Would anyone care to suggest a suitable replacement for the HP 1853-0063? used as the 75V series pass regulator in the HP 3400A? or perhaps has one lying around in the spares drawer?? My search of eBay failed to find one.

Mine seems to be C-E short ... which is sub-optimal

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The specifications from the Microfiche that CuriousMarc scanned, no generic replacement listed.



David

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Hi Robin,

Opened up my 3400 which has a 1218A prefix and has Q1 and Q2 both as 1850-0038 (date codes 1975 and 1976).? ? ? ?
My manual which covers much older prefixes, 401 with backdating for earlier 3400A's and shows Q1 to be an 1850-0098 but with Q2 as a -0038.?
I believe these are Ge transistors with the 1850 prefix as opposed to Si with the 1853 prefixes.? I have the old Bench Briefs but as you and others have found, there is no cross reference.??

Based on voltages in a manual for prefixes 401 and below, Q1 has 16.5 V across it with about 100 mA flowing thru it for a dissipation of about 1.65 W.??

Maybe this info will give you some more data to work with.??

My 3400 is still working but shows the meter fluctuating a small amount (no matter range or input level) which, reaching back 30 plus years when I worked on 3400's might be due to the chopper.? ? If I remember correctly there was a modification kit to replace the old photo chopper circuits.??

I miss working at HP (1977-2001 at the Eastern Region Repair Center in Paramus and eventually Rockaway, NJ)!?

Regards,? ?Pete

?

I took a quick look at the circuit, and I think the Q1 spot should have a medium-high Vcbo transistor, like 120-150V, for best reliability. I don't know what the original part was. Under normal conditions or partial overload, Q1 doesn't need to drop much, but with the +75V (or the "+77V") supply shorted to ground, the bottom of the raw supply (C1's can), hence Q1's collector, will need to go to whatever the raw voltage is, maybe -100V or more. Such an event could have caused the C-E short mentioned in the OP, if the part couldn't handle it. I'd find it hard to believe that HP would not have used a suitable part for Q1, that could take a common fault like this. That's also why Q1 and Q2 are different - if Q1 didn't need to be higher voltage, they could likely have been the same part type. Alternatively, you could just say so what if Q1 breaks down when +75V is shorted. If it's stout enough, it will take a momentary short, or perhaps blow the line fuse, and not be damaged. I'd rather have it act more civilly, and just recover when the fault is removed.

The parts list says Q1 is silicon. It looks like the normal load on the +75V is around 25 mA, so modest power then. It looks like the circuit including CR703 and CR704 provides some foldback limiting, but I think the maximum would be around 50 mA, so worst case power dissipation maybe 6W, which should be no sweat for a properly chassis-mounted TO-3.

I'd recommend looking for a TO-3 Si PNP with Vcbo or Vceo over 150V, and DC-SOA rating better than say 100 mA at 150V.? I vaguely recall there are some medium-high voltage TO-3 PNPs used in the HP141T, that may be suitable, and very common, but don't recall the number (-0140- or -0138- seem to pop into my brain, but maybe entirely wrong).

Like Dave W. mentioned, you'd probably want something fairly slow, but simple circuit mods can fix any instability if choices are limited. Also, it doesn't necessarily have to be TO-3, since it's fairly easy to fit T0-220 and others into the footprint. I'd recommend not using ones intended specifically for SMPS use, without first checking the hfe spec at low collector current - sometimes they're quite low, which could cause circuit problems.

BTW my assessments are based on a manual I found online: 03400-90013, 1 December 1986. Others may show different circuits and changes.

Ed

My data base shows that this transistor has been replaced by 1853-0059. PNP, VCE-55V, IC-5A, PD-150W, FT-3MHZ, SI TO-3.

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