TDS520 weird channel offset - Page 1
Forum section: electronics / testgear

author: rvalente
author_rank: Frequent Contributor
timestamp: October 10, 2020, 02:54:32 am
content: Hello mates,Trying to fix this 520 (no letter), the scope boots with two errors.Regarding the capacitor plague, all of the SMDs were to be previously repaired, someone tried to fix this before me.I washed the acquisition and cpu boards thoroughlyThe power rails seems fineWhen the 1Khz probe signal is feed, channel one reads and triggers just fine, but changing the channel vertical position ads a bunch of crap noise in some positions.Channels 2 and AUX 1 and 2 have this noise all the time.I've checked power rails in the majority of ICs (bunch of TL074s) and others, seems fine.I've checked the offset voltages to the pre-amp and seems fine.I've tried to measure signals on the +SIG12AB | -SIG12AB, +SIG34AB | -SIG34AB with two channels in subtract mode (poor mans differential), I believe my measurement setup was to noisy, the signal itself on the TDS was more clear than the one in the other scopeWell, I'm kinda lost, what should I look at?This will be my first nice digital scope so I'm really looking forward to fix and send this baby to calibrationGreetings

author: rvalente
author_rank: Frequent Contributor
timestamp: October 10, 2020, 11:21:58 am
content: Could not attached the pictures previously

author: Jwalling
author_rank: Unknown
timestamp: October 10, 2020, 04:51:13 pm
content: Looks like bad memory. Run the full self-test from the utility menu, then check the error logs for memory failures.

author: rvalente
author_rank: Frequent Contributor
timestamp: October 10, 2020, 06:42:43 pm
content: I'm afraid my errors are not that useful. No ram testing address failureThe weird thing is, I'm getting this signal "noise" in all channels but, a little less in chan 1, why does it change with vertical position?

author: rvalente
author_rank: Frequent Contributor
timestamp: October 10, 2020, 09:22:26 pm
content: With the dips open in position 7 and 6, (expand error log) diagnostics I get a error at add 0x730006.https://forum.tek.com/viewtopic.php?f=568&t=141921&__cf_chl_jschl_tk__=c64219b2de6fd864063daf80ddcd0f21e050ec77-1602357695-0-AXiZHOI1LkkrmE7KoDW0_6dWzjMCzSfwzq0TcVIZ_HZQMpiCCfjchCMF90W6e86IxAarZi0YVyHC3Ql_PQ7lMQJBb7dxZzNR8AqcUpgn9aUZpLkyS8Ikax155h3IErK_7Y2yLP9gD18-1gBFQax5det00lpdKesEsvhm-C_jI2zT4sJ92oWyNH8yVfFaQZBLjGI2bnXRfiJBieXNQn2Om0GYjvLdRAW6o8XqwDdH_5Q0CG5elZD8iRk1HxoyUcTvDM7sZjoScb3a4ok1nBzYnt8p87FzfL_FlmATjV6hNKVLJBTqx6Y3szvJqXsjGhQE5JdUqFz1CCUO8cczq9i9FakYDWuBEvB-4mwFdtBXr3mm0ixl4LwBndJ-ZetRIZUZ2wSo, maybe a bad ram chip? I'm suspecting the U201, as its the first address on 0x73xxxxI've tried to lift some ram chips legs and the "noise" effect gets worse, but could not isolate the problem.Is there a way to test the hole ram bank?Is there a bank mapping to know which IC does which address?

author: Jwalling
author_rank: Unknown
timestamp: October 11, 2020, 08:30:43 am
content: Withbothchannels active, do you see the noise in both channels?

author: rvalente
author_rank: Frequent Contributor
timestamp: October 11, 2020, 02:05:31 pm
content: Quote from: Jwalling on October 11, 2020, 08:30:43 amYes, the noise is always there no matter what 2 channel combination I do.While probing I've noticed that when I probe the ADC MSB bit, the noise on screen increases.When I turn off some ram bank, it also increases.So I'm trying to understand how is the channel vertical position related to this noise appearing on screen.About the RAM failure:I probed the RAM to the demultiplexer (as tek calls it) and the traces are well conected, as well the resistance to ground is the same in all address pins. But I've noticed the that the LSB bits of the first ram chips traces and vias runs very near a electrolytic cap, although traces does not seems to be corrodeDoes anyone knows how to test the RAM banks? Is there any internal procedure for that?