Agilent 33250a Blown Output Hybrid

[kcbrown]

Do you require a floating output? Very few "off brands" offer that.

Probably not because I'm not sure what you are referring to.

A 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.

[amspire]

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

[philipz]

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

[daveyk]

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

Either 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.


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[james_s]

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.

[TheSteve]

PM sent - I actually need a front face for a 33250A.

[daveyk]

PM sent - I actually need a front face for a 33250A.

Just sent a response.


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