Help with HP 54201A Digitizing Oscilloscope

[Jacques021]

Good day

I obtained this scope from a friend a while ago and one day while I was working with it, it start displaying alot of exclamation marks and a fatal error message. It does this when you switch it on. I could not find much on google but I replaced the battery and read / tested the 8 intel rom chips with my eeprom reader/writer but I did not find any problems with them as far as I can tell... Problem still exists and I really want to fix this boat anchor. 

[JFJ]

Have you tried a 'Key-Down Power-Up' reset? It's quick and easy, so always worth a try:

[Jacques021]

Will give it a try

[Jacques021]

Will give it a try

Did not work at all, no response from any of the keys.

[Lorenzo_1]

I suggest you remove, clean and reseat the boards. I have a working 54201a and a similar vintage 1630d logic analyzer that has quite a bit in common when opened up. My 1630d showed garbled screen junk intermittently when first acquired. Board reseating fixed that nicely.

I’m pretty sure the screen in both has the same text-based segments on the LHS and top side and then a graphics field taking up lower RH area for waveform display. It also writes text over the graphics field in some modes. If you get some keyboard response after reseating, worth confirming that all fields are displaying correctly. It’s possible for text to display fine across the screen but graphics field to be blank/faulty. The 54201a service manual is a bit thin but the 1630 manual has lots of detail on exactly how the screen is driven and I suspect you’ll find it useful for your scope.

[Jacques021]

Dear Lorenzo_1, thanks alot for the kind reply, I also will look at the HP 1640D logic analyzer, thanks for pointing me in that direction.
I have removed the boards, cleaned and reseated them a couple of times with no luck at all. I also reseated the IC's wich are in sockets and made sure they make contact.
The CRT wont come on now for some reason.

[Lorenzo_1]

Hi Jacques.  Do you have the 54201A service manual?  You can get it from the Keysight website (This is the link I used - hhttps://www.keysight.com/en/pd-54201A%3Aepsg%3Apro-pn-54201A/300-mhz-digitizing-oscilloscope?cc=AU&lc=eng).  The 1630D manual is available here too (https://www.keysight.com/en/pd-1630D%3Aepsg%3Apro-pn-1630D/logic-analyzer-with-43-channels?cc=US&lc=eng).

54201A manual has the troubleshooting procedure set out in Section 6A and, unsurprisingly, starts with checking power supply voltages and then checking for board faults.  Take heed of the several warnings about dangerous voltages present in the power supply and CRT circuit of course. Do you have the right equipment for testing?  I wouldn't recommend anyone tackle a power supply/CRT related fault unless they were confident they had the skills, knowledge and equipment needed.

I'd suggest, if you're confident you know what you're doing, stepping through the first part of this troubleshooting procedure on the LV supplies and board removal tests and see if you can narrow down the cause of failure. 

[Lorenzo_1]

Here's a couple of shots of my 54201A power supply which is working fine - u may find them useful.  There have been a few boards modifications over time - this one has LED indicators for thermal shutdown, primary current limit and over-voltage failure as well as NORM[al] (lit here).  See manual p6A-11 for LED failure troubleshooting if you have others lit up. Comparing the 1630D and 54201A power supply schematics indicates that the late model versions at least have the same power supply and I suspect much of the CRT display driver circuitry will be the same - so the extensive detail in the 1630D manual may prove very useful if you do have a board level problem.  Note also there's a very useful vertical connector in front of the PS board that can be used to plug in the display board etc for testing/debugging (just visible in the photo shadow) - if you get to that stage.

[Jacques021]

Hi there, yes im more than confident working with high voltage circuitry as I work on valve/tube radios and equipment. I recently bought a Tek 545B scope still in very very good condition and working perfectly.
I have the original service and user manual of the HP but could not find any schematics of the CPU board, however the one resistor near the display connector burned out aswell as a few 74 IC’s while troubleshooting yesterday.... Dont know what happend as nothing was shorting out. Can you maybe help me with the value of the resistor as I cant find the schematic, but I guess it should be in the 300 ohm range? Maybe I must just try and buy a working CPU board from someone but they are very scarse like chicken teeth

[Lorenzo_1]

Hi Jacques. Thought I'd posted this but it doesn't seem to have gone through. The resistors are 33.2 Ohm 1% and 1.8 Ohm 5% nominal on my unit - both are close to the mark when measured. See photo for the locations.  I have my CPU board fitted in the service slot and will leave it there while u work through your diagnostics in case you have any further queries.

I've also attached a picture of the whole CPU board and the parts layout for the similar board used in my 1630D logic analyser.  There are differences, but the similarities of layout and component footprints are striking and I suspect you could find the very extensive functional descriptions and schematics in the online 1630D manual informative about the 54201A, with a bit of research into the differences. For some reason the logic analyser manual is far more detailed and has full schematics for all the boards and very good descriptions of how the CRT is driven.  The online 54201A manual is the same rather thin version as my hardcopy original version.  If you do find a more comprehensive service manual I'd be pleased to hear about it.

I did have to debug and replace a couple of 74 series chips on my 1630D when the display became garbled.  Came up good and has worked fine since, so they are repairable even by a non-expert like me.

 

[Jacques021]

Hi Lorenzo
Thanks for helping, really appreciate it alot! Hopefully I will get to the electronics wholesaler tomorrow to buy the new chips/IC's.
Will keep you posted!