HP 8133A 3GHz pulse generator - teardown and repair - Page 1
Forum section: electronics / testgear

author: bson
author_rank: Unknown
timestamp: November 08, 2018, 02:54:16 am
content: So I picked up a cheap HP 8133A pulse generator, non-working as is, on eBay. The photos showed something on the display, which led me to think it might be a decent repair project.On the bench, power it up, it mostly worked. Output was limited to 2.4GHz after which is faded quickly. I left running for a few hours, and it didn't seem to have any thermal issues. So I moved it around a bit, which made it stop working (no output), then fiddled with it a bit more and the output returned. So, clearly something mechanical, like a loose connector or corrosion.First step was to open it it up to see what kind of messes there might be inside. Corrosion, debris, dust piles, devulcanized rubber, leaky caps, exploded tants... who knows until you take it apart.Off comes the top panel, side panel, and the top shield, exposing the MCU (MC68HC008 @ 8MHz) and main control board. Under the second shield is the card cage. Under the logic board is the power supply. I removed 9 screws to get this far; this thing is seriously solid iron!Battery looks super clean, measures 3.14V, so I'm not going to bother desoldering or replacing it for now. In fact, there was barely any dust on it; this unit must have spent its life (it was a rental unit) in immaculate lab conditions! Some of the MCU board chips are dated wk 39 1999, so I suspect this unit shipped around 99/00 or so.Now I've removed the cage top cover, exposing the card cage. The board seen is the channel 1 output (opt 002/003 add a channel 2, which requires a different front panel). Underneath this is the main timing board. I didn't get to that board today; the card cage is accessed by removing the front panel, and I only got to removing the rear to check the power supply. Maybe I'll pick up a nicer front panel and replace it; at that point I'll take a look at the timing board. 14 screws held the cage cover, and we're now up to 23 screws removed.Super squeaky clean, like the MCU board. Just a hint of dust, which I vacuumed off.On the bench, service guide on the laptop.Overview of the channel 1 board:Output switches. It has complementary differential outputs that can be individually turned on/off.Delay unit. This has traces for 8 coarse delays. Finer delays are created with a DAC biasing a diode, routing the signal through one of the traces in the delay unit.The top board position is used for opt 002 or 003. Adding it isn't as easy as just dropping in a board though, as it requires a front panel and the proper connectors and output assembly.MCU board removed. 6 screws. Checking the voltages is easy since they're immediately accessible on popping the shield. All good here.Pulling the PSU requires removing the back portion of the chassis. 10 screws. Plus 4 on the left (seen from the rear) to free the rear frame. 37 screws removed so far.Detaching the PSU from the chassis. Another 9 screws. 46 screws.And there, finally it is. Super clean, all Nippon Chemi-Con, all looking just fine. No leakage or problems. I took notes to order replacements to re-cap at some point.That's a monster toroidal... Clearly no expense was spared in the design of this thing!A small machined block for the power trannies.So, cleaned out all internal dust and reassembled it. 46 screws... and some fiddling to get the cage cover to fit right. Nothing overly difficult, but just took a bit of care. It's one of those jobs where you wish you had a third hand!Cleaned off the front panel and checked that it still works. Now it gets to 2.9GHz before fading. I couldn't get it to drop the output either, so I seriously suspect it's the jumper in the back that's a bit crusty and could use a good cleaning. I had to remove the rear block assembly to get the rear off, and just removing and reinstalling the jumper probably scraped some of the crust off. I didn't think the cause was going to be anything related to the front switches since both complementary outputs were identical and faded similarly. The fading was measured with my Signal Hound SA. So next on the agenda after this is to give the rear block a good cleaning. The instrument specifies 3GHz minimum, 3.5GHz typical, so it's still a bit on the low side.What's with patching up panels with stickers? Okay, I can see a calibration sticker someplace, but how about removing old ones? Ick. So much nicer without that mess on the front.Spec is 0.2% frequency accuracy, so good enough.Gave the outside panels a good IPA scrub too, followed by a wet rag (water).

author: xrunner
author_rank: Super Contributor
timestamp: November 08, 2018, 03:01:27 am
content: Nice project. I haven't seen that unit before. Looks good now that you cleaned it up.You got a nice lab bench setup too.

author: bson
author_rank: Unknown
timestamp: November 08, 2018, 05:29:47 am
content: Cleaned off corrosion on the rear link, and no improvement - still loses signal with frequency past 2.9GHz. Down 20dB by 3.5GHz. So, I suspect the thick-film hybrid output driver.But lo and behold, someone is actually selling these on eBay! So, I grabbed the last one.https://www.ebay.com/itm/122299927620I figure if this is all that's needed I'm still doing pretty well here...Might still get cosmetically nicer looking panel and replace both at the same time.

author: bson
author_rank: Unknown
timestamp: November 15, 2018, 06:42:11 am
content: Received the replacement amplifier today...So, time to get back into the instrument. Popped the cover(s) off, desoldered the amplifier and detached it, removed the bracket holding it in place.This is when I realized it's screwed onto the tray and can't be replaced this easily.In fact, getting to it requires removing the entire card cage, which requires removing the PSU and front panel assembly, then the tray from the cage and the PCB from the tray. It also required removing the rear link block from the rear panel - some pretty serious disassembly. Took me about 1.5 hours to take it apart. At this point the instrument is spread out in a HUGE pile of parts.Removing the channel 1 board and tray reveals the main board.Cage rear, after removing the link block:The channel board after removing it from the cage. It's attached to this tray, which seats into the cage.Side view; the hybrid chips are screwed directly onto the tray.Removing the board from the tray finally provided access to the output amplifier!Output amplifier removed...Bottom side:Replacement amplifier in place, about to lock it down with the bracket and solder the ground lead (?) onto it.Back onto the tray and into the cage.And, then, another hour putting it all back together. A quick smoke test as soon as it was sufficiently reassembled to power on.Looks good on the SA too, now getting solid output all the way up to 3.5GHz.Tried to apply hydrogen peroxide (40V gel) to the front panel, which had no effect after a full day in sun. Turns out there's a clear plastic film on top of it. The panel itself seems to be the main adhesive side... Not sure what to do about that, maybe I'll just discard the clear film and remove the adhesive.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 15, 2018, 07:43:46 am
content: NIce catch, and good job. Thanks for the teardown pictures.

author: xrunner
author_rank: Super Contributor
timestamp: November 15, 2018, 02:17:56 pm
content: Ha - what a job. After all that you hope the thing works! Thanks for the pics.

author: jhenderson0107
author_rank: Regular Contributor
timestamp: November 15, 2018, 06:52:16 pm
content: Thanks for the teardown. It will be of help to me, since my 8133A requires a replacement amp for channel 1.

author: bson
author_rank: Unknown
timestamp: November 15, 2018, 11:25:08 pm
content: Quote from: jhenderson0107 on November 15, 2018, 06:52:16 pmHeh, yeah, would have saved me a bunch of time...Just be careful and take your time, nothing requires force. Attach the new amplifier (loosely) to the links before inserting the screw posts into the board. This can be tricky.Getting the cage top back on: there are two standoffs in the rear that need to slot into it. This cover was designed by the devil. Three hands would not be enough, you really need five to get it back on. Plus an assistant inside!The PSU screws are threaded differently from the other chassis screws, and are also a little longer, so keep those separate.If it seems you killed one or more LEDs on the panel you probably didn't insert the W100 ribbon cable fully, until it locks in place.There are toothed piece of copper shield foil strips spanning the chassis and rear link block; carefully remove these (if you don't want to get them all wrinkly) before detaching the block itself. I assume HP used a conductive adhesive here, so I left this on and just reattached the little strips when reassembling. If I had known I would have researched a good substitute adhesive before, but they were still sticky enough to reinstall, so no problem really.If you detach the plastic front power rod/button you can get the PSU out by sliding it back slightly to release it from the connector, then lift it up and out. I could never get the power button rod off and didn't want to brute force it (who knows how hard that would be to replace) so just did a bit more assembly to slide it out the back. If you know what you're doing it will probably come right off. (Being an idiot it also didn't occur to me to check how to do this either of the times I had the PSU out and I could get a good look at it.)Tools list:7.9mm socket (8mm worked fine) for SMA connector lock nuts7.9mm wrench (again I used an 8mm one) for SMA links (the service guide says 7.9mm torque wrench!)Needle nose pliers and tweezersTorx T15 for the casePhillips screw driver (I used a Wiha PH 1x60)Razor blade to peel the conductive foil strips over the rear and terminal blockIPA for cleaningNon-marring pry tools

author: Xtremexp
author_rank: Regular Contributor
timestamp: September 27, 2023, 06:03:28 pm
content: Hi bson,Can you reattach the pictures? I got mine and during first boot it showed error 008 frequency counter failed. what it may be?But after several reboots I do not see that message anymore. Not sure what is going on. I can see pulse coming out from channel 1 output (I can measure upto 500MHz) may be higher frequency is not working.regards,Xtreme