The TDS-420 is one of Tektronix's first oscilloscopes in the TDS line and while, at 150MHz, it is somewhat old and slow by today's standards it can now be had for relatively little money and it uses the modern TDS interface, making it popular with hobbyists and in education. We use them at MIT in one of the teaching labs and getting them all going again has been a pet project of mine on and off for a couple of years now. Since I am just now (finally) finishing the last of these beasts I thought I would share some of the common ailments and their cures with everyone so that others may get some benefit from my (many) long hours of troubleshooting. The service manual for this beast is available at the link below, however, it is all-but-useless since it does not contain schematics and the calibration instructions amount to "Install calibration software on computer. Connect oscilloscope to computer. Follow on-screen directions."
http://i3detroit.org/wi/images/0/0e/460-ServiceManual.pdfCapacitors, Capacitors, Capacitors!The first thing to know about these oscilloscopes is that they suffer from the capacitor plague of the early 1990's. For some reason, almost all of the SMD electrolytic capacitors from the early 90's are failing now, seemingly irrespective of use and storage conditions. I have heard tell that there was an incident with counterfeit capacitors made using a stolen electrolyte recipe that was missing a key corrosion inhibitor, but this may just be a story. The ones in this oscilloscope are especially bad as when they fail they leak corrosive electrolyte all over the boards. This electrolyte rapidly corrodes away traces and, due to electrolysis, this process occurs much more rapidly in the presence of a DC bias. Thus, if you have a TDS-420 DO NOT PLUG IT IN until you have replaced all the SMD electrolytic capacitors.
Here is a photo of the type of capacitor that needs replacing (note that the picture was actually taken after the parts were replaced):

I recommend the following part numbers (available from both Mouser and Digi-Key) for replacement capacitors:
25x 33uF@10V: Nichicon UUA1A330MCL1GS
18x 10uF@35V: Nichicon UUA1V100MCL1GS
2x 3.3uF@50V: Nichicon UUA1H3R3MCL1GS
Note that you will need to replace capacitors on the following boards:
-Attenuator board
-Acquisition board (this is the main board at the bottom of the oscilloscope)
-Video Board
-Serial / Parallel board
-Front panel board
My procedure for replacing the capacitors was as follows:
1) Remove capacitor
2) Use solder and wick to thoroughly clean pads
3) Check the continuity of all surrounding traces. The damage from corrosion can occur right where a trace meets a pad where it is almost invisible, so continuity checks are the only way to go.
4) Remove, clean, and test any surrounding resistors and capacitors that show significant corrosion. (Remove anything that looks black from corrosion - a bit of a mat finish on the pads is probably OK.)
5) Scrub the surrounding area with alcohol and acetone using q-tips.
6) Install the new capacitor and re-install any removed components.
If any of the little brown 1206 SMD ceramic capacitors are corroded beyond help, they are 0.1uF@50V and are replaceable with AVX part number 12065E104ZAT2A.
Take it slow here. If you miss a damaged trace, it will be HOURS of work to find again later.
tbiRampTest FAILAll of the units which I have worked on so far failed the power on self test with an 'ACQ - FAIL' message before I replaced the bad capacitors. In the error log (look it up in the service manual if you can't find it) the errors have all been related to the 'tbiRampTest.' Replacing the capacitors did not fix the problem on this last one I was working on and the error log said "tbiRampTest FAIL - It appears the ramp didn't stop". In operation, the symptom was that when triggered the oscilloscope would rarely or never refresh the waveform shown on the screen unless the 'Force Trigger' button was manually pushed. However, if no signal was present (allowing the waveform to 'roll') the scope would seem to work normally. With no schematics it was a real bear to find the source of this problem. The only reason I was able to do it was because I had a working unit to compare with. First, I swapped boards between the units and determined that the problem was on the acquisition board. Not too surprising as this is the board with the most capacitor leakage damage on it. I then put the working unit and the broken unit side-by-side on my bench and compared the waveforms on each pin of each transistor and IC while marking the signals that were incorrect on the broken unit with red sharpie. You can see these marks in the picture below. After many, many hours of work on this I was able to determine the following information about this ramp test. Armed with this info, it should not be too hard to troubleshoot similar tbiRampTest problems.
I don't know exactly what the ramp is for, but here is (somewhat of a simplification of) how it works.
1) The ECL control logic generates a positive edge (first waveform picture) which, after going though some other transistors, appears on the base of the ramp generator transistor.
2) The ramp generator transistor, operating as a constant current source, charges a capacitor to form a ramp waveform (second waveform picture). This waveform appears on the collector of this transistor.
3) The ramp appears on the '+' input (pin 2) of the LM311 ramp comparator. The other input of the comparator (pin 3) is held at ground and so when the ramp reaches zero volts, the comparator triggers producing a positive edge.
4) This signal goes though another few transistors and ends up going back into the ECL logic at the point I am calling 'TP1'.
5) The ECL logic brings the base of the ramp generator transistor low again, ending the ramp.
Note that there is much jitter in the length of the ramp. This is (apparently) normal.

Waveform at the base of the ramp generator transistor:

Waveform taken at the collector of the ramp generator transistor (the ramp):

Waveform taken at TP1:

In the end, the problem with my unit was a bad ramp generator transistor. This transistor is marked 'M3J' and is a Motorola 'MMBTH69' for which a datasheet is available here:
http://www.datasheetarchive.com/dlmain/Datasheets-21/DSA-408130.pdfThe only place I could find which carries this part is American Micro Semiconductor:
http://www.americanmicrosemi.com/CMOS BatteriesAnother weird problem with this oscilloscope was that, even when turned off, the fan would 'twitch' - every second or so the blades would jump a bit. After much board swapping, I determined that this problem is caused by a low, but not completely dead, CMOS battery on the CPU board. I replaced all the CMOS batteries in all of the TDS-420 oscilloscopes I have worked on, so here is some info on how to do that.
First, there are two CMOS batteries in these oscilloscopes. Both are held in clips on the chassis and connected with wires to the boards. One is connected to the CPU board and the other to the DSP board. The CPU board is identifiable because it in in the first slot from the top of the unit and has the GPIB connector on it, and the DAQ board is identifiable because it is shorter then the CPU board, is in the bottom most slot and has no external connectors on it.
The battery on the CPU board may be disconnected for replacement with no ill effects. However, I recommend that before replacing the battery on the DSP board, you connect a 3V power supply in series with a 1N34A diode to the battery terminals on the board. This will assure that the contents of the memory are preserved while the battery is being replaced. If the contents are lost, it is not a major problem (the calibration data is safely in EEPROM) but it will cause your error log to be cleared and replaced with a single battery low error. The error log can't be cleared without special software (which I have never been able to find) so it is nice to keep it 'clean'.
Each of these batteries is replaceable by Panasonic BR-2/3AT2SPN. However, to prevent the sharp clips from cutting through the thin insulating material on the batteries and shorting them to the case, each battery must be covered, like the originals, in another layer of insulating material. The correct material is 31mm (flat) by 4mil thick clear vinyl heat-shrink ('battery-wrap'). I bought mine from eBay. Use a 1-5/8" long strip and shrink it around the battery with a heat-gun, then transfer the battery leads from the old battery and insulate them with heat-shrink like the originals.
Adjusting the displaySometimes the displays get dim or raster lines start showing up. This is usually a simple adjustment. Follow the procedure in the service manual. Note that it seems some scopes, even when properly adjusted, will show raster lines for a few minutes until fully warmed-up. I think this is just aging CRTs.
Signal Path CompensationIf nothing else works, or you are having some really weird problem, try running the signal path compensation. This is available in the 'Cal' menu and has worked magic for me a couple times. First I discovered that when one of these appears to be 'missing samples' when running in equivalent time (ET) sampling mode running signal path compensation will fix it right up. The picture below shows the problem. Very odd.

Second, I tried running signal path compensation before fixing the tbiRampTest FAIL problem. This put the scope into a weird mode where even after the original problem was solved, it still wouldn't refresh the screen when triggered. Running SPC again solved it for good.
Finally, if the DC balance is off (the trace moves up and down when changing the attenuation setting, SPC should fix it.
Attenuator repairsIf someone manages to blow the 50-Ohm resistor in the attenuator(s) of one of these beasts, the higher voltage ranges (10V and 20V I believe) won't work. This can be fixed as I described in a previous thread:
viewtopic.php?f=8&t=216792Hope this help someone!
-Matthew