Tektronix 565 repair

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** Surge tolerant resistors rely on thermal inertia of the resistance material - so there is not much change in conductor temp during a short current surge.

Wire wound types are the obvious choice, though solid composition may do as well if you can find any.

If you have been blowing thick film resistors then try the same rating in WW.

.... Phil

Thanks. These things make a pop and a little hole.

Greg

Yes, that definitely sounds like a film resistor.

Jon

You might want to measure the capacitance of the first filters off the rectifiers. Tolerance on lytics allow for much higher than rated values and it can cause seriously higher (or longer duration) inrush current.

And 10 ohms seems a bit high. Are you sure that is the correct value ? Sometime there are mistakes on the print or parts list.

Today I was measuring voltage(I) across each of 3 resistors by quickly flicking variac up. I expected a big spike, but not much to look at, or blow a resistor.

Greg

Pretty sure they are 10 ohms. I have already replaced a couple caps with slightly higher values. I don't have an easy way to measure high uf. The 3 main caps are 400 uf 250 volts. Two separate transformer windings.

Greg

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** No way you will ever create a significant inrush surge by doing that.

.... Phil

Thought not, but I expected to see a lot more. It's not immediate break down. It takes 1-3 seconds. I'm still inspecting parts down the line.

Greg

g

If you are talking about R 660 and 680 I would be suspicious of C 661 and 6

1. There is a slight possibility that one of them, maybe more likely 661 h as a weird failure mode in that there is leakage at zero volts which is "bl own" clear once the voltage gets to it. It is a bit rare but at the age of that thing you can get failure modes that are not usual, as with anything.

Looking at the circuit I see nothing else that could be the problem. And as for them being the wrong type of resistors, why did the originals fail in the first place ? i don't know about you but I haven't replaced many resist ors in that place in a circuit without finding some sort of a short.

The direct, unregulated supplies go through R 677 and 697 which limits curr ent to where it should not blow your inrush limiters even if all four outpu ts are shorted. That would be 500 ohms and being much higher than 10 ohms t hose resistors would blow first.

The only other thing hanging off those supplies are tube plates, and you ca n eliminate them as a possibility by simply removing V 667, 677 and 697. Ev erything else is to high value resistors and no matter what the possibility of those shorting out is so remote it isn't even worth consideration.

The possibility or the wrong replacement resistors being the trouble while remote, is a possibility, but only if the replacements are smaller in size and therefore have less thermal mass. Some newer resistors can take higher temperature thus being able to dissipate more power when they are physicall y smaller. That situation might be ameliorated by using with 2 5 ohms in se ries or 2 20 ohms in parallel.

In theory raising the value should not work as overall dissipation will be higher, and lowering the value which might save the resistors will put more stress on the input filters and/or the rectifiers.

So the first thing I would do is to disconnect the original filters and wir e in some 470 uF @ 250 volts and see what happens.

Worth a shot I would say.

Wait, it is blowing all three ? Hmm, that stickies up the wicket now. That makes it more likely that you will have to go with the dual resistors in each place.

Much more likely, to the point where I wish I could edit the other post. That is now the most likely.

It might be alright to go with higher wattage resistors, it still has a fuse.

In this case the only other possibility is the transformer is putting out too much current and that, well, think you can win the lottery two weeks consecutively in a row ? (playing only one number each time)

I forget the sequence, think one for the 420 volts went first, then later after I replaced a carbon, that was off ohms, then it too has blown for the

100 volts section. Both upper 400uf caps replaced with 470s. I will be changing the other 200 volt section shortly. Everything goes to resistors except tubes. I can do process of elimination. They don't all blow out at once. Usually just one, but another may go off tolerance. I'll do more checking. The 106 voltage reading low with some kind of blip. I got some bigger wirewounds and enough other metal oxides to turn 4 into one.

Greg

The original caps are still in machine just disconnect. I was only able to get ESR check, and that seemed low of about .1

Greg

Just meandering thought here, what is the actual line voltage ? You know these old things were made to work on 110, not 125.

And as usual you are wrong. Per the manual: The line voltage can be selected from 110, 117, 124, 220, 234 or

248 vac. 50-60 cycles.

--
"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com

120 normal here, and 125 up in building. I'll check that. I just saw a 500 vdc on power supply diagram. Not sure where that came from.

Thanks Gregj

500 was changed to 300 vdc in my schematic.

Greg

Been extra cautious with this thing. Been checking everything. Replaced a VR tube, and moved a lead from VR adjust pot was too close to chassis. Replaced all 3 10 ohm resistors with wire wounds. Put current sensing LEDs with drops across each. With hard turn on, the one has a pretty huge current spike. I think that one was the original problem, pretty sure at top of 420 volt section. Failure of it may have taken out one on lower supply, guessing.

This scope may be replacing one in research setup, that uses external connections, and an extra mod for a sawtooth output.

Years ago, another 565 from the same lab was mounted at the top of a rack with wheels. I was pulling out a big strip chart recorder near the bottom. The rack started tilting forward, and strip chart extending. I decided to move out bumping into table, and loud crash. Was not pretty. They kept using that wheeled rack, but put lead weights in bottom.

Greg