Author Topic: Tek 485 - how to test power transformer and rectifier board out of circuit  (Read 1170 times)

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Offline Lorenzo_1

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My Tek 485 was working fine till a couple of weeks ago when in use for some diagnostics.  It was on standby when screen display suddenly contracted to a small and very fuzzy square (perhaps 1" by 1").  Then completely blank.  At this stage the balance node was going positive during inverter start-up suggesting I had no 3KV supply - all the other power supplies checked out fine.  Couldn't test -2950V supply at that stage (now have an HV probe for that) but HV current sense (TP1958) indicated a short.   Subsequently, however, all the DC supplies went out across the board.  All I have now is the warning neon on the Inverter board flashing (but none of the usual ticking or neon flashes from the power section of the Horizontal Amp board), regular start pulses at TP1835, and Q1900 & Q1902 working fine. Power looks to be getting to the T1960 primary but perhaps nothing out the other side. Inverter is trying to start up but not succeeding.

I've repaired this Tek a few times (new U1600 6x multiplier a few years ago).  Several resistor/cap replacements to fix DC supply issues.  But I figured at this stage I needed to check what happening at the power board and particularly the T1960 transformer - which is a fair bit of dissection.  Nothing obvious now I have it out, but would appreciate advice as to how best to test the transformer and rectifier circuits while it's out.  The caps and resistors on the rectifier board check out OK but there's a couple of 12kV diodes there that I likely need to test.  There's no obvious sign of breaks in any of the transformer primary or secondary circuits, but that's a very limited test and unlikely to tell me about shorts. I have a Variac, so can gently power it up and look for output on the HV and low voltage lines out (very carefully!), but not sure whether that's wise with no load on the transformer output.  Grateful advice.

Re the 6x multiplier, it looks fine with no signs of cracking or shorting.  I think I recall someone testing this by feeding in a low voltage (25V or so) square wave and testing output at far end.  Is that the way to go?

Keen to try and get this debugged out of circuit as far as possible as it's a big job to get it out and then reinstall, only to find it's still not working.  Really need to know it is doing its job before I can look for any other points of failure.
 

Offline tggzzz

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Have you checked the Yahoo! TekScope group's archive?
https://groups.yahoo.com/neo/groups/TekScopes/info
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online tautech

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Checked this?
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Troubleshooting chart in the service manual?
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