Racal Dana 1992 counter blows fuses

[gpsdo_nut]

All,

First off, Merry Christmas and happy holidays to all!

The other day I fired up my once, perfectly functional 1992 after being powered off for quite a while, and the display lit up for a couple of seconds then went blank.  I checked the power supply fuse and noticed it was blown.  Put in another 250V 0.5A fuse and instantly blew that one.

I'm not proficient in electronics and certainly lack real experience in fixing stuff, but I looked around the internals and didn't see anything too alarming other than a slightly blistering/bulging 10000uf (C49) cap.  Armed with a DMM I started to look for shorts to ground.  The 1992 uses what looks like a linear power supply, and the primary side of the transformer looked fine, or rather no shorts to ground, as far as I can tell.  However, I can't figure out what's going on with the secondary windings.  All of the secondary windings all read shorted, or "practically" shorted (0.8 - 1.3 ohms) to ground...  I removed a couple of filter caps. (which after removal were not shorted) and removed all of the AC sides of the two bridge rectifiers (D11 and D12).  Even so, the secondaries still read shorted to ground.

I guess my question here, are the secondaries supposed to be low resistance to ground?

If my assumption to the above is 'no', and at this point, I'm out of ideas on what else could go wrong other than condemning the transformer.  One oddity was that the bridge rectifier D11 indicated a direct short from one (but not the other) AC line to the (-) output - think of the diodes may have blew in there.  Diode checks around D12 tested fine and no direct shorts around any of the diodes.  Could the potentially low-performing 10000uF cap cause D11 to fail and, in turn, blow out the transformer secondaries?

Would appreciate any thoughts/pointers, as I don't really know what else to look for!

[coromonadalix]

Racal dana 1992 schematics are available, check at ko4bb  website  in the RACAL section

First i would carefully desolder any rectifier diode / bridge

i have 2 of theses counters,  but they have the stuck keypad switches, for one of them i removed all the switches,  and now the counters are sitting duck
i could do unpowered measurements ...



http://www.ko4bb.com/getsimple/index.php?id=download&file=Racal/Racal_Dana_1991-1992-ServiceManualSch.pdf


Racal_Dana_1991-1992-ServiceManualSch.pdf    figure 9   page 15


Be very careful,  this counter has some unobtanium parts, use anti static protection


The transformer secondaries are all common, they share a ground pin, pin#8

One occuring problem was the regulators legs would simply cut under mechanical stress, sometime its almost impossible to see them broken

[gpsdo_nut]

WELP....that would certainly explain why all secondaries measure the way they do with respect to ground...haha.  My assumption for the fault at wrong place.  Thanks a bunch for the link to the drawings - I've completely forgot to check the KO4BB site for additional schematics!

Well, with the legs of D11 completely removed from the mainboard, I am measuring a dead short between legs 2 and 4 of D11 so far.  Sounds wrong, correct?  I certainly hope that didn't fry the stuff downstream from the bridge, namely Q6 and IC316, and nothing was connected to PL21.

D12 continues to check out okay for all diode tests.

I'll continue to check later today.

[coromonadalix]

if you did say the right pins numbers   for d11

remove c49 and c50 and redo a test

[gpsdo_nut]

Before I pull out other components, let me clarify what I was testing:

With D11 completely off the board, pins 2 and 4 of D11 itself are shorted.

With my DMM negative on the metal case:

Pin 1 on mainboard -> case = 0.3 - 0.5 Mohms
Pin 2 on mainboard -> case = 0.4 ohms (thru the secondary coil I guess)
Pin 3 on mainboard -> case = 0.5 ohms (also thru the secondary coil I guess)
Pin 4 on mainboard -> case = 0.3 to 0.5 Mohms

Should I still try to pull C49 and C50 off the board and redo the mainboard pin tests?

[coromonadalix]

unsolder / remove any solder around the xformers secondaries pins, leaving them floating,  the xformer can cause bad readings


remove the gpib board, the counter can work without it, you don't have the battery pack do you ? if so, remove it

[gpsdo_nut]

Is there an easy way to do that?  Looks like the whole transformer is anchored down to the board and I'd have to de-solder the 10 pins...

[eevcandies]

Note the 1992 has a battery power option...so you should be able to find the tie-in point & power it up from your bench supply (to check the counter itself).

Also that means you should likely be able to  separate out & test the supply itself without  "blowing" the counter.

Note the SKEGACA010 is a perfect replacement switch (at least for the Racal1996...I believe will work for the 1992, see photos)...fits PCB perfectly & the keycap goes right on, NO MODS....it's final height may differ by 1mm, but is not visible if you replace all the switches.    you can buy 100 of them for $23 USD, I  might buy another batch just to have.
https://www.eevblog.com/forum/repair/racal-repair-idea-quick!/msg2427723/#msg2427723

[TerraHertz]

I had a Racal Dana counter that would blow fuses AND one of the PS rectifier diodes repeatedly. Every time I replaced the fuse and diode and tried again to find the problem, pop went the fuse and the diode instantly.

I found the problem by powering it via a variac, and starting with almost no mains voltage, increasing and seeing where the current went.

It was a tantalum cap on one of the DC rails, I forget which. Dead short, no visible sign of damage to the cap, but able to take everything the power supply could give it.

This is typical for old tantalums, especially if left unpowered for a long time. But usually they have the decency to visibly smoke a bit. Or a lot.