Tektronix TDS 640 Aquire Board - Page 1
Forum section: electronics / repair

author: Treehouseman
author_rank: Unknown
timestamp: October 03, 2018, 05:27:26 pm
content: I'm looking to see if anyone knows where I can get a reasonably priced aquisition board for a TDS 640A, the mux chip on mine seems to have died, when the scope switches sampling modes going from 100us/div to 40us/div it gets psuedorandom noise on all 4 inputs, I've checked the attenuator, buffers, and the adc, all seem to check out, as well as all my voltage rails. Only thing left in the circuit is the mux chip. I'd be happy just to get a replacement one of those. As far as I can tell the substrate voltage on the mux chip is the issue, something internally is dragging it to +3V when according to the schematic it should be a negaitve rail. It comes out of an opamp and into a resistor divider to -5v with caps for filtering, with the caps showing ground above them, not below.The interesting this is just how it seemingly doesn't care for the most part, it works fine aside from the noise. I can turn on averaging and get rid of most of it, but that is only useful when I'm looking at static waveforms, can't do averaging when looking for noise on a signal, and I won't be able to see it with the noise the scope is generating itself.

author: Jwalling
author_rank: Unknown
timestamp: October 03, 2018, 06:22:36 pm
content: You've cleaned and re-capped the entire board, right?

author: Treehouseman
author_rank: Unknown
timestamp: October 03, 2018, 06:47:59 pm
content: Is there a guide for what needs to be recapped? I've seen people metion recapping these but the supply in this one is fine and just randomly recapping the enitre board is a massive process. I found no noise anywhere but the mux chip it seems, none of the voltage rails looked noisy or out of spec. This unit was last calibrated in 2011 and still passes all self tests. the unit was also kept in a very dust free environment, there were only a few places on the case that needed dusting, the rest blew off.

author: nctnico
author_rank: Super Contributor
timestamp: October 03, 2018, 08:06:48 pm
content: What is the date of the copyright shown on the start-up screen? Does it also pass the SPC test? What happens with the input shorted? Be aware that is oscilloscope doesn't have peak detect so a signal can alias into all kinds of weird signals.

author: Treehouseman
author_rank: Unknown
timestamp: October 03, 2018, 08:42:37 pm
content: It passes all the self tests, the date shown on the boot screen is 1993, version 3.8.5e. Just ran a signal path test and it passed just fine.

author: Treehouseman
author_rank: Unknown
timestamp: October 03, 2018, 08:55:23 pm
content: Well that's something, the signal path test seems to have fixed it... or at least whatever the issue was is now gone. I guess I'll go through and order caps, from what I've seen it's just the electrolytics that need to be replaced. Hopefully this issue stays gone. Now to just find a serial board (or make one from a schematic I found), image the nvram, and maybe find a floppy drive.

author: nctnico
author_rank: Super Contributor
timestamp: October 03, 2018, 09:01:57 pm
content: If it is from 1993 it is likely your scope isn't affected by leaky electrolytics. From my own limited experience only earlier models had issues. You can check by using a lamp at various angles and see if you can see an oily/greasy area around a capacitor. Also check for corrossion on legs of components near the electrolytics. If your scope doesn't have these symptoms then I'd leave it as it is. Replacing the capacitors isn't an easy job and it is easy to damage the circuit board. If it ain't broken then don't ruin it. I used to own a TDS510A and TDS744A from 1994/1995 and these didn't have the leaky capacitor problem.

author: Treehouseman
author_rank: Unknown
timestamp: October 03, 2018, 09:15:21 pm
content: Yeah, all of the boards were spotless, no corrosion or discoloration. I may still pull off one or two of the capacitors to check them with my esr meter to see if they're making their way to failure just to be safe.

author: Treehouseman
author_rank: Unknown
timestamp: October 03, 2018, 09:50:26 pm
content: Looks like the plot thickens more, the time division options changed! Previously it started having issues when going to 40us/div from 100us/div which was one step, now there's no more 40us/div, it goes to 50us/div after 100us/div, as well as previously the fastest division was 2ns/div and now it's 500ps/div. Guess this might have been a software issue then, something was configured wrong and messed up the time base options and used a different patten than it was supposed to.

author: Jwalling
author_rank: Unknown
timestamp: October 04, 2018, 09:35:34 am
content: Quote from: nctnico on October 03, 2018, 09:01:57 pmThe TDS744A couldn't have. They used all ceramic and tantalums AFAIK.

author: nctnico
author_rank: Super Contributor
timestamp: October 04, 2018, 05:23:59 pm
content: Quote from: Jwalling on October 04, 2018, 09:35:34 amI'm pretty sure my TDS744A had some electrolytics in it. Maybe just on the acquisition board or on the processor board. My TDS510A definitely had electrolytics on all boards and these didn't leak.

author: Jwalling
author_rank: Unknown
timestamp: October 04, 2018, 07:45:50 pm
content: Quote from: nctnico on October 04, 2018, 05:23:59 pmI was talking about only the acquisition board. Sure there's electrolytics everywhere else. Here's a 724A (2CH varient) that I've only stolen a few chips off of with serial prefix B010.Maybe it was a 744 no-suffix? Those I'm not entirely sure about...