Electronics > Repair
HP 3478A Multimeter - Parts for overvoltage protection circuit
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11mrjones:
Hello all- First repair of any type of instrument and I need some advice on replacement parts. Meter had a problem with 3K ohm range not reading accurately and unable to successfully cal it. Found the problem was the ohm current source being pulled down by the over-voltage protection circuit. The schematic is attached. So may need to replace any or all transitors Q201-Q205. From the parts list in the service manual. I can see Q201-Q204 are 2N6520, so no problem there (see attached parts list).

However, I am having trouble identifying Q205, it is supposed to be a JFET n-channel, depletion-mode device. The parts list gives just the H/P P/N as 1855-0298. Looking at a HP P/N cross-reference (https://xdevs.com/doc/HP_Agilent_Keysight/HP%20Master%20Xref.pdf) it looks like it would be either: 2SJ2472 (a jfet p-chan), MBR2545CT (a schottky barrier rectifier), MJ12005 (a npn xtor, hi curr, hi. voltage). So none of these are a JFET n-channel, depletion-mode device.

On the Q205 physical transistor itself, markings are just last digits of the HP P/N, ie,  National Semi Logo and 204 5 5 0298. 

Does anyone know the generic part number for Q205 , or what I can use as an available replacement for it? I can't find the datasheet for it since I don't know the P/N, so I can't get the specs to try and find an equivalent replacment either. In the end, testing it in circuit looks like it is ok, but I would like to order a Q205 part (and replace it with new) as I need to order Q201-Q204 (p/n 2N6520) anyway.

Thanks for any advice/help.
Victor
Wallace Gasiewicz:
This fellow has some HP and Tek Parts:   

https://www.eevblog.com/forum/profile/?u=141173;area=showposts;start=75   

But I do not think he has your JFET.   I would try a common JFET in this position.  If you get too much "leakage" then we can look for another low leakage Jfet.  The transistor is used as a diode and is probably a low leakage part since they did not use a common diode.
pqass:
I couldn't find a datasheet for J2472 nor a substitute. I just found a product obsolescence notification here.

You may be in luck as there still could be new-old-stock here.  The owner passed away and the site is winding down.

If that fails, the same protection circuit is in the 34401A and a lookup of the schematic (9-10 or pdf page 153) and parts list (page 129) shows: "Q211 ... Transistor J-FET N-Chan D-Mode ... MMBF4117A".  Its still available from Digikey here.

IMSAI Guy video #1818 (sort of) explains how the circuit works.
squadchannel:
There are still NOS products on fleabay.
https://www.ebay.co.uk/itm/233385727562
Kleinstein:
The JFET is used as a low leakage diode. So the exact type does not matter. The MMBF4117/8/9 should be OK as small and low leakage types.
There is no need for a specific, NOS type.
If THT is needed one could also consider a dedicated low leakage diode (may need a cover when in glass case).

For a quick test one could also use the base collector junction of a transistor (e.g. 2N3904, BC547,...) as a relatively low leakage diode.
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