All fixed!
All fixed!
Faultfinding on a: Dick Smith Video Monitor (Model X-1198)
Just an update to close off this thread. The monitor is now fixed. It had two faults.
Fault 1:
I had indeed correctly identified the wire from the video amp & found the amplifier transistor (Q302
2SD415). Rather ominously the collector voltage was only 15V whereas the good monitor showed 95V. However the base & emitter were 3V higher (8.6V vs 5.6) on the bad unit - was the transistor being driven too hard? Also I considered whether the the B+ (is this correct term for the 95V supply?) was good, or possibly a bad transistor dragging the 95V down. An ohms check of the transistor in-circuit against the good unit (also in-circuit) revealed same readings. Not content I removed the two transistors and rechecked - same readings. I concluded the video amp transistor was fine & reinstalled.
Over the course of 10 hours (spread over 7 days) I pursued the theory that the video amp was being driven too hard as the base signal (on scope) was higher than it should be. This lead me all over the board trying to find the source of this increased signal. Eventually I found myself back at the video amp.
I then suspected perhaps B+ was weak at its source, so went through that circuit. Then removing the video amp transistor, B+ lifted to over 100V. Finally suspecting this transistor maybe internally
shorting, I touched the C & E pins to their connections and B+ fell. I then offered up C & E pins of a
known good video amp transistor - B+ stayed high. Soldering in the new transistor - Bingo - good
picture.
How nasty - the transistor was breaking down only in its high-voltage environment. Jeepers, if only I had swapped the two transistors when I first had it out of the board...
Fault 2:
While the picture was good, I did notice a jitter occasionally (every 5-10 secs or so) and knowing Tezza would like his monitor to be 100%,
I thought I better have a look at that.
With no signal going into the monitor I scoped about. I could see the jitter in the 'base?' signal
everywhere I tested. (I don't know the proper term for 'base' signal - monitor noobie here). I
suspected another transistor breaking down so removed a couple. While these checked out fine, it
enabled me to isolate what area was generating the problem. I found myself at the Input transistor
(Q301 2SC184) with a resistor-capacitor circuit. The capacitor connected to earth, but scoping it on
the earth side produced a strange signal compared to that of the identical (good) monitor. I'd expect a flat line, but the good monitor on that part of the board produced a steady base signal.(Perhaps it was inducing RF into the probe or something). On the other side of this cap (on the bad monitor) was the jitter. I removed the cap & held a substitute on the solder-pads. Rock-steady signal - Bingo! Soldered the new cap (C133) in & perfect picture.
Philip