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Old 20th Mar 2019, 11:03 pm   #1
high_vacuum_house
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Smile HP 1222A oscilloscope vertical input FET transistors

Good evening,
I am currently playing with an HP 1222A oscilloscope with a defective channel A input and channel B not working correctly. Having a poke about with a working oscilloscope and a signal generator it appears to be the vertical input dual FET transistors which are at fault. These are the first active semiconductor components from the BNC sockets and it looks like they could have succumbed to a high voltage being applied on the channel A+B BNC sockets.

These devices are dual N channel JFET's. Each can holds 2 separate FET devices. One device for the A or B channel input amplifier and the other device used as a voltage source for the next stage in the amplifier. To prevent cross coupling, one device is used per channel. The part numbers show that they are pretty much unobtainable as you would have guessed!!

Could 2 standard N channel JFET's be used to replace the oddball dual FET can?. What types could be used. The vertical bandwidth of this oscilloscope is 15MHz

Many thanks,
Christopher Capener
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Old 20th Mar 2019, 11:14 pm   #2
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Smile Re: HP 1222A oscilloscope vertical input FET transistors

The JFET is on the right hand side of the A2A2 amplifier. The other device in the dual JFET package is used later on in the amplifier as a voltage source.

You get a healthy looking signal on the gate with a signal generator connected to A channel but on the source output resistor there is just DC. B channel has a signal on the source though it is fairly small and believe this JFET has been damaged as well.

Christopher Capener
Attached Files
File Type: pdf A channel input.pdf (380.5 KB, 114 views)
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Last edited by high_vacuum_house; 20th Mar 2019 at 11:24 pm.
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Old 21st Mar 2019, 6:54 am   #3
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Default Re: HP 1222A oscilloscope vertical input FET transistors

"Could 2 standard N channel JFET's be used to replace the oddball dual FET can?" AFAIK they need to be matched and bonded thermally Chris, also AFAIK you can still get dual tranny's, probably SMD package, wether within the same spec as those I've no idea, do a search on mouser/digikey.

Andy.

PS - https://www.digikey.co.uk/products/e...?k=dual%20fets
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Old 21st Mar 2019, 10:17 am   #4
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Default Re: HP 1222A oscilloscope vertical input FET transistors

It's certainly worth experimenting. I had a similar problem with a special Tektronix dual FET in a 7A22 vertical plugin (the one with the legendary 10 microvolts/cm differential sensitivity). It was a bit leaky, which meant that the trace wandered off the screen at higher gain settings.

I bought a load (about 20) of run-of-the-mill J113 FETs and made a simple circuit to measure the gate voltage at the drain current at which the circuit operated. I found two that were acceptably close and put them in the plugin. It works fine, and the performance is now indistinguishable from another 7A22 with the original dual FET in.

The Tek plugin has a little plastic hat which fits over the FET socket, presumably to keep draughts away from it. The thermal connection of two single FETs will be nowhere near as good as a dual one, of course, but gluing their packages together would probably help a bit.

Chris
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Old 21st Mar 2019, 1:29 pm   #5
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Default Re: HP 1222A oscilloscope vertical input FET transistors

A U440 dual-FET might be suitable. Data readily available on the www. This item is available from various, incl. Farnell.

Al.
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Old 21st Mar 2019, 1:53 pm   #6
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Default Re: HP 1222A oscilloscope vertical input FET transistors

I second Chris' suggestion. I used 2 x MPF102 to replace the input stage of the external trigger input on a Tektronix 7B80 timebase plugin with no problems. A quick check shows that the trigger sensitivity exceeds the spec by a large margin, at least to 250MHz which is far as I can easily test it. One reason for choosing the MPF102 is that they mount flat face to flat face in the Tek six pin socket, you could make a small brass clip to hold them together.

Regards,

Roger
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Old 21st Mar 2019, 2:29 pm   #7
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Default Re: HP 1222A oscilloscope vertical input FET transistors

Do you have the HP part number or OEM part number. I've got about 30 dual JFETs here. Might have a direct replacement!
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Old 21st Mar 2019, 2:41 pm   #8
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Default Re: HP 1222A oscilloscope vertical input FET transistors

Something like his (with leads soldered on!) should work:



On Semiconductor

CPH6904

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Old 21st Mar 2019, 4:07 pm   #9
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Default Re: HP 1222A oscilloscope vertical input FET transistors

This was known in HP as 'The plastic scope' designed to try and capture some of the educational market and service bench market for ultra cheap instruments (well, relative to normal HP prices )

There were no fancy parts in it, things already set up were chosen.

CRT accelerating potential is rather low, the trace dim, and the spot could be driven off the screen by rubbing the plastic implosion guard.

THere was one kicking around in our lab. Never used for work, but occasionally signed out by someone wanting one for a few days to fix their TV. It did that OK, but was less good than a telequipment dual channel job.

Probably a pair of 2N5245 or something of that ilk with a bit of selection and a metal temperature link between them ought to do.

David
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Old 21st Mar 2019, 8:30 pm   #10
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Smile Re: HP 1222A oscilloscope vertical input FET transistors

Good evening,
Many thanks for your replies. This oscilloscope is certainly very basic on the front panel controls side and also on the inside and looks as if it was intended for educational/low performance work use. I noticed it has a low EHT with all of the connections made to the CRT base, their being no anode stud or deflection plate pins in the CRT bulb envelope.

This oscilloscope was got at Harpenden for £5 and was in a rather poor state. I have quite a few massively better/ performance oscilloscopes than this one. I just wanted the challenge of seeing if I can get it going!!

I might try a pair of MPF102 JFET devices or a 2N5245 device to see if I can get any results as I can get these on Ebay fairly cheaply and these devices will be working within their maximum ratings. The CPH6904 dual FET has an internal common connection and therefore probably not desirable in this design.

Christopher Capener
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Old 21st Mar 2019, 9:43 pm   #11
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Default Re: HP 1222A oscilloscope vertical input FET transistors

I'd have bought it for a fiver.

Coincidentally I came across this earlier while looking for something completely different....

https://www.hpl.hp.com/hpjournal/pdf...Fs/1974-02.pdf
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Old 22nd Mar 2019, 9:04 am   #12
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Default Re: HP 1222A oscilloscope vertical input FET transistors

Quote:
The CPH6904 dual FET has an internal common connection
Sorry, didn't notice that!


You might be able to reverse D & S (I think that's the case with JFET'S?)


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Old 28th Mar 2019, 12:55 pm   #13
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Default Re: HP 1222A oscilloscope vertical input FET transistors

Quote:
Originally Posted by cmjones01 View Post
It's certainly worth experimenting. I had a similar problem with a special Tektronix dual FET in a 7A22 vertical plugin (the one with the legendary 10 microvolts/cm differential sensitivity). It was a bit leaky, which meant that the trace wandered off the screen at higher gain settings.

I bought a load (about 20) of run-of-the-mill J113 FETs and made a simple circuit to measure the gate voltage at the drain current at which the circuit operated. I found two that were acceptably close and put them in the plugin. It works fine, and the performance is now indistinguishable from another 7A22 with the original dual FET in.

The Tek plugin has a little plastic hat which fits over the FET socket, presumably to keep draughts away from it. The thermal connection of two single FETs will be nowhere near as good as a dual one, of course, but gluing their packages together would probably help a bit.

Chris
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greetings
Martin
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