| Electronics > Repair |
| Transistor Jfet HP equivalent |
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| Wallace Gasiewicz:
Thank you for correcting me on the 310 on resistance. I must have been thinking of something else. They vary quite a bit in their on resistance even in the same batch. Maybe 40 to 180 ohms. They did work in my repair of a 3456. |
| aronake:
--- Quote from: HUGOMAN on January 29, 2017, 01:16:04 am ---Hi to All, it's long time i look for equivalent of Jfet 1855-0743, even though i found a list by google but it lacks this reference, please indicate with what we could substitue this transistor by near one or what. Thank you in advance for the support Best Regards --- End quote --- Did you find some replacement that worked? Or how did you solve (or not solve) this? |
| Bill158:
First, before I start this subject again I want to state that I am not interested in debating why I chose this device to replace the HP 1855-0743. But I will state why I used a 2N5458 as a replacement. I had a failed device in my 3458A A1 board. It was in the current measuring circuit and after troubleshooting I found that A1Q204 had very high IGSS. I had to pull A1Q201 thru A1Q205 to find which FET was bad. While I had them out I measured the Rds @ 1ma, the Id(off) @ -7v gate voltage ( the logic "off" voltage at the Gate ), the Vgs @ 1ua, and the IGSS @ -7v. The Rds was about 180-200 ohms. The IGSS was under what my 576 CT could measure using the "emitter leakage". The Id(off) was under what I could measure also. The Vgs was about -4v. This was for the other 4 1855-0743 devices. My choice of the 2N5458 was based on: 1. I had a few on hand along with other N channel J-FET types. 2. Measuring the 2N5458 it had similar characteristics. Mainly the Vgs(off) @ -4v. The Rds was 220 ohms. I again couldn't measure the IGSS or the Id(off) using the "emitter leakage" position of the 576. So I reasoned "WTF, let's give this a try and see what happens". I could have tried other FETs if this didn't work. As far as I am concerned the 2N5458 works just great. I did a CAL 10.000E3 and no issues. ACAL just the same. I confirmed the DCI as best as I could comparing the readings with another 3458A. Resistance measurements are good also. The specs on this HP device are simply NOT available as all have found out. HP/A/K will NOT sell you this device and their stock is marked "Protected Stock". HP/A/K will sell you a replacement 03458-66501 board for US$4900!!! For a home hobbyist I am just as happy as a duck in water! For those who are in business, go for the HP/A/K repair at what ever the going rates are now. Just one man's experience. Bill |
| Wallace Gasiewicz:
Since my last post, I think I have learned something. In the HP3456 the aforementioned accursed FET was used in switching and also as a "clamping diode". As I recall I replaced some of the switching diodes (not the ones used as diodes) with common FETs and that worked. I then obtained a DAS 45 voltage reference and had it Calibrated. Using the calibrated values, I found out that the low volt (100 mV) scale on my 3456 units would not work correctly. My Blissful Ignorance was unceremoniously ended..... I Believe-------( I am just a hobbyist, not an Engineer )------- The problems I had with the 100 mV range were related to the use of the FET as a clamping diode with source and drain tied together. Eventually I procured some Thompson 2N4117 FETS and installed some in the clamping positions....they worked, These are extremely low leakage FETS with advertised gate leakage of 120 femto amps, I do not know of any diodes that are of such a low leakage value. Perhaps there are some but HP used the FET. They test in my BSide as two diodes as do some of the original FETs. This is just an observation, I do not know it's significance. I also had to replace the "pre charge" FET. At Present I have been able to calibrate the two repaired 3456 units with my General Resistance DAS 45, so I conclude that they are working. I have a third board that has the same trouble with low volt range that I have not fixed. At Present, my working theory is that the choice of FET for "switching" is not as critical as the Clamping Function that they are also used for. There is another extremely low leakage diode (not a FET) used in the protection of the resistance function that I had to replace on one board, Cycle Collector on For Sale forum has some. As I Recall, this diode caused high resistance values to be off. The test for this is easy and is elucidated in the manual. |
| Kleinstein:
I some cases the same type of JFET us used as a clamping diode (usually towards to positive side) to get some compensation of the gate leagate current. The switches when off get a negative gate voltage and thus the positive side for compensation. So there can be cases where parts from the same series / batch would be preferred. Changing an input switch may effect the precharge adjustment. There are low leakage diodes, especially BAV199 (there is also a glass case THT type, that however needs protection from light). The test specs are not very tight, but the typical (nearly all samples) leakage is really good. The testing for sub pA currents is expensive and not found with lower cost parts. |
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