OK took the scope apart again and worked a bit on it.
Probed a bit more in the acquisition area. Checked that the main acquisition chip (the one that interfaces with the CPU, not the other ACQ chip that handles the RAM data). That chip does get its 20MHz clock, and I see that its R/W line, presumably driven by the CPU, is very active. So I took that as a good sign that the CPU was asking the ACQ ASIC to acquire some data. So it was reasonable to thus expect to see activity on the 3 ACQ RAM chips, no ?
So I probed for that. Zero activity on the address and data buses, same on Chip Select pin. At least it's consistent, no chip select means the ASIC doesn't intend to use the RAM, so why would you have activity on the address bus.
So I looked at what provided that chip select signal for the RAM. As you can see from the schematic posted earlier, there is a bit of glue logic there, in the middle of the ACQ chips and RAM chips : a NOR gate U52b, and U56, a quad two input MUX.
The first ASIC doesn't drive the chip select directly. Instead it feeds it to the input of that NOR gate, whose output then drives the second ASIC, the R/W of the RAM, as well as pin #1 of the MUX / U56. Pin #1 selects between the two inputs of the MUXes. Two of the 4 outputs of the MUX feed back to the first ACQ ASIC, and the two other drive... yes, the Chip Select of the RAM chips. So it's all very much intertwined...
Now I probed the NOR gate. Nothing coming out of it, so no Chip Select, normal. Maybe the gate is dead ? Nope.. its two inputs are low, and show zero sign of activity. The first ASIC drives one of the inputs, the other input as you can see is connected to ground via a RC network. Not sure what that's supposed to do... maybe a crude method to delay the signal a bit before the NOR gate transfers it to the second ASIC and RAM chips, no idea.
But whatever... you will notice on the diagram that the output of that NOR gate is precisely where I had a broken track ? Remember, that pervert one, hidden under that "block dot" right next to the output of that gate. So I switched to "if it looks like a coincidence... it probably isn't ! " mode.
So I wondered : what if that broken trace was AGAIN another factory mod, like the cut trace on the bottom side of the board, that was cleanly drilled ?
The "dot" was indeed very circular/clean too, and concave as well. So I thought maybe again they drilled that track on purpose... and decided to cover the hole with a black marker / pen, to keep it from corroding or god knows what, not really important I guess....
I wasn't sure how that would cure my problem, because for all I see, the only thing driving that line, is the gate ! So if I remove my mod wire that bridged the broken track... there would be nothing left to drive that line, it would be floating ! But I thought.. to hell, it's been already demonstrated several times that this schematic just isn't reliable, so maybe on the actual board there is some chip other than that NOR gate, that's there to drive the line. So I decided to remove my mod wire and give it a go !
Glad I did :
ACQUISITION NOW WORKS !!!! 
So for the second time, big thanks to Shakalnokturn for suggesting that the bottom track could have been a factory mod ! The other track on the top side was too !!!
I was so happy, put the scope together AGAIN... and AGAIN I shouldn't not have done so : I AGAIN have more problems, so AGAIN I will have to take it apart ! GRRR !!!!
What's wrong now ? Well, acquisition works but... only on channel one. I still get the hard-coded " test pattern " on the other 3 channels.
ALSO, I have now lost channels 2 / 3 / 4 in ANALOG mode too, were they used to work before ! Oh no ! :-(
So this scope is very depressing indeed... one step forward and one step back ! Not sure I will ever see the end of it... but well, I guess I am making progress none the less, eh ?!.......................
Of course the first suspect is that the mod wire I removed somehow caused this problem, because in this scope the digital and analog stuff are so closely intertwined, it's a complete mess, nothing is very clear, it's all fuzzy. So I thought oh no, it that mod wire cures the acquisition but makes the analog channels disappear, it's a lost cause... I give up.
But before I give up, lemme put the wire back, to see if the 4 channels come back to life in analog mode : NO THEY DON'T !
GREAT ! Phew ! So the wire didn't cause the problem, it's a new issue altogether, somehow I prefer that !
It was just a coincidence then, just back luck.
So I guess I made some progress after all : found out that the black dot was a factory bodge, re-activated it and now I have acquisition working, albeit only on Channel 1. But if it works on one channel, then 90% of the chain is working and the rest is bound to be a more minor and less complicated issue, I can hope.
The way I am attacking this is : since I lost channel 2/3/4 on both analog and storage modes, a reasonable assumption is to think that there is one single problem, common to both modes. So at first I would rule out any problem with the acquisition subsystem entirely, and focus more on the analog stuff that's common to both modes.
I attached the overall block diagram for the scope, highlighting the signal paths that I suspect most.
As one of you mentioned, most obvious is to check that the signal traces are not simply way off screen. So I checked carefully, thoroughly for that, in both modes of operation.
In analog mode there is no " Beam Finder " button like one is used to see in almost every scope ever made. It's a GOULD after all, it's exotic, so I guess they didn't want to do like the rest of the crowd.
So all I could do was to set all channels to GROUND coupling, trigger set to AUTO of course, then set the attenuators to the least sensitive setting, in this case a whooping 20V (most scopes are 5V usually), then played ad nauseum with the vertical position up/down control switches... no luck. No trace to be seen other than channel #1 that works fine. Tried running the auto calibration routine that's there to get rid of any offset and gain errors in the front end... no joy. Tried the dreaded " Autoset " button, for once in my life... no luck either.
The only hint of an offset/off-screen trace, I found only in storage mode, and only on channel #2 : if I press the "select trace" button, it turns on the cursors and places one of them, which " follows " the trace, on the selected trace. So using that control, I can see that I am able to select channel #2, because the read out tells me the cursor is using it. Also, if I move the cursor along the trace, well an invisible trace of course, I CAN see the bottom half of the cursor sticking out of the top edge of the screen, suggesting that the trace might indeed be there, off screen at the top. Also, the readout shows a couple arrows that point upward, suggesting that the trace might be indeed up there off screen. So that would indeed point to a vertical offset problem, a huge one... but only for channel 2. Channel 3 and 4, no suck luck.
So I think I may indeed have a vertical problem on channel 2, so will investigate that. But it can't explain why channel 3 and 4 are missing, never mind why in analog mode 2 to 4 are missing.
So I think I have a second issue there. Trying to figure out how I could have channel one working fine in both analog and digital, and no trace at all on all 3 other channels... your guess is as good as mine, well better actually, but my own reasonable guess would be.... how does the scope show 4 traces at the same time, with a single electron gun at its disposal ? Yes, if the " beam switching " mechanism fails, it would explain everything...
As we can see on the block diagram, this mechanism is a mess like the rest : it's done in THREE parts ! ... Apparently the scope was designed as a two channel scope originally, so the circuitry for the first two channels is on the main board, then to implement channel 3 and 4 they had to add that large daughter board that sits on top of the main board. It handles part of the stuff for the last 2 channels. Some of the acquisition, and some of the analog stuff.
So there is a switch for channel 1 and 2, then another for 3 and 4, then a third switch that switches between the output of those first two switches ! Lots of switches !
So I guess I need to take the scope apart yet again (I am kinda getting the hang of it now... ), and start probing those switches...more generally leave the digital part of the scope, move on to the analog part of the board, and follow the signal path from the BNC inputs down to the output of that final switch. Hopefully I am right and I will find something interesting en route... wish me luck
Home
Help
Search
Login
Register
