Re: Curve Tracer 576 - current consumption


Bob Koller <testtech@...>
 

Hi Matthias,

It sounds like you have changed all the caps I indicated. I have worked on and sold dozens (literally) of 576's over the years, and only recall this problem once. You may be on to something with C863. It is a tantalum, and looks like it would control the startup. Also, check the film resistors connected to any significant voltage, like R864. These are known to fail when connected to higher voltage. When it does work, does it regulate? Also, did you measure the AC ripple on the LV supplies?


From: Dr. Matthias Weisser
To: TekScopes@...
Sent: Thursday, May 3, 2012 12:29 AM
Subject: Re: [TekScopes] Re: Curve Tracer 576 - current consumption

 
Hi Bob,

I am not sure what will happen changing the transistor.

I changed the 100uF 150V cap, which did not help. What helped
was adding just another 150uF cap. The voltage rise then
would be less steep and this helps. I am still not sure about
those 22uF caps on the board. May be I should have a
look at them. They seem to be used for a start up condition?

The transformer is a black silicone one, no brown semi
hard material. I changed the top main filter cap but found it
looked ok. Just a higher ESR. The others are still original
except for the 4 output types that I changed. The values
that I used for this are now higher than what was there. I did
not have 5uF and used 10uF instead. I also did not have 15uF
and used a 33uF instead. For the 39uF I used 47uF.

I also did change the 100uF filter cap near the HV board with
a lower esr type. Which bypass caps do you mean?

Matthias

Am 02.05.2012 23:39, schrieb Bob Koller:
 
You can try a new Q851, but it may not solve the problem. I recall seeing this once before on an instrument in for repair. I think it has to do with the HV transformer. There is something not quite right, and if the supplies come up in a way that the HV circuit doesn't like, the oscillator doesn't start properly, and overloads the 100V supply. From your serial number, the unit was made with the black silicone potted HV transformer. The early type transformer was potted in brown semi-hard something, I think urethane. These all failed, with various failure modes. If your trans is brown, someone replaced it, this is then, the problem. As I recall, I ended up replacing the entire HV module, as I could not get the offending one to work reliably. You might also check all the main filter caps for ESR, and replace the four output caps, also check the bypass cap on the HV module, these do go bad, and may cause problems.

Keep us posted!

Bob



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