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

[ik1zyw]

Hello.
I've come across an object marked Tektronix VX1405 Mainframe whose slots are filled in with what seems to be a complete test-set system: arbitrary function generator (HP E1445A), two channel oscilloscope (HP E1428A), waveform analyzer (Tek TVS625) and a command module (HP E1406A). See the attached picture.

After doing some research I understand that this is a VXIbus system that speaks HP-IB with the outer world. It should have its configuration onboard, unless that is stored in a battery-backed RAM that has certainly faded away (will need to check the battery too!). The more I read the command module E1406A manual, the more I understand how to proceed (and render this post unnecessary).

Let me see if I get it right. I can use the RS-232 interface with a dumb terminal to check the system status and configuration. If it looks in proper working order, to use its modules I need a computer with HP-IB interface and software. Right? So, unless I locate the computer that came with the mainframe, what does "software" translate to in this case? I think they are/were all proprietary to exploit all functions? It looks like there is free software that runs on modern hardware, right?

I come from an almost-all-analog home lab, certainly stand-alone instruments. Should I consider this collection of instruments still useful in a home/hobby lab?

There may be obvious answers to these questions, or the post just fall into oblivion until someone searches the same mainframe here on eevblog.

Thank you for reading this far.
Paolo

PS: looking at the picture I notice now some screws holding modules are not tightened. I'll play with it anyway.

[coromonadalix]

since they claim  LXI  support,  you should be able to find bits  in Mathlab  or even Labview ??

BUT as you may gessed, normally you need the host computer somewhere in the rack,  or a sub rack with link cables / interfaces

good luck


Edit

the last module / bottom has, i think an gpib connector ....   i think it's the brain ?  or some sort of controlling card

you should try to find any spec sheet infos and try to decipher this mainframe to some extent

[Roehrenonkel]

Hi ik1zyw,
 
a nice system you got there!
But in a kids mainframe. ;-)))

If the drivers are still present in the E1406 you should be able to control (at least) the basic functions of the plugins.
Get the manuals so you know what SCPI-commands to send.
Replace the leaking NiCad-Pack in the 1406-cover.
I use my VXI-system as a curve-tracer (for tubes/valves) with indeed very special software.
If you need further help, feel free to contact me.

@coromonadalix: WHO claimed LXI-support??

Good luck

[gslick]

Hello.
I've come across an object marked Tektronix VX1405 Mainframe whose slots are filled in with what seems to be a complete test-set system: arbitrary function generator (HP E1445A), two channel oscilloscope (HP E1428A), waveform analyzer (Tek TVS625) and a command module (HP E1406A). See the attached picture.

From a quick look online it appears that the HP E1445A and E1428A and the Tektronix TVS625 are all VXI message based instruments. That means that you should be able to send SCPI commands to those instruments as documented in the programming manuals for those instruments through the GPIB or RS-232 interfaces of the HP E1406A VXI controller, without needing to download any instrument specific drivers to the HP E1406A VXI controller.

When things get tricky is when the VXI instruments are register based. Then instead of sending plain text SCPI message based commands to control the instruments, the VXI controller needs to know the register layout and bit definitions of the instruments in the VXI address space. Sometimes VXI instrument vendors never provided the register programming information and only provided drivers to control the instruments through the register interface. For those types of instruments, if you can no longer find the drivers, the instruments are basically unusable. I have such an instrument, an Analogic DBS2050A 2.4 GS/s Dual-Channel Arbitrary Generator, which is unusable due to lack of drivers and no other way to program it.

In addition to being able to send SCPI messages to VXI instruments through the GPIB or RS-232 interfaces of the HP E1406A VXI controller, some versions of the E1406A can run Instrument BASIC directly on the E1406A, with a serial terminal as the user interface. Those versions of the E1406A do not seem to be too common. I managed to find a couple of those versions.

[ik1zyw]

Thank you for your expert insights. It will take some days before I can put my hands on the baby rack again. I might even locate the documentation and software.

I will report on my findings.
Paolo

[ik1zyw]

Alright. After spending a week anticipating the power up of the instrument, I finally brought it home. The "baby" rack weights 20.5 kg.

First things first, I checked the NiCd battery in the command module. I had just started leaking the salt. The battery pack is secured to a panel of the module so, if it is stored in vertical position the leak will fall into nothing causing no damage.

Both fuses are intact. I fitted the command module back in, plugged the AC cable resulting in a small spark (that's normal) and pressed the ON button. Nothing. I then proceeded to remove a side panel to see the power supply and get room to push plugins outwards. The attached picture shows the side of the baby mainframe. I tried to power the instrument without plug-ins and still got nothing. No fans, no blown fuses and no obvious failed parts. It doesn't smell bad either.

Switching power supplies are not (yet?  ) my thing and I have no other mainframe to test the plug-ins. It will return to the storage with my notes for whoever will decide to try a repair in the future.

End of the journey.
Paolo

PS: plug-ins AS-IS might be put up for sale, but that's not my decision.

[Roehrenonkel]

Ciao Paolo,
 
...sad to hear that.
But don't give up so easy.
Is the power-button okay?
Is there something on the rear side?
The bigger Tek-Mainframes have remote On/Off.

Best regards and good luck

[ik1zyw]

You are right Roehrenonkel about not giving up. So I tried again, as the fault might be something really stupid like the power switch.

After removing tons of screws I managed to move the two power supplies few centimetres, but they would not come out of the chassis. The power button and its wires are buried so deep that I have no idea how to get to it without disassemblying the whole chassis.

Unless someone comes up with a scan of the service manual (should be 070-7725-00), or I can get it fairly cheap(*), it will wait for few months before I will try again opening it. I have put it back together and I am left with 10 screws, of which only two I know where they should go but were too inaccessible to be installed again.

(*) there's one for sale at 50USD. While it might be worth it, the instrument is not mine and the owner doesn't need to put it into service.

Paolo

[Roehrenonkel]

Hi Paolo,

sometimes a good source for information is artisantg.com:
"https://www.artisantg.com/TestMeasurement/50373-2/Tektronix-VX1405-5-Slot-Horizontal-VXI-Mainframe"
under "Manuals" you'll find what you're looking for (for free). :-)

Btw: By "kids/baby"-mainframe i didn't mean any disrespect.
I have a HP E1421B (6-slots) myself also.
For saving rackspace or maybe transportable applications.
The smaller mainframes tend to be much more expensive than the full-size ones.

Did you check if there is current flowing into the psu?
Does it change if you push the power/standby-button?
Any ticking sounds from the psu?

Best regards

[ik1zyw]

Thank you for the artisantg tip!

Did you check if there is current flowing into the psu?
Does it change if you push the power/standby-button?
Any ticking sounds from the psu?

It takes 12W at 220V regardless of the ON/StandBy front panel button. No sounds at all from the PSU (actually there are two PSUs providing the various voltages). Fans don't spin.
Next step will be to probe for voltages as described in the service manual. I've also learned that a faulty PSU disables the other one and this model does not feature the remote power control.

Regarding the "baby" comment, I never intended/understood it as a sign of disrespect but rather with the "cute" meaning. It's really a cool and versatile box of instruments, from 1990's.

Paolo

[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