REFURBISHMENT and VERIFICATION of the HEATHKIT TT-1
TUBE TESTER
Kent Nickerson
Dedicated to the memory of Lloyd Generoux, who introduced me to the wonders of old technology.
The Heathkit TT-1 Assembly and Operating manuals are arranged logically enough
for the original kit builder but since the TT-1 is likely acquired in an aged and already
assembled state, refurbishment and verification become the first issues. It can be
tough to say which of refurbishment or verification comes first. If you are impatient
(like me) to verify and wish to fire up a unit for the first time in decades before
refurbishment, make sure that the proper (2 Amp) fuse is installed before turning it
on. Supply caps in particular can become quite cranky after many idle years.
Disclaimer: Dangerously high voltages (up to a few hundred volts)
exist at accessible points in the Heathkit TT-1 chassis. Only those
competent in high voltage electronics practice should remove the
TT-1 chassis from its case. This document is the author’s personal
reference and report. Effort has been made to check for accuracy
and safety, but ANY READER WHO WISHES TO ADOPT THIS
DOCUMENT AS A GUIDE DOES SO AT THEIR OWN RISK!
PART 1: REFURBISHMENT
Most components in the TT-1, especially capacitors, resistors and contacts, are
subject to age and will require checking or replacing before operation (coils,
transformers and the meter should stand up to age pretty well). A list of relevant
parts is included at the bottom of this document. Borrowing conventional advice for
antique electronics, I did the following:
• Replaced all electrolytic (polarized) capacitors with modern electrolytics, and
paper capacitors (such as the black plastic tubular types) with film or ceramic
types. This meant pretty well all the caps in the TT-1 but fortunately good modern
caps are cheap and easy to find. All the electrolytic caps in the TT-1 (i.e. with
values larger than a few microfarads (uF)) are for supply filtering, so they can be
replaced with caps of at least the original voltage rating and up to twice the
capacitance. I chose high temperature (105 deg. C.) electrolytics for greater
reliability. Most of the other capacitors are for AC coupling, and can be replaced
with caps with at least the original voltage rating and within 25% of the original
value. The exception to this is the 0.22uF calibration cap located behind the
calibration switch: it is a 2% precision type that can be verified in situ with a
capacitance meter if the “Calibration” knob is turned to “Bias” or “Operate”. It
looks like a quality part and checked fine on my TT-1.
The 0.02uF 600V cap sets the AC signal frequency (5kHz nominal, but is 7kHz on
my TT-1), and every 20% change in value will change the frequency about 10%. The
0.1uF cap to the meter will affect calibration somewhat. Small shifts in frequency
and meter response are compensated in calibration, however, so +/-25% is still
plenty accurate for replacing these two components.
• Disconnected the AC capacitor. This AC cap is used as a series element to set
the 300, 450 and 600mA filament supplies, and this relies on a high tolerance
(better than 5%). Not only did the AC cap in my unit triple in value with age
(which would have accordingly tripled intended filament current), but it started to
heat up and bulge in minutes when loaded! Since I see few circuits with current
driven tube filaments, however, running them from an appropriate voltage instead
should be sufficient. If the AC cap must be replaced, the best choice is probably
polystyrene film types which are AC rated for 150 VAC or more (non AC rated caps
may overheat and do ugly things!). These can also be paralleled to yield exact
values, but such caps are not particularly cheap or compact.
• Replaced out of spec carbon resistors. Carbon resistors tend to change over
time toward the high side. Fortunately most are electrically open on at least one
end and can be checked in situ with an ohm-meter. Any resistor found outside its
tolerance (most are 10%) can be replaced with a new resistor of equal power
dissipation rating (metal film types if possible). Be aware that carbon resistors
that initially check fine may increase their value significantly after being heated,
such as when an adjacent component is soldered, so recheck any old resistors
you leave in after you’ve changed parts! Happily, I found that the precision
resistors on my TT-1 were still within their 1% tolerance.
• Replaced diodes. The original CR-101 silicon rectifiers are not very rugged, and I
found a couple of bad ones. Modern 1N4007 diodes are cheap and superior. I
just clipped one end of the original diode away and bridged the leads with the
new part (Note: the red end of the old diode is the cathode!) For purists who
want to leave the old diodes in, a good CR-101 measures about 0.45V with 1mA
forward current (same as typical silicon diodes).
• Cleaned all contacts. Contacts get covered with oxides and gunk over the years.
I sprayed switches and inside potentiometers with contact cleaner, then rotated
them over the full range a few times. An ohmmeter placed between one end of a
pot and the centre wiper terminal will show smoothly changing resistance as the
control is turned if the pot is clean (this is where a needle type meter is great) and
jump around in value if not. Most pots have a strip of carbon inside and shouldn’t
be treated too roughly, but rheostat windings (such as on the LINE SET and
METER pots) can be scrubbed with a Q-Tip and contact cleaner (I removed the
back plate off of the METER control to clean it as well). The slide switches on my
unit were stiff and balky at first, but a few sprays of flux cleaner and contact
cleaner inside, applied over a few days along with some periodic actuation, got
these working very smoothly (mechanically and electrically). Where convenient,
switch contacts should be checked for a very low contact resistance that doesn’t
change when the switch is wiggled sideways. Tube socket contacts were cleaned
with a pipe cleaner (something a bit tougher would have been nice) and contact
cleaner. Any switch or control that remains intermittent after a while should be
replaced, as these can damage a tube under test with transient conditions.
• Replaced tube sockets. In my case, the miniature 7 and 9 pin sockets were worn
to the point of loose, intermittent contact during test. I could not readily find a
replacement for the 9 pin socket, so I replaced the contacts with those removed
from a junk box socket.
As for cosmetics, bolts on the aluminium instrument panel may have a white crust
(“metal mold”), and I’m told that nickel plate does this over time with humidity when in
contact with aluminium. This can be removed with a stiff brush.
Following is a list of components taken from the Heathkit TT-1 Operational Manual:
Qty. Value
Resistors
1 47 ohm ½ watt
1 2.7 Kohm ½ watt
1 4.7 Kohm ½ watt
1 150 Kohm ½ watt
1 18 Megohm ½ watt
1 1.8 Kohm ½ watt
1 100 ohm ½ watt 5%
1 620 ohm ½ watt 5%
1 100 Kohm 1 watt
1 2.0 Kohm 1 watt 5%
1 4.3 Kohm 1 watt 5%
2 3.0 Kohm 1 watt 5%
1 47 Kohm 2 watt
1 10 Kohm precision (1%)
1 200 Kohm precision
1 1.0 Kohm precision
1 225 Kohm precision
1 500 ohm precision
1 710 ohm precision
2 250 ohm precision
1 190 Kohm precision
1 133 Kohm precision
1 2.24 Kohm precision
1 360 ohm 7 watt
2 2.0 Kohm 10 watt
Controls (potentiometers and rheostats)
2 10 Kohm linear (tab mount)
1 10 Kohm linear (screwdriver adjust)
1 7.5 Kohm linear
1 7.5 Kohm línear (semi-precision)
1 350 ohm 25 watt
Capacitors
1 .02 uF 600 volt
2 .5 uF 400 volt
1 .1 uF 400 volt
1 .05 uF 200 volt
1 .22 uF 200 volt 2%
3 20 uF 350 volts
1 40 uF 150 volts
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