Keithley 236 blowing fuses

[jogri]

Hi guys,

i got myself a broken Keithley SMU that blows its fuse when it gets switched on (nearly instant, as soon as i pushed the button i heard the fuse pop). My conclusion was that this was due to a short somewhere on the secondary side of the transformer (not that much on the primary that could short out), and i found that R73 (a 2W resistor) as well as a Zehner (VR7, between R73 and the ground rail) were busted (resistor had ~30ish Ohms instead of 15k, the diode was a dead short).

Is this just a case of "replace the broken parts and you're good to go" or do you think that it might have damaged something else when it blew? Those components were responsible for the negative rails of the analog board, i really don't want to poke around in there searching for possible faults if i don't have to.

I also encountered another weird thing: The 236 has a 115/230V selector switch and i measured ~500-1k  between the bridged contacts. Is that to be expected when measuring it with a DMM or should i also replace that part?

[alm]

Funny, in a Keithley 237 unit I also had a 15k resistor fail. I don't remember if it was for the positive or negative rail. In my case, the resistor failed open. So either the resistors they used were not very reliable, or overstressed.

I imagine that the resistor failed first, and the decreased resistance increased the current through the zener, killing it.

I'm worried that the -15V rail was pulled down to -150V through R73. So I would be worried that everything connected to that rail could be blown. I would test by desoldering R73 and connecting a 15V bench supply across the -15V rail (watch polarity!). The unit should then be operational. Then test the various ranges, positive and negative output voltages / currents.

I don't remember how the mains voltage selection works in this unit, but I would expect the resistance of the switch to be < 1 Ohm when the contacts are closed. So that switch sounds suspicious to me. Try cycling it a couple of times, and measuring it again.

[jogri]

Thanks for the help guys,

looks like the shorted 15k resistor was the reason for the blown fuses. I replaced both of them with 5W resistors (old ones were 2W, they both looked like they were properly cooked) and it powered up, so it doesn't look like the -15V line of the analog board was pulled to -150V for long enough to kill something (can't say that for sure as i haven't tested the unit properly yet, i am still looking for triaxial cables and connectors...). The connectors on the 115V/230V switch had a nice oxide layer, but a bit of scrubbing with steel wool took care of that.

[Ice-Tea]

Quite common.

As far as I know, chain of events is: fan dies, resistor cooks off, sometimes the zener cooks as well.

Check the zener. And perhaps redo the other +/- 15V rail as well.

[jogri]

So it absolutely needs a good fan/airflow to avoid cooking itself? That might be a bit problematic for me as my unit came without covers and a fan. Do you by chance have a product number/specs of the factory fan? I just bought the cheapest 230VAC fan i could find and now i'm a bit worried that it can't provide enough airflow (mine claims to have 40m^3/h).

(The service manual doesn't help here, it just lists the fan as "FAN" without further infos. Gee, thanks)

Edit: Is it normal for those two 15k resistors to be extremely hot? They both reached ~60°C after two minutes of letting the 236 run idle.

[Ice-Tea]

Have a look here:

https://www.eevblog.com/forum/testgear/keitley-238-restoration/

I also used

https://be.farnell.com/cgs-te-connectivity/er5815kjt/res-15k-5-7w-axial-wirewound/dp/1751897?CMP=i-ddd7-00001003

more recently. Been using these fans:

https://be.farnell.com/multicomp/sf11580a-1083hsl-gn/fan-80mm-115vac/dp/9606599?CMP=i-ddd7-00001003

[JxR]

The original fan as far as I'm aware: ETRI 126LF2182