Heathkit IO-12 Toasty Resistor

[Ra]

Hi there,

First time poster, long time fan!

I picked up a Heathkit IO-12 recently from a friend/antiques dealer.  Turns on, worked .. more or less alright. 

Cracked the case on it the other day and discovered a toasted resistor.  Appears to be a 100 ohm resistor, but nowhere in the parts list is a 100 ohm resistor specified. 

I didn't get the manual with the unit but I was able to find a condensed manual with schematic and parts list.  The smallest resistor that I see is 1k ohm.  Just want to make sure of the correct value before I replace it.  Makes sense that if the resistance is an order of magnitude lower than the spec that it would burn out.

Confirmation would be much appreciated.

Cheers!

[Radio Tech]

What is the number under the resistor? Silk screen on pcb.
Looking at that burnt resistor you can tell it is a bit out of place. Newer type than rest of the ones on board.
You are probably correct with the 1k value.

[Alex Eisenhut]

Yeah I was about to say it looks like the value is under the part!

[Brumby]

As already noted, that resistor is newer than the rest - so it is obviously a replacement.

But now that replacement looks like it could do with replacing, which begs the question - why this resistor ... again.  (There is a possibility that it may have been replaced several times.)

Either the power rating is inadequate or the there is a problem somewhere causing excessive stress on this resistor.  If the former, then a higher rated resistor should fix this.  However, if it's the latter, there might be something that should be addressed as it might be affecting the operation of the equipment.  I'd be inclined to try and track down where it is in the circuitry and find out.

[Paul Moir]

1K.  From my IO-12 which happens to be sitting next to my computer with the case screws out.  Power of the internet eh? 

If there's anything else you need let me know.  Mine is not exactly very original anymore and I was planning on changing out the knobs and a few other things if I ever got around to it.

For those playing along at home, this resistor is the lower 1K at the output of the vertical amplifier push-pull (top middle, a bit to the left).  The fact that it's heating along with the 3.3K along side it suggests that the +INT signal is drawing too much current for some reason.
Mine shows no evidence of these resistors ever getting hot, though the 2700 is stood-off and does show a little PCB discolouration around the leads.

+int should go straight to the sync selector switch along with -int.  If this was the problem, you would think that the -int resistor would be cooking too.  Since it's a heathkit, I'm thinking the +int wire is simply miswired or shorting out.

[AF6LJ]

If I had to guess that is a One Watt resistor, also watch the voltage rating, that little replacement may not have been able to hold off the voltage across it.

[Paul Moir]

If I had to guess that is a One Watt resistor, also watch the voltage rating, that little replacement may not have been able to hold off the voltage across it.

It is a one watt resistor (see the schematic, you don't even have to guess), but it is getting hotter than it should.

[Ra]

Wow!  Darn you guys are quick.  God bless the interweb!

Well I'll be sourcing a 1W 1k resistor.  As there are no electronics supply stores around where I live I might as well order a load of replacement capacitors as well so I'm not paying $8 for shipping a single part.  Would you guys recommend just replacing the electrolytics and leave the rest alone?

Cheers!

[Tomorokoshi]

Would you guys recommend just replacing the electrolytics and leave the rest alone?

Cheers!

The large electrolytic muti-cap in mine sizzled and smoked a few years ago. I will get around to replacing it eventually.

[Paul Moir]

One of the two beown .1uF HV capacitors is leaking something on mine.  I was thinking maybe these:
http://www.digikey.ca/product-detail/en/R76TN3100SE40K/399-7673-ND/3465821
You feel like ganging up on a parts list?

[Tomorokoshi]

One of the two beown .1uF HV capacitors is leaking something on mine.  I was thinking maybe these:
http://www.digikey.ca/product-detail/en/R76TN3100SE40K/399-7673-ND/3465821
You feel like ganging up on a parts list?

That would be a nice thing to have around. I have two variations of that scope. I don't have any manuals or schematics, although they are around on the web somewhere.

[Paul Moir]

Values from the schematic followed by actual capacitors from my oscilloscope.

Big can:
40uF / 450v - 40uF / 450v
20uF / 450v - 20uF / 450v
20uF / 450v - 20uF / 450v
50uF / 300v - 50uF / 300v

Small can:
20uF / 200v - 20uF / 250v
20uF / 250v - 20uF / 250v
20uF / 250v - 20uF / 250v

Axials:
100uF-60v / 100uf-50v
40uF-100v / 40uF-150v
40uF-150v / 40uF-150v

Replacements:

http://www.digikey.ca/short/37mb30

This is the best I could come up with so far.  All are ~10000h/105C rated caps, but that didn't add much to the cost.  The 22uF capacitors are pretty cheap for their high voltage rating and lifespan as they are targeted for fluorescent ballasts.  I also went higher voltage ratings on some of the caps because it seems to me if you start pulling tubes there's nothing to keep them from being exposed to higher than maximum voltages.  Maybe if you loose tubes you've got bigger problems but I didn't really like those 250v rated caps hanging off the 415v line.  Perhaps if I understood the unmarked tube to the right of them better I wouldn't mind.  Also 250v caps were ~0.94 while the 450v ones are just $1.21 so not a huge cost.

The idea is to replace all the 20uF with the 22uF, and parallel two of them for the 40/450v and two more for the axial 40/150v on the plate amplifier board at the back.  You get a spare capacitor this way.  Alternatively you could order 11 of them and drop the 39/160v cap, or order 14 and drop the 47/450v cap. 

The axials are about 65mm pin to pin, so the leads on the capacitors will be far too short.  I think you can form them out to something like 30mm at best.  I had a brief look at new axials but they were all old technology and too expensive.