Hello, RATS and Radio+phonos! Seasons Greetings!

I recently had the chance to compare two vintage TC-2's on the same
set of half a dozen or so used tubes. One TC-2 was from our company
"museum" and the other just acquired through the good offices of a
fellow Canadian "radio+phono" - I'll call them "Company's TC-2" and
"My TC-2", respectively. They each tested quite differently, by as
much as 20% on the meter depending on type and condition of tube but
all the same way, i.e. my TC-2 indicated all tubes as much worse.

My TC-2 also had a problem of not allowing the Line Test to be set
within the limits of the Set Line knob - you could not turn it down
enough. Our local line voltage is quite high at 122 VAC much of the
time (Heathkit mentions this in the instructions and suggests a extra
resistor in the AC line.) I think it was nearer 125 VAC when I was
doing this late at night.

Further examination of my TC-2 showed the resistors around the "Type"
switch were way off the schematic's values, well out of the supposed
+/- 5% range specified, e.g. the 1500 ohm resistor was about 1750 ohms
and the 820 was 932 ohms. Neither was I impressed with the Se
rectifier used to make the 1 mA meter into a voltmeter for Line Test
(it has no other function.)

So, surgery was resorted to, viz...
1. Fix the high voltage problem with a small voltage bucking
transformer from the spares box.
2. Replace Se rectifier with a Si diode from the spares box and
re-calibrate the Line Test circuit.
3. Fix the resistors around the Type switch.

Below I've pasted the documentation for my records (and kept
with the my TC-2 unit in the box so the next custodian is not too
spooked!)

As a result the two units now test a bit "closer" but still not
identically as the Company's TC-2 "Type" resistors are a bit off,
too - but I'll trust mine now!

Cheers

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada

Heathkit TC-2 Tube Checker

Power Supply
The standard TC-2 mains transformer ratios do not always allow
"line test" meter setting when the AC line voltage is on the high
side, e.g. over 120 VAC. This was made worse (and quite wrong, too)
when I replaced the Selenium rectifier (used only by the meter) with a
silicon diode (see later, this also affects meter calibration.)

To fix this I have add a voltage bucking transformer on the primary
side of the standard mains transformer. I used a small 5 VAC
"wall-wart" plugged into a modified dual three pin receptacle, all
mounted under the panel. I cut the copper piece between the two
sockets on one side only and wired the 5 VAC secondary of the
wall-wart between them (in the correct phase to subtract voltage) -
see schematic below:

[Pencil sketch - not in e-mail. Quite simple: the wall-wart
transformer secondary is in series with the TC-2 AC line with
phase chosen to subtract voltage. Primary is across the supply, of
course.]

To remove this modification, either:
A. remove the wall-wart and restore the unit to original circuit, or,
to make it reusable later,

B. Remove wall-wart secondary (and tape it off safely) then simply
bridge the cut link on the socket.

A useful further modification (not done) would be to wire a SPDT
switch to the link and wall-wart secondary (break before make) to
allow it to be switched in out of the circuit as needed.

Replacing meter Selenium rectifier
Having a spare silicon diode I decide to replace the obsolete Se
rectifier. Doing this caused the meter to read high so I put in a
meter calibration potentiometer (a Helipot.) See circuit below:

[Pencil sketch - not in e-mail. Very simple: the 5K Helipot
simply replaces the 1200 ohm resistor and the slider goes to the
diode. The 75K stays the same.]

To allow the meter to read exactly at the "Line test" point (actually
0.5 mA through the 1 mA FS meter) when the "Set line" knob is
correctly adjusted, proceed as follows:

A. Adjust front panel "set line" pot until the filament voltage is
correct as measured by an accurate external DVM - I used a Fluke model
111. You can use any of the filament switch settings (with no tube in
place, of course) but I interpolated the best readings on the 5, 6.3,
12.6 and 110/115 VAC settings since the tapped transformer windings do
not produce exactly these voltages at the same time, i.e. with a given
primary voltage. For the 110/115 VAC aim at 112.5 VAC.

B. When the "Set line" pot is at the best point, simply adjust the
Helipot so that the meter points exactly at the "Line test" mark (0.5
mA). That's all, meter is now calibrated.

Resistors around the "Type" switch
These were all off value by a significant amount, i.e. most by more
than the alleged 5%. I have used series and parallel connection of
spare resistors to hand to get as close a possible to the specified
values (see schematic for these values). I plan to replace them later
with exact values (820, 1500, etc.) when I get around to it.

_____________________________

Roger Jones. December 2003

"Engineer" wrote in message ...
Hello, RATS and Radio+phonos! Seasons Greetings!

I recently had the chance to compare two vintage TC-2's on the same
set of half a dozen or so used tubes. One TC-2 was from our company
"museum" and the other just acquired through the good offices of a
fellow Canadian "radio+phono" - I'll call them "Company's TC-2" and
"My TC-2", respectively. They each tested quite differently, by as
much as 20% on the meter depending on type and condition of tube but
all the same way, i.e. my TC-2 indicated all tubes as much worse.

My TC-2 also had a problem of not allowing the Line Test to be set
within the limits of the Set Line knob - you could not turn it down
enough. Our local line voltage is quite high at 122 VAC much of the
time (Heathkit mentions this in the instructions and suggests a extra
resistor in the AC line.) I think it was nearer 125 VAC when I was
doing this late at night.

Further examination of my TC-2 showed the resistors around the "Type"
switch were way off the schematic's values, well out of the supposed
+/- 5% range specified, e.g. the 1500 ohm resistor was about 1750 ohms
and the 820 was 932 ohms. Neither was I impressed with the Se
rectifier used to make the 1 mA meter into a voltmeter for Line Test
(it has no other function.)

So, surgery was resorted to, viz...
1. Fix the high voltage problem with a small voltage bucking
transformer from the spares box.
2. Replace Se rectifier with a Si diode from the spares box and
re-calibrate the Line Test circuit.
3. Fix the resistors around the Type switch.

Below I've pasted the documentation for my records (and kept
with the my TC-2 unit in the box so the next custodian is not too
spooked!)

As a result the two units now test a bit "closer" but still not
identically as the Company's TC-2 "Type" resistors are a bit off,
too - but I'll trust mine now!

Cheers

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada

Heathkit TC-2 Tube Checker

Power Supply
The standard TC-2 mains transformer ratios do not always allow
"line test" meter setting when the AC line voltage is on the high
side, e.g. over 120 VAC. This was made worse (and quite wrong, too)
when I replaced the Selenium rectifier (used only by the meter) with a
silicon diode (see later, this also affects meter calibration.)

To fix this I have add a voltage bucking transformer on the primary
side of the standard mains transformer. I used a small 5 VAC
"wall-wart" plugged into a modified dual three pin receptacle, all
mounted under the panel. I cut the copper piece between the two
sockets on one side only and wired the 5 VAC secondary of the
wall-wart between them (in the correct phase to subtract voltage) -
see schematic below:

[Pencil sketch - not in e-mail. Quite simple: the wall-wart
transformer secondary is in series with the TC-2 AC line with
phase chosen to subtract voltage. Primary is across the supply, of
course.]

To remove this modification, either:
A. remove the wall-wart and restore the unit to original circuit, or,
to make it reusable later,

B. Remove wall-wart secondary (and tape it off safely) then simply
bridge the cut link on the socket.

A useful further modification (not done) would be to wire a SPDT
switch to the link and wall-wart secondary (break before make) to
allow it to be switched in out of the circuit as needed.

Replacing meter Selenium rectifier
Having a spare silicon diode I decide to replace the obsolete Se
rectifier. Doing this caused the meter to read high so I put in a
meter calibration potentiometer (a Helipot.) See circuit below:

[Pencil sketch - not in e-mail. Very simple: the 5K Helipot
simply replaces the 1200 ohm resistor and the slider goes to the
diode. The 75K stays the same.]

To allow the meter to read exactly at the "Line test" point (actually
0.5 mA through the 1 mA FS meter) when the "Set line" knob is
correctly adjusted, proceed as follows:

A. Adjust front panel "set line" pot until the filament voltage is
correct as measured by an accurate external DVM - I used a Fluke model
111. You can use any of the filament switch settings (with no tube in
place, of course) but I interpolated the best readings on the 5, 6.3,
12.6 and 110/115 VAC settings since the tapped transformer windings do
not produce exactly these voltages at the same time, i.e. with a given
primary voltage. For the 110/115 VAC aim at 112.5 VAC.

B. When the "Set line" pot is at the best point, simply adjust the
Helipot so that the meter points exactly at the "Line test" mark (0.5
mA). That's all, meter is now calibrated.

Resistors around the "Type" switch
These were all off value by a significant amount, i.e. most by more
than the alleged 5%. I have used series and parallel connection of
spare resistors to hand to get as close a possible to the specified
values (see schematic for these values). I plan to replace them later
with exact values (820, 1500, etc.) when I get around to it.

_____________________________

Roger Jones. December 2003

Just a small point, Roger. Usually the instructions say to adjust the
"line" when the tube IS plugged in. Since the line pot adds some
impedance to the primary of the transformer and degrades it's
regulation, all output voltages are affected by the filament load, so
a 6L6 has significantly more effect than a 6AU6 for example. Thus, one
should check the filament voltage WITH a tube plugged in to calibrate
the 'line' indication on the meter, although it will only be truely
accurate at one filament power. I think I would set it with a
6.3V-300mA filament since that is probably the most common filament
power for radio tubes.
Have you looked at the resistors in the 'company tester' for
comparison? It would be nice to know why there is still a significant
difference between the two. I suppose that is why, in my "Precision
612", they use instrument 'spool' resistors in all the measuring
circuits.

Neil S.

See below in text...

"Neil S." wrote in message
om...
"Engineer" wrote in message
...
Hello, RATS and Radio+phonos! Seasons Greetings!

(snip)

So, surgery was resorted to, viz...
1. Fix the high voltage problem with a small voltage bucking
transformer from the spares box.
2. Replace Se rectifier with a Si diode from the spares box and
re-calibrate the Line Test circuit.
3. Fix the resistors around the Type switch.

(snip)


Just a small point, Roger. Usually the instructions say to adjust
the
"line" when the tube IS plugged in. Since the line pot adds some
impedance to the primary of the transformer and degrades it's
regulation, all output voltages are affected by the filament load,
so
a 6L6 has significantly more effect than a 6AU6 for example.

Yes, I'm aware of that, but it I could still not reliably Line Test
zero it even with a large tube plugged in if the line voltage was over
125 VAC. With the 5 VAC "bucking" it zeros both with and without a
tube in, but at different pot settings, of course.

Thus, one
should check the filament voltage WITH a tube plugged in to
calibrate
the 'line' indication on the meter, although it will only be truely
accurate at one filament power. I think I would set it with a
6.3V-300mA filament since that is probably the most common filament
power for radio tubes.

That's a different matter, relating to setting up the TC-2 for
operation. But changing the diode and recalibrating the meter can be
done with out without a tube in as it just reads the current AC
voltage, loaded or not.

Have you looked at the resistors in the 'company tester' for
comparison? It would be nice to know why there is still a
significant
difference between the two. I suppose that is why, in my "Precision
612", they use instrument 'spool' resistors in all the measuring
circuits.

I checked them and they were also a bit off but not as much as mine -
I'll likely change them out at the same time I do mine for posterity!
Certainly your 612 is superior!

Cheers,

Roger

Neil S.