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HP-1404A - Question About Power Supply/Distribution

 

I've been doing some troubleshooting with a few aspects of my 1404A and am confused about something regarding the power rails. Various schematics reference +15V (some have a +15V(A) and +15V(B) designations); however, I'm not finding where those are sourced.

The mainframe supplies various voltages to the two plugin jacks but those don't include 15V. I think there may be a regulator of sorts on the an A13 board but, so far, I have not found that schematic. Furthermore, Paragraph 8-21 Section d states there is supposed to be various voltages present on the A13 board as well as a "VR1" that provides +15Vdc but I have not found that regulator. Figure 8-28 shows an image of the A13 board (which, like the rest of the scanned images, is very poor) but I can't find a schematic.

Is the A13 schematic missing from the online PDFs? Is there another board that contains VR1 and, for that matter, a better scan of the manual somewhere?

Thanks,
Barry - N4BUQ
 

If HP is doing what Tek does, the (A) and (B) are isolated (by choke) and filtered (caps) power rails derived from the 15 volt supply.? Tek's preferred method is to take a 1/2 watt resistor and wind a few turns of wire around it, making a choke.? Should the wire ever burn out, the resistor (typically 10 to 100 ohms, IIRC) will still give some voltage.? Likely not designed to do that as a failure mode, but possible.

Look to see where the 15 volt rail comes into the board.

Harvey

On 3/26/2024 11:48 AM, n4buq wrote:
I've been doing some troubleshooting with a few aspects of my 1404A and am confused about something regarding the power rails. Various schematics reference +15V (some have a +15V(A) and +15V(B) designations); however, I'm not finding where those are sourced.

The mainframe supplies various voltages to the two plugin jacks but those don't include 15V. I think there may be a regulator of sorts on the an A13 board but, so far, I have not found that schematic. Furthermore, Paragraph 8-21 Section d states there is supposed to be various voltages present on the A13 board as well as a "VR1" that provides +15Vdc but I have not found that regulator. Figure 8-28 shows an image of the A13 board (which, like the rest of the scanned images, is very poor) but I can't find a schematic.

Is the A13 schematic missing from the online PDFs? Is there another board that contains VR1 and, for that matter, a better scan of the manual somewhere?

Thanks,
Barry - N4BUQ
 

The +15V supply is split into three rails that have small-value series resistors and 0.01uF caps on the other side of those resistors. That's done on the A10 board. The A9 board has a similar arrangement but without the caps. I just can't find where that +15V supply comes from. I assume on the A13 board but I can't find a schematic for that board.

Thanks,
Barry - N4BUQ

If HP is doing what Tek does, the (A) and (B) are isolated (by choke)
and filtered (caps) power rails derived from the 15 volt supply.? Tek's
preferred method is to take a 1/2 watt resistor and wind a few turns of
wire around it, making a choke.? Should the wire ever burn out, the
resistor (typically 10 to 100 ohms, IIRC) will still give some voltage.
Likely not designed to do that as a failure mode, but possible.

Look to see where the 15 volt rail comes into the board.

Harvey


On 3/26/2024 11:48 AM, n4buq wrote:
I've been doing some troubleshooting with a few aspects of my 1404A and am
confused about something regarding the power rails. Various schematics
reference +15V (some have a +15V(A) and +15V(B) designations); however, I'm not
finding where those are sourced.

The mainframe supplies various voltages to the two plugin jacks but those don't
include 15V. I think there may be a regulator of sorts on the an A13 board
but, so far, I have not found that schematic. Furthermore, Paragraph 8-21
Section d states there is supposed to be various voltages present on the A13
board as well as a "VR1" that provides +15Vdc but I have not found that
regulator. Figure 8-28 shows an image of the A13 board (which, like the rest
of the scanned images, is very poor) but I can't find a schematic.

Is the A13 schematic missing from the online PDFs? Is there another board that
contains VR1 and, for that matter, a better scan of the manual somewhere?

Thanks,
Barry - N4BUQ
 

From wire 902 (white-black-red), which goes to the stud mount Zener diode VR1 (1N2979) on the chassis, at the back of the plug-in. The pdf is missing circuit details for the chassis mounted parts, one of the larger resistors nearby will almost certainly be dropping the +100V rail for the Zener based +15V supply.



David
 

I took a look at that during my lunch time.? I didn't realize that was a Zener but it makes sense.? If I'm not mistaken, the A13 board and the A14 board are back-to-back and, apparently, there are some transistors(?) that make contact between them.? I still haven't found a schematic for either board but I'll just have to do without it.

Thanks!
Barry - N4BUQ

From wire 902 (white-black-red), which goes to the stud mount Zener diode VR1 (1N2979) on the chassis, at the back of the plug-in. The pdf is missing circuit details for the chassis mounted parts, one of the larger resistors nearby will almost certainly be dropping the +100V rail for the Zener based +15V supply.



David

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