Tek 465 Blows mains fuse - Page 1
Forum section: electronics / testgear

author: Tek-er
author_rank: Newbie
timestamp: February 17, 2021, 10:44:15 am
content: I’m in Australia & just came across your site hoping someone may be able to help me with an old analogue Tek 465. - (Not A)Love this CRO but, - mains fuse (240 v .75A), blew while working with it. (Thought we had a blackout)I have downloaded the Cct. & after careful checking, all filter caps. seem OK - (just checked with a multimeter across them in circuit & no low readings) Replaced 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. - ( Would also appreciate any help with this too). - 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.

author: tautech
author_rank: Super Contributor
timestamp: February 17, 2021, 08:15:07 pm
content: Check the 15V rated tantalums on the 12V rails as they commonly go short. Replace with 25V versions or use an electrolytic if you haven't got tants on hand.

author: Tek-er
author_rank: Newbie
timestamp: February 17, 2021, 10:49:59 pm
content: Thanks tautech.I'm a little miffed about the tantalums for replacements, as I regard them as un-reliable as do many others, plus I had bad experiences with them years ago when I used them in equipment thinking they would be more reliable than plain electro's. However, not so, as they were the first to fail & when replaced with common electro's there was no further trouble years later.Hence my concern about replacing tants with tants in a Tek. I would prefer to put in higher rated voltage electro's & generally higher capacity values as these components are now much smaller. - Further, in these instruments I would also "form up" the electro's over a few days before using them as I find I get very low leakage from these if that is the reason they were used by Tek in the first place.

author: tautech
author_rank: Super Contributor
timestamp: February 17, 2021, 11:01:36 pm
content: New technology tants are much better that those from decades ago where as relatively new technology designers didn't know to sufficiently derate them which of course they learned later and is still good practice. I have no reservations in replacing tants with modern tants with an adequate voltage derating.

author: med6753
author_rank: Super Contributor
timestamp: February 17, 2021, 11:17:59 pm
content: Chances are CR1451 isn't your issue. Most likely a shorted tant on the +15V bus. Towards the rear of the service manual is a very good procedure for troubleshooting the PSU as well as isolating the various boards in order to narrow down the short. I suggest you give that procedure a go first before condemning any parts.Also, as tautech said....replace tantalums with same type, just go higher in voltage.

author: Tek-er
author_rank: Newbie
timestamp: February 18, 2021, 02:29:36 am
content: Thanks Med6753,FWIW, the 15v rail measures 175 ohms with or without the Tant connected & the tant itself measures 3.7 megs., (this is easy to get at !), also, the resistance to ground from the o/p of CR1541 reads 700K so I'm still suspicious of the CR1541 itself.Further, I also thought about CR1512, - also on the o/p of CR1541 but it is NOT fitted. It IS shown though on the PCB overlay but there is no reference or correction noted in the manual ??So I'm confuscious !!

author: med6753
author_rank: Super Contributor
timestamp: February 19, 2021, 04:30:50 pm
content: OK, I opened up one of my 465B's (I don't have a functional 465) and recorded the typical resistance readings for each supply voltage. The 465 and 465B differ in that the Trigger, Vertical, and horizontal boards are totally different but the PSU is 99% identical. So the readings given should closely match your 465.These readings were taken at the voltage test points on the main board.+110V 19.57K+55V 1.024K+15V 109.8 ohms+5V 82.2 ohms-8V 45.3 ohmsHope this helps.

author: Tek-er
author_rank: Newbie
timestamp: February 20, 2021, 04:44:28 am
content: Many thanks med 6753. - This was helpful. Mine reads roughly 5 - 15% higher on all except the 110v which reads about 9% lower.From this I would think that they are in the ball park.Are you able to read directly from the un-reg. 15v CR1541 bridge o/p, - (the large one near the side), across C1542 please ?Thanks.

author: med6753
author_rank: Super Contributor
timestamp: February 20, 2021, 10:52:40 am
content: Quote from: Tek-er on February 20, 2021, 04:44:28 amOK, later today I'll get those measurements for you. I'll have to pull the 465B back out of the case again but that's no big deal.

author: med6753
author_rank: Super Contributor
timestamp: February 20, 2021, 03:33:52 pm
content: Measuring across C1542 I get the expected reading of about 500K then slowly increases as capacitor charges. I also checked the AC side of CR1541 and get about 1 ohm.You might want to check Q1544, Q1548, and especially the pass transistor Q1516.

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2021, 04:18:32 pm
content: Quote from: Tek-er on February 17, 2021, 10:49:59 pmProperly derate the replacement solid tantalum capacitors, which was not but should have been done in the initial design, and there will be no problem.Or use aluminum electrolytic replacements with about 4 times the capacitance to get the same impedance at high frequencies.

author: Tek-er
author_rank: Newbie
timestamp: March 01, 2021, 04:49:22 am
content: Thanks David & med6753, - Fixed at last !It was was not the 15v bridge CR1541, (which I had already replaced), but Bridge CR1511 - 120v supply.I also replaced the 15uf tant on the 15v reg. with a 47/50v electro & a 33uf 20v tant with a 47uf/50v 105 degree for good measure as they were very accessible.Interesting your comment David about replacing with larger values, but I read somewhere about NOT changing the value of one particular electro (10uf I think), but now I can't remember what particular one it referred to. Otherwise I think it's a good suggestion.Thanks again to all who responded.