The screws in these units, like most 70s and 80s era HP instruments, are Pozidriv, not actually the same as a standard Phillips screw. A Phillips screwdriver may work in a pinch, but a special Pozidriv screwdriver works much better, with less chance of stripping the screw heads.
A lot of people don't distinguish it, it can still be done carefully by hand, but it's worse with a screwdriver, where devastation is almost certain, Pozidriv doesn't just have the auxiliary protrusions, mainly it has a different approach angle.
I actually own 6 of these 400F/FL/GL meters (don’t ask; I may need professional help). I opened each one up to look for that diode. They all have the pads, with the plated through holes, at that location, but none of them have a diode soldered in there. Very curious. It seems like that diode location is across the base-emitter junction of Q15, similar to CR17 which is across Q14. But it’s not on the schematic, and the boards are not populated with that part.
You need to get the latest edition of the service documentation, HP sometimes made changes in the connection and put these changes at the end of the manual. My 3400 has about 20 of those changes - I haven't counted them exactly yet.
I have no insight into this particular item, but what is the mystery diode connected to? Find those parts on the diagram? My hunch is it's not connected to the wall outlet.
Sure you got the right schematics for your version of the 400F? On the older PCB's C31 was at the position that the diode is in yours, so maybe they relocated C31 to make room for the diode. A late revision or modification of the board perhaps? To improve sensitivity, or reduce noise or something...
With the meter adjust, could you use a fine chisel tip on your soldering iron to put a slot in it for a small screwdriver? Could the mystery diode be CR26 ? It's near Q14 & Q15 on the schematic.
If the mystery diode is CR26 then there might be an interesting note in appendix C / backdating changes of the manual (rev. July 1974). It says: Delete diodes A2CR24 through A2CR26 (or A2CR28? having a bad quality PDF...) from Figure 7-2, Figure 7-3, and Table 6-1.
I hate these rotary wafer switches. They are a nightmare to clean and repair. I have an Exact function generator from the 70s which has a few of these switches and none of them works properly.
Please, share the schematic so people can pitch in and review the circuit together. One thing one can notice from PCB, it seems at the time the CAD software for designing PCB was not available and the PCB has been done with marker on transparency. Also, it might be instructive to know that what is the highest frequency that this AC Voltmeter is capable of. I guess we have power meter that goes all the way to microwave frequencies with 50 Ohm input line, and you can inferred the voltage from power given 50 Ohm impedance of input. P=( V^2)/R so V=SQRT(50*P)
I love those die-cast chassis parts, with the holes and die-casting and all the holes! So much more elegant than stamped sheet metal.
I love the old HP construction.
-- the case of the Standing Diode --
Found cr19,20,21 in the lower left corner of the parts layout.
The screws in these units, like most 70s and 80s era HP instruments, are Pozidriv, not actually the same as a standard Phillips screw. A Phillips screwdriver may work in a pinch, but a special Pozidriv screwdriver works much better, with less chance of stripping the screw heads.
A lot of people don't distinguish it, it can still be done carefully by hand, but it's worse with a screwdriver, where devastation is almost certain, Pozidriv doesn't just have the auxiliary protrusions, mainly it has a different approach angle.
there are 4 little lines on the screw to tell you it is pozidrive
I actually own 6 of these 400F/FL/GL meters (don’t ask; I may need professional help). I opened each one up to look for that diode. They all have the pads, with the plated through holes, at that location, but none of them have a diode soldered in there. Very curious. It seems like that diode location is across the base-emitter junction of Q15, similar to CR17 which is across Q14. But it’s not on the schematic, and the boards are not populated with that part.
You need to get the latest edition of the service documentation, HP sometimes made changes in the connection and put these changes at the end of the manual. My 3400 has about 20 of those changes - I haven't counted them exactly yet.
Many capacitors are TANTALUM which are probably Okay.
That diode is a puzzle...cheers.
Hey Andy, you're missing Mr Brown. started 16/10 at 22.05
I have no insight into this particular item, but what is the mystery diode connected to? Find those parts on the diagram? My hunch is it's not connected to the wall outlet.
Luckily I didn't buy it, now I would have an extra diode 🤣
Nice day 🙂 Tom
I don't see any traces going to it either. My guess is it's a spare diode.
Sure you got the right schematics for your version of the 400F?
On the older PCB's C31 was at the position that the diode is in yours,
so maybe they relocated C31 to make room for the diode.
A late revision or modification of the board perhaps?
To improve sensitivity, or reduce noise or something...
With the meter adjust, could you use a fine chisel tip on your soldering iron to put a slot in it for a small screwdriver?
Could the mystery diode be CR26 ? It's near Q14 & Q15 on the schematic.
If the mystery diode is CR26 then there might be an interesting note in appendix C / backdating changes of the manual (rev. July 1974). It says: Delete diodes A2CR24 through A2CR26 (or A2CR28? having a bad quality PDF...) from Figure 7-2, Figure 7-3, and Table 6-1.
@@c-a-r-s-t-e-n That's interesting. I wonder why they did that. (?)
I hate these rotary wafer switches. They are a nightmare to clean and repair. I have an Exact function generator from the 70s which has a few of these switches and none of them works properly.
Please, share the schematic so people can pitch in and review the circuit together.
One thing one can notice from PCB, it seems at the time the CAD software for designing PCB was not available and the PCB has been done with marker on transparency.
Also, it might be instructive to know that what is the highest frequency that this AC Voltmeter is capable of.
I guess we have power meter that goes all the way to microwave frequencies with 50 Ohm input line, and you can inferred the voltage from power given 50 Ohm impedance of input. P=( V^2)/R so V=SQRT(50*P)
www.pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/20_HP_Agilent/HP_400F_FL_Op-Serv/HP_400F-FL_Op-Serv_1966.pdf
More probably broke off - not broken off!
it done busted