Fixing Heathkit Oscilloscope

"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

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.

50, but I would like to get this one going. Tools available are VOM, VTVM and one hand safely in the pocket.

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

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

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

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

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 ;-(

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

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

You illiterate moron.

Prior to split bobbin types, Triad & Stancor both used to sell scope filament transformers rated for 2500 volts between windings. I would guess that Merit and a couple dozen other transformer companies had them in their 'Universal Replacement' lines. I used some in the '60s on old Eico & Heath scopes.

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 keep s blowing, where would be a good place - method - to start diagnosis? I di d get a manual and schematic with the scope.

ut $ 50, but I would like to get this one going. Tools available are VOM, VTVM and one hand safely in the pocket.

e

-

'

Heathkit Model 10-18 Oscilloscope Placed 2 amp fuse (only one available) and does not blow. Here is a link to the 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 grou nd. 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

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:

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

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.

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 about right, too.

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

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

the power supply portion of the schematic.

(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?

The model is 'IO'-18 (EYE-OH), not 'Ten'-18. I may have a junker out in the garage that got wet. It still has all the tubes, but the case is missing. I would have to dig it out to find the model number. I also have three NOS Westinghouse 1V2 in storage.

I don't remeber ever seeing a 1V2 light up. They were designed for use as Focus Rectifiers in TV sets. The other common focus rectifier was the 2AV@, that diud emit a little light in a dark room. The set was drawing too much current if you could see it under normal light. Your schematic shows that the filament voltage is .625 volts YO can't expect to read that difference if the meter is referenced to ground.

Jon Kirwan wrote in news: snipped-for-privacy@4ax.com:

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.

--
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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