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Author Topic: Heathkit Chronicles: HO-10 scope fried  (Read 3826 times)
KJ4KET
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Posts: 73




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« on: June 09, 2009, 11:25:25 AM »

Well after you all helped me fix a transmit hum problem with my old Heathkit HW-101, I now have a fried HO-10 monitor scope.  It was sitting on the bench when it crackled, there was a very small amount of smoke, and it was gone.  I switched it off and on, and tube 1V2 glowed like a searchlight.  I quickly switched off again, and tested that tube on an old Eimac tube tester (which, knowing my luck, will be the next to break down).  The tube tested good.  I put it back, and now the tube seems to light normally, but all I get is a very faint fuzzy dot the size of a penny, instead of a regular dot.  

I am trying to figure the best procedure to eliminate possible issues.  Does this sound like the focus is gone, hence, maybe a problem with the high voltage from the transformer?  Maybe the CRT tube itself? There is no sign of any caps having burned through or anything like that.

Although I am in despair, you did all help me to fix my hum problem with my HW-101, so I am minded to say "Yes, We Can!" while awaiting your expert advice concerning my HO-10. Anyway, I have no choice -- I have no budget for new equipment!

Best,

David M
KJ4KET
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AA4PB
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« Reply #1 on: June 09, 2009, 11:54:49 AM »

The 1V2 is the high voltage rectifier. The high voltage is related to both the focus and the intensity. The plate of the 1V2 should have a negative 1500VDC on it. BE CAREFUL!

You might check the two 0.15uF capactors (C30 and C31) in the 1V2 plate circuit as well as the resistor between them. One of the capacitors might be leaky and loading down the high voltage. Smoke would make me think that something has cooked, like the resistor.
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WA3SKN
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« Reply #2 on: June 09, 2009, 12:00:07 PM »

As I recall, the 1V2 is a recifier tube.  It sounds like a power supply trouble.  Break out a voltmeter and remove all the tubes not associated with the power supply, then measure the DC voltages getting to the tube sockets.  I'll bet one or more voltages are way off... or not there at all!  Correct the power supply problem, re-install the tubes, and see if it works.
This sounds like an electrolytic cap went bad.
Lucky you do not need a monitor scope to operate that rig!
Good luck!

-Mike.
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KJ4KET
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Posts: 73




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« Reply #3 on: June 09, 2009, 01:27:11 PM »

Well using an ohmmeter, capacitor C-30 appears to be an electrical short.  That would probably explain why IV2 lit up like a searchlight.  I guess the question now is what damage has been done.  If I can figure out which wires, maybe I should measure the AC voltage across the HT winding of the transformer?  If that is fried, I might as well throw this baby out and buy another.
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AC5UP
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« Reply #4 on: June 09, 2009, 02:26:16 PM »

Be very careful when measuring the HV tap on the power transformer as it's a rare meter that can tolerate more than 600 VAC on the input... The typical DVM comes in at 1000 vDC and 600 vAC max. If you can scrounge or borrow a high voltage probe that would reduce the chances of smoking the front end of your meter. I should also mention the -1500 volt rating is a ballpark, typical value as your AC line voltage will have a major affect on the HV tap. In other words, -1200 isn't necessarily "wrong".

Since you found a shorted cap I'd replace that cap and see what happens. Note the voltage rating on the cap as it's not uncommon to find big ceramic disks in the HV section with a 2 kV rating on them.

As for the focus circuit, it's generally a very high value pot (2.5 megs) arranged as a voltage divider with some high value fixed resistors top & bottom. Check everything in the string for continuity one part at a time. And, like the HV check, don't expect precision readings. On a scope the Focus and Intensity pots tend to attract crud and unless they're sealed units may need some dusting, cleaning and exercise. Given what happened with your scope I wouldn't be surprised if one of the pots now has a dead spot where the coating arced over.

As for the 1V2 HV rectifier being brightly lit, yeah, that's not good, but it also tells me the filament didn't open. The tube may have lost some of the thorium coating on the filament due to overheating which could be a problem if it was soft to begin with, but unless you're a purist it can be replaced with a pair of 1N4007's in series or a HV rectifier stick liberated from a junker CRT computer monitor.

Back when that scope was built HV silicon was unobtanium. Today, not so much... Wink

BTW: What was the fix for the HW-101 Huh
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KJ4KET
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Posts: 73




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« Reply #5 on: June 09, 2009, 02:37:33 PM »

Well it seems I probably don't have to measure the AC voltage on the HT winding after all.  I took a chance and plugged the scope in with the bad capacitor removed altogether, and I got a trace, albeit a little out of focus (and not a straight line, more of a sawtooth pattern, suggesting other problems - but that was an issue previously anyway).

The bad capacitor and one other in the same circuit are 0.15mfd, 1600v.  The best I can do is get some 0.47mfd, 600v and wire three in series for each old capacitor.  That should come out close to 0.15mfd with a 1800v rating, no?

Once I do that, I will see what happens.  I do have some replacements for some of the other capacitors as well, but not for the power filter capacitors, as those are in a can multi-capacitor, and those are expensive if you can even find them these days.

As for the HW-101, I resoldered every connection in the mic wiring.  Apparently, I do still have a hum, according to the latest report, but after extensive diagnosis with another ham, who played my audio back to me, it does seem like a grounding issue with the mic.  I can replace the cable with a new one and/or get a better mic, but the hum is not too bad at this point.
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KA5N
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« Reply #6 on: June 09, 2009, 05:05:22 PM »

I don't reccomend using several capacitors in series.
The voltage across each capacitor may not divide equally and one cap may have its voltage rating exceeded and ker-pow.  You will notice that when several electrolytic caps are used in series they always have equalizing resistors across each cap.The type capacitors you need for the 1600 volt jobs are "Orange Drops" a polyproparol insulated metal foil type.  These are available for about $1.50 each and most antique electronic suppliers have them.  They are made by Sprague and other makers have similar caps.  These caps will last a very long time compared to the old paper and wax caps of early days.
Allen
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KJ4KET
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« Reply #7 on: June 11, 2009, 06:32:21 AM »

The problem is that I can only locate 1600v orange drops at 0.05mfd and at $8.00 or so each.  I would need 6 of them at this value and the scope isn't worth it.

What if I use a 10meg resistor across each series capacitor?  That would be 30meg across a voltage of 1500 volts, for a fairly minimal power consumption.  The capacitors (and hence the resistors) would run from the diode cathode (-1500v) to ground.

David
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WD8AJY
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Posts: 63




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« Reply #8 on: June 11, 2009, 11:43:23 AM »

try digikey this part might work

940C16P15K-F
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KA5N
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Posts: 4380




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« Reply #9 on: June 11, 2009, 03:27:03 PM »

Radio Daze has .o15 @ 1600 volts for $1.35 each.  They have a minimum of about $10.  
Allen
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W9GB
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Posts: 2659




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« Reply #10 on: October 22, 2013, 09:07:07 PM »

Mouser shows a CDE Type 940C capacitor replacement in stock (75 units):
http://www.mouser.com/ds/2/88/940C-25653.pdf

Mouser PN: 5984-940C16P15K-F
http://www.mouser.com/ProductDetail/...hhoBWsMg%3d%3d
Description: 0.15uF/1600V polypropylene film capacitor
Mfr: Cornell Dubilier (CDE)
Mfr PN: 940C16P15K-F
Each: $4.16
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KD0REQ
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Posts: 1050




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« Reply #11 on: October 24, 2013, 08:25:28 AM »

those 0.15 uF caps were all over Heathkit scopes, and they are a widely known failure point.  I put the CDEs mentioned in a SB610, working fine.  they should be lifetime caps, where the end-fill stuff is always going to leak and blow in time.  many a 610 transformer has blown the HV winding when they short.

all of the focus string resistors (like all the lowest-bidder Heath carbon resistors) are known to age poorly, I'd shotgun those with modern metal film jobs at twice the wattage, just because.
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