Keithley 2000 Blown Fuse

[floobydust]

Transformer's primary winding consists of 4 sections - White 100V Blue 20V Violet 100V Gray 20V Red  in series.
For 100 and 120V "White 100V Blue 20V Violet" section used 0.175-0.18mm wire, for 220-240V additional sections "Violet 100V Gray 20V Red" - 0.125mm wire or so, so their resistances won't match (number of turns the same - 612 + 122 turns) - no issue with that.

So we can at least conquer the transformer's schematic- is this even close? I don't quite follow what you mean by 20V tap.
You know the # of turns, you must have taken one apart I'll add the secondary windings to the schematic later.

edit: updated pic, looks right I'm still confused about the primary connections, they aren't making sense fixed

[Hendrik]

The problem seems to be the transformer .. 
44.3W draw with just the transformer plugged in.

Does this seem like a suitable replacement?: https://www.ebay.de/itm/404036085934
Anyone knows what the #A6-3 in the title of the ebay article means?
The images seem stolen from the other ebay.com post previously mentioned.

Maybe this one: https://www.ebay.de/itm/304113286672

Edit: I've ordered the second ebay link as it had own images and a faster estimated delivery time.

[Per Hansson]

That transformer will probably work but read Reply #22 by floobydust on the previous page if you did not already.
I would put up a email notification on eBay for a search for such a transformer, sometimes they are parted out of parts units so you would get an original transformer eventually.
P.S: It might be worth to connect only the 120v or 100v winding via a stepdown transformer.
If the short is not on the secondary but on the primary it might just work fine that way.
Maybe you have a small bathroom "shaver socket" 110v transformer available: it should suffice for a no-load test.

[Kleinstein]

There is not much improvement expected from using only part of the windings.  The problem is likely with a few shorted turns. Thease act like a load on the transformer and it does not matter if the related winding is used for something else. Not using part of the primary may lead to a slightly (e.g. 10%) lower voltage and thus slightly less overheating, but it would still not really help.

The windings are quite tight coupled, so not much that can be done, short of taking the transformer aprat and redoing the windings.
The coupling is usually so good that it get hard to tell which winding has a few shortet turns. With the extra shields the damage could even be with one of the shields, creating a short.
It would be a lucky case if the damage is more to the outside - often the inner turns run hotter and thus are more likely to be damaged.

[floobydust]

It looks like the transformer design had issues, OP's transformer seems to be labelled TR-299 REV.B1  Notice the oddball connection as if to a thermal-fuse but it's embedded in the secondary windings, which is wrong.
There is REV.B6 seen in posts, and eBay listing "REV.B4" (and misspelled "BRISTOL, WI" with "BEISTOL, WL") enspires such confidence in the eBay beijing A6-3 store

I have no idea what Keithley is like for obtaining parts. OP could PM the other people who were looking for one to see what they found.
Six revisions to the part implies a certain clown car with Northlake Engineering Part # K1-433P, Standex bought them.

[chekhov]

Thermal fuse is embedded into mains winding - connected to White wire - start of all windings, but it's for excessive long-term heating, not for case when load accidentally increased a lot and very thin wire just melted underneath ...
Thermal fuse - 2.5A 145°C !!!!

[floobydust]

OK I got your PM and understand you would like to do a detailed drawing of the transformer schematic and construction.
I've updated mine above just for people to be able to do continuity tests. I can't make sense of the white (common) wire having the thermal fuse and the "20V" you mention.
Your measured (load) voltages seem too high i.e. yel-yel 39.6VAC and OP got 32VAC as well as 14-0-14V for orange/grey and you got 19.6VAC. I think 14V is correct.

[chekhov]

Just in case, very quickly measured resistances (mains just via mains, switching selector via all values), transformer has revision B:

Code: [Select]

Primary
 64.70  White Blue
 80.45  White Blue Violet
241.85  White Blue Violet Gray
282.97  White Blue Violet Gray Red

White  Blue    64.70
Blue   Violet  15.75
Violet Gray   161.40
Gray   Red     41.12

Secondaries
Brown  Brown    0.757
Yellow Yellow  58.870

Green  Green    1.335
Gray/red Orange 5.240
Gray/red Orange 5.316

Mains winding consists of 4 sections, all connected in series, White is always connected to mains, selector then selects from the reset - Blue 100V, or Violet (+20 = 120V), or Gray (+20+100 = 220V), or Red - (+20+100+20 = 240V).

Maybe when I measured voltages I had 220V selector and 230+ in the socket, don't remember, but these voltages are always much higher than rails that we have (and in some cases they are used - if you look in reversed schematics you could find +20V tap going before +15V regulator), that's one of reasons these regulators are hot, and then caps leak ...

With this thread my wish to make single detailed post seems to fail ...

Edited: fixed the latest resistance, accidentally put 2+ ohms

[chekhov]

Indeed, it seems long ago I used 220V selector, taking advantage of currently opened unit, I measured AC voltages again:

Code: [Select]

K2000 transformer voltages,    235.1 VAC input (may vary a bit), 240V selector

                  No load    Loaded
4-pin connector
brown-brown           7.9       7.11      7.24 (No FP)
yellow-yellow        38.0      35.5      36.1  (No FP) 

5-pin connector
green-green           8.74     8.07
gray-orange          18.83    17.47
gray-orange          18.84    17.48


[floobydust]

[...] Indeed, it seems long ago I used 220V selector, taking advantage of currently opened unit, I measured AC voltages again:
235.1 VAC input (may vary a bit) [...]

Is this with power on the 220V or 240V selection (windings)?

With those ohmmeter numbers, end-end on the primary 240V (white-red) you measured 283Ω and OP measured 172.8Ω, 220V (white-grey) you measured 242Ω and OP measured 146.2Ω
So there seems to be a problem seen just looking at the ohmmeter (primary) readings, OP's are 60% of yours.

[Hendrik]

Just for completeness I meassured all resistances:

primary:
White-Blue: 64.9Ω
Blue-Violet: 13.2Ω
Violet-Grey: 67.9Ω
Grey-Red: 26.4Ω

secondary:
Yellow-Yellow: 82Ω
Brown-Brown: 0.85Ω
Grey-Orange1: 4.61Ω
Grey-Orange2: 4.51Ω
Orange1-Orange2: 9.09Ω
Green-Green: 1.48Ω

Seems like I am missing ~100Ω from Violet-Grey and 15Ω on Grey-Red.

[chekhov]

Unfortunately that means that very likely somewhere inside we have something like that:


Still opened question is what took this transformer down, so there should be something on main board, or maybe front panel (less likely if it lights up). While looking for transformer replacement, topic starter will have plenty of time in order to find initial issue.

[Kleinstein]

There may be nothing wrong on the main PCB to cause an isolation failure in the transformer.  A likely cause is a somewhat weak point in the isolation (e.g. a kink in the wire or wires crossing) and than some overvoltage spike in the mains supply (e.g. a distant lightnig strike of transformer disconnect).  It is not very common, but transformers occasionally fail for no obvious reason on the output side.