577 Curve Tracer Problem


henasau@...
 

No, R754 didn't shorted through. Part of the resistor touching -30V trace burnt. Its resistance decreased. I think it changed to about 150k.


Buzz Boxx
 

Nice detective work Henry. Are you saying that the resistor shorted through
and then formed a connection with the underlying 30v trace?

I'm just beginning to collect old 575 curve tracers. My second purchase is
a model 575 122C and it is roached with humidity damage....lots of rust and
other oxidation. Aside from reducing conductivity and oxidizing components,
what else can excessive humidity damage/cause?

Thanks,
-Barry
On Jan 9, 2015 6:14 AM, "henasau@... [TekScopes]" <
TekScopes@...> wrote:



I would like to draw attention to some possible nasty problem with 577
Curve Tracer.
Since I bought mine a few months ago, very intermittently, I could hear
arcing noise somewhere at the back of the unit. It lasted only for a
fracture of a second and the 577 continued to work afterwards. I thought
this was in anode supply but couldn't find anything wrong.


A week or so ago, after another arcing incident my 577 stopped working;
+30V was gone and 200V went down to about ~120V.
The C767 tantalum cap got shorted and all three transistors in +30V rail
failed. I replaced all faulty components and....... here we go again,
horrible welding sound returned. This time I could see it with my own eyes.
It appeared the short was somewhere at the back of low voltage PSU board
where power transistors are mounted. All three transistors in +30V rail
failed again.


I couldn't see anything obvious so I decided to clean the board with
alcohol. I was just about to turn it on again when my eyes caught something
suspicious about R754. From the top looked like new but I could see some
debris underneath. When I unsoldered it, I found the resistor was burnt
underneath and the track was evaporated.


To cut the whole story short this resistor connects to +200V rail and the
track underneath is -30V rail. Possible difference of potentials is 230V.
The resistor during the assembly was soldered tight against the board. Over
the years, time, dust and humidity did its magic. This time I soldered new
R754 way above the board.



Henryk



[Non-text portions of this message have been removed]


[Non-text portions of this message have been removed]


Tom Jobe <tomjobe@...>
 

Thank you Henry, for sharing this excellent tip about R754 and its possible problems!
I'm going to have a look at my 577 before it does the same thing.
tom jobe...

On 1/9/2015 4:14 AM, henasau@... [TekScopes] wrote:
I would like to draw attention to some possible nasty problem with 577 Curve Tracer.
Since I bought mine a few months ago, very intermittently, I could hear arcing noise somewhere at the back of the unit. It lasted only for a fracture of a second and the 577 continued to work afterwards. I thought this was in anode supply but couldn't find anything wrong.


A week or so ago, after another arcing incident my 577 stopped working; +30V was gone and 200V went down to about ~120V.
The C767 tantalum cap got shorted and all three transistors in +30V rail failed. I replaced all faulty components and....... here we go again, horrible welding sound returned. This time I could see it with my own eyes. It appeared the short was somewhere at the back of low voltage PSU board where power transistors are mounted. All three transistors in +30V rail failed again.


I couldn't see anything obvious so I decided to clean the board with alcohol. I was just about to turn it on again when my eyes caught something suspicious about R754. From the top looked like new but I could see some debris underneath. When I unsoldered it, I found the resistor was burnt underneath and the track was evaporated.


To cut the whole story short this resistor connects to +200V rail and the track underneath is -30V rail. Possible difference of potentials is 230V. The resistor during the assembly was soldered tight against the board. Over the years, time, dust and humidity did its magic. This time I soldered new R754 way above the board.



Henryk


henasau@...
 

I would like to draw attention to some possible nasty problem with 577 Curve Tracer.
Since I bought mine a few months ago, very intermittently, I could hear arcing noise somewhere at the back of the unit. It lasted only for a fracture of a second and the 577 continued to work afterwards. I thought this was in anode supply but couldn't find anything wrong.


A week or so ago, after another arcing incident my 577 stopped working; +30V was gone and 200V went down to about ~120V.
The C767 tantalum cap got shorted and all three transistors in +30V rail failed. I replaced all faulty components and....... here we go again, horrible welding sound returned. This time I could see it with my own eyes. It appeared the short was somewhere at the back of low voltage PSU board where power transistors are mounted. All three transistors in +30V rail failed again.


I couldn't see anything obvious so I decided to clean the board with alcohol. I was just about to turn it on again when my eyes caught something suspicious about R754. From the top looked like new but I could see some debris underneath. When I unsoldered it, I found the resistor was burnt underneath and the track was evaporated.


To cut the whole story short this resistor connects to +200V rail and the track underneath is -30V rail. Possible difference of potentials is 230V. The resistor during the assembly was soldered tight against the board. Over the years, time, dust and humidity did its magic. This time I soldered new R754 way above the board.



Henryk