where in places that did not actually effect the main rails), found on both the logic cards and the main supply, including the processor reset filter.check for opens or large value shifts on any carbon compound resistors, found may 1-10 Ohm resistors measuring 80-300 ohm, causing things to be on edge.And had a leaky diode on the main rectifier,Other than that the logic did not have any major issues on mine, it ended up being a blown out CRT so had to settle with the video output, but the issues where the above, might help you narrow down a fault

author: ioprev
author_rank: Contributor
timestamp: April 11, 2020, 09:46:51 pm
content: Well, its been some time that I didn’t had the chance to work on this but hopefully during lockdown updates will increase in frequency.So, trying to understand this 3.8V signal I decided to tear the PSU apart. As seen in the previous picture, the PSU consists of a main board and a ‘control’ daughter-board where the SMPS control circuit is located. By first look its not something over-complicated, just a NE5560 and an LM358 with two comparators and some diode logic driving the MOD pin. This control board is powered from a separate transformer on the main board that together with 2 diodes generates 22V DC. The components on board are powered by a 10V line coming directly from a 78L10 regulator.The control daughter-boardNow, back to our mysterious 3.8V signal on the main board there is a simple circuit driving this from 5V with a 1N751A zener (I suppose for protecting the logic board ?)The base of the transistor in main board is driven from a trace that is coming from the control board where again a simple circuit, will enable this signal when the power to the board falls below 12V.With all that, I conclude that it seems to be a power failure signal, as srb1954 said, that is active when LOW.Meanwhile, having desoldered most of the caps on the PSU I decided to replace all the electrolytics with new ones before reinstalling it back into the unit.Investigation will proceed with the main board assembly of the generator

author: ioprev
author_rank: Contributor
timestamp: April 11, 2020, 10:07:06 pm
content: Quote from: srb1954 on April 11, 2020, 10:59:29 amIn case you haven't seen that already, I found some info for your supply here:https://store.dbgweb.net/max-354-9920Pin 5 is indeed the power failure pin, but seems to be active LOW from what I’ve seen on mine.Meanwhile, I measure 24 Ohm resistance on mine, between 5V and GND which I don’t think is a good sign...Im going to tear down the main board assembly tomorrow, desolder all caps and check if there is a short somewhere. There are around 15 tantalums on main board which need my first attention.Apart from the tantalums, near every single logic chip, there is an A5E104M, which by the name I suppose are 0.1uF caps. They look like a rectangle version of a ceramic cap.. Are these also failing that often?

author: Rerouter
author_rank: Super Contributor
timestamp: April 11, 2020, 10:19:09 pm
content: Ceramic caps are fairly safe, I have not yet seen one in a decoupling role fail, there voltage rating is usually the highest on the PCB, value shift yes, fail partially shorted no.

author: srb1954
author_rank: Super Contributor
timestamp: April 12, 2020, 12:49:26 am
content: From your sketches the circuit appears to be a simple level detector on in the incoming supply to the regulator so it forms a crude early warning power fail indicator.Looking back at the picture of the main CPU board the red arrow points to pin 32 on the MC68230, PC2/TIN. This pin can function as either a general purpose I/O line or a timer clock input. It could be that the S/W configures this input as a timer control to generate an interrupt signal out of the MC68230 with activity on the PF line, possibly after a short time delay.Check to see if there is any activity on the PC3/TOUT line (MC68230 pin 33), which can be configured as an IRQ output from the timer subsystem. If this IRQ line is stuck low then the main processor is being continuously interrupted and won't be able to run normally.

author: ioprev
author_rank: Contributor
timestamp: April 12, 2020, 08:04:53 am
content: Quote from: srb1954 on April 12, 2020, 12:49:26 amThank you a thousand times for this!!! This evening I had somehow convinced myself that pin 32 was ~CS...Now it makes much more sense!

author: ioprev
author_rank: Contributor
timestamp: April 12, 2020, 08:11:19 am
content: Quote from: Rerouter on April 11, 2020, 10:19:09 pmIndeed, they are ceramic caps from AVX.. Never seen this package beforeVoltage rating is 50V (!)

author: ioprev
author_rank: Contributor
timestamp: April 12, 2020, 02:37:53 pm
content: Started debugging the main logic board today, however it's a bit difficult to take measurements on the board while it is inside the board drawer.Then noticed that there is a 64-pin header on the back side of the main logic board so I decided to follow the traces from this pins; most of them are directly connected to the 68000 processor socket.For anyone that may visit this in the future, here is the pinout of this debug headerNow I quickly noticed that on mine the clock is running but ~RESET and ~HALT lines are kept at zero volt. Will not continue searching on the reset circuit.Here is the clock signal:

author: fcb
author_rank: Super Contributor
timestamp: April 12, 2020, 02:48:06 pm
content: Not familar with that MCU, but would expect _RESET to be high when running. If you trace that line back, you might find a watchdog/brownout circuit. This might well give you a clue as to what voltages to expect in a correctly functioning unit.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 12, 2020, 03:20:20 pm
content: I have a 2020 which should be ok. As I also have the 2045, that is the one that gets used. Another user has aquired at least a 2045, or more. But none of us has a service manual.I could provide measurements on the 2020, but not instantly.

author: duak
author_rank: Super Contributor
timestamp: April 12, 2020, 05:37:58 pm
content: I've attached a simple description of the RESET and HALT lines. I would trace out the circuitry attached to these lines to see if there is some extra logic that asserts these signals when starting.I remember the AVX DIP caps from the early 80s. AVX claimed lower impedance and were easier to insert by automatic assembly equipment.

author: srb1954
author_rank: Super Contributor
timestamp: April 12, 2020, 10:59:11 pm
content: The ~RESET and ~HALT lines should both be high for normal running of the MC68000 processor. I think you need to hook up an oscilloscope to look at the sequence of signals on these two lines from a power up.The ~RESET line should start out low on power up and transition high when the power supplies and clock oscillator stabilizes. The fact that it is stuck low may mean that the reset circuit has not recognized the main power rails as having settled. A quick look at the PCB shows there is a LM393 comparator (Z22)and a LM385 voltage reference (CR1); these could be part of a voltage detector for the main +5V. I think you need to trace out the reset circuit and verify it is working correctly.Another reason the ~RESET line is stuck low could be a watchdog time-out. If this is the case the ~RESET will transition high after power up and then transition low again after a time delay, probably in the range 100us to 10ms. I don't see a dedicated watchdog timer IC on the PCB but the equivalent function could be built into one of the PAL chips. Normally a 68000 design would have a watchdog timer or bus time-out circuit to recover from a processor hang if a device on the bus doesn't respond.The ~HALT line being stuck low probably indicates that the processor has detected an bus error condition and is unable to continue running. This could occur if some device on the bus is not responding or if there is a failure in the handling of an interrupt signal e.g. from the PF circuit. Trace out the circuit around the interrupt encoder chip (Z51 74LS348) and see if there is an active low IRQ signal on any of the input lines to this chip or if there is an active low signal on any of output lines to the the processor interrupt inputs, ~IPL0, ~IPL1 and ~IPL2.

author: srb1954
author_rank: Super Contributor
timestamp: April 13, 2020, 02:11:20 am
content: Thanks.This connector pin-out will be very helpful if I ever have to connect a logic analyzer to the system.

author: srb1954
author_rank: Super Contributor
timestamp: April 16, 2020, 12:40:58 am
content: The attached schematic extract may help your investigation. This isn't for the Data Precision 2020 AWG but is from the Analogic/Data Precision 6000 Universal Waveform Analyzer. This is also an MC68000 based design so it is likely to share some design features with the 2020 logic.This reset circuit incorporates both a supply voltage detector and a watchdog time-out, both of which override the reset circuit and force the CPU into reset. Helpfully, there is a jumper link which can be used to disable both the voltage detector and watchdog and allow the reset circuit to only function as a normal power-on reset. If you can identify a similar link in the 2020 reset circuit it should greatly facilitate debugging the circuit if you can disable the watchdog and voltage detector circuitry.Note the reset voltage detector is likely to be completely separate from any power fail signal from the PSU; the PSU power fail signal will provide an early warning of impending power failure and allow the CPU to save critical data before the power fails totally. The reset voltage detector will force an immediate reset on the CPU and unsaved data will be lost.Sorry about the poor quality of the scan but the original itself is barely readable.

author: srb1954
author_rank: Super Contributor
timestamp: October 19, 2020, 10:41:22 am
content: Quote from: ioprev on April 12, 2020, 02:37:53 pmDid you ever get your 2020 going? I managed to get mine going after repairing the PSU and another fault in the reset circuit.It is a really interesting piece of test gear to play with but you need to spend a lot of time reading the manual to figure out how to best use it.

author: ioprev
author_rank: Contributor
timestamp: April 07, 2024, 04:56:34 pm
content: Coming back to this thread as I got another scrapped 2020 unit. Fortunately the logic board was good on this one so just swapped it to the old one that has been 4 years in storage… And it finally booted!Now, out of curiosity, started swapping the PALs between the logic boards and the one named CZ5 seems to have done the thing. I suppose the old one was gone.I think I already know the answer, but is there any way to backup the config of these PALs? Tried hooking on up in TL866 and all I got is a bunch of 0xFF. That should mean that they are protected, right?Also found a user manual here:http://manuals.repeater-builder.com/te-files/MISCELLANEOUS/ANALOGIC%202000,%202020%20User.pdf

author: ioprev
author_rank: Contributor
timestamp: April 08, 2024, 11:01:24 am
content: Another question (maybe a noob’s one): My other unit has the MAX354-9920 PSU installed in it. In terms of voltage, current and ripple measurements it works fine though every time it is turned off I can hear a clicking noise for like 20 secs from it; it kind of reminds me the noise old CRTs were making.Any idea what could be the source or the noise? Is yours making the same noise when power is cut?

author: srb1954
author_rank: Super Contributor
timestamp: April 09, 2024, 07:40:04 am
content: Quote from: ioprev on April 08, 2024, 11:01:24 amNo obvious noise from my unit although I didn't take the lid off to get a closer listen and my 70-year old ears aren't so great these days.I would take a look at the capacitors in the PSU as, with the age of the unit, some of these have probably dried out. In my unit a 100uF capacitor on the supply to the SMPS controller chip had dried out and caused all sort of strange start-up symptoms.