Author Topic: 2246 distorted readout characters  (Read 500 times)

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Offline yannis

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2246 distorted readout characters
« on: May 29, 2021, 09:44:52 pm »
2246 passes all diagnostics successfully. Readout character dots are misplaced and appear distorted. Any suggestions or help how to fix the problem would be greatly appreciated.
 

Offline rvalente

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Re: 2246 distorted readout characters
« Reply #1 on: May 31, 2021, 07:06:35 pm »
I'd check if the eprom is correctly set in the socket as well as firmware checksum.
 

Offline yannis

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Re: 2246 distorted readout characters
« Reply #2 on: June 02, 2021, 06:11:09 pm »
Dear RVALENTE thank you for the reply. I reseated both eprom and rom chips that were on bases and check the chesksum of readout rom as well as system rom through diagnostics menu and they are ok. Unfortunately the problem remains. I can see a change in the misplaced readout dots (they move but still out of place) when I scroll the underline in the service menu window.
 

Offline james_s

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Re: 2246 distorted readout characters
« Reply #3 on: June 02, 2021, 07:40:23 pm »
The interesting thing is that the distortion is not consistent, the letter E for example is distorted in different ways in different positions. I wonder if it's related to the position of the character on the screen? I've seen vaguely similar behavior in old arcade game boards when there was a stuck bit on the output of a character RAM.
 

Offline yannis

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Re: 2246 distorted readout characters
« Reply #4 on: June 06, 2021, 10:38:25 am »
James_s thanks for the comment.
The problem is so weird that I cannot figure out where to start from. Same characters are not only distorted differently on the same screen but they chance every time I move the underline with the up or down cursor. Initially I thought that the problem might come from PS capacitor noise since 33 years had past from the day I bought the scope from TEK as a student in the states. I recaped the PS but the character problem remained.         
 

Offline philwong5176

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Re: 2246 distorted readout characters
« Reply #5 on: June 07, 2021, 01:03:24 am »
I want to remind you to replace ALL 17 ZM diodes. See thread below:

https://groups.io/g/TekScopes/message/170804?p=,,,20,0,0,0::relevance,,17+diodes,20,2,0,76287924

2245, 2246, and 2247 Tek scopes all use the same power supply.

I have a 2245A and a 2246.
 

Offline yannis

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Re: 2246 distorted readout characters
« Reply #6 on: June 10, 2021, 07:11:46 pm »
Thanks for the tip. I ordered the diodes and the old ones will be replaced as proposed.
 

Offline james_s

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Re: 2246 distorted readout characters
« Reply #7 on: June 11, 2021, 04:25:29 am »
How does the readout circuit work on these things anyway? Does it just step through eprom addresses and output the data to a pair of DACs?
 


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