HP 427A Voltmeter Repair – Q6 FET SU527 Replacement

[Jon Chandler]

I purchased an HP 427A voltmeter from eBay but it's not working.  Inspection showed that Q6, an SU527 from Siliconix, had one lead connected to the circuit board, one lead floating in air, and the third lead broken off flush with the case.  Since this is the impedance converter, I guess it's not too surprising that the meter indicates mid scale no matter what's connected.

I can't locate any information on the SU527 – the only web reference is to the HP 427A service manual.  Can anyone suggest an alternative part?

The attachments show the damaged part and the section of the service manual schematic.

Thanks in advance!

[david77]

I'd just try a generic FET like the 2N7000 or a BF245 or something.

[fpliuzzi]

Hello,
I noticed in the HP427A servive manual that the HP part number for the SU527 P-channel JFET is 1855-0023. There's a downloadable 143 page HP components cross-reference listed at this link...

www.sphere.bc.ca/test/hpparts.html#cross

Unfortunately, of all of the thousands of components listed in the pdf, HP part number 1855-0023 is not included there. On page 16 of the 143 page pdf HP states that components that were made exclusively for HP (presumably having certain characteristics such as this JFET maybe having a particulartly low Cgs for example) are not included in this xref.

(you may still want to download the pdf file because it may prove helpful in the future)

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It would be pretty pricy when you include shipping, but it seems that Tucker Electronics has the HP 1855-0023 in stock at $5 each.

www.etestparts.com/Search.aspx?Search=1855-0023

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Also, like david77 previously mentioned, you very well may be able to get away with using an appropriate generic p-channel JFET for Q6. The meter may work fairly well with a substitute Q6, but just not quite meet all of its published performance specifications.

By the way, I really like HP meters from that era. My HP410c's 700MHz bandwidth really comes in handy sometimes. Good luck with your meter repair.

Regards,
Frank

[Grayfox]

http://www.ctg123.com/rapid-quote-system/request/?p=SU527