Agilent 33250a Blown Output Hybrid - Page 1
Forum section: electronics / testgear

author: daveyk
author_rank: Frequent Contributor
timestamp: April 18, 2017, 07:06:41 pm
content: Anybody ever able to fix or replace the output hybrid? Agilent will not sell the part; they swap generators for about $3500.I would love to have someone design A discrete amplifier to replace the hybrid. I would only worry about 100KHz - 30MHz. The output amplitude doesn't have to remain flat as it would be monitored with a scope. It needs to drive 10vP-P in to 50 ohms. It would be great if we could fit a little add-on board inside.DaveSent from my iPhone using Tapatalk

author: TheSteve
author_rank: Unknown
timestamp: April 18, 2017, 07:42:42 pm
content: I've never seen anyone try to replace/repair the hybrid. I know I was warned to watch out for dead hybrids when looking to buy a 33250A to repair as it was game over if it was dead.

author: daveyk
author_rank: Frequent Contributor
timestamp: April 18, 2017, 07:58:17 pm
content: Quote from: TheSteve on April 18, 2017, 07:42:42 pmWell this was from dumpster diving at work. The sine wave coming out of the attenuator network is pure.The output must have been hit with a UT square wave pulser. The protection diode is also shorted. The diodes for the output monitoring circuits have been pulled so that it's possible to work on the generator. It thinks it's working. The hybrid is not. What I want is a replacement output amplifier. It only needs a 30MHz bandwidth, not 80MHz.Maybe I should look to the schematic of an old Wavetek 166 for inspiration but was hoping an engineer on here may have already tackled this.DaveSent from my iPhone using Tapatalk

author: TheSteve
author_rank: Unknown
timestamp: April 18, 2017, 08:04:35 pm
content: One option is to sell it for parts on ebay - even with an advertised blown output you'll probably still get enough to buy a nice working 20-30 MHz AWG.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2017, 08:32:41 pm
content: I really wish companies that make this sort of stuff would product a bunch of spare hybrids. In general I really hate hybrids and custom silicon, at least the latter is usually fairly reliable, hybrids seem to be a lot more problematic.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 18, 2017, 09:27:31 pm
content: The 33250A sells new for over $ 6000 !I agree with TheSteve, sell it as broken and you might get as much money as you need to buy a working 30 MHz 33220A for around $500Can you show us a picture of the broken hybrid?I don't have a 33250A, So I have never seen inside one.

author: daveyk
author_rank: Frequent Contributor
timestamp: April 18, 2017, 10:16:26 pm
content: Quote from: HighVoltage on April 18, 2017, 09:27:31 pmWhen I get home. It's probably 1"x2" with a heat sink bonded to it.I wrote software around the 33250a, but if the 33220a had glib I could adapt the test program. I'll look in to it; thanks.DaveSent from my iPhone using Tapatalk

author: daveyk
author_rank: Frequent Contributor
timestamp: April 18, 2017, 10:18:09 pm
content: Quote from: HighVoltage on April 18, 2017, 09:27:31 pmAh, the 33220a is only 20MHz, not 30Mhz. Bummer.Sent from my iPhone using Tapatalk

author: TheSteve
author_rank: Unknown
timestamp: April 19, 2017, 03:00:28 am
content: Here is a picture of the hybrid and inside - there is no shortage components and there are more on the bottom side of the PCB, on the front panel and of course the power supply.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2017, 04:41:55 am
content: What's the other side of the hybrid look like? Any chance of repairing it? Some hybrids are far more complex and specialized than others. While it may be unlikely, it's possible a printed resistor was burned out or something else simple.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 19, 2017, 08:31:50 am
content: Quote from: daveyk on April 18, 2017, 10:18:09 pmAhh, my bad, sorry.I mixed it up with my 33522B that goes up to 30 MHz.But the 33220A has GPIB

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 09:25:39 am
content: Quote from: TheSteve on April 19, 2017, 03:00:28 amThan you; it's square. I was thinking of another one from a different instrument when I described it as rectangular, but that's it.Sent from my iPhone using Tapatalk

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 09:36:37 am
content: Quote from: james_s on April 19, 2017, 04:41:55 amI don't recollect being able to see anything. I have nothing to lose, so I guess I could pull it and look. Thanks for the suggestion.I am thinking of looking at the output amplifier of a Wavetek 166 and seeing if I couldn't built that on a perf board and cobble it in there. Yea, it wouldn't have the performance it should but it might work. I guess no one has ever really gotten past a fried hybrid in these things.I did have some good news this morning. I finally fixed my Agilent E3632a power supply from the same dumpster (another thread). After replacing the bridge rectifier and both pass FETs, it still deregulated at 14volts. So thyristors showed up yesterday. I replaced them. No fracking luck. This morning something came to me in a dream so I checked it out when I got up. The wire connection from the transformer tap to the pcb was bad. That transformer tap read 6k to the other taps. The crimp connector was bad! I soldered it and now almost zero ohms between taps and the PS works perfect. Good Lord, several weeks and about $50 in parts and it was a bad wire crimp!So that proves it; I'm dumb. I scoped and metered that thin for several weeks and couldn't figure it out but my subconscious in a dream could. I need to hire my subconscious full time. (Sigh).DaveSent from my iPhone using Tapatalk

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 09:41:08 am
content: Quote from: HighVoltage on April 19, 2017, 08:31:50 amNo problems; that's still a great idea, but I'm not seeing any on eBay yet. I'll keep looking. It would probably work fine for me and be a bit less money.DaveSent from my iPhone using Tapatalk

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 09:47:14 am
content: Quote from: HighVoltage on April 19, 2017, 08:31:50 amI guess if I have to, I could use an HP8116a. The output is not accurate compared to what you tell it to be, but the scope can monitor it and then I can have the program send commands to tweak the generator. I think that will be a last resort, but they are dirt cheap.P.S. - I don't know if HP-IB is compatible with GPIB, so that might not be a good idea.Sent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2017, 03:44:43 pm
content: HP invented HP-IB, later other manufactures started using it and the name changed to GPIB, it's the same interface.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 19, 2017, 06:34:54 pm
content: Once it is known what the exact job of this hybrid is, I don't see why it should be impossible to recreate its function on a piece of FR-4 PCB with components available from digikey.It will never deliver the same performance as the hybrid, but it'll at least save the entire waveform-generator from the scrapyard.Is there a pinout available? Or can one be determined by probing around in the unit? If the rest works, the incoming signals should give a hint to what is happening on that hybrid.Also a picture of it might be helpful to see the componentsOr am I completely mistaken and there's such high precision required in the manufacture of this thing that it's impossible to copy its function with anything less than ceramic substrate and laser-trimmed resistors?

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2017, 06:45:39 pm
content: It may even be possible to duplicate the original performance with some care. What required a hybrid in the past may be reasonable to do on a standard PCB with modern components today, even if you have to hand select parts. Then again it may use custom silicon which makes the prospect of replicating it far more daunting.

author: pigrew
author_rank: Frequent Contributor
timestamp: April 19, 2017, 06:52:36 pm
content: Quote from: SaabFAN on April 19, 2017, 06:34:54 pmThe service guide has the pin-out. It is a 10x gain, >80MHz amplifier, that can drive a 100 ohm load. There is a extra pin for adjusting the DC offset. It should be somewhat doable.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2017, 06:56:48 pm
content: Certainly sounds possible, not trivial but a good analog designer could come up with something that would do it I'm sure. Careful attention to layout will be required.

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 08:45:58 pm
content: Quote from: james_s on April 19, 2017, 06:56:48 pmIt's just an amplifier, but there must be an awful lot in that small package. I was thinking of using the output power amplifier of the Wavetek 166 as a guide but when I looked at it, there is a huge amount of parts and it would be a huge build. There should be room inside that case if it did go bigger.Apparently, this is a well known failure of this excellent generator. Why Keysight won't sell the part, I dunno. If it was a $1000 generator I could almost understand, but a $6000 instrument and the only repair option is a $3500 swap-out?If the problem is that prevalent , someone could design an adequate board and sell it. I doubt it would be worth anyone's time or investment.I could bring the "good" signal to the front panel out of the internal attenuators (which are before the power amp, or the majority of them are), if there was an off the shelf RF amplifier one could buy relatively cheap, but that's probably too much of a cobble.Just a nice, simple amplifier/driver would be nice. Myself, I don't need over 30Mhz. Output amplitude variance can be compensated for.I'm sure this is a lost cause and I will just buy a used one (with return policy) off of eBay for about $2500.00. The other generator mentioned might be an idea too.Since they can't be repaired, even a Chinese brand in the $1000 range may be an idea and write some code for it, but they don't do GPIB, probably just network and the rest of the test equipment I use is still GPIB.I need to do some more searching....Sent from my iPhone using Tapatalk

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 08:56:18 pm
content: Quote from: james_s on April 19, 2017, 06:56:48 pmhttps://www.ebay.com/itm/300331081153There's a possible alternative and it even has GPIB.Does anyone have experience with RIGOL? That may be early RIGOL.Sent from my iPhone using Tapatalk

author: TheSteve
author_rank: Unknown
timestamp: April 19, 2017, 09:05:52 pm
content: Do you require a floating output? Very few "off brands" offer that.

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 09:21:17 pm
content: Quote from: TheSteve on April 19, 2017, 09:05:52 pmProbably not because I'm not sure what you are referring to. I use a trigger, or Gate, input, sinewave 10mv to 10vPP. A built in accurate delay function is nice, but not a deal breaker.DaveSent from my iPhone using Tapatalk

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2017, 09:23:38 pm
content: Quote from: TheSteve on April 19, 2017, 09:05:52 pmOkay, DC offset? Not really unless when using a pulse or square wave output.Sent from my iPhone using Tapatalk

author: kcbrown
author_rank: Frequent Contributor
timestamp: April 20, 2017, 05:03:13 am
content: Quote from: daveyk on April 19, 2017, 09:21:17 pmA floating output is an output that has an independent ground. Most inexpensive function generator tie their output ground to the unit's power supply ground, which results in that ground effectively being shared across devices in most cases.Having a floating output can be handy if you need to inject a signal into a part of the circuit that isn't itself ground-referenced (but where some other part of the circuit is), or a part of a circuit where the point where you'd connect the negative side isn't itself ground. If you were to do that with a normal output, you'd end up tying the negative connection point to chassis ground. Needless to say, things can blow up spectacularly if you do that under the right conditions.Not having a floating output limits where you can inject signals. It must not be too terribly limiting, else I expect we'd see a lot more lower-end signal generators with that capability, but it seems to me that having it would make life quite a bit simpler.

author: amspire
author_rank: Super Contributor
timestamp: April 20, 2017, 07:32:10 am
content: The 33120A has a discreet power amplifier that is probably not too hard to build. I could take a photo of the PCB layout. The 33120A has a lower frequency output so I do not know how much bandwidth margin the amplifier has. Uses an opamp to zero the DC offset.https://ese.wustl.edu/undergraduate/Documents/lab_support/HP_33120A_Function_Generator_Service.pdf

author: philipz
author_rank: Contributor
timestamp: April 20, 2017, 09:11:18 am
content: Other option is output amplifier from 8112/8116 - they have bandwidth 50 MHz, especially 8112A which capable to produce 5nS rise and fall time. Maximum output amplitude is +-14V on 50 Ohms. Both have more or less the same output amplifier. There may be 2 issues - original schematic require +- 22V and use some transistors which has only HP part number (for example 1853-0354 and 1853-0357)Attached is a schematic build from 2 sheets available in net for easy reading

author: daveyk
author_rank: Frequent Contributor
timestamp: April 20, 2017, 12:09:42 pm
content: Quote from: philipz on April 20, 2017, 09:11:18 amEither way it's a lot of odd ball parts to gather up. The 8116a looks s little simpler but still a lot of work. It's just a damn shame that Keysight won't sell that hybrid. It's frustrating when you know what's wrong and the fix would not be that difficult but can't get the part.I don't know if the front panel or power supply is of value to anyone or not. The Power supply is repairable and the front panel probably never fails. I'm not sure it's worth the eBay fees. Otherwise this 33250a may end up in recycling.I may buy a used one for critical work and a 8116a for everyday work. If I remember the 8116 had a signal processing hybrid that had problems but you could buy those. I remember replacing one years ago in an 8116a. I see that chip for sale on eBay for $50, so it must be the common failure of that generator.I remain frustrated... but life must go on.Sent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2017, 03:55:05 pm
content: Well if you get to where you're just going to recycle it, I'd pay shipping to take it off your hands. Looks like it would be interesting to play with.

author: TheSteve
author_rank: Unknown
timestamp: April 20, 2017, 04:16:28 pm
content: PM sent - I actually need a front face for a 33250A.

author: daveyk
author_rank: Frequent Contributor
timestamp: April 20, 2017, 10:45:47 pm
content: Quote from: TheSteve on April 20, 2017, 04:16:28 pmJust sent a response.Sent from my iPhone using Tapatalk