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Tektronix VX1405 mainframe and its plugins (VXIbus, HP-IB)

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ik1zyw:
So, the mainframe has two power supplies. I've plugged it in without modules, just the mainframe, and I see the power input going from 12W up to 26W when I press the ON button. The ON button doesn't light up as it should, according to the docs. In the operating configuration there is no difference in input power.

Since I've tested it in complete silence I could now hear a weak repetitive clicking sound like a common furniture beetle. Even a fan woke up at one point.

Without the plugins I could probe the voltages at VXI sockets as detailed in the manual. The +12V, -12V and +5V rails produce garbled readings on the DVM and never reach their expected values. Outputs from the other PSU, +/-24V, +5.2V and -2V are there.

This means that I probably have a failure in PSU #1 which, BTW, powers up also the fans. The service manual says the PSU needs to be "replaced", which I read "repaired".  :)

ik1zyw:
I finally managed to extract the PSU #1. The service manual available online does not mention some parts, so either an overzealous assembler added brackets and screws, or my specimen is newer than the manual. I would say that the additional mechanical parts provide better robustness so they were probably added in later releases. For the records: PSU #1 (+/- 12V and +5V) assembly is attached to the bottom layer with two screws, barely visible through the bundle of cables going to the backplane, and not mentioned in that service manual.

Anyway, I attach a picture of PSU #1. Now, experts, what is the black goo under the two large electrolytics (top left of the picture)? Is it a glue against vibration? Other components are glued with a yellow compound. Or is it some kind of leak for large caps? It is not present in PSU #2 that has slightly smaller caps in that position.

Feeding AC to this PSU alone it takes 5W and there's no output. I think some sequence might be needed to bootstrap it so next time I will reconnect GND and the "REMOTE CONTROL" like so that I can use the front panel ON/OFF button. Unless it's the big caps that failed, I will need advice how to proceed troubleshooting this circuit.

Thank you.
Paolo

ch_scr:
Anti-vibration goop. Maybe the 2 power supplies came from different manufacturers, using different materials? Usually the smaller the electrolytic, the shorter the lifespan. So I'd leave the big ones alone, for the moment. With the "repeat clicking" it might be the supply failing to start up - look for a small electrolytic near the main switching regulator (about 50V/47uF), it's the "general go-to culprit" as it provides the energy to start the supply. But with a more complex (and hard-to-access) beast like this, there's really no telling until you go in there and have a close look. With switching supplies, a good general strategy is to look at the big power components (mains rectifier, filter, switching elements, transformer, rectifier, filter again). Compare to the topologies listed here, try to account for all of the big components. Good luck!

ik1zyw:
Hello.
I've managed to get the board out of the rack so that I can work on it more easily on the bench. As I've stated, for future reference, in my version of the rack there are more parts to remove and screws than stated in the service manual that's available online: take your time and study the situation.

I've been reading about switching PSU topologies and considering the high-current output (35A at 5V) I think I am trying to repair a flyback. Besides the custom inductors, the circuit has something that looks like a custom-made IC as Tek used to do (in my experience with them) even if the power supply came from a different supplier. That component is the controller IC as there isn't a chip on the AC side. Under the larger transformer there is a smaller one that is probably powering the control circuitry.

I have located an interesting document from Texas Instruments "Common Mistakes in Flyback Power Supplies and How to Fix Them" that guides through troubleshooting a unit and includes how's, why's and waveforms. As long as I'm learning new stuff, I'm fine. Here's a slideshow for those of us that don't need full-text explaination: https://www.ti.com/seclit/ml/slup398/slup398.pdf

In order to troubleshoot the AC-side circuit I'll need to "lift from earth" either the PSU or the oscilloscope, keep one hand in the pocket and adopt all high-voltage safety procedures. OTOH for the output side I can operate normally without the insulating transformer.

Update. Doubtful that the PSU would need to see a load to regulate (fans), I added a resistor on the +12V and an LED on the +5V output. No changes, but while poking around with the DVM I noticed that 10 minutes after power up the LED started flashing. That could mean only one thing: heat makes it work. I confirmed my theory beaming a hairdyer to the circuit and sure enough, after a bit of "coughing", I got a steady output that vanished when temperature returned towards 20°C ambient. I'm not sure how warm the board got, probably in the range of 40°C.

I think I am after either a cold/unstable solder joint (unlikely as it would react to physical "stress" of the board), or a component that fails at room and lower temperatures. I'm not sure tantalum capacitors do this, but electrolytics for sure.

Paolo

ik1zyw:
Hello.
While trying the "heat me up" method to identify the probable culprit of the failure I understood that the failing part is in the AC/switcher side. Probing around the DC side there are no voltages until it begins trying to start up. On the other hand, the AC side drains 6W so it slowly heats up until the failing part reaches a point where the whole circuit begins sending groups of pulses to the switcher.

I am now quite confident that the failing part is inside the covered vertical board, the only masked component. It might just be a PCB in heatshrink, but I doubt I can remove it for easier troubleshooting. My endeavour ends here. I might try to retrofit an external PSU to see if all the rest is working, and a repurposed ATX power supply should be able to provide enough current on the 5V line (I cannot go above 5-6A at 5V in my lab).

I will talk with the owner if we keep the instrument for display only or it will be sold as spares (if you're reading this in March 2024 and needs the plug-ins listed in the first post, contact me privately).

It has been a positive learning experience that instilled the curiosity to probe inside a working switch mode power supply.

Thank you for reading and helping me out.
Paolo

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