A quick search brought up this thread in which I appear to have posted 3 replies, offering sage advice on protecting the mains transformer (I don't even remember writing them). I'm resurecting it to document my own carelessness and stupidity... and not following my own advice!
I was using my trusty Tek475A a couple of days ago to monitor a DDS sig gen and thought I would let it stretch its legs for a while as it hasn't had much exercise recently. After an hour or so, I left the room for a few minutes and came back to a completely dead scope. Initial checks showed the 750mA input fuse blown, but not in a dramatic style.
Mistake #1: Having checked that the fuse for the EHT supply was intact, I assumed that the input fuse must have aged and replaced it (luckily there was a nice packet of original BUSS fuses still in the pouch). This immediately blew, in the same non-dramatic fashion. Unnecessary stress on the transformer.
Mistake #2: I checked for shorts on the DC rails to rule out Tants and
thought I had checked all of the bridge rectfiers. I also checked the two primary windings for equal resistance. By this point I was beginning to suspect the mains transformer. This time I slowly powered it up on the variac so that I could monitor the DC rails coming up. Before I had finished though, a nasty varnish smell alerted me to the heating up mains transformer. Damn! By this time I was seriously worrying that the transformer was fried. The only way to know for sure would be to disconnect the secondaries. Unfortunatly these are inaccessible without removing the trigger board. Time to sleep on it.
Mistake #3: After pondering overnight, I resolved to systematically re-check everything. This time I discovered that the bridge rectifier, CR1412, supplying the Unreg +50V, 110V, and Reg +50V was showing a low resistance fault between all four legs (how did I miss that?!). Ok, faulty bridge rectifier. As it was flush to the board, I crushed it and unsoldered the pins (to get to the underside of the PSU/Horizontal board you need to remove the vertical amp board and trigger board, plus knobs, heatsinked transistors etc.) and patched on a replacement.
Without checking, I again started to bring the scope up on the variac. Oh no, varnish smell again!
Getting it right
at last: Checking carefully around CR1412, again what appeared to be a 4 wire low resistance short. The Unreg +50V was still showing no short (after initial cap charging), confusing. After looking carefully at the schematic I realised that only shorted C1412 or C1414 could cause the symptoms, sure enough, C1412, arrowed in Red showed a dead short! After solder sucking the anode pin and isolating it with slivers of wooden coffee stirrer the short on the rectifier had dissapeared. Bodging on a 100uF 200V electrolytic rewarded me with a fully working scope with clean trace and all supply rails spot-on.
What had confused me so much was the configuration of the rectifier. The -ve output is grounded and the unreg +50V is taken from the transformer centre tap, with the +110V taken from the +ve. The capacitor short, together with the secondary winding resistance and grounded -ve pin made it look like a dead bridge rectifier. I've run the scope for several hours now and the transformer seems to have survived the abuse. Needless to say, the trigger board is coming off and all reservoir caps replaced before it goes back in its case!
Learnings:
1. Check
everything possible at least twice before attempting to power up anything after a blown fuse.
2. Bringing up things on Variacs
can be a really bad idea!
3. Ageing electrolytics can fail short as well as drying out (even though I've owned the scope for over 40 years and it has low hours).
4. Learn to recognise your own declining faculties as you age and compensate by slowing down and allowing more thinking time.
5. Follow your own advice...
even if you can't remember giving it!
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