Electronics > Repair

Power Transistors for old HP-6920b

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Hi All

I could not resist purchasing this old HP calibrator for <$50 AU

Before turning it on I pulled the series power transistors which are PNP germanium hp part 1850-0168. I think they may be Bendix or HLAB branded according to the manual but after several nights searching I cannot find any specifics about them, somone indicated they were 70v high gain 5A. Two seem ok, one is dead and one looks really leaky. I was looking for advice on a suitable replacement. There looks to be a shortage of TO-3 transistors or perhaps they are fading away naturally. I have some MJ15004 or 2N2955 I could use as replacements ? They are Q8, Q9, Q14 and Q15 in the attached schematic.

Any help appreciated as always.



Kim Christensen:
The best I could find was this info which is pretty close to what you found.

A bit pricey, but they have 2 at Sphere.ca in stock. I have ordered stuff from them about 7 years ago but nothing since but all was OK that time.


I have 2 versions of the service manual and it appears a 2N1545 may be interchangeable for the 1850-0168, at least on the lower pcb board anyway. I will try to search based this transistor also.

The change notes seems to suggest they used a matched pair for Q33/34 from s/n prefix 1105A, these are being used as output rectifiers.

Q14/15 are the inverter switching transistors, the inverter may be damaged if the external AC reference signal is lost, the manual does mention this.


The Q14 / Q15 part could be a bit tricky as the transistors will also see reverse operation of some kind.

The Q8 and Q9 part looks like a more normal linear operation and chances are good that normal silicon types (e.g. 2N2955) should be OK too.


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