TDS500/700 video clock

[Tantratron]

Hello, I'm trying to match a LCD driver with 640x480 display directly to the VGA connector J88 found on most color or mono TDS500/600/800 main board (video section). In order to fine tune the LCD driver internal scaler vuideo chip, I need to know precisely the pixel or dot clock generated by the TDSxxx. Unless I'm wrong, my understanding is that quartz near ADV471 does this job at 24 MHz (printed on the quartz) but when measuring ADV471-40 pin (chip U199), it shows 25 MHz.

I've tested this value both on a mono-color TDS520D and color TDS784D, both generate 25 MHz instead of what is printed on the quartz (24 MHz) near to the U199 chip.

Do you know the pixel clock value is generated by another ASIC or dedicated tektronix chip or what could the explanation of 1 MHz difference ?

Note about color model, there is another quartz 80 MHz to drive specifically the shutter found on the CRT, here I'm only interested on the VGA output signal found either on J88 internal connector or external VGA connector.

Thank you, Albert

[TerrySt]

I don't have a scope open right now, but I remember my TDS-524A used a 25MHz pixel clock for the BT471 RAMDAC chip that drives the VGA connector on the rear of the scope.  That clock came from an oscillator over near the battery-backed ROM chips.
The video signal that drives the CRT came from a different RAMDAC chip that got a clock from somewhere else.
I have never found a complete schematic for the 524A, so it is hard to be more clear.  And I never understood why Tek used two RAMDAC chips in this design.

Terry

[Tantratron]

Thank you Terry to confirm the pixel clock from the U199 RAMDAC does operate at 25 MHz which helped now successfuly VGA drive with a RTD2660 scaler chip LCD driver.

Maybe it can help others or maybe someone can provide an explanation, here are two logic boards (TDS500/700/C/D serie) pictures, one mono-color and other color. We see the 24 MHz quartz, no idea how the U199 RAMDAC generates 25 MHz pixel clock. Indeed there are 2 RAMDAC (ADV471) for mono version and one RAMDAC is replaced by another chip (TLC34075) which is clocked 80 MHz. From what I remember, the color CRT uses a shutter requiring row sequence shifting of a shutter (old tektronix patent) so they triple the VGA frequency plus requires another connector J62.

[TerrySt]

The 24 MHz oscillator for the vga RAMDAC isn’t shown in either of your pictures.  It is over near the ROM chips (the battery backed ones).   I couldn’t find a schematic so I used an ohmmeter to trace it down to a buffer chip and oscillator over on that side of the board.  Way far away from the ramdac chips. 

Terry

[Tantratron]

The 24 MHz oscillator for the vga RAMDAC isn’t shown in either of your pictures.  It is over near the ROM chips (the battery backed ones).

Please check again both my pictures on their top left corner, you'll see the 24 MHz quartz (it is printed on its metal case)

[TerrySt]

Yes, I see that in your pictures now.  My TDS-524 was different.  Those you point out are almost certainly the vga clock sources.

Terry

[Tantratron]

I'm now bit confused by another post you wrote a while ago, see here where you first mention 25 MHz then many times 24 MHz. Maybe the TDS524A is different than later TDS700C/D like mine but again I really wonder why mine have 24 MHz quartz near the RAMDAC but probing RAMDAC-40 shows 25 MHz.

[CaptDon]

My TDS644B had VGA output on the back apron but had no internal CRT display although the actual CRT unit was proved to be functional. The trouble was a dead can oscillator on the main board and it was an oddball 74.xxxx MHz. I found a replacement and the internal display now works.

[TerrySt]

I'm now bit confused by another post you wrote a while ago, see here where you first mention 25 MHz then many times 24 MHz. Maybe the TDS524A is different than later TDS700C/D like mine but again I really wonder why mine have 24 MHz quartz near the RAMDAC but probing RAMDAC-40 shows 25 MHz.

Sorry for the confusion.  Like I said, I don't have a scope open right now that I can look at, so I am going by memory, but I'm almost certain that on my 524A the BT471 chip uses a 25MHz clock.  That clock came from a 25MHz oscillator that is located in the rear (facing the 'scope) area of the board.
I am attaching a picture I found on ebay that shows what I think is my the version of the board in my scope.  The BT471 is circled in red.  The oscillator is in yellow and the buffer that the 25MHz clock goes through on the way to the BT471 is in green.
Maybe it is not the same in later versions of the board.

Terry

EDIT:  I found a clearer picture of the board, and the oscillator I circled is clearly labeled 25.000 MHz.

[Tantratron]

I am attaching a picture I found on ebay that shows what I think is my the version of the board in my scope.  The BT471 is circled in red.  The oscillator is in yellow and the buffer that the 25MHz clock goes through on the way to the BT471 is in green.
Maybe it is not the same in later versions of the board.

Terry

EDIT:  I found a clearer picture of the board, and the oscillator I circled is clearly labeled 25.000 MHz.

Bingo Terry, now I get the real picture thanks to your picture 

It happens that I have a spare full TDS510A which seems to have more or less the same CPU board as your TDS524A so I see now both quartz (32MHz and 25 MHz). However notice the presence of the 24 MHz which I have queried before. I got misleaded by the fact it was near the RAMDAC but you are right, the 25 MHz comes from further away which explains why probing RAMDAC pin 40 shows 25 MHz.

Attached 3 pcitures with circles: TDS700C/D with new design including 68040 processor and additional 80 MHz quartz for NuColor shutter CRT again near RAMDAC as your TDS524A then its mono-color version TDS500C/D and finally same type of your logic board, this time running with 68020.

Many thanks for helping solve this intriguing 24 MHz near RAMDAC whereas pixel clock is really 25 MHz

[Tantratron]

My TDS644B had VGA output on the back apron but had no internal CRT display although the actual CRT unit was proved to be functional. The trouble was a dead can oscillator on the main board and it was an oddball 74.xxxx MHz. I found a replacement and the internal display now works.

Yes now I do remember once purchasing a TDS784C from Belgium in 2021, the external VGA worked but no CRT. I did fix as you mention, namely changing the 80 MHz quartz near the J62 connector and all worked fine again.

As a matter of fact, I had to purchase many quartz replacement to repair my TDS784C so i've still many spares if people are interested to purchase some stock (see attached the model which i've used as a replacement).