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/o4tybr81ce...A_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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On Fri, 18 Oct 2019 14:48:36 +0000, Cursitor Doom wrote:

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.

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!

Cursitor Doom wrote:

---------------------
Greetings, gentlemen,


** Very funny ...


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/o4tybr81ce...A_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.


** 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

On 10/18/19 7:05 PM, Phil Allison wrote:
** Have you checked the junctions of each transistor with
an analogue multimeter on the ohms X1 range?

*laughs* Maybe a tube tester will help.


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

Fox's Mercantile Lunatic wrote:

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

Phil Allison wrote:


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


*laughs* Maybe a tube tester will help.



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

Make my day.

If you have nothing but a brainless snipe

then go **** yourself



...... Phil

On 10/18/19 7:56 PM, Phil Allison wrote:
Fox's Mercantile Lunatic wrote:

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

Phil Allison wrote:


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


*laughs* Maybe a tube tester will help.



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

Make my day.

If you have nothing but a brainless snipe

then go **** yourself



..... Phil


Aw come on Phil, it was a ****ing joke.



--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

Fox's Mercantile wrote:

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

Fox's Mercantile Lunatic wrote:

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

Phil Allison wrote:


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


*laughs* Maybe a tube tester will help.



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

Make my day.

If you have nothing but a brainless snipe

then go **** yourself




Aw come on Phil, it was a ****ing joke.


** Really ???

On whom ?

About what ?

How exactly was it meant to be funny ?

You know my opinion of commercial "tube testers" but an analogue ohmmeter on the X1 range is invaluable as a primary check on PCBs with faulty BJTs - cos it works fine in circuit.

DMMs do not.


..... Phil

On Fri, 18 Oct 2019 20:56:56 -0500, Fox's Mercantile wrote:

Aw come on Phil, it was a ****ing joke.

No it wasn't. It was just plain stupid.

On 19/10/2019 4:46 pm, Chris wrote:
On Fri, 18 Oct 2019 20:56:56 -0500, Fox's Mercantile wrote:

Aw come on Phil, it was a ****ing joke.

No it wasn't. It was just plain stupid.

So given the standard of posts jhere who gives a rats arse ??

On Saturday, 19 October 2019 09:46:58 UTC+1, Chris wrote:
On Fri, 18 Oct 2019 20:56:56 -0500, Fox's Mercantile wrote:

Aw come on Phil, it was a ****ing joke.

No it wasn't. It was just plain stupid.

jokes generally require some content that's amusing.