tek 475 unknown correct parts values and types and try out to repair

[yo0]

hello, i have a non working tek 475, i think it is a transitional specimen (serial B271XXX), cause it has the 475a  A9 board inside. it maybe has been repaired before, the problem is i am not sure if the parts inside are the correct ones, a burned resistor (i think is 10 ohms) a dead fet 2n3819 and a "blue" tantalum capacitor 10uF 35v dead too, i cant find these parts in none of the schematics, sm and parts lists i´ve found in the net. somebody with a 475A can help me to identify the correct parts please?

the first two photos are from TekWiki 475a  A9 board, the resistor seems to be 10 ohms.

any help will be appreciated.

best regards.

Pio

[yo0]

ok after removal, the fet lettering appears on the pcb, Q920 = 151-1025-00 = KE4416(PN4416), now just the tant cap value remains a mistery to me  .



best regards

PIo

[bd139]

Tant is probably non critical. They’re only used for supply decoupling. Stuff a voltmeter across where it came out and measure voltage. Then pick one at least 4.7uF with 10v of overhead. Usually I just stick an electrolytic in if I’m honest. 22uF / 35v.  Tek have some pretty big holes in their manuals.

Also if there’s one tant that’s gone, there are going to be others. I think I replaced about 9 so far in my 465.

[yo0]

Tant is probably non critical. They’re only used for supply decoupling. Stuff a voltmeter across where it came out and measure voltage. Then pick one at least 4.7uF with 10v of overhead. Usually I just stick an electrolytic in if I’m honest. 22uF / 35v.  Tek have some pretty big holes in their manuals.

Also if there’s one tant that’s gone, there are going to be others. I think I replaced about 9 so far in my 465.

thank you!, do you replace all nine with 22Uf / 35v electrolitics without issues? if yes thats pretty interesting and practical.

best regards.


Pio

[bd139]

Yeah. You usually take the tantalum capacitance value and double it to replace. They used tantalum originally as electrolytic capacitors were pretty crap back then but the new ones are fine for this.

I've repaired quite a few one these series of scopes. Some threads for reference:

https://www.eevblog.com/forum/testgear/yet-another-465-repair-thread/

https://www.eevblog.com/forum/repair/tektronix-465b-repair-97327/

https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/

Worth scanning through if you get any problems.

[yo0]

Yeah. You usually take the tantalum capacitance value and double it to replace. They used tantalum originally as electrolytic capacitors were pretty crap back then but the new ones are fine for this.

I've repaired quite a few one these series of scopes. Some threads for reference:

https://www.eevblog.com/forum/testgear/yet-another-465-repair-thread/

https://www.eevblog.com/forum/repair/tektronix-465b-repair-97327/

https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/

Worth scanning through if you get any problems.

Thank you!!!


Pio

[yo0]

tek 475a sm with schematics and parts list, found it! no good quality but good enough.

http://docmesure.free.fr/manuels/Tektronix/Serie%20400/TEK%20475/TEKTRONIX.%20475A.%20SCOPE.%20INS.%2014678.pdf

by the way the tant cap C961 is 2.2Uf /20v, the resistor R961 is 10 ohms 1/4w 5%, and the fet Q920 is 151-1025-00 = KE4416/PN4416

hope the SM helps somebody.

best regards.

Pio

[tggzzz]

Yeah. You usually take the tantalum capacitance value and double it to replace. They used tantalum originally as electrolytic capacitors were pretty crap back then but the new ones are fine for this.

Personally if I was replacing a tant bead with an ordinary electrolytic I would double the capacitance, but if I was replacing it with another tant bead I would make sure the rated voltage was double the operating voltage. Especially in the case of a 15V tant bead on a 13V line (in 485s).

I wouldn't replace a "silver axial" tant used in integrators such as a timebase, unless it had leaked.

[bd139]

Agreed.

The integrator caps are encapsulated ceramic rod units i think. Hence the somewhat extreme size. Never had to replace one. I did crack one open off a spare timebase and it wasn’t a tantalum unit at least.

[tggzzz]

The integrator caps are encapsulated ceramic rod units i think. Hence the somewhat extreme size. Never had to replace one. I did crack one open off a spare timebase and it wasn’t a tantalum unit at least.

IIRC (but it is early in the morning) I had to replace such a tant in an HP1740a timebase. But that is unusual.

[bd139]

Interesting. Worth keeping an eye on the little buggers then.

[tggzzz]

Interesting. Worth keeping an eye on the little buggers then.

The worst were in my first Tek 1502 TDR switching PSU - they spewed acid across the PCB and destroyed tracks. Lucky that was an easy repair, the most difficult part being finding lots of axial electrolytics to fit in the restricted vertical space.

I haven't seen that on any of my other[1] Tek1502s, but I replace those particular caps on sight.

[1] Sad. Bordering on a fetish. But they are just fun, are wonderful to watch, and do things small scopes can't. For example the 3cm resolution means you can measure connector VSWR, and their portability means I'm remembered at hamfests for measuring secondhand cables before buying them Must sell some of my surplus ones.

[bd139]

Nasty. At least it didn't destroy the board irreparably. I've seen a couple of boards pretty badly ruined now (including that D83).

I'd like an excuse to relieve you of one of your 1502s but I can't really justify it. Also, contrary to the church of TEA, I'm downsizing at the moment. Probably so I can get more crap in later

[tggzzz]

Nasty. At least it didn't destroy the board irreparably. I've seen a couple of boards pretty badly ruined now (including that D83).

I'd like an excuse to relieve you of one of your 1502s but I can't really justify it. Also, contrary to the church of TEA, I'm downsizing at the moment. Probably so I can get more crap in later

Since they are pleasingly small and light (and robust), you could get one and still physically downsize. They give you new capabilities that are difficult with a scope. And watching a twinkling sampler's output is ... hypnotic.

There are many available on fleabay, but none are shown working. The PSU is very strange (includes a programmable UJT!) and operates in subtle ways, so they aren't as easy to get working as you might think.

Mine do work, but I'd have to mark them as "parts only", since I didn't re-install the waterproofing when reassembling them

[yo0]

well, some advance in my repair, now i have trace and A-trigger in both channels, remain problems: unblanking in the three fastest speeds (left part of the trace fades out), also in A-intes and b-dlyd buttons press just shows a hidden single dot, need to move horizonal knob to see it, and it no pass middle screen. any clues or sugestions? by the way all voltages and ripple are ok (almost official values), just in the +50v unrregulated one 420mv AC, ah! and trigger view button just shows barely at the very bottom of screen the very superior part of what i think is a trace.

any help will be really appreciated.

best regards


Pio

[bd139]

Unblanking being too slow... had that.  See here: https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/msg1330121/#msg1330121

Good to hear the rest is going well.

[yo0]

thank you!, do you think all the above problems are tantalum related? 


best regards!


Pio

[bd139]

For me, most of them. Also one of the coax connectors needed resoldering.

[yo0]

For me, most of them. Also one of the coax connectors needed resoldering.

What is your prefer technic to find bad caps?


Best regards

Pio

[bd139]

Properly dead ones are short so DMM. Dying ones you can’t find easily so I tend to replace all the caps around problem areas to see if it makes a difference. A lot of the time it does.

[BFX]

For me, most of them. Also one of the coax connectors needed resoldering.

Replace all tantalum caps, they will fail soon or later.
For electrolytics use some simple ESR meter published here or Chinese component tester. 

[med6753]

For me, most of them. Also one of the coax connectors needed resoldering.

What is your prefer technic to find bad caps?


Best regards

Pio

As was mentioned by others...when it comes to the beaded tantalums just do a mass replace. I just replaced 20+ on a similar 465B. It's worth the effort and you'd be surprised what little issues it fixes. (Ripple on the traces was my fix).