Repair Keithley 2100

[arrenio]

A student of mine accidentally connected a Keithley 2100 to a 240AV source when the selector was set to 120V line. Now, if we try to power-on the instrument the fuse blows. I have opened the case and disconnected the transformer by the circuits (there are a long connector, and two short connectors). Again, the fuse blows. So I think that it is the transformer.
Someone knows the exact transformer I need to try to replace the broken one?
Thank you a lot

[exe]

> Again, the fuse blows

I'm confused. If it's the tranny, then the fuse should not blow when it's disconnected...

[arrenio]

Thank you for the reply.
If I disconnect the transformer the fuse does not blow.
If I connect the transformer (which is disconnected from the instrument) the fuse blows. It seems that the input coil absorbs too much current..

[Mr. Scram]

I think Keithley sells transformers but they tend to be stupidly expensive.

[arrenio]

Yes they do, but I was wondering if someone knows the model of this trasnformer. Looking for the codes printed on the transformer gives no results..

[exe]

Just in case, did you try to put selector into 240V?

From past experiences of forum members, connecting a 120V tranny into a 240V power grid is usually fine if there is a proper fuse... May be the fuse was of incorrect current rating, and the tranny is damaged.

[arrenio]

The fuse blows when the selector is set to 240V (my line is at 240V). I think that the transformer is damaged: i.e. the selector was set to 120V and the 240V power line was pugged in...

[MadTux]

But if the 120V fuse with double current rating is installed and selector is set to 120V, but instrument gets 240V, then the transformer and most things get most likely toasted with lots of overvoltage.

If you're lucky, only transformer and some linear regulators are toast, if overvoltage somehow got past the linregs, the instrument is probably beyond repair, if like all parts on 5V rail are killed.

[Kleinstein]

Finding out the correct voltages for the transformers would not be that difficult. It reasonable easy to calculate back from 7815 or similar regulators what voltage is needed. The tricky part can be that one ideally wants a transformer with reduced capacitive coupling, maybe an extra shield and possibly higher insulation rating than normal.

In addition chances are one would need something like 2 x 18 V and 1 x 9 V, which is not a common combination. So one may not find this as an of the shelf transformer.

[Chris56000]

Hi!

Will this manual I've attached here help?

There are five secondaries:·

1) 30V @ 100mA :– +37V VFD Anode supply regulator;

2) 1.55–0–1.55V @ 250mA – VFD filament supply – VFD filaments are typically 3.15V nominal;

3) 10V @ 1.5A:– Front Panel, Main board Digital and "in guard" analogue supplies;

4) 18–0–18V @ 250mA:– "Out–guard" (floating) ±18V analogue voltage supply regulators;

5) 10V @  250mA:– "Out–Guard" (floating) digital and interface circuit regulators.

Windings 1 to 3 and 4 & 5 need to be screened from each other and insulated to withstand 1.5kV between them, as the "in–guard" (microprocessor, display and logic controller) need to be insulated from the "Out–Guard" (floating input, amplifier and a/d conversion) circuits are at the potential of the floating input terminals, which are rated for ±600V relative to true earth!

If you go on the UK Vintage Radio Restoration Forum, (you will have to register to post) and contact Member Ed Dinning, he can rewind your transformer for about £80– £120 for you, or you might like to contact Array with the transformer details given in the parts list they provide – I believe they're willing to provide parts on application!

I will be beginning the job of tracing full circuit diagrams of my 2100 in the (hopefully) not too distant future, but until I have the time to begin this, you might like to look on the "xdevs" Keithley and HP repair site, schematics of the 2000 can be obtained from there!

Chris Williams

[Chris56000]

Hi!

Using the information I've given, you might be able to temporarily wire in substitute transformers to see if the meter circuits have been damaged in any way before you commit yourself to trying to get the exact replacement – one with two 9V secondaries rated about 25VA will power the digital parts, secondaries 3 and 5 in my previous post, a transformer from an old microwave oven or clock radio etc., that had a vacuum display will do for secondaries 1 and 2, whilst one with 2 X 15V or 2 X 18V at 15–20VA will do for the analog supplies, secondary no. 4!

If you get really mega–stuck, use 5 X PP3 (16904) the batteries in series to feed the input of the 37V VFD regulator, two 1.5V alkaline AA batteries with a 1k resistor across each to provide an artificially tapped VFD filament supply and about 5 more PP3 (1604) batteries to power the other regulators, unplugging the transformer secondary plugs first of course – this will only last a few minutes running but should be enough to see if enough of the meter has survived to make it worth your while to go any further!

Chris Williams

[arrenio]

Dear friends,
thank you a lot for your answers. Next week I will give a look to the instrument according to your suggestions. I will update the discussion with the results of my tests. Best regards.