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Problematic Fault-Finding

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:

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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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Reply to
Cursitor Doom
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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. :/

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Some mothers do have 'em!
Reply to
Al

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** Very funny ...

** 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

Reply to
Phil Allison

*laughs* Maybe a tube tester will help. 
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"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com
Reply to
Fox's Mercantile

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Phil Allis>

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

Make my day.

If you have nothing but a brainless snipe

then go f*ck yourself

..... Phil

Reply to
Phil Allison

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