Author Topic: RAM replacement on GOULD scope (4ch) - distorted traces  (Read 1718 times)

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Offline rg58

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RAM replacement on GOULD scope (4ch) - distorted traces
« on: January 14, 2018, 09:06:13 am »
I need to replace RAMs in an oscilloscope. These are HM6264ALP-10 and TC5565APL-12 for the signal aquisition. As a replacement for both I consider LH5164D-10L.

Now, looking at the datasheets, all 3 are PIN compatible. However, the internal organization for the address decoders varies in all three in the block diagrams (attached).

For example, for column address they use:

HM6264: A1, A2, A0, A10, A3
TC5565: A0, A1, A2, A3, A4
LH5164: A0, A1, A2, A10, A11

Are these 3 interchangable? Would it actually matter how the internal addressing is being done? I struggle to come up with a conclusive answer to this myself.

Other than that, are there any other parameters that I should pay attention to when replacing RAMs, appart from pinout, speed, voltage and maybe power consumption? This is my first time that I need to replace RAMs with substitutes.
« Last Edit: January 14, 2018, 08:34:34 pm by rg58 »
 

Offline Alex Eisenhut

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Re: RAM replacement in oscilloscope - compatibility question
« Reply #1 on: January 14, 2018, 05:04:42 pm »
Why do you need to replace the RAM? These chips never go bad IME.
Are you sure they are SRAMs and not NVRAMs?

They are all the same from the outside. Sure, you can probably find some minor timing differences and pin naming differences, but as long as the access time and supply voltage is the same, it'll be the same.

The inner workings are not important since you can only access the chip through its pins... which are all the same.
 
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Offline RoGeorge

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Re: RAM replacement in oscilloscope - compatibility question
« Reply #2 on: January 14, 2018, 05:17:54 pm »
Looking up HM6264, it's a SRAM, as in static RAM, so no refresh is required. Also, I don't see a block read/write capability, all address lines are available at once at the pinout, so it shouldn't matter if the address lines are swapped. It doesn't have RAS and CAS, as a DRAM has. Same for TC5565. Sometimes (very rare) a faster memory cannot be used to replace a slower one, but most of the time it doesn't matter if the replacement is faster than the original.

What are your concerns regarding the swap of some address lines?
« Last Edit: January 14, 2018, 05:24:49 pm by RoGeorge »
 

Offline rg58

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Re: RAM replacement in oscilloscope - compatibility question
« Reply #3 on: January 14, 2018, 07:39:45 pm »
@Alex Eisenhut
All 4 traces are distorted in storage mode (see screenshot) but flat in non-storage mode. I was also under the impression that the bit errors on Ch1/2 happen in the same set of 'blocks' and on Ch3/4 in another set of 'blocks', but now I'm not so sure anymore (see the 2 screenshots with 2 traces ch1/2 and ch3/4). I initially suspected the ADC, so I swapped it on the Ch3/4 addon board - without any improvement. It's not much left there anymore other than the RAM and the controller. I have seen RAMs going bad in music equipment several times before (effects), so this does happen. What you get in this case is mostly noise or very strange, random sounds from the speaker on top of the original sound. So I'm thinking it could be the same here, just for video data this time.

@RoGeorge
The more I think about it, the more I realize that it might really not matter. The memory controller writes data to a specific address XXXXh and reads if back from the same offset XXXXh again. I was confused, thinking the actual (physical) address would matter and that the internal structure effects this. Or are we thinking too simplistic now?

I attached the relevant part of the schematic, the block diagram and more pics of the unit, as well as of the addon board before the ADC surgery. It's a Gould btw.

Couldn't this be caused by faulty RAMs? Or what else could cause that? The ADC for Ch3/4 had already been swapped as I said.
 

Offline RoGeorge

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #4 on: January 14, 2018, 09:17:41 pm »
I would try to swap the CS lines between the 3 SRAM chips, or to keep them disabled one by one, and try to identify which one is the defective RAM.

If you have a good spare RAM for tests, you could try to put all pins "in parallel", except the CS pin, by simply pushing the spare RAM on top. I use to do this trick without de-soldering the existing chip, just plug the new one "on top" of an existing chip, as you are plugging in a socket. The pins alone (without soldering) are springy enough for a good contact during testing.

Of course, these tests are not 100% significant (because all the chips are in parallel, except for the CS pin), but it might help.
« Last Edit: January 14, 2018, 09:20:49 pm by RoGeorge »
 
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Offline Alex Eisenhut

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #5 on: January 14, 2018, 10:29:43 pm »
Hmmm. Can you reseat that PLCC?
 
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Offline stj

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #6 on: January 14, 2018, 10:43:57 pm »
yes, plcc sockets can be an issue,
and srams do fail.

if you have an eprom programmer, most can test sram.
although it's only 99% trustable because it wont test the ram at full speed.
 

Offline Alex Eisenhut

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #7 on: January 15, 2018, 12:15:34 am »
Oh, and don't worry about the internal architecture. The manufacturer might be using a single larger capacity die to make an entire family of SRAMs so the smaller 8kx8 only uses part of the die.

Who cares? You're not seeing the forest for the trees. It's irrelevant. You have 13 address lines, that's 2^13=8192 bytes. That's it, that's all.

Besides all that, have you checked the usual suspects first?
Power supply
Reset line
Clock

Bad power supply can lead to erratic operation. Many SRAMs enter a low-power data retention mode below a certain voltage. If you have a weak power supply, it might be triggering the chips in and out of that mode.
Bad reset could lead to improper startup state of state machines, for example. A reset pulse that's too short could cause that.
Clock, check the crystal oscillator. Two things to check are of course the frequency and voltage, but also start-up time.

Mostly you want to avoid desoldering chips and ripping things apart before checking the basics.
 

Offline rg58

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #8 on: January 15, 2018, 05:22:20 pm »
Thanks for all the good advice! Of course I would like to avoid unnecessary soldering and messing with parts, but this error isn't that easy to troubleshoot. Trust me, unsoldering that DIL-28 from this double-sided board with all legs bent over was no fun, it probably took well over an hour (as I tried to save the ADC which usually costs $25-55). I'm surprised the IC survived the heat from my big iron for an extendend period of time. I put it back in the socket and it still works as before. And of course I do check power supply first in every repair.

Here's what I've done today:

1) Re-checked voltages, this time directly on the IC pins of the RAMs and the ADC, and some on the plug. All voltages are clean with no visible ripple.

+5V   => 5.3V
-5.2V => -5.0V
+12V  => 11.9V
-12V  => -11.4V (moving towards -12V on startup)

2) Confirmed clock frequency 20MHz (5V) on CONV input of ADC

3) Removed and re-seated the PLCC (SRAM controller) on the 4ch board

4) applied 3.6V to the missing memory battery contacts before my new battery arrives

5) ordered some 6264-7 that I found cheaply (70ns!)

Nothing obvious was found.

@RoGeorge
Swapping CS lines isn't that easy to do, unless I scratch tracks and run wires. Will give it a think. But thanks for your idea to put another one on top for testing. I might try that once my order arrives. In the fault case however, what would happen is that opposite bits 'collide' (H + L level). This could cause damage to the outputs inside the RAM, but maybe for a few seconds of testing it's OK.

@stj
I don't own an EPROM programmer (yet)

@Alex Eisenhut
No worries, I understand now that there's no issue with the internal architecture of the RAM. I have done everything you suggested, except for the Reset part. I'm not exactly sure where to probe that. On the main CPU?

What I also noticed is that those erroneous trace blocks look different and almost disappear in some Time/Div settings, but then I get random things like $$$$$ and dots on the readout. The menu seem to have a permanent issue now as well with bit erros (missing or wrong letters). This wasn't happening before I took out the old battery for replacement and cleaned up some leakage (that trace error was there before).

One more thing the scope does since I got it is that on startup the top of the readout (and traces) start almost in the horizontal center of the screen and looks completely fuzzy for 20-60 seconds during warm-up, with some stuff swirling around the letters and traces. Looks very weird.

Will need to investigate further once my order has arrived.

Another question I have:

The scope is rated as 20MS/s and I confirmed the CONV input signal of the ADC to be 20MHz as well. This would be equal to 50ns per period/sample. Why 100ns and 120ns RAMs are being used for signal acquisition and why are they fast enough?
 

Offline RoGeorge

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #9 on: January 15, 2018, 07:01:01 pm »
...put another one on top for testing. I might try that once my order arrives. In the fault case however, what would happen is that opposite bits 'collide' (H + L level). This could cause damage to the outputs inside the RAM, but maybe for a few seconds of testing it's OK.

It will probably won't damage the chips, but this is not what I was trying to say. Put the good chip on top except the CS (Chip Select) pin. So, first cut the CS trace on the PCB, put the onboard memory on disable by tying its (now floating) CS to +5V (disabled), and rewire the CS trace to the chip plugged on top. Other words said, disable the onboard chip, and rewire the CS PCB trace to the top chip.

Anyway, since you desoldered one, you probably put the nice AUGAT socket I've seen in one of your pics.

About the desoldering difficulties, back in the days I was using a syringe needle to desolder ICs with many pins. It was working better than a manual desoldering pump. Works great with bent pins, too. It's useful tool and easy to DIY by cutting a hypodermic needle with a file or a rotary tool. Syringes or needles alone are always on stock at any nearby pharmacy, and dirt dirt cheap. Solder does not stick to the syringe needle.







Of course, nowadays you can buy dedicated desoldering needles.

About the power lines, apart from the absolute value, you need to look at them for excessive ripple or spikes during functioning. A DMM put on the AC mode can give some hints (ideally you should see 0 Vac), but an oscilloscop in AC mode will be the proper way to look for power ripple (if you have a spare oscilloscope).

About the 20MSa/s (50ns) with 100ns RAM speed, it seems from the schematic that the memory controller is buffering some ADC samples, then spit them all 3 samples * 8bits at once (that is why it has 3 data buses, one from each chip to the memory controller). Other said, 3 consecutive samples from ADC will be stored at only one write cycle, each sample in a different chip but at the same chip address.
 
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Offline stj

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #10 on: January 15, 2018, 10:37:40 pm »
can you get to the ram while it's running?

if you can, scope the address lines etc
 
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Offline Alex Eisenhut

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #11 on: January 15, 2018, 11:30:30 pm »
Has this scope ever worked before? Are there some UV EPROMs with the scope's firmware on it maybe? Those things can go wonky, maybe you simply have rotten code running?
 

Offline Alex Eisenhut

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #12 on: January 16, 2018, 02:02:43 am »
Oh, and 5.3V sounds a bit "hot" to me. Is that a value specified in a manual somewhere?
 

Offline RoGeorge

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #13 on: January 16, 2018, 09:42:04 am »
5.3V sounds a bit "hot"

Indeed, for TTL it should be 5V +/-0.25V
5.25V is the upper limit in all datasheets. Memories are a little bit more tolerant, but any delays in MOS devices varies widely with the supply voltage. So, the closer to 5.00V, the better. Again, look for spikes and ripples on the +5V supply near memory pins.
 
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Offline rg58

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #14 on: January 16, 2018, 10:34:22 pm »
I did check voltages with multimeter AND scope, hence above I said 'with no visible ripple'. Just checked the 5V line again with two DMMs, one showing 5.26V, the other one 5.34V. They are using a L296 switching regulator for the 5V. The switching frequency is 85-110kHz according to the datasheet. I also just checked again for high-frequency ripple on the 5V line. When I check on the plug from the power supply I get <100mVpp of 'noise' (refer to attached pics). When I probe the Vcc of the RAMs directly, I get about 150-200mVpp, mostly noise and 20MHz clock, no spikes. I don't think that's too much of an issue with the ripple to be honest.

@RoGeorge
Now THAT's a great idea with the needles, thanks! Why I didn't think of that!? So I went to 4 different pharmacies/medical supply stores this morning, only 1 of them had them in stock and selling small quantities. The only sizes they had were 0.45mm and 0.9mm, so I took the 0.9mm ones, hoping they will fit through the holes on the board. Pretty cheap, just 0.10 EUR each. Took a file and made one of them flat already. Yes, I did put in that nice green IC socket.

And regarding the outputs, you are right. Those RAMs have a 3-state output - the 3rd state being high Z, so no problem to connect them in parallel to another RAM 'on top' as long as we make sure that all outpus are in fact in the high-Z state. Telling from the truth table of the HM6264, this is possible in 3 different ways (I'm learning a lot here, so bear with me). !CS1=H or CS2=L or !OE=H. So when cutting the !CS1 tracks from the RAMs I should ideally also pull them High (can be done by running a short wire to CS2 in this circuit as it's always HIGH). Still waiting for my RAM, will try as soon as it arrives. Take another look at the screenshot of the 4 traces that I took tonight. 1&2 looks different from 3&4, so we must assume the error happens in seperate stages of 1/2 and 3/4. In the block diagram that would be block 6.15 (Beamswitch & ADC) and 6.11 (Data Control Array, Max-Min, RAM) as they join together in the next stage 6.11 (Acqusition Controller, Beamswitch Control, Ramp Control) -> hence my reasoning: ADC first, RAM next (after I already made sure that the power supply is clean).

@stj
Haven't probed address lines yet, might be of limited value. Unless... there's a short or interruption on one of the address lines. OK, makes sense. Will do tmrw. Was a very busy day...

@Alex Eisenhut
Well, I assume the scope must have worked once, but I got it in this bad condition, pretty messy and full of issues, but still worth repairing. If only for the learning experience (I do own other scopes). You're right about the 5v issue, it should be 5.0v not 5.3v. No UV EPROMS in this scope, but I was already considering corrupt firmware, not sure. 

ALRIGHT, things start to make sense to me now! I have probed R/W and CS1 lines of the RAMs. R/W was 500ns low/500ns high. As those 3 R/W pins are connected they read/write from all 3 RAMs at the same time. The 13 address lines are in parallel too, that's where the CS (chip select) pin comes in I suppose. On !CS1 I saw 100ns low (which means THIS Chip is Selected). I also have a 500ns cycle on top of it (see pics).

Attached what it looks like on startup.
Stage 1) Turn on: the screen top is too low with fuzzy readout characters (less than 1 minute)
Stage 2) The srceen suddenly pops into place, still fuzzy readout, menu and traces
Stage 3) The fuzzy stuff slowly goes away, gone after about 2 minutes
Stage 4) Scope becomes usable from about 2-3min, although the traces are still distorted in storage mode only (that's what I'm working on right now)
 
Good nite!

P.S. Today no missing/wrong characters in the readout.
 

Offline RoGeorge

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #15 on: January 17, 2018, 12:47:23 am »
That is a very big undershoot of about -2...-3V on the R/W, CS1 and CS2 lines, basically on any digital line showed in the pics.

Is the probe properly compensated? Please connect the probe to the square calibration signal provided by the good scope, and check if the displayed waveform is square and clean. All the "corners" of the reference waveform should be at 90 degrees, without overshoot or undershoot spikes, without ringing, and without "rounded corners".

Another cause might be the ground point for the measuring probe. Ideally would be to connect the GND of the probe as close as possible to the GND pin of the measured memory chip.

If the probe compensation and the GND point are OK, then those spikes of -2..-3V seen in the previous pics are much bigger than I was expected to see. It might be because of a strong reflections of the signal on those lines. Maybe some terminator is missing, maybe on the other end of the wires (on another board) somewhere else, or some bad contacts in the cables (I see a ribbon connector that seems to be wired to the memories).
« Last Edit: January 17, 2018, 12:48:56 am by RoGeorge »
 

Offline james_s

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #16 on: January 17, 2018, 01:23:57 am »
A trick I've used in the past that can sometimes help is piggyback a known good chip of the same type over a suspect chip, as in all pins connected 1:1. If there is any change in the behavior then there is a good chance that the suspect chip is in fact bad.

If the behavior changes suddenly as it warms up a can of freeze spray is a useful troubleshooting aid. Those cans of air duster work if you hold them upside down and those tend to be much cheaper than stuff sold as freeze spray.
 

Offline Alex Eisenhut

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #17 on: January 17, 2018, 01:49:04 am »
"Well, I assume the scope must have worked once, but I got it in this bad condition, pretty messy and full of issues, but still worth repairing."

Ugh. Now you have a scope that may have been in the hands of a butcher... That -5.2V (Warning: scope contains ECL!) is "cold". I'd rather see the voltages tweaked a bit first.

But you seem to be plowing ahead nicely.

I'm hoping you don't have a weird ASIC failure.
 

Offline rg58

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #18 on: January 17, 2018, 08:28:02 am »
@RoGeorge
The undershoot is in fact that big. Just now I have re-adjusted the probe compensation which was only a little off. Like last night, I also connected to the closest ground point I could access, which is the minus side of the blue capacitor that you see in the picture of the 4ch board (for ch3/4), just about 3-5cm away from the RAM Pin 20 (!CS1). It does make a huge difference though as I had my scope ground connected to the case before, but changed it to the closer GND before I took above pics (we're dealing with square waves of up to 20MHz here, I'm well aware of that). Take a look at the pic with newly adjusted probe compensation and closest ground point - the undershoot doesn't get much better.

@james_s
Had been suggested by RoGeorge before and I followed up on his suggestions in my last post. But thanks anyway!

@Alex Eisenhut
In the service manual it states -5.2V has to be in the range of -4.8V to -5.2V, so I'm right in the middle with my -5.0V. Although funny that the target voltage -5.2V is stated as the minimum at the same time. Conversely, the +5.0V has to be in the range 4.8V-5.2V which I am exceeding a tiny bit (by about 60-140mV). Yes, ASIC failure would be the worst.
 

Offline rg58

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #19 on: January 17, 2018, 09:11:57 am »
Alright, seems I found a bad address line! On A9 (Pin 24) the levels are out of specs. The other 12 look proper, level-wise at least. I could have figured this out much earlier. Now the trace cutting starts, if this is easily possible at all on this 4 layer board... But again, it might come down to a failed RAM. What do you think?
 

Offline mikerj

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #20 on: January 17, 2018, 09:37:45 am »
Alright, seems I found a bad address line! On A9 (Pin 24) the levels are out of specs. The other 12 look proper, level-wise at least. I could have figured this out much earlier. Now the trace cutting starts, if this is easily possible at all on this 4 layer board... But again, it might come down to a failed RAM. What do you think?

Do the logic levels on A9 change when the scope starts behaving after power on?  Before any trace cutting I would give the freezer spray/hot air gun a try.  Monitor A9 and then go around every device connected to the address bus.
 

Offline RoGeorge

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #21 on: January 17, 2018, 11:03:21 am »
I'm afraid the A9 coming from the ASIC is bad, as in the ASIC is bad. If you are lucky, maybe it's just a RAM chip. I wish you the best luck.

Before cutting traces, measure with a DMM the contact resistance between the A9 ASIC pin and the A9 RAM pin. Compare the measured value with another address line. This is just to be sure there is a good contact between the ASIC A9 pin and the socket.

Identifying between a bad ASIC or bad RAMs would be easy to check by swapping A9 with some other address line near the ASIC (as in swapping all A9 from the RAM at the same time). If ASIC's A9 is still bad after swapping, then it's the ASIC. If the A9 recovers and the new swapped line becomes bad, then it's the RAM or something else on the board.

Even if the ASIC is bad, there is still hope you can repair the scope without replacing the ASIC, but let's hope for the best. Maybe it's just a bad RAM chip.

james_s suggested a slightly different method for piggybacking, which is cleverer than how I was doing it, because it does not need to cut traces. He was saying to connect _all_ pins in parallel, including the CS. Anyway, since you identified the A9 problem, it's all clear now. No need for piggybacking any more.
« Last Edit: January 17, 2018, 11:08:56 am by RoGeorge »
 
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Offline rg58

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #22 on: January 17, 2018, 02:28:55 pm »
Good news guys! I fixed the problem!  :-+  (now only 5 or so left)

It was in fact the A9 address line. Even before reading @RoGeorge's post, I went more or less the same way as he suggested with a few more steps in between. Looking at the schematics, I realized that the other 3 RAMs on the main board are in parallel on their address lines as well. So when I removed the 4ch-board and probed Pin 24 (A9) there it showed the same problem, so the problem must be on the main board. I then tried to measure resistance between the ASIC Pin 28 and RAM Pin 24 but that failed -  the reason being that the pinout did no match the schematics at all. Even the Vcc and Ground connections were different for the most part, so they must have re-designed the board layout and possibly changed to another ASIC in later revisions. Later I found out that pin 20 ASIC connects to Pin 24 RAM.

Long story short, I pulled the ASIC (U44, Acquisition Controller) from its PLCC socket and re-seated it. After that A9 looked proper on the scope and the problem was SOLVED! I have reseated the other 2 PLCCs on the mainboard now as well, but this didn't improve the other issues. I should have re-seated them all in the first place, not only the RAM controller on top of the addon board. The other 3 are sitting underneath that board, so my excuse is that they were not easily accessible  :P

Next I will investigate the ripple on the vertical deflection that you can see in the screenshots.

Anyway, thanks a lot guys for helping me out here! Appreciate it very much and learned a lot in the process. This in-depth digital stuff is still quite new to me. I'm more into analog stuff.

P.S. If there are other issues I'm not able to solve by myself I will open another thread. For example, I believe the CRT might be on its way out. That fuzzy stuff might indicate a vacuum or HV issue, not sure. Just this morning when I had my window open at outside temperature of 2°C the CRT wouldn't show ANYTHING for at least 2-3 minutes before it slowly heated up and dim traces appeared.
 

Offline RoGeorge

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #23 on: January 17, 2018, 03:38:38 pm »
Well done, congrats!  :-+


Offline james_s

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Re: RAM replacement on GOULD scope (4ch) - distorted traces
« Reply #24 on: January 17, 2018, 11:16:14 pm »
A CRT problem is highly unlikely to be related to ambient temperature, there are many potential issues with the circuitry driving the CRT that are more likely. A worn CRT will normally be dim all the time and get blurry when the brightness is turned up. It will typically take a long time to warm up and become usable but it will not be intermittent or affected by room temperature. The cathode is heated red hot in operation, a few degrees of room air isn't going to make a difference. If there is any air leakage into the CRT you will get plasma and/or arcing in the neck, it takes very, very little air to completely stop it from functioning.
 


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