[tryppyr]

The CRT never lights up. That's why I focus so much attention on the HVPS module. Reading the circuit theory has me wondering whether one of the trigger conditions has cause the protection circuit to kick in and shut off the CRT. However, none of the switches are set to cause that to happen (e.g. not in Deep Erase mode).

On the front panel of the unit, the LEDs light up correctly when various buttons are pushed, so power is flowing through the system.

 

[KrisBlueNZ]

OK, here are a few things you can try.

With the unit powered down:

Disconnect the feed from the transformer (pin 7 I think) to the "HV MULT" unit (tripler) that feeds the final anode of the CRT. Triplers can be unreliable and a faulty one will stop the oscillator.

Pull the PCB out and check for dust, dry joints, signs of arcing or tracking (creepage), and discoloured areas.

Lift one end of CR3 and measure its resistance in the reverse direction (positive meter probe to cathode) on the highest resistance range to check that it's not leaky.

With the unit powered up:

Check the voltages on the rails coming into the supply: +15V UNREG (used by the oscillator), -15V (used as a reference voltage in several places), +120V (must be right otherwise the oscillator is halted).

Check that the base-emitter voltage on Q3 (the one whose collector connects to Q1's base) is less than about 0.6V. If it's more, the protection circuit around Q2 thinks there is a problem and is preventing the oscillator from running.

Check the voltage on pin 6 of U1. With no oscillator output, it should be high - something like +10~12V. If it isn't, report the voltages on all pins of U1.

 

[tryppyr]

I had to flip the unit over and check the rail voltages from the LVPS, as that's where they are marked. These were the points closest to the primary transformer outputs feeding the rails to the HVPS. Here's what was observed:

Pin : Measured
-15 : -14.8V
+15 : +14.9V
+156 : +31.2V
+15UN : +25.5V
+15Ung : 0.0V
+48 : 26.1V
+120 : 4.8V

Obviously, I need to find out what's going on with the transformer.

A visual inspection of the HVPS card showed no obvious signs of problems on either side. But with feed voltages like these, that's not too surprising.

- Greg

 

[tryppyr]

I just checked the outputs directly from the transformer:

Pins 1 and 2 (which feed a 120V line) has 141.8V
Pins 3 and 4 (which feed a 48V line) has 58.8V
Pins 5 and 6 (which feed a 15V line) has 18.0V
Pins 7 and 8 (which feed a 5V line) has 9.5V
Pins 10 and 11 (which feed the 15V Unreg and 15V Unreg Gnd) has 20.4V
Pins 12 and 13 (which feed the -100V line) shows 124.0V
Pins 14 and 15 (which feed the 156V line) shows only 60.7V

These measurements were taken straight off the plug that feeds the LVPS board.

Obviously the one that concerns me most is the reading on pins 14 and 15. However, taken together with the readings of the DC volts provided at the rail, I would also be inclined to the opinion that the bridge rectifier on the 120V line (or something in that line between the transformer and the rail) has failed.

 

[KrisBlueNZ]

Yes, that seems likely. If you can unplug the secondaries from the board they feed, you should see the voltage increase to normal.

Post the LVPS schematic here if you like.

 

[tryppyr]

Here's the A16 (LVPS) schematic.

 

[KrisBlueNZ]

Are the references and part numbers readable on those schematics in any of the versions you have there? If so, could you post readable versions of both (A15 and A16) schematics please.

 

[tryppyr]

Sadly, that's about the best I can do. I don't have an original of the manual.

 

[KrisBlueNZ]

Have you measured between transformer plug pins 14 and 15 with the plug removed from the board?

 

[tryppyr]

Yes, that is how the measurements in post 24 were taken.

 

[KrisBlueNZ]

OK, it looks like the +156V DC rail is created by adding voltage to the +120V rail, so 60V AC across 14 and 15 is probably right.

It's really hard to be sure because the markings on the schematic are not really readable.

So the +48V and +120V rails are far too low. I would start with a careful visual inspection of the LVPS board. Look for resistors that look like they have been hot, and discoloured areas on the PCB which are another sign of heating. Flip the board over and check for dry joints and any other problems.

It's really impractical trying to work from that schematic. Does the manual have other readable information that you could post? For example a list of voltages on various connectors, and/or test points, would be really helpful.

Can you upload the whole manual to a file hosting site and post a link to it?

 

[tryppyr]

Here is a link to the entire manual.

http://cp.literature.agilent.com/litweb/pdf/01741-90911.pdf

 

[tryppyr]

The last couple of weeks have been busy for me. Only today was I able to get the LVPS board removed and examined. I saw no strong evidence of component failure, but there was some discoloration around the capacitors that are fed by the transformer.

However, since the last test of the transformer was done with it unplugged from that part of the circuit, I seriously doubt my problem is there.

I need to trace the circuit and see what else might have been inline when the last test was being run. During that test I left the small plug plugged in, and the large plug (rail) pulled out. So if there was something still confounding the circuit, it would have been on the small plug side.

 

[(*steve*)]

In that third image, some of the soldered connections along the bottom look dodgy to me. (Edit: the last three in particular - are the solder joints fractured?)

Is it possible that your problem is as a result of poor/failed solder joints?

It's not something I would expect of HP equipment of this era though.

 

[tryppyr]

Steve, anything is possible at this point. I noticed the dodgy solder joints too. Looks to me like someone has already had a go at restoring this unit.

Another image... this one backlit so I can see the traces on both sides at once. I'im trying to ascertain what (on the small plug side) could have limited the voltage on the 120V line.

 

[(*steve*)]

I'm just trying to be an extra pair of eyes. Kris has already suggested looking for dry joints, and it appears there are some. It may be worth scanning the board for others.

A very silly suggestion, but have you confirmed that the heater is powered?

Can you find a way of measuring the current being drawn from the 120V supply?

 

[tryppyr]

I think we already confirmed that the protection circuit is in effect because the 120V lead on the rail is WAY under voltage, so I don't think the heater is powered.

Yes, I will examine the board for more dry joints. Honestly, though, I find it easier to do by taking photos and looking at them. For some reason I see more that way than I do just looking at the objects.

 

[KrisBlueNZ]

I agree there's a problem with the joints on that connector. I've seen this before on the end pins of long headers when vibration, and usually, high currents, are present. I haven't seen it before on a double-sided board though.

Is that wide connector where the secondaries from the transformer come in to the board?

As Greg says, the CRT heaters won't be doing anything; they're powered from the HV transformer, and (we think) the HV oscillator/driver is not running.

It could be useful to measure the 120V rail at the power supply with all the other boards disconnected, although running a power supply with no load at all can give misleading results. The best option is as Steve suggested, to measure the current coming out of the 120V rail.

You can measure this across the current sense resistor. I think it's R2 but it's hard to be sure. Follow the negative output from CR1 down and across. Where it goes under the three small transistors, there's a small series resistor - R2 I think, 6.8 ohms I think. Calculate the current through it by measuring the voltage across it and using Ohm's Law: I = V / R where I is the current, in amps; V is the voltage across it, in volts, and R is the resistance, in ohms.

 

[tryppyr]

Thanks for the follow up Kris.

Yes, the large plug is where the outputs from the transformer enter the board.

After staring at the board for a while I have become suspicious of the red components nearest the plug. If you look at the back of the board, wherever those parts are placed there's flux residue around the joint, suggesting these are replacement parts. Like Kris, I'm finding it hard to read the schematic to determine what those parts ought to be. What is immediately obvious is that they are not rated for the same voltage as the blue ones, and they are not X7 rated.

Obviously, I can't say for sure these are problematic, but part of me wants to undo what the previous "restorer" did.

 

[tryppyr]

Before I reassemble the unit (which, by the way, is very time consuming) for any further tests, I wanted to address the dry solder joints on the plug.

As I look at the solder joints for the red capacitors, I see even more evidence of shoddy work. I think I need to address that before reassembly, as well.