# Fixing Heathkit Oscilloscope

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#### Ivan Vegvary

Jan 1, 1970
0
Picked up the above for $10. Fuse was blown. Don't have a 125V 1A fuse but used 125V 1/2 amp fuse instead. Front indicator light went on and 30 seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps blowing, where would be a good place - method - to start diagnosis? I did get a manual and schematic with the scope. I realize that I could probably buy an equivalent "working" scope for about$ 50, but I would like to get this one going. Tools available are VOM, VTVM and one hand safely in the pocket.

Thanks, Ivan Vegvary

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#### Phil Allison

Jan 1, 1970
0
"Ivan Vegvary"

Picked up the above for $10. Fuse was blown. Don't have a 125V 1A fuse but used 125V 1/2 amp fuse instead. Front indicator light went on and 30 seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps blowing, where would be a good place - method - to start diagnosis? I did get a manual and schematic with the scope. ** But we did not. What model number is it ?? I realize that I could probably buy an equivalent "working" scope for about$ 50, but I would like to get this one going. Tools available are VOM, VTVM
and one hand safely in the pocket.

** You need a scope to fix a scope.

..... Phil

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#### ehsjr

Jan 1, 1970
0
Picked up the above for $10. Fuse was blown. Don't have a 125V 1A fuse but used 125V 1/2 amp fuse instead. Front indicator light went on and 30 seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps blowing, where would be a good place - method - to start diagnosis? I did get a manual and schematic with the scope. I realize that I could probably buy an equivalent "working" scope for about$ 50, but I would like to get this one going. Tools available are VOM, VTVM and one hand safely in the pocket.

Thanks, Ivan Vegvary

Well, based on the available information, it's guesswork.
That said, check the electrolytic capacitor(s) in the DC supply
with your VOM set to measure resistance. If you don't know what
you should see on the meter, get a new 'lytic of the same
value and measure it to get an idea. You want to leave the
meter connected for a while to watch the measurement climb
up toward infinity. 'Lytics often fail with age and can
show leakage on that test.

Ed

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#### Michael Black

Jan 1, 1970
0
Picked up the above for $10. Fuse was blown. Don't have a 125V 1A fuse but used 125V 1/2 amp fuse instead. Front indicator light went on and 30 seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps blowing, where would be a good place - method - to start diagnosis? I did get a manual and schematic with the scope. I realize that I could probably buy an equivalent "working" scope for about$ 50, but I would like to get this one going. Tools available are
VOM, VTVM and one hand safely in the pocket.

Thanks, Ivan Vegvary
People have said in the past that the transformers on the Heathkits were
pretty cheap, and did often burn out in the long run. I have no
experience, but I can imagine that. The problem being of getting a
replacement that not only has the needed windings, but fits the sapce.

SO that's something to look into.

Michael

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#### Michael Black

Jan 1, 1970
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Not always. I've fixed several with just a VTVM and a calibrated
eyeball.
My Tektronix from 1959 (the model came out that year, don't know the date
mine was manufactured) developed ripple in the trace. So I opened up the
case, and put the probe on each of the filter capacitors in the power
supply. At one point, the ripple is worse, so that's the bad capacitor.
I was surprised (this was about 20 years ago) that the local "surplus"
place had a capacitor of the right value, since it was higher capacitance
than the average tube-era power supply filter capacitor. I put in the new
capacitor and the trace was nice and flat like it had been before.

Michael

F

#### Fred Abse

Jan 1, 1970
0
Picked up the above for $10. Fuse was blown. Don't have a 125V 1A fuse but used 125V 1/2 amp fuse instead. Front indicator light went on and 30 seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps blowing, where would be a good place - method - to start diagnosis? I did get a manual and schematic with the scope. I realize that I could probably buy an equivalent "working" scope for about$ 50, but I would like to get this one going. Tools available are
VOM, VTVM and one hand safely in the pocket.

Thanks, Ivan Vegvary

A perennial problem with tube 'scopes is the CRT heater winding on the
supply transformer going leaky or short to frame, or to another winding.
The heater is connected to the cathode, to take the stress off the
heater-cathode insulation, and is floated at negative HV, hence the
transformer insulation must stand several thousand volts DC.

The only cure for that is to fit a separate heater transformer with
sufficiently good insulation, such as one of those that used to be sold to
fix mono TV heater-cathode shorts. They used a split bobbin.

BTDT many times ;-(

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#### Phil Allison

Jan 1, 1970
0
"Michael A. Troll"
Not always.

** There is no "always" in my comment.

You illiterate moron.

I

#### Ivan Vegvary

Jan 1, 1970
0
Picked up the above for $10. Fuse was blown. Don't have a 125V 1A fusebut used 125V 1/2 amp fuse instead. Front indicator light went on and 30 seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps blowing, where would be a good place - method - to start diagnosis? I did get a manual and schematic with the scope. I realize that I could probably buy an equivalent "working" scope for about$ 50, but I would like to get this one going. Tools available are VOM, VTVM and one hand safely in the pocket.

Thanks, Ivan Vegvary

I

#### Ivan Vegvary

Jan 1, 1970
0
30 seconds sounds about right for a tube to heat up and go over current.

1/2A may not be enough fuse, so blowing the fuse may just be a diagnostic

that the fuse is doing its job -- get the right fuse to start with.

With Heathkits, you always need to ask the question "did it ever work?".

Then you need to ask "did it ever work _right_?". Then "was it

modified?". Then, finally, open it up and visually inspect everything

not just for the obvious like burnt components or wires, but for the less-

obvious assembly errors like cold solder joints, wires broken because of

improper assembly, wires put in the wrong places*, etc.

Inspect with your eyes _and_ your nose -- if you get a whiff of "that

expensive smell" then track down what's burnt. When you turn it on (out

of the case, of course), go over it with your nose again -- your nose not

only smells, but it's a pretty good heat sensor, sometimes you can

"smell" warm air coming from a too-hot component before it actually

starts smelling burnt.

If it still blows the same way with a 1A fuse then start checking tubes

for proper bias and shorts and stuff. Then check all of the power supply

voltages to make sure they're correct.

If it's solid state other than the CRT this'll be easy. I would think

that if the CRT is pulling too much current that you'd have seen a honkin'

bright spot, but I suppose that if the biases were really all shot to

hell then you may have just lit up the whole screen and not noticed it --

power it up in a dark room the next time and see.

And tell us what model it is, and other essential things like how many

tubes!

* I'm partially color blind in green and red, so my Heathkit oscilloscope

got assembled with a green wire swapped for a gray wire. One was a 160V

bias, the other was a 5V signal. I made expensive smells with that one.

But then, I fixed it, too.

--

My liberal friends think I'm a conservative kook.

My conservative friends think I'm a liberal kook.

Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software

http://www.wescottdesign.com

Heathkit Model 10-18 Oscilloscope
Placed 2 amp fuse (only one available) and does not blow. Here is a link tothe power supply portion of the schematic.

The 1V2 tube does not light. Tube has no continuity between pins 4 and 5 (with tube removed). Voltage at either pin 4 or 5 reads circa 1065V to ground. However, I can discern no voltage between pins 4 and 5. Is this OK? Do I have a bad transformer? Should I go ahead and order a new tube?

Thanks, Ivan Vegvary

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#### Jon Kirwan

Jan 1, 1970
0
The 1V2 tube does not light. Tube has no continuity between
pins 4 and 5 (with tube removed).

Yeah, there should be continuity between 4 and 5. The heater
voltage is supposed to be 0.625VAC with current at 0.3A, from
what I see at the following link:

This suggests, hot, that the resistance is about 2.1 ohms.
But according to this Wiki:

http://en.wikipedia.org/wiki/Vacuum_tube

the operating temperature of oxide-coated tungsten filaments
was about 700 C. The following chart on tungsten:

http://hypertextbook.com/facts/2004/DeannaStewart.shtml

gives a resistivity of about 24.93 at 1000 K and about 5.65
at 300 K. So about a factor of 4.4. From that, I'd expect to
see slightly less than 1/2 Ohm, or so.
Voltage at either pin 4 or 5 reads circa 1065V to ground.
However, I can discern no voltage between pins 4 and 5.
Is this OK? Do I have a bad transformer? Should I go
ahead and order a new tube?

The voltage difference should be, as cited above, about
0.625VAC. Reading the difference, though, may be difficult if
you are measuring each lead individually referenced to
ground. (600 ppm isn't an easy determination.) You may need
to measure across the two socket pins.

The tube appears bad. But I'd want to make sure, before
plugging in a new one, that the filament voltage appears

You could avoid worrying about measuring AC voltage at high
potential by instead testing the filament voltage by placing
a resistor across the two pins and powering up, feeling for
some heating effect. 2.2 Ohms there would be close to the
nominal value and would create less than 1/4 watt of
dissipation. With a 1/2 watt resistor, it should be enough to
feel and not enough to be worrisome.

Jon

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#### Jon Kirwan

Jan 1, 1970
0
<snip>
You could avoid worrying about measuring AC voltage at high
potential by instead testing the filament voltage by placing
a resistor across the two pins and powering up, feeling for
some heating effect. 2.2 Ohms there would be close to the
nominal value and would create less than 1/4 watt of
dissipation. With a 1/2 watt resistor, it should be enough to
feel and not enough to be worrisome.

I should have added: But only feel the resistor AFTER turning
the system off and safely removing the resistor, beforehand.
Please do NOT touch it while operating the system.

Jon

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#### Ian Malcolm

Jan 1, 1970
0
I should have added: But only feel the resistor AFTER turning
the system off and safely removing the resistor, beforehand.
Please do NOT touch it while operating the system.

Patch in a 1.5V maglite bulb - it should glow noticibly on 0.625 Vrms and
you can get a rough check of the voltage by visual comparison with an
identical bulb powered from an adjustable low voltage DC supply.

OTOH a floating measurement with a good quality multimeter and leads isn't
really a problem. Attach lead clips with power off of course.

If the meter is rated less than Cat III 1500V, I would advise putting it on
an insulator - e.g. a sheet of at least 4mm glass or clear plastic and
making sure the multimeter lead insulation does not touch any other lead,
component, metal or conductive object. Use nylon cable ties to dress the
leads if you have to and do NOT touch the meter or leads with power on FOR
ANY REASON.

I

#### Ivan Vegvary

Jan 1, 1970
0
OK. It is an IO-18 that I have sitting in the garage, if you need any

parts.

Thanks Michael,
Very kind of you to go through the trouble. As you will see in a new post, I did get the scope working.
Thanks again, Ivan Vegvary

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