Author Topic: Tektronix 2456B without beam  (Read 1494 times)

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Offline RidghTopic starter

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Tektronix 2456B without beam
« on: July 01, 2015, 10:37:36 am »
Hello ALL!
I need - as far as I know - a major help and assistance:
I've bought 2 scopes: 2465A and 2465B. I've tested them prior purchasing for about half an hour and they worked perfect.
After a while, on the 2465B, I've observed that the trace and readout are dimming after approx. half hour up to disappearing completely. I found a friend that told me to change the capacitors on PSU and I did it.
The new capacitors are on specs, but maybe it was mentioned on this forum that 2 capacitors are interchanged on Layout. After this "incident", I got no trace or readout.
The 'scope does several tests (the 4 channels are tested, and at "ADD" it remains stuck). After pressing A/B Trig, the panel lamps are working and responding, there are "clicks" when they should be (compared to the A version, but nothing appears on CRT.
Just after several hours it is disconnected (5-24 hours), the beam appeared, without readout.
I understood that A5 PCB has a problem, and indeed I've found several leakages from electrolytic caps. There was a lot of "salt" on the solder, and as well a 10k resistor (0.1%) burnt. I exchanged them with what I had on my lab and ordered new ones that are supposed to arrive within 2-3 days. All the voltages at J119 are as specified on the service manual. The DAC shows me that I have inputs as they should be, the microprocessor pin 40 is high (it should be), and I have a good stable clock at pin 39. The DAC has normal measurements at pin 16 and 19.
The U800 is not too hot (approx. 50*C after 20 minutes).
These are the details so far. From time to time, (long time...) the 'scope gets a beam, but it surf down the screen up to ~70%. It can be moved up or down, right and left. However, if I turn it off even for a second, I can see nothing again on screen.
The recall button works, I can recall the saved configurations. The beam finder doesn't work.
I'll be more than thankful if any of the members from this forum may help or get an idea for what I am supposed to look for further.
This is the summary, and I have no idea  what to do further...
Thanks in advance,
Ridgh 
 

Offline poot36

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  • Posts: 678
  • Country: ca
Re: Tektronix 2456B without beam
« Reply #1 on: July 03, 2015, 12:29:02 am »
I would try reseating all of the connectors in the scope and check that you are getting the proper voltages to the crt.  It could be the blanking circuit is not working correctly and is always blanking the screen.
 


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