Flickering Tektronix TDS684A - Page 1
Forum section: electronics / repair

author: -FA-
author_rank: Newbie
timestamp: December 28, 2020, 04:21:27 pm
content: Hello guys,i have got a Tektronix TDS684A with a defect. When I get booting the scope, i got an error with "Cal Intialization". After that the signal of the channels 1 & 4 are flickering completely confusing without aplling any axternal signal. On channel 4 the problem sometimes occure, it's not alays there. Firther I cannot adjkust the offset, it only works for channel 1. So i read out the error log as you can see in the pictures.https://upload.pokefans.net/m142_a2oogw782.pdfFirst i tap on the attenuator board under the metal case directly at the connector. So I tried to switch the two boards from channel 1 & 2. But that doesn't have any effect, the probelem still remains on channel 1 & 4. So I think there could be another problem direct on the board (A10?). Do you maybe having any guesses?

author: nctnico
author_rank: Super Contributor
timestamp: December 28, 2020, 04:45:12 pm
content: I'd start with checking the electrolytic capacitors.

author: madao
author_rank: Frequent Contributor
timestamp: December 28, 2020, 05:38:48 pm
content: TDS684A is old serie, probably eletrolytics leak plague.But TDS684A is unit with 1 GHz bandwith, it souldn't have old acquisition board with eletrolytics (same as TDS644A ). Can you take good pictures of lower board ?Pleas don't power it anymore and look excatly, how are solder joint of SMD eletrolytics. If it smell after solder after urine and sometime -> this is a alarm sign.greetingsmatt

author: -FA-
author_rank: Newbie
timestamp: December 29, 2020, 09:17:12 am
content: Hello,thank you for quite fast your answers. Good so far I dind't breathe something like a urine smell, but I will check the caps this afternoon. Here I have a picture from the lower board, I hope the quality of this upload will be good so far:greetings aaron

author: madao
author_rank: Frequent Contributor
timestamp: December 29, 2020, 10:24:54 am
content: Oh, this is new board, but yet with smd eletrolytics.Yes, pleas check him and solder it for check of smells.greetingsmatt

author: -FA-
author_rank: Newbie
timestamp: January 09, 2021, 07:00:30 pm
content: Hello guys,thank you for your responses.I checked the caps, but there wasn’t something noticeable. Further I checked the supply-voltages, here I haven‘t also noticed something.Then I continued to check the channels itself. I determine, the flickering signal is only on channel 1. I guess the timewise flickers on channels 2 to 4 are only side effects of the fault. First some notices I made:- On initliasation only the attenuators of channel 1 are clicking, from channels 2 to 4 there is nothing to hear- the flickering signal of channel don’t change its amplitude when I change the resolution of the y-direction.As I currently said I switched the attenuator boards under the metal case directly at the connector from channel 1 & 2 each other. Noticeable at that was, the mistake behaviour remains at channel 1. Next I took a signal generator and applied an low sinus voltage at the normal BNC-connectors at the front. With another oscilloscope I checked the voltages at the red marked IC in the picture. I measured at the two pins, which are connected to this pair of track, which go to the attenuator board unter the metal cover. They marked in orange.On channel 2 to 4 I measure the externally applied sinus-signal at these pins, so I think they would work in principal. On channel 1 I didn’t measure the signal. So I think there would be a defect, which prevents the signal’s flow through the attenuator board. But this itself is working, resulting from the exchange of channel 1 and 2. But what or which controls the attenuator boards? There are some ICs to find. For same I got data sheets and for some I didn’t find really much information.But I think it would be good to know what’s these ICs function in principal are.- Red market IC: Unfortunately, you can't read the designation out of the heat sink. My guess, that it‘s the sample & hold device, be right? Do you maybe know the type of it?- Green market IC: On its case you find the descriptions: W9418, ADG286D, MM9423-ACEMy guess this would be the A/D-converter.- Blue market IC: This is indeed the most mystical IC for me. Its description: imp I10412-01; 9404-FTNI don’t find really much information about that. I only read, it is called as “daculator”. My guess, its in principle the control unit / IC for each channel. Do you maybe have some further information or experience about this IC? Is that IC maybe also a weak point in theseoscilloscopes, esp. when somebody “shoot down” a channel, as he put in a too high voltage or what else? Probably if this IC “controls” the attenuator boards and the attenuator boards themselves work okay so far, then it would be suspected.And so far, as I see, schematics or something like that are not really available from Tektronix. Or do you maybe know a source for that?Link to picture:https://upload.pokefans.net/m143_a3b3rc3j3.pdf

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 09, 2021, 07:31:33 pm
content: Check the relays on the hybrids. They wear out.