LeCroy LT264M repair, does not power on. Update: powers on but broken trigger

[Jontm]

Hi!

I got my hands on a LeCroy LT264M that have spent the last 8-10 years in a semi open shipping container exposed to the elements 
All the other equipement i found in this container seems to be working ok, but the scope has some problems powering on. Fan turns on and standby led light ignites then i press the power button, but the display does not turn on. If i turn it off and try to turn it on after 5+ minutes, the screen turns white for a second before turning off and again getting stuck on the standby led.
From what i can tell the standby light indicates start of self test. I guess it does not pass the self test and gets stuck.

It did start one time, but i have not been able to reproduce this. When it started, i was able to run a test on all the channels to ensure that they where working ok. Everything was working ok. Except for the fact that the next time i tried to turn it on, it gave the same short white flash on the screen and stuck on standby led.

I have opened the unit up and it looks mostly ok without any obvious signs of damage or black spots. I have measured all the voltage rails and they looks to be ok both on the CPU board and the main acquisition board. Tried reseating all the ribbons and connectors, including the flash card and SD-RAM.

I have a feeling that the issue is on the CPU board, but i have not been able to pinpoint it yet

I have focused my main efforts on this circuit that should control voltage to the display, picture attached.
From what i can see, the service manual for this unit does not contain a schematic for the CPU board. I found the schematic for the CPU board on the waverunner 1, but it seems like the VGA circuitry for power to the display is slightly different on the waverunner 2 series. I have however identified that Q10 (and Q21) does turn on for a fraction of a second when i hit the power button, before turning off again. I have however not been able to identify where the signals to turn on and off Q10 comes from, it does not go to the IC86 as on the waverunner 1 schematic.

Anyone have any experience with this scope or perhaps have any ideas where to look next ? Is there any way to bypass the self test or find out what step in the self test it fails on ?

[shakalnokturn]

One of the first things to check when experiencing intermittent power-up fails other than PSU voltages and ripple on LeCroy scopes (among other TE) is the RTC backup  battery voltage.

[Jontm]

One of the first things to check when experiencing intermittent power-up fails other than PSU voltages and ripple on LeCroy scopes (among other TE) is the RTC backup  battery voltage.

Thanks for the input. I checked the battery voltage and it looks fine, just over 3 V.
Is there any point in trying to connect to the serial port ? Maybe it posts some init logs to serial when i try to boot it ?

[Jontm]

I have done some more work on this and it seems like there is no signals on at least the external 232 port of the scope during boot. At least with the baudrates that i have tried with.
Looking further into the CPU, it seems like that the scope has a JTAG connector with some perhaps useful signals for further analysis:

NSRESET - Measured to be at 5 V
NHRESET - Measured to be at 5 V
NCHECKSTOP - Measured to be at 5 V

Based on this i would assume that the CPU is at least not in a checkstop condition ? And not in a hardware or software reset condition when the scope is stuck with black screen and the standby light illuminated. This might not be of any use or relevant to check, i am unfortunately not familiar with embedded processors and what might be relevant to check.
Is there any other relevant pins to probe to assess if the processor has started properly and is operational ? Are anything specific known about the power on self test ? Such as steps the processor go though and possibly any probing locations that might be relevant ?

[shakalnokturn]

Considering the scope's history you may want to inspect for corrosion on the FPC and SDRAM contacts.

In normal conditions the standby LED comes on during splash screen then goes off, blinks briefly during the single beep when the graticule is displayed and stays off after that.
If screensaver is enabled the standby LED comes back on while screen is disabled.

Check that no reset is stuck for some reason IC4 (CPU), IC86 (LCD power, VGA reset), IC2  (Bus control).
That CPU clock is running IC3, IC2.
You should be able to use the unpopulated SW1 if you need to make repeated hardware resets.

After that probe around for various bus activities on reset that may point to where it gets stuck.

The CHIPS VGA controller is known to fail on the LeCroy LC series but those do run hotter than the LT.

[Jontm]

Thank you for your valuable input and knowledge. The scope was in much better condition visually than what i expected based on the prior history. No visible corrosion on any contacts and such. Mostly just some old dust/mold on the main board inputs. Cleaned most of it away.

I did some probing around and from what i can find NRESET is high (5 V). The NRST on Q6 is however only 1.4 V, i am not sure if this is how it should be. I figured it should be either 5 V or 0 V, but i do not understand the circuit well enough yet... The clocks are looking ok.
Probing memory activity on the CPU and bus activity on the VGA controller it seems like all activity stops around 800 ms after power on. This seems a bit odd. If i short SW1, there is about 100 ms with activity and then nothing. I the stop in memory and bus activity seem to coincide with the output of the reset circuit IC86 going high.
My problem is that the only schematics that i have of the CPU and VGA if from the service manual for the waverunner 1. It looks like there have been changes on this exact circuit on the waverunner 2 series.
Q10 does not connect directly to the output of the IC86 on my scope. There is at least one extra transistor that switches the connection between IC86 and Q10. I have however not been able to find anything that connects to the base of this transistor.
I think i will do some more digging tomorrow, got a feeling that i should investigate the NRST signal and IC86 a bit more.

[shakalnokturn]

FWIW & if it can be of any help it should be possible to boot the scope from an adequate SRAM card containing the firmware, useful to recover from a bad flash or when in doubt on the onboard flash IC's themselves.

This was at least possible with the 93xx and LC series scopes, unfortunately there is no documentation around on that feature for the LT series.
I assume it involves jumpering somewhere on P2 and that the last four lines in the table at the bottom of sheet "BUSCONTROL (33)" on the WR1 service manual also relates to that although I can't see how to apply it to the WR2.
I'd have gone out experimenting on my LT but I'm stuck for a way of getting the firmware image onto a SRAM card.


Concerning NRST: 1.4V sounds OK, R17 and R18 voltage divider same on WR1 and WR2.
NRST goes low either on a SW1 press or driven by Q6. I assume Q6 is part of the front panel key combination software reset.

[Jontm]

Ok. Thanks! I have looked into the possibility of flashing new firmware, but only have a CF adapter in my PCMCIA slot right now. Would it work with a CF card ? I also have no idea on where i should start looking to get a copy of the firmware even if i could figure out how.

I cannot find any bus activity out from the VGA controller, but i guess that might also be due to none being sent from the CPU. Nothing on pin 53 (activity indicator) of the VGA controller either.
On the CPU there is only about 100 ms of activity between the RAM and the CPU after every reset. If i disconnect the RAM, then there is no activity at all and the scope does not even reach the stage where the standby led turns on. Disconnecting the ribbon cables to the main board also have no effect. Still only 100 ms of activity when reset is triggered.
Based on everything so far, it looks to me like a problem either with the RAM or the CHIPS VGA controller. I think i will try to get hold of a new stick of RAM and possibly also order a new VGA controller if i can find some cheap on AliExpress or something.

[shakalnokturn]

For the firmware and other advice subscribe to the LeCroy owners on groups.io

If you don't want to bother you can get the firmware here until I need to free some storage space:

http://eninz.free.fr/wr2-9.3.0/WR2_(Blue_Cabinet)_ver_9.3.0.zip

I don't think you can update or rescue boot from CF.
You'll need a minimum of 2MB PCMCIA SRAM which are expensive and not that easy to come by, after that you still don't know the jumper config to start from the card.

Definitely give changing the SDRAM a try first that should work-out cheap.

[Jontm]

Thanks for all your valuable help. I am happy to i form that i finally found the fault. It looks like it was just a bad memory stick. I hope that the information in this thread can be of help to anyone else that have similar issues.

My only issue now is that the trigger calibration fails and that the scope freezes up and displays an error message if I hit the panel button. Not a huge issue for me at least. Will do some more digging to see if I can find out. Does this scope have a self test menu or similar that I can enter to test all systems ?

[shakalnokturn]

Lucky for the SDRAM.

Well... a working trigger is a useful feature on any scope, does it just fail calibration or not trigger at all?

The maintenance menu and error log is accessed by exiting any menu by pressing the return key a number of times.
Pressing and holding 3rd menu key, pressing and holding 4th menu key, pressing and holding 5th menu key, releasing 3rd then 4th then 5th.

[Jontm]

Thank you! That menu looks very helpful although a bit overwhelming. Yes, a working trigger is pretty essential

I enabled calibration logs and found that all channels fails the trigger test on both positive and negative, AC and DC coupled.
From some further testing i have confirmed that the trigger function is entirely broken. It is not possible to get any of the channels to trigger on anything.
I guess i will have to do some more probing to figure out what is wrong with the trigger circuit.

Will start with checking all the power rails for MST429A, the _RESET and the different clocks. The MST429A has a -2.3 V rail that i have not probed before. Luckily the trigger circuit looks pretty well documented in the service manual, i have a decent hope of finding the problem. I really hope that the MST429A is not broken, in that case i might be at the end of the road...

[shakalnokturn]

I've had the problem of the -4.5V (VEE) supply to the MST412 ASIC intermittent due to a bad Melf diode on a LeCroy 9354, the diode has been changed to a different setup on the LT acquisition board but I'd definitely check that top of the list. (Including the filter 11FL1)

[shakalnokturn]

Sorry, wrong SM... I was referring to WR1, didn't remember there was the schematic for that section in the WR2 SM.
Anyway the same applies... Check common power supplies involved, right up to the ASIC if possible. Then you get the specific ones VEE (-4.5Vmst), -2.3V Vtt(ECL), I think Vbb is generated by the MST429A itself.

If you must probe around the acquisition board make sure you get some airflow on it if running more than a few minutes.

[Jontm]

The voltage rails looks to be ok, i have however found one potential issue. I cannot seem to pick up the 125 MHz clock. I am able to pick up the 500 MHz clock.

Probing on 12Q5 collector i only pick up -3.4 VDC. Probing on pin 7 on 12IC5 i only pick up -1.75 VDC. No clock signal that i can pick up at least, just 10-20 mV noise.
Probing the 500 MHz clock going into 12IC5, i am able to pick up a weak (150 mv pk-pk) signal with a 1.3 V DC offset. The reason for the weak signal is however likely due to the fact that i only have a 350 MHz bandwidth scope available to probe around with right now. I don't really know what signal level to expect for the 500 MHz clock.
I will check at work if i there is a faster scope that i can borrow, but for now i am assuming that the 500 MHz clock is OK. It does however looks to me like the 125 MHz clock is mostly used for calibration and perhaps the adjustable calibration output. From what i can see, a bad/missing 125 MHz clock should not result on trigging functionality not working ? Except if the scope disables all trigging if the trigger calibration fails, that seem however seem a bit far fetched.

[Jontm]

Grabbed a better scope from work to check the 500 MHz clock. Unfortunately I do not have any better probes on hand than the LeCroy PP006 500MHz probes that comes with the LT264. I have attached a plot of the 500 MHz clock. It does seem a bit too low to me. And that might also be the reason for the 125 MHz clock also not being present. Hope that it is one of the resistors or capacitors on the 500 MHz clock that has failed short. If not, then I guess the output driver for 500 MHz clock on the 14IC2 (MCG426) could be bad. Let's hope not.

[shakalnokturn]

I'd call that good, you're really on the edge of what you can probe that way...
For comparison here's what I got with the same probes you used and a DDA-120 as display while looking at the CKL/ _CKL inputs to the MST412 I was on about previously.
I had doubts at the time too but it turned out the clock input was good.

[Jontm]

Ok, thanks. That makes sense. Did some reading and the clock is referenced to VBB (between -1.35 and -1.25 V in this case), i guess that explains the DC offset of around -1.3 V. It might be that the 500 MHz is ok yes. The divider circuit does however not produce a 125 MHz CLK, at least i am not able to capture one with my setup. I might order a MC10EL33 in case that i need to replace it.

[shakalnokturn]

For probing at the fast ECL signals you'd probably be better off with a piece of 50 Ohm coax and 50 Ohm termination on the probing scope.
I'm not sure about this but if the "125MHz CLK" is not active all the time I'd expect it to be at least during power on reset and during an automatic recalibration if that's enabled.
Do you ever see "INTLV CAL" change state during POR?

[Jontm]

INTLV CAL goes high for about 200 ms on startup before going low again (+700 mV, - 700 mV). It stays low during the self test and initial calibration.
That being said, i know nothing about ECL circuits. For all i know - 700 mV is high and the 200 ms of +700 mV is just some power on thing. From what i understand in the datasheet for the MC10EL33, a high reset pin (INTLV) will cause the chip to not output a clock. I am however a bit confused on how to understand the logic high and low signals on a scope with ECL signals. I guess the signaling is current based and therefore a not so simple to measure in terms of absolute voltage levels.

It is very easy to pick up the 500 MHz, at least when i borrowed a faster scope. The 125 MHz i am not able to pick up at all.
It might be wrong, but the way i see it right now one of these three is true:

1. 500 MHz is too weak to for the MC10EL33 to pick up (maybe doubtful seeing that i can pick op the signal and it looks clean. There are also no oblivious shorts, it charges up to between 5-20 kOhm when measuring the resistance to ground on both the 500 MHz and the 125 MHz)

2. I am misinterpreting the INTLV/Reset signal (- 700 mV is high) and therefore the chip is set to not output the 125 MHz signal

3. The MC10EL33 is bad and needs replacing, i have ordered one just in case.

I guess it is also another option that i am completely wrong and that the missing 125 MHz clock has nothing to do with all this. After all, it does not look like the 125 MHz clock is used to anything other than the generation of the calibration signal for the self test. I would assume that it would fail on other things than the trigger calibration if it does not have any calibration signal at all ?

[shakalnokturn]

I don't see why you'd see any negative values for "INTLV CAL", something is wrong. Is your scope DC coupled and reference for probing GND?

I'd expect Low as around 0.3V, High around 3.8V (being pessimistic) and high impedance to rest at 1.8V, that translates to around -2.8V, -1V and -1.3V for high impedance at the MC10EL33 reset input which just about fits the logic requirements.

MC10EL33 would be reset for most positive values (>-1V on pin 1).

Have you tried setting your scope to single trigger on the "125MHz CLK" during sartup or tried looking for one of the "CHx CAL SIG"?

[Jontm]

Yup. DC coupled and ground reference.
Tried setting the input impedance to 50 Ohm, then the INTLV CAL is stuck at 0 V. Never goes high (if that is >0 V)
Tried to single trigger on both 125 MHz and the channel cal signals and I get nothing. Measuring the 125 MHz I get a steady DC voltage of about 3.3 V.

In summary, I measure the INTLV CAL to be stuck at 0 V when probing with 50 Ohm input and around -700 mV when probing with high impedance. Is that sufficient to set Reset on the MC10EL33 and thereby not output any 125 MHz ?

[shakalnokturn]

My comment on probing at 50 ohm coupling was only for the fast ECL outputs.

Regarding "INTLV CAL" (16IC6 pin 7): Don't confuse it with the pin 1 input to 12IC5.
Considering values of resistors 12R82, 12R83, 16R53 on schematic it shouldn't be possible to have a negative voltage at "INTLV CAL". 16IC6 is supplied between +5V and GND so the only case I can imagine to have a negative voltage there is 16IC6 being high impedance or having a defective open output and 16R53 being open. Although that situation should lead to 12IC5 outputs being active and not in a reset state...

Per datasheet 12IC5 divider should be active when its pin 1 is in the -1.5 to -5V range and that would only happen when 16IC6 pin 7 is held low.
If schematic is correct and for the sake of checking the functionality of 12IC5 without desoldering it, you should be able to force it active either by removing 12R82 or "shorting" 12R83 (parallel a low value resistor).

[Jontm]

Yes, that makes sense, my mistake. Probing on the actual "INTLV CAL" signal gives me a steady +5 V. This is constant even during power on self test. It looks like reset is indeed high.
Shorting out 12R83 does produce a 125 MHz clock, i guess proving both that the 500 MHz is ok and that MC10EL33 is working correctly.
I will have to do some more investigation on what should make the "INTLV CAL" go low. I would assume that "INTLV CAL" should go low during auto calibration, but i assumed that auto calibration runs during power on self tests ? I also tried to do a test from the service menu (picture attached) and at no point during this test did "INTLV CAL" go low.
That being said, i still don't know what the circuit connected to the 125 MHz actually do, other than supply the clock for generating CHx CAL SIG. Or in other terms, i still do not know what CHx CAL SIG does. I assume that it is a for a calibration routine, but all other items than the trigger passes the self test. Only thing i know is that it seems like CHx CAL SIG is not being generated during power on self test and the self test in the service menu. Although, for all i know that might be normal. I still does not know what triggers the scope to enable the CHx CAL SIG or INTLV CAL...

[shakalnokturn]

Sheet 17 (page 149/155) shows the 16bit DAC that is responsible for generating the variable control voltages for most of the frontend and trigger control.
DAC's buffered output "DCG CHARGE" is then sent to be MUXed out to the various places it's needed, this is done by three 4051's on sheet 18.
One of these MUXed outputs becomes "INT CAL REF", which after going trough 16IC1 and 16IC2 becomes "INT CAL".
"INT CAL" is sent to every frontend where it can be switched in place of the BNC input to calibrate the attenuator, BWL, and amplifier sections.
(I'm not quite sure why they have "INT CAL" and "CAL OUT LVL" on 16IC4 unless that's a diagnostics function available on the front panel CAL OUT.
Anyway that's how it could pass other stuff and fail trigger, the main calibration signal is "INT CAL"...

On sheet 18, the DDS (16IC8) should be able to generate an output frequency up to 60MHz, because this goes through an ECL buffer then series capacitor I assume the "CHx CAL SIG" that go to frontend HFE428's are only used for timing and not amplitude. If it is for calibrating the interleave when combining channels as "INTLV CAL" name may suggest I'm not sure how it would be used on the LT264 as it doesn't have the ability to combine channels anyway. However not having RTFM I'm a little lost here.

Back to the trigger:
If you're not getting any change on "INTLV CAL" at any time, check activity and signal integrity on 16IC6 pins 11, 12, 14. Better still to scope all three at the same time and you may have a chance of reading if the LT264 actually loads the data to enable "INTLV CAL".
Also check 12Q2A, 12Q2B if that's fried it could be holding the line high.

Another suspect if you have no trigger at all including on EXT Trig. is 18IC3 (page 150/155) as this MUX is responsible for all the MST reference voltages. Analog MUXes are known to fail once in a while. Sheet 12 gives the voltage ranges you should get after the buffer OP-AMP's (+0.56V to -3.07V).
Following that route it would also be interesting to check if you can get any output on the front panel CAL OUT ring. If you do have some output try changing the amplitude setting in the scope's menu that should give you some clue on whether 18IC3 is completely dead or not.