Customer Repairs
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Two difficult Marconi
repairs
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I bought a faulty model
2432A Marconi Frequency Counter some time ago. It was fairly
cheap, being described as faulty; and as I'd also picked up a
manual for it, I thought it would be a good repair proposition. |
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I'd tackled it twice
previously before settling down to get to grips with it for this
(third) time.
On the first occasion I'd discovered
I hadn't been the first to try and fix it.
Also, the thing was without
its outer case and it had obviously been stored somewhere damp
allowing the circuit board to become tatty-looking and the soldering
to take on a cloudy look.
The power supply seemed to be
faulty. This used a couple of 741 op amps, the connections to
which had been really badly damaged, by the last repairer, with
missing pads and scruffy bits of wire added.
After a couple of fruitless
hours I'd given up.
The second time I'd looked at
it I'd replaced the two op amps and tidied up the soldering.
I'd also fitted a couple of replacement regulator transistors
as these were short-circuit.
I remembered that powering up
produced nothing. The two main power rails, 12 volt and 5 volt
were very low, in fact close to zero volts. Back it went to the
pending pile as a large TV appeared at the door gripped by a
pair of white knuckles.
This third time... I was determined
to make progress.
Powering up produced what I'd
remembered 6 months previously. I looked at the circuit diagram
and discovered that the 12 and 5 volt power supplies were interdependent.
With no 5 volts the 12 supply wasn't regulated. I didn't think
too much of the implications of that at that moment.
I decided to check the 12 volt
rail for resistance to ground. It was around half an ohm. Too
low... no wonder the regulator transistor voltages were odd.
The circuit board is unusual.
It pre-dates plated-through hole technology and instead relies
on metal rivets to connect front and back circuit tracks. The
power tracks were all over the place, ducking and diving throught
the board, first one way then the other, then back again and
so on. Each time using a soldered through-link.
As the 12 volt rail had countless
tracks going different ways to feed different parts of the circuit
I decided to cut the tracks at specific points to isolate the
short.
After half a dozen cuts I eventually
found a small red tantalum capacitor that measured 0.7 ohm. I
removed it and after fitting a modern equivalent, and soldering
across the cut tracks, powered up and saw some red displays illuminated.
A meter across the 12 volt rail
said 13 volts and a measurement across the 5 volt rail showed
4.9 volts. I tweaked the pot and got exactly the 5.1 volts stipulated
in the manual. No separate adjustment for the 12 volt rail is
provided as this seemed to follow the regulation of the 5-volt
rail. This is a serious design fault and I'd like to give the
recalcitrant Marconi engineer a piece of my mind! What was Marconi
QA thinking about when the circuit had been released as being
sound! More later!!
I applied a signal to the low
frequency input socket expecting to see a series of digits appear
on the display... but no.
I switched on my Marconi TF2008
intending to feed a couple of hundred megs into the HF socket.
It wouldn't switch on. I checked its fuse. Open-circuit. I fitted
a new fuse but that didn't help. I groaned and set it to one
side then reverted to the low frequency audio signal and there
was the row of zeros again. |
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Above: The innards of
a TF2008 revealed |
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I scratched my head and
at that moment the display of zeros disappeared to be followed
by a whisp of smoke from somewhere on the main circuit board.
I waited a moment for whatever it was to roast a little then
switched off and felt around the components. A second red tantalum
capacitor had clearly failed as a bead of solder had appeared
on its outer surface and its jolly red colour was now a dirty-looking
brown.
Why had it failed?
To cut a long story short. The
metal through-links had developed dry joints. One of the links
in the 5-volt feed was clearly worse than others and was now
open circuit. A quick measurement of the 12-volt rail revealed
the reason for the capacitor failure. The 12-volt rail, without
the aid of the 5 volt rail, was sitting close to the full 16
volts of the regulator input. The idea of setting the 5.1 volts
and the 12 volts via a single potentiometer was not a good idea
as the lower the voltage on the 5-volt rail, the higher that
on the 12-volt rail. Minimum on one and the maximum on the other!
Tanatalum capacitors are pretty fickle things and if rated at
16 volts maximum, that's the maximum... 16.5 volts and failing
capacitors! I resoldered all the through-links in sight, replaced
the second faulty tantalum capacitor and switched on again.
I was rewarded by a good 5 volt
rail and a good 12 volt rail. Unfortunately I was still reading
either zeros or just a decimal point.
I set the counter on one side
and pulled down the Marconi signal generator.
The TF2008 is an interesting
beast. Inside there are maybe 50 or 60 metal boxes carrying bits
of circuitry. Some of the boxes are fitted inside other boxes
and carried on large hinged metal structures to allow servicing.
A quick check revealed a short on the 12 volt power supply rail.
Funny I thought... I've just been here. The power supply and
some of the DC control equipment is fitted on open circuit boards,
but of course these proved blameless.
A couple of hours later, and
after opening and testing countless boxes I'd got to the box
with the faulty component. Yes... you've proabably guessed...
a small tantalum capacitor.
This time the rail was sound.
It hadn't risen to a voltage higher than the capacitor rating.
In fact the latter was 35 volts so plenty in hand here. Just
an end-of-life failure I suppose? I fitted a modern electrolytic
and reassembled everything. A couple more hours and I'd discovered
I'd forgotten to resolder a wire in the heart of the thing! Another
30 minutes this was remedied and all was well.
I applied a hundred megs or
so to the counter but was rewarded by nothing other than a decimal
point.
There ends the story until another
period of slackness arrives.... |
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