Problematic Fault-Finding

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Greetings, gentlemen,

This should have been an easy fix given everything I have on my side,  
however it's been anything but. I'm about all out of ideas as to how to  
proceed.

In this section of a board there's a fault:

https://www.dropbox.com/s/o4tybr81cefbk5n/8565A_amp.GIF?dl=1

The fault is somewhere in the "voltage to current input amplifier" part.  
This is the x-amplifier board that generates the horizontal sweep (a  
"sawtooth") for the x plates of the CRO display of an RF spectrum  
analyser. Normally I would simply probe from one stage to the next until  
the waveform vanished and then I'd know which stage was at fault. I can't  
do that with this circuit due to two big obstacles:

The stages are all directly coupled.
They're in vertical pairs with shared supplies.

This makes them all *interdependent* on each other. Wherever the fault  
lies is impossible to isolate because owing to this interdependence, ALL  
6 transistors are showing 'impossible' DC voltage readings such that none  
of them can be expected to function properly and pass a signal on to the  
current-to-voltage section.

Here's another real kicker: the instrument uses a *Y* amplifier board  
*identical* to the X one in every way. They're interchangeable. So I've  
tried doing comparative passive resistance checks between the two boards  
expecting to find the faulty area that way, but even the resistance  
values on the faulty board are all over the place as well! I would have  
thought having an identical board to compare with would have saved my  
arse, but it seems not!

Any suggestions? I don't really want to pull individual transistors at  
random for out-of-circuit testing on a 40 year old board if I can avoid  
it.



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Re: Problematic Fault-Finding
On Fri, 18 Oct 2019 14:48:36 +0000, Cursitor Doom wrote:

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How about.... power the board up and hold a thermocouple up against each  
of those transistors in turn looking for one that's got a higher or lower  
temp than the others?
Or..... measure the voltages on the pins of each of those 6 transistors.  
Determine the transistor with the least out-of-whack voltages then pull  
the one _furthest_ away from it. So for example say Q13 is closest to  
normal expected values..... then pull Q10 out for testing.
Best I can come up with. :/





--  

Some mothers do have 'em!

Re: Problematic Fault-Finding
Cursitor Doom wrote:

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** Very funny ...  

  
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** Have you checked the junctions of each transistor with an analogue  multimeter on the ohms X1 range?  

Sometimes you will find one or more with an open B-E junction or higher than normal resistance readings. Was very common with high voltage types in TO92 or TO5 packs from that era.  

I used to pull dozens from Phase Linear amplifiers back in the day.  

Check all the resistors too, not impossible for one to go high or open.  

If any of the transistors show signs of heat stress, they should be your prime suspects.


....   Phil  




Re: Problematic Fault-Finding
On 10/18/19 7:05 PM, Phil Allison wrote:
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*laughs* Maybe a tube tester will help.


--  
"I am a river to my people."
Jeff-1.0
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Re: Problematic Fault-Finding
Fox's Mercantile Lunatic wrote:

---------------------------

Phil Allison wrote:

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** If you have a point - then post it.  

  Make my day.  

  If you have nothing but a brainless snipe

  then go fuck yourself



.....  Phil  


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