Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Tektronix 565 repair

Been playing with this scope for some time. Original problem, blowing 10
ohm 2 watt surge resistors in supply. Replaced a couple caps so far, not
sure if they had problems. I can bring up on variac and run, no problems.
200 ma. Draw on each of 3 resistors. I still burn resistors on full power
up. Not sure if replacement resistors are suitable. Thinking now of upping
power from 2 watt types.

Greg
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GS wrote:

--------

Been playing with this scope for some time. Original problem, blowing 10
ohm 2 watt surge resistors in supply. Replaced a couple caps so far, not
sure if they had problems. I can bring up on variac and run, no problems.
200 ma. Draw on each of 3 resistors. I still burn resistors on full power
up. Not sure if replacement resistors are suitable. Thinking now of upping
power from 2 watt types.



** 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

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Phil Allison wrote:
GS wrote:

--------

Been playing with this scope for some time. Original problem, blowing 10
ohm 2 watt surge resistors in supply. Replaced a couple caps so far, not
sure if they had problems. I can bring up on variac and run, no problems.
200 ma. Draw on each of 3 resistors. I still burn resistors on full power
up. Not sure if replacement resistors are suitable. Thinking now of upping
power from 2 watt types.



** 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
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Default Tektronix 565 repair

gregz wrote:


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

Yes, that definitely sounds like a film resistor.

Jon
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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.


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gregz wrote:
Been playing with this scope for some time. Original problem, blowing 10
ohm 2 watt surge resistors in supply. Replaced a couple caps so far, not
sure if they had problems. I can bring up on variac and run, no problems.
200 ma. Draw on each of 3 resistors. I still burn resistors on full power
up. Not sure if replacement resistors are suitable. Thinking now of upping
power from 2 watt types.

Greg


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
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wrote:
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.


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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GS wrote:

----------

Today I was measuring voltage(I) across each of 3 resistors by quickly
flicking variac up.



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



..... Phil


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Phil Allison wrote:
GS wrote:

----------

Today I was measuring voltage(I) across each of 3 resistors by quickly
flicking variac up.



** 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
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On Monday, March 12, 2018 at 10:53:38 PM UTC-5, GS wrote:
Been playing with this scope for some time. Original problem, blowing 10
ohm 2 watt surge resistors in supply. Replaced a couple caps so far, not
sure if they had problems. I can bring up on variac and run, no problems.
200 ma. Draw on each of 3 resistors. I still burn resistors on full power
up. Not sure if replacement resistors are suitable. Thinking now of upping
power from 2 watt types.

Greg


If you are talking about R 660 and 680 I would be suspicious of C 661 and 681. There is a slight possibility that one of them, maybe more likely 661 has a weird failure mode in that there is leakage at zero volts which is "blown" 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 resistors 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 current to where it should not blow your inrush limiters even if all four outputs are shorted. That would be 500 ohms and being much higher than 10 ohms those resistors would blow first.

The only other thing hanging off those supplies are tube plates, and you can eliminate them as a possibility by simply removing V 667, 677 and 697. Everything 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 physically smaller. That situation might be ameliorated by using with 2 5 ohms in series 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 wire in some 470 uF @ 250 volts and see what happens.

Worth a shot I would say.


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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)
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wrote:
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
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gregz wrote:
wrote:
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
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Just meandering thought here, what is the actual line voltage ? You know these old things were made to work on 110, not 125.
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gregz wrote:
gregz wrote:
Fox's Mercantile wrote:
On 3/21/18 10:45 PM, wrote:
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:
http://w140.com/tek_565.pdf
The line voltage can be selected from 110, 117, 124, 220, 234 or
248 vac. 50-60 cycles.




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
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