[KrisBlueNZ]

I wouldn't worry about the capacitors. The solder does look new, but the joints look OK to me.

Nice work on the plug pins. Personally I would resolder _all_ of them; I think of the other dry joints as a warning sign.

Before you reassemble the unit, can you power up the power board with the transformer connected and measure the rails? Also, if access to the board is restricted, you could solder a couple of thin wires across the current sense resistor I mentioned, so you can measure the voltage across it (and the voltage on it, relative to ground) once the unit is reassembled.

 

[tryppyr]

Kris, let's just say after pondering it for a bit I still haven't come up with a way to power up the board short of full reassembly. The likely problem here is that I'm not terribly clever about such things. One issue is that the leads coming from the power plug are only about 1.5 inches long.

 

[(*steve*)]

I see even more evidence of shoddy work. I think I need to address that before reassembly, as well.

Agreed.

 

[tryppyr]

At Kris' suggestion I reflowed the other posts from the larger plug, then did the same for the leads from the smaller plug. I also resoldered some of the capacitor connections.

While doing some examination, I found what look to me like solder bridges on two of the voltage regulators. Now I'm wondering if these might have been part of the problem.

On further examination it appears the solder bridges are intentional.

 

[(*steve*)]

Get out the solder wick and see if it's a bridge or if it's a trace under there.

(I reckon there's a trace under there -- it kinda looks unlike a solder bridge) But yeah, I'd check!

 

[tryppyr]

I just finished the reassembly. It wasn't as hard as I expected it to be, but still took some time. Anyway, power up didn't show any change in behavior. But at least now I'm in a position to check voltages again. This time I checked them at the connector that leads down to the HVPS.

At the +156 post I get 30.2
+15UNGND ............... 0.0
+15UN....................... 25.3
-15 ...........................14.7
+120 ......................... 4.9
+48 ........................... 26.8

 

[(*steve*)]

Are you saying that the -15V rail is at +14.7V?

I would be taking a serious look at those low voltage rails first. They concern me more immediately than the high voltage rails being low.

 

[tryppyr]

Are you saying that the -15V rail is at +14.7V?

I would be taking a serious look at those low voltage rails first. They concern me more immediately than the high voltage rails being low.

Sorry... my bad... -14.9

 

[(*steve*)]

That's better

 

[tryppyr]

I'm having some trouble identifying these components. None of my Google searches turn up anything useful. When I search for the transistors listed in the replaceable parts, all I get are vendor web sites offering to quote me (plus one that sells them for $10 apiece, plus plus).

Can one of you tell me what these are and perhaps give me a search string I could use to get a proper data sheet?

 

[KrisBlueNZ]

Those markings are probably HP-specific part numbers, used by the actual manufacturer at HP's request. The actual components are probably standard devices, perhaps members of the TIPxxx family, just badged with HP's part number instead of the standard one.

If the HP documentation doesn't state a generic part number, the best option is to choose a generic part based on the requirements of the circuit, but without readable markings on the schematic, this is tricky.

But even before that, I wonder why you want to replace those transistors. There is a fair chance that they're faulty, being power components, but that's not certain. You need to identify the fault, but to do that, you need to find a readable schematic.

 

[tryppyr]

I'm not so much keen on replacing them as identifying them.

The replacement parts sheet gives each component a specific part number, and identifies its function. I suspect these are among the A16Qxx parts, but none of the numbers line up with what I see on the replacement part sheet. I suppose I could just pull each one and test it in my transistor tester, but I'd rather know in advance what I should see as results.

 

[tryppyr]

Of course, the other strong likelihood is that there is something wrong in one of the voltage regulators or its accompanying circuits.

 

[tryppyr]

Since I can't provide a readable schematic, I transcribed the LVPS circuit theory.

SCHEMATIC 1

LOW VOLTAGE POWER SUPPLY• CIRCUIT THEORY

GENERAL The low-voltage power supply provides regulated +5 V, +15 V, +48 V, +120 V, +156 V, -15 V and -100 V for operation of various circuits in the instrument. Most low voltage supplies are referenced to the +15 V supply, directly or indirectly.

+15-VOLT SUPPLY. The ac input is applied through transformer Tl to full-wave, diode-bridge rectifier Al6CR5. The rectified voltage from AI6CR5 (nominally +21 Vdc) is applied to integrated circuit A16U1 and series regulator Q4 to produce the regulated +15 V output. A16U1 contains a compensation reference circuit (pin 4) and a differential amplifier with a Darlington output (pin 6). The compensation reference circuit is applied to the non-inverting input of the amplifier.

The Darlington output drives the base of series regulator Q4. The emitter output is applied to a voltage divider network consisting of A16R25, R26, and R27. +15 V ADJ potentiometer A16R26 is adjusted to compensate for variations of the reference voltage so that with an output of +15 volts from the supply, the inverting and non-inverting input voltages to A16U1 are equal.

IC A16U1 includes an output current limiting circuit consisting of an NPN transistor whose collector is connected to differential amplifier and input to the Darlington amplifier located within the IC. The emitter and base connections for the NPN transistor are pins 1 and 10 respectively. When the load current through A16R24 produces a sufficient voltage drop, the NPN transistor conducts, pulling the input to the Darlington pair toward the emitter potential of Q4. This limits the output current.

-15-VOLT SUPPLY. The -15-volt supply, consisting of A16U3 and Q6 operates in the same way as the +15- volt supply except that the non-inverting input to Al6US (pin 3) is the sum of the +15 V and -15 V outputs (nominally zero volts).

+5-VOLT SUPPLY. The +5-volt regulator A16U2 functions in the same way as the + 15-volt regulator except that the reference voltage is provided by the attenuated output of the + 15-volt supply. The attenuation network consists of A16R28 and A16R29.

+120-VOLT SUPPLY. The dc output of full-wave, diode-bridge rectifier A16CR1 is filtered by A16C3. A +15- volt reference is applied to the base of A16Q1 through A16R1. A16Q1/A16Q2 form a differential amplifier with the base of A16Q2 connected to a voltage divider network across the +120-volt output circuit. If the output falls below +120 volts the base of A16Q2 becomes less positive causing it to conduct harder. The collector of A16Q2 is directly coupled to Darlington pair A16Q4 and Q2. When conduction through A16Q2 increases, conduction through the Darlington pair increases, resulting in an increase in output voltage. When the output reaches +120 volts, conduction through A16Q2 is such that equilibrium is reached. Transistor A16Q3 and resistor A16R2 form a current limiting circuit. As current requirements increase toward the limit of-supply capability, the voltage drop across A16R2 is applied to the base of A16Q3 which conducts and limits current drain from the Darlington pair.

+48•VOLT SUPPLY. The +48-volt supply consists of A16CR2, A16Q5-A16Q8, and Q3. It operates in the same manner as the +120-volt supply.

+156-VOLT SUPPLY. The +156-volt supply consists of A16CR3 and emitter follower A16Q11. Its output is referenced to the +120-volt output and is used in the astigmatism and auto-focus circuitry. It is also used in the storage mesh and collector mesh circuits.

-100-VOLT SUPPLY. The -100-volt supply consists of A16CR4 and A16VR4. It is not referenced to any other supply. Its output is regulated by zener diode A16VR4 and is used in the storage mesh and floodgun grid circuits.

LINE FREQUENCY. The line frequency sync signal is developed in the same secondary winding of input power transformer T1 that is used for the +120 volt supply. The signal is applied through A16R40 to HF REJ switch A7S2C on assembly A7 (see schematic 7).

 

[KrisBlueNZ]

There are many components there that could have failed.

The 120V circuit doesn't use an IC; it's all discrete transistors.

I've read the circuit description. Without being able to refer to the schematic, I don't find it very useful at all.

Perhaps you could try to find vintage electronics groups who might have an original manual you could scan at a higher resolution.

 

[tryppyr]

Okay, I think I have what you need. I printed the PDF on a large format printer at the office, then scanned it back in as JPG. It is much, much easier to read.

http://greg-davis.com/images/LVPS.jpg

 

[KrisBlueNZ]

Oh, I see now. There are two copies of the schematic in the service manual! That one is the second one, and it's very clear. The first one is scanned at such a low resolution that it's unreadable.

OK, now we have the schematic. Are you able to measure voltages in that circuit? Can you power it up with everything plugged in and measure the voltages on both ends of R7, for example? And the voltage across it?

 

[tryppyr]

I measured -179.5 on both ends, 0.0 across. The image shows what I believe to be R7.

 

[KrisBlueNZ]

Aha! Progress.

Can you measure the voltages on the following points with the negative multimeter lead connected to the right hand end of R2 (that is, the bottom end of R4, bottom end of R7, and emitter of the off-board Q2).

1. Q4 base
Edit: 1.5. Q3 emitter

And can you measure the voltages on the following points with the negative multimeter connected to 0V.

2. Q1 base (should be ~-15V)
3. Q1 emitter (should be ~16V)
4. Q2 base (that's Q2 on the LVPS board)

With those voltages I should be able to work out what's wrong.

 

[tryppyr]

The attached image shows the components I tested.

Q1 Base = 183.7
Q1 Emitter = 182.2
Q2 base = 0.0 (electrically the same as - test point)
Q3 emitter 182.9
Q4 base 0.2

Somehow, I'm certain I screwed that up.