Agilent 81104A 80 MHz pulse/pattern generator (fixed!) - Page 1
Forum section: electronics / repair

author: TheSteve
author_rank: Unknown
timestamp: February 25, 2016, 06:28:03 am
content: Another late night surfing ebay lead to the purchase of an Agilent 81104A 80 MHz pulse/pattern generator with two 81105A 80 MHz output modules. It was sold for parts and as not working as the output modules both failed the self test. There is no service manual that I can locate but the price was right so I clicked that buy it now button.When it arrived a few days later I decided to give it a quick power on to verify the condition. Sure enough the output modules failed self test, there was also a bit of a "hot" smell from the unit. Anyone who has dealt with a lot of electronics knows that smell.Switched the power off and opened it up. It is very pretty inside, lots of very nice looking hardware, excellent build quality as we all expect from Agilent. It didn't take long to spot a problem - one tantalum cap was a nice roasted brown color instead of its factory yellow. This brought a huge smile to my face - could it really be this simple??? In a word - YES!I swapped out the cap and it now passes all self tests and generates lovely pulses and patterns. Part of me wishes it was something a little more challenging, the rest of me is thrilled with a stupid simple repair.At this point I have been playing with it as I've never used a pulse/pattern generator before. I cranked the outputs up to 80 Mhz and took a look at it on my scope. If I had a scope that had more then 100 MHz of bandwidth I'd post the pics, as I don't you get to see a shot or two from the spectrum analyzer instead. The Rigol DS1074Z-S does tell me the rise time is around 4ns when in reality it 3ns but it obviously didn't look anything square. The 81104A lets me adjust rise time in 10 pico second increments starting at 3ns minimum. All in all it's a neat device but I will likely relist it on ebay as it isn't something that will add real value to my lab. Perhaps the funds can go towards a better scope - speaking of a scope, that is the one piece of gear that is near impossible to find cheap/broken(a newer Agilent anyway) on ebay.If anyone wants any further pics or details on the unit let me know, there is very little info on this beasts around.

author: TheSteve
author_rank: Unknown
timestamp: February 26, 2016, 05:27:39 am
content: And one pic of what should be a square pulse as displayed by a Rigol DS1074Z-S running at a blazing 100 MHz.The 81104A pulse is spec'd at a rise time of 2.80ns and a width of 5.88ns. The Rigol does get the pulse width bang on even if it can't keep up with the rise/fall times.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: June 10, 2021, 11:54:50 am
content: Sorry for resurrecting this old thread, but I would like to add some information regarding the repair of the 81104A pulse generator, which might be useful for other people attempting the repair of these units.I recently dealt with one such device showing a multitude of self test errors:- Hw:1 Measured frew. too big- Hw:1 Low and High level error, with no, positive and negative offset, 50 Ohm and 1 kohm- Hw:1/ VCO 8 failed test for 360 MhzThe device was not outputting anything from the output, which was sitting at around - 5V DC, and the trigger out was also only working consistently if using the external 10 Mhz clock input.This entire slew of errors was just due to a single failed tantalum capacitor on the -31 V rail, on bottom side of the 81105A output module, close to the AD817.This single capacitor was dragging down (well, up) the -24 V and - 31 V rails to around -6 V. The capacitor was not a dead short and did not appear exploded, but was getting super hot during operation.I have planned the re-cap the entire output module because most likely the other capacitors are on their way out as well.I hope someone might find this information helfpul.