Hi All,
Been looking at a Tek 475 on eBay with no display.
I know this can be a hundred a one things, but does anyone know if this is a common fault with the 475 and if so, what it may be. I vaguely remember seeing something on Gooogle a few months back, but I can't find it again.
Cheers...Bob
"bob_e1947" wrote in news: snipped-for-privacy@m73g2000cwd.googlegroups.com:
power supply failures are very common.Bad caps and/or bridge rectifiers. Just because the front panel lamps light does not mean the PS is working right.
Then there's HV/Z-axis failures,sweep failures,bad deflection circuits...
Possible worst;a dead CRT. or a motherboard that leaking electrolytics ruined.
-- Jim Yanik jyanik at kua.net
Like Jim said, most common is power supply capacitors, then PS diodes, then shorted tantalum capacitors, then bad HV power supply, then maybe bad CRT.
Or a combination of the above. One 475 I worked on had all of the above. No ait, the CRT was only weak, not totally dead.
A service manual with the detailed trouble shooting diagram is useful for a repair . Check bama.sbc.edu or it's faster mirror for a free download.
hth, Andreas
Hi JIm - seeing your name reminded me of a discussion about a Tek 475 we had yonks ago - it had a blown eht generator transistor. It was suggested that the closest equiv. to the TEK original was a 2n3055., I think SPHERE list this. Experimented a bit - found a common TV horizontal output transistor worked - cant remember what it was, but its a BUX something - 70's vintage Philips - TO3 case- out of a junked chassis sitting under the bencg.... Worked this out by using external current limited power supply to feed cct, and looked at waveform on base of transistor, as per diagram in manual - the TV one gave the proper waveform, the 3055 had big spikes (why, I dont know - I am not a engineer) The other problem was that huge film type resistor (300MegOhm?) in the EHT feed back cct had gone LOW in value - this one stumped me for a while, never seen a resistor go low, usually high. physics is (apparently) copper migration from terminals, only happens in HV circuits.
Anyway, now goes OK - last fault is, after an hour or two, the trace intensity starts varying......one day, when everything else is done, will tackle it....
Is it a nice scope - YES!!!!! Would I do up another one NO!!!!!!!! (Chewed up heaps of time and money ) - a previous fault (it was given to me as a junker) had me replacing the EHT transformer, the EHT multiplier, the CRT, and the IC that drives the vert plates........too open ended.......
Andrew VK3BFA.
"Andrew VK3BFA" wrote in news: snipped-for-privacy@e3g2000cwe.googlegroups.com:
The original 151-0140-00 was a wierd,ancient manufacturing-process type,long discontinued now. Nobody at TEK ever figured out -why- other types would not work. (probably T&M division's drift away from service support.) And that xstr was used in lots of different TEK products,7000 scopes,400 series,528 and 520/A NTSC TV instruments(and PAL versions,too)
It's nice to know about that BUX-type HO-xstr,I'll try to remember that. Thanx!
IIRC,30 megohms or in that range.The "thick-film ceramic". It was HV divider on one side,and focus divider on the other.
Yeah,but generally,they were pretty durable. I don't think TEK ever expected those scopes to last so long.
IIRC,there's a 1uf/150V al.electrolytic cap on the wiper of the grid bias pot that would go bad,and the DC restorer diodes were always a common failure item in the grid bias circuit.But you need a curve tracer to test them.(it could also just be a dirty or worn-out intensity pot,or grid bias pot.)
-- Jim Yanik jyanik at kua.net
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