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Newbie - Tek 465 Help Please

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Vintage-er
Posts: 1
Joined: February 12th, 2021, 7:26 pm
Country: Australia

Newbie - Tek 465 Help Please

Post by Vintage-er » February 12th, 2021, 8:02 pm

Hello,

I’m in Australia & hoping someone may be able to help with an old analogue Tek 465.

Love this CRO but, - mains fuse (240 v .75A), blew while working with it. (Thought we had a blackout)

Careful checking all filter caps. (OK), & replacing with a 1 amp fuse, (no .75A readily available), & slowly turning on via a variac, shows that raw 15v dc o/p supply from rectifier CR1541 seems to clamp at about 1.6 volts when applied mains voltage goes from 30 to 60 volts. C1542 seems Ok as it reads high on Ohm scale to earth.

I was not game enough to turn the variac any higher, so I am assuming it is the bridge rectifier CR1541. Only problem, - how do you replace it without virtually dismantling the whole CRO to get to the underside of this huge PCB. (Why didn't they put just the P/S on a sep. PCB ?)

Checking all other voltage rails & HV caps. with a multimeter shows no shorts or suspect low resistances. 1.5 A F1419 (feeding HT supply ??), is OK. Also, I have been putting up with a DC shift & slight jittery problem on Ch 1 for some time but can live with it. - I use this scope to look at very low residual levels from a THD null & also X-Y nulling. ( My Chinese “you beaut” digital scope is useless for this work !)

Any of you cluey TEK guys our there have any suggestions or experience with this sort of problem. I would really appreciate some expert advice as I'm not really into dismantling TEK CRO's !

Thanks,

Ron.

jrseattle
Posts: 2
Joined: January 23rd, 2021, 1:45 pm
Country: United States

Re: Newbie - Tek 465 Help Please

Post by jrseattle » February 14th, 2021, 4:06 pm

Remove the Trigger board and you'll have access to the bottom of the main interface PCB, at least the portion where the filter caps and bridges are.

JdR

daveca
Posts: 4
Joined: June 7th, 2021, 11:06 am
Country: United States

Re: Newbie - Tek 465 Help Please

Post by daveca » August 31st, 2021, 9:00 am

"slowly turning on via a variac"

That is NOT a valid procedure for working on regulated power supplies, especially SMPS.

"Running it up on a Variac" is a popular Old Wives tale. It was useful back in the Vacuum Tube days, to find a power supply with an excessive load that would open a fuse or breaker with full line voltage applied, but it is USELESS with regulated or SMPS supplies.

"Suddenly clamping" is indication of an active power supply. Not a passive one that can be "run up on a variac"

Sorry, but after 15 years on the bench, I fear youre not qualified to work on this unit.

KMittaaja
Posts: 3
Joined: September 3rd, 2021, 12:08 am
Country: Finland

Re: Newbie - Tek 465 Help Please

Post by KMittaaja » September 7th, 2021, 2:12 am

The service manual
https://w140.com/tekwiki/images/3/32/07 ... ervice.pdf
See page 36 for powersupply
See page 126 for powersupply schematic

Generally, if the +15V supply is blowing the fuse, I would check any shorted capacitors first.
Some emit the whitish smoke of death (Canberra MCA tantalums are spectacular!!) but most are not seen visually bad.
If you have access to thermal camera, look what capasitor is heating up due to short-circuit current going through it.
If you don't, consider buying one. They are quite cheap and make a huge difference finding where the short-circuit energy goes..
The ones for smartphones with about 160x100 resolution are good enough.
I've replaced LOTS of shorted tantalum caps on 30+ years old circuitboards to resurrect electronics.. Low ESR electrolytic caps are the usual replacement.

But you really have to find what is blowing the fuse. It might be something else than a shorted cap.

As others mentioned, running up with variac can have strange effects with active circuits.. See the schematic at service manual and consider how it should behave. The +15V supply is not that complicated, but do realize it uses the +55V reference. Simulating it with free spice, like "switchercad", can be a good idea to get an idea. If there is an "easy" shorted DC filter cap, it could heated up with lower voltage and be found.

Summary: the magic repair help is thermal camera to find possibly shorted components. Very hot capasitors heated by short-circuit current are obvious.

inotarobot
Posts: 17
Joined: April 25th, 2017, 6:37 am
Country: Australia

Re: Newbie - Tek 465 Help Please

Post by inotarobot » September 14th, 2021, 7:00 pm

Hi there,

did you solve your problem?

I am in Melbourne and still have a 465 DM 44 (currenly not working] and 475 DM44 working.

PM me here is you wish

regards
David

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