Test Equipment
If you're going to collect and/or repair vintage equipment then you
need the appropriate test gear. The photo below shows my test bench during
alignment of the Eddystone 770R . Some of the individual equipment is discussed
later. The equipment required need not be expensive or modern, often the
test gear produced around the time the equipment under repair was made
is the most appropriate; possibly backed up with a modern DVM, oscilloscope
and frequency counter.
I have a lot of Heathkit equipment, it is cheaply obtained at rallies
and was originally aimed at the hobbyist market so normally has all the
features required. It is also relatively easy to upgrade/modify. Two things
to look out for with British Heathkit are the extensive use of (leaky)
Hunts capacitors and transformers that are wound for 220V or less. I have
fitted many of my units with additional wirewound resistors to drop 240V
down to 220V or so, bringing the LT down to 6.3V, in some units it has
been as high as 7V.
Heathkits were, as the name suggests, mainly sold as kits so the build
quality is variable. They did however come with excellent manuals so most
units should be reasonable. The British arm of Heathkit was known as Daystrom
and based in Gloucester for many years. The heyday of kit building was
in the sixties, with the arrival of the 70's Heathkit's core markets of
Hi-Fi, test equipment and Ham radio were under attack from the far east.
This, coinciding with the widescale adoption of transistors and PCB based
construction, meant that kit building no longer offered any real money
saving.
British Heathkit test equipment seems to have had four different styling
phases which can be used for approximate dating: The earliest equipment
from about 1958 onwards is shiny grey with white text (see AV-3U), then
in the midi sixties came grey and silver (see IM-13U, in my opinion these
units look the most modern/professional), in the late sixties the finish
changed to brown with black lettering (see IM-58U) and finally the knob
styling changed (see IM-48).
Return to Index.
Heathkit AV-3U AC millivolt meter.
This little AC millivoltmeter was the first piece of Heathkit gear I purchased.
Its minimum sensitivity of 10mV FSD and its frequency response up to 400kHz
make it ideal for audio testing. The damping of the meter movement
is just right too. As bought the lowest range (10mV) wasn't much use due
to high levels of background noise. Changing the rectifier to a bridge
and fitting an extra stage of smoothing resulted in a meter which reads
0mV with no input. A BNC socket has been added for easier connection to
screened leads.
Status: Working
Restoration Problems: Leaky capacitors.
View Heathkit Data Sheet (156k JPG)
Heathkit IM-13U Vacuum tube voltmeter.
On the face of it the vacuum tube voltmeter would seem to have been made
obsolescent by the modern DVM, however it still has some advantages: The
1M resistor in the DC probe means that relatively high impedance circuits
can be probed without disturbing the circuit under test - this can be useful
when probing around valves etc on powered radios, it was particularly useful
on the Volksempfänger. The Ohms ranges also go up to 1G! The AC accuracy
is however poor. A slight oddity of the design is that although it is mains
powered the Ohms ranges still require a battery!
Status: Working
Restoration Problems: Leaky Capacitors
View Heathkit Data Sheet Page1(183k JPG)
Page2(245k JPG)
Heathkit IM-58U Harmonic Distortion Meter.
If you're serious about audio then at some point you'll need a Harmonic
distortion meter. It took me some time to find one of these. The minimum
FSD is 1% meaning easy readings of levels down to 0.1%. Main problem in
using one of these is that it also requires a low distortion oscillator
to feed the amplifier under test. A BNC input socket has been added for
convenience.
The meter uses a Wein bridge filter network to notch out the applied
fundamental leaving only the residue distortion products.
Status: Working
Restoration Problems: Scratchy pots:
Heathkit AG-9U Audio Generator.
Ideal compliment to the above, output distortion in the order of 0.05%,
frequency range 10Hz - 100kHz. Also features very versatile output attenuator
with meter, output can be adjusted from about 1mV to 10V RMS - a useful
piece of equipment.
Status: Working
Restoration Problems: Out of tolerance capacitors.
View 1962 Hi-Fi News AG9-U review (323k JPG)
View Heathkit Data Sheet Page 1(165k JPG)
Page 2(137k JPG)
Heathkit IM-48 Audio Analyser.
This is a rarer unit combining three functions in one box, namely AC Millivolt
meter, IM distortion meter and AF power meter with built-in load resistors
rated at 25W. This particular example seems to be American built for the
European market (note no U for UK in the model number). I have never actually
used the IM distortion meter, but the power meter is useful. For a millivolt
meter I prefer the AV-3U. Heathkit also made a standalone AF power meter
(the AW-1U), but the example I have is tatty and only useful for spares.
Status: Working
Restoration Problems: Weak valve in meter circuit.
Heathkit RF-1U RF Signal Generator.
This RF signal generator covers a wide frequency range from 100KHz to 100MHz
(200MHz on harmonics). Unfortunately it also shows the limitations of the
kit approach since screening between sections is poor and the attenuator
ineffective at higher frequencies. The 30MHz - 100 MHz span is covered
by a dedicated oscillator to avoid the strays that would be introduced
by switching. I wonder if this was added to an existing 30MHz design and
the stretch didn't really come off. Note also that the output is not metered
in any way, so no use for applications requiring a constant O/P over a
range of frequencies.
The modulator circuit in this unit is unusual and seems to result in
an uneven modulation envelope.
Status: Working
Restoration problems: Burnt out resistors in attenuator (a common problem
in RF generators, normally caused by them being connected to IF strip anodes
without a DC blocking capacitor).
View Heathkit Data Sheet Page1(100k JPG) Page2(153k
JPG)
Heathkit C-3U R-C Bridge.
This unit is rather useful for testing capacitors for leakage as well as
measuring R and C values. The latter functions are easily provided these
days by a DVM and a capacitance meter, but no modern equipment provides
for versatile leakage testing. This unit can apply a test voltage from
5V to 450V, any leakage is indicated by closure of the magic eye. The magic
eye is also used to show when the bridge is in balance during component
measurement. These units seem comparatively rare, I think firstly because
they still offer a useful function and secondly because of their use of
the rare EM34 magic eye: often they are bought simply to be stripped for
this part. Note that the bridge applies about 50V RMS to the device under
test and so is not useful for measuring modern low voltage components.
This unit is fairly pessimistic on leakage due the high input impedance
of the magic eye circuitry.
Status: Working
Restoration Problems: Leaky capacitors, slightly weak rectifier.
Heathkit IP-20U Power Supply.
This unit is a transistorised 0-50V 1.5A power supply. Designed in the
early 60's this unit gets around the lack of then available power transistors
with high Vce ratings by spliting its output range into 10 separate ranges
of 5 volts, eg 0-5, 5-10 etc. Each range is driven from its own tapping
on the transformer ensuring that the series pass regulator transistor never
sees excessive voltage. Variable current limiting is also offered, and
uses its own separate series switching transistor. The voltage reference
in this unit is a beefy Zener diode, but the supply to the diode is pre-regulated
by an OA2 style neon regulator tube, making this unit not entirely solid
state!
When obtained the unit wouldn't work properly with outputs above about
25V. This was found to be due to a previous owner having replaced the comparison
transistor (which does see the full output voltage), with something
not having high enough VCE rating. The main output transistor had also
failed (possibly due to poor insulation of the heatsink from the chassis).
In light of these discoveries I decided to replace all the Germanium transistors
responsible for voltage regulation with modern Silicon devices. This cured
all the problems at a stroke and improved the load regulation performance
as well, the voltage adjustment pot also required some contact cleaner;
which cured slightly unstable output voltage.
Status: Working
Restoration Problems: Faulty transistors
Marconi Instruments TF913 FM Receiver Tester.
This unit is described as an FM receiver tester and contains an AM/FM generator
+ an AF power meter in one box. It was purchased to help me line up the
Eddystone 770R. Frequency range is from 20 MHz to 168 MHz and the output
is metered. It also contains a crystal reference for accurate setting of
output frequency. This unit dates from about 1952 and is built to a high
standard incorporating plenty of screening.
Status: Working
Restoration Problems: Leaky capacitors, dirty contacts.
Taylor 62A AM-FM signal generator.
This unit doesn't actually belong to me, but was lent by a friend to help
with the 770R line-up (thanks Terry!). It is also an AM/FM generator, maximum
output frequency is 120MHz. On range 1 of the 770R this was a problem since
its 165MHz could only be reached on harmonics - it was very easy to set
the local oscillator below rather than above the tuned frequency. This
unit also can produce sweeps in conjunction with an oscilloscope, this
is very useful for lining up FM discriminators. Up to three reference crystals
can be fitted for accurate frequency setting.
Status: Working
Restoration Problems: Burnt out resistors in output attenuator.
AVO MKII Valve Characteristic Meter
A lot of frustration and unnecessary expense can be saved with a valve
tester and the series made by AVO are probably the best known in Britain.
Units of this type use AC voltages to simulate the results that would be
obtained with ordinary DC operation. This means that variable DC regulated
power supplies are not required and so cost and size can be reduced.
If buying a unit ensure it has the settings book in the tray underneath.
The earlier two panel units are not so useful because they have fixed
grid voltage making it difficult to match results with valve manufacturers
data for 'modern' valve types (although they can still be tested).
The later units (such as the MKII in the picture) do allow valves to
be tested against manufacturers data. This is useful as the supplied settings
books sometimes contain errors, even on common valve types.
Status: Working
BC-221M Frequency Meter
These frequency meters are a fairly common sight at rallies, but often
not in this good condition. This particular unit was made by Bendix in
the USA. Originally battery powered it has had a military standard regulated
PSU added at some point. Calibration markings seem to show that the unit
was last in regular use in 1981 - not bad for a unit built around 1944!
This particular unit was one of those re-calibrated by the British government
in the early sixties. The new book dial settings are printed onto photographic
paper and stuck over the old readings.
Status: Working: Comparisons with a frequency meter show a very high
level of accuracy.
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