Tektronix 565 repair

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
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

Re: Tektronix 565 repair
GS wrote:

--------
Quoted text here. Click to load it

** 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


Re: Tektronix 565 repair
Quoted text here. Click to load it

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

Greg

Re: Tektronix 565 repair
gregz wrote:


Quoted text here. Click to load it
Yes, that definitely sounds like a film resistor.

Jon

Re: Tektronix 565 repair
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.  

Re: Tektronix 565 repair
Quoted text here. Click to load it

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

Re: Tektronix 565 repair
Quoted text here. Click to load it

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

Re: Tektronix 565 repair
GS wrote:

----------
Quoted text here. Click to load it


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



....  Phil  



Re: Tektronix 565 repair
Quoted text here. Click to load it

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

Re: Tektronix 565 repair
On Monday, March 12, 2018 at 10:53:38 PM UTC-5, GS wrote:
Quoted text here. Click to load it
g

If you are talking about R 660 and 680 I would be suspicious of C 661 and 6
81. 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.

Re: Tektronix 565 repair
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)

Re: Tektronix 565 repair
Quoted text here. Click to load it

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

Re: Tektronix 565 repair
Quoted text here. Click to load it

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

Re: Tektronix 565 repair
Just meandering thought here, what is the actual line voltage ? You know these old things were made to work on 110, not 125.  

Re: Tektronix 565 repair
On 3/21/18 10:45 PM, snipped-for-privacy@gmail.com wrote:
Quoted text here. Click to load it

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.



--  
"I am a river to my people."
Jeff-1.0
We've slightly trimmed the long signature. Click to see the full one.
Re: Tektronix 565 repair
Quoted text here. Click to load it


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

Re: Tektronix 565 repair
Quoted text here. Click to load it

500 was changed to 300 vdc in my schematic.

Greg

Re: Tektronix 565 repair
Quoted text here. Click to load it

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

Site Timeline