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Fixing Heathkit Oscilloscope

Started by Ivan Vegvary November 20, 2012
Picked up the above for $ 10.  Fuse was blown.  Don't have a 125V 1A fuse b=
ut used 125V 1/2 amp fuse instead.  Front indicator light went on and 30 se=
conds 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, VT=
VM and one hand safely in the pocket.

Thanks, Ivan Vegvary
"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



On 11/19/2012 11:05 PM, Ivan Vegvary wrote:
> 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
On Mon, 19 Nov 2012, Ivan Vegvary wrote:

> 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
Phil Allison wrote:
> > "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.
Not always. I've fixed several with just a VTVM and a calibrated eyeball.
On Tue, 20 Nov 2012, Michael A. Terrell wrote:

> > Phil Allison wrote: >> >> "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. > > > 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
On Mon, 19 Nov 2012 20:05:37 -0800, Ivan Vegvary wrote:

> 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
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
On Mon, 19 Nov 2012 20:05:37 -0800, Ivan Vegvary wrote:

> 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 ;-( -- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)
"Michael A. Troll"
> Phil Allison wrote: >> >> "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. > > > Not always.
** There is no "always" in my comment. You illiterate moron.
Fred Abse wrote:
> > On Mon, 19 Nov 2012 20:05:37 -0800, Ivan Vegvary wrote: > > ? 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 ;-(
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.