Heathkit IO-103 oscilloscope

[pchilson]

Thanks Bill.

[Enzo]

For anyone else wanting a Heath cross reference, here is a link.

HCross.RRW

[pchilson]

Ok, I'm not smart enough to move forward on this. I'm a hobbyist not a seasoned pro.

When I first got this machine I checked all the transformer windings and came up with 0 vac on the 36 vac tap. I assumed a bad transformer and put the unit away.
When I pulled it out the other day and revisited it, I disconnected the 36 vac winding and found when disconnected from the circuit that I did have voltage on that tap but it read 47 vac so apparently the transformer is ok.
Smart or not I got the idea to hook my variac up to the board in place of the 36 vac tap and immeadiatly blew the fuse in the variac. So I figured a short downstream as the power transformer was not involved.
Thats when I started probing around the rectifier and found continuity all around the bridge. That is not correct eh? I replaced the rectifier diodes with the UF4007s I had laying around. Like a fool I didn't check the continuity to see if that had changed before powering up. But I did power up and same thing, fuse goes. So this can't be the rectifier but somewhere else downline.

Anyone can steer me on how and what to look at? How do I track down the short if I can't power the circuit?
Thanks for any help and guidance you can provide me.

The schematic in pdf format.

[52 Bill]

So this can't be the rectifier but somewhere else downline.

I wish that I could make more sense out of that schematic.

First test all of the parts connected to the output of the bridge, C405, Q401, ZD404. If any of these parts are shorted, they will cause the fuse to blow. I can't tell what else is connected to this supply yet, but that's a good place to start.

The negative side of the bridge is not connected directly to ground so don't let that fool you. And check R413 as well.

[pchilson]

Hi, thanks Bill.

I have a working scope and a sig generator. Can I pump in a sine wave at the front of the bridge and use the scope along the way to find anything? Would that hurt the sig generator or the viewing scope having a short in the circuit? Or would the short make it a useless effort?

Oh, and I hear you about the chopped up schematic. Its like trying to figure out a puzzle in a puzzle.

[Tom Phillips]

...I have a working scope and a sig generator. Can I pump in a sine wave at the front of the bridge and use the scope along the way to find anything? ...

With such a "hard short" that blows the fuse instantly your first best approach is to just take resistance readings and try to find the low resistance across a part where it should be a high resistance. Scope powered off and all caps drained of course. The problem could be a part or even bad wiring since the scope has never worked since it has been in your possesion.

[pchilson]

Following Bills suggestion I started checking downstream components...
I see messed up components.

Three of the 4 rectifier diodes, D405 thru D408, were short, one was not. I pulled them all assuming bad and didn't note which position the good one came from. My inexperience.
R412 - 210R 5w - ok @ 218R
C405 - electrolytic 500uF @ 50vdc - no real way to test this. its old, its electrolytic, assume its bad
ZD404 - zener diode - short
Q401 - transistor - short
R405 - ok
R406 - ok
R407 - ok

....

R408 - 470R - high resistance reading of 881R. See pics

[52 Bill]

Following Bills suggestion I started checking downstream components...
I see messed up components.

Three of the 4 rectifier diodes, D405 thru D408, were short, one was not. I pulled them all assuming bad and didn't note which position the good one came from. My inexperience.
R412 - 210R 5w - ok @ 218R
C405 - electrolytic 500uF @ 50vdc - no real way to test this. its old, its electrolytic, assume its bad
ZD404 - zener diode - short
Q401 - transistor - short
R405 - ok
R406 - ok
R407 - ok

....

R408 - 470R - high resistance reading of 881R. See pics

It doesn't really matter which diodes were shorted just replace all for with 4 matching ones. The cap may be bad, but the test you need to do here is to see if it is shorted. Just use your ohm meter and see if it charges and doesn't read a constant short or low resistance.

The shorted transistor and the shorted diode need to be replaced, but more importantly if R408 has burned up, then whatever circuit it feeds needs to be checked as it would seem that it was drawing too much current and may have been the cause of the original failures.

Edit: R408 is on the power pass transistor, so that's why it cooked. Check R409, R411 and R412 as well, as they draw from the same supply.

[pchilson]

Is there any speculation from whats known so far which direction the "surge" came from? Downstream from the power supply or upstream from the input?
I guess I'll see when I start testing the amplifier boards but do you experienced guys see indicators from this?

Thanks

I tried stitching together the pieces of the schematic available in the pdf above. Its not perfectly aligned and some of the power supply is a bit off but it gives a much better perspective of the whole.

Large image file of the schematic.

[pchilson]

Another thought. Does the 47 vac measured at the 36 vac tap concern anyone? Could this be the issue? Even at +/- 20% that seems awfully high no?

[Tom Phillips]

Another thought. Does the 47 vac measured at the 36 vac tap concern anyone? ...

Not really. the 36 VAV is the reading after it is hooked to the circuit and thus loaded down. The measurement will be lower when you get the regulator circuity all hooked up to that tap. Also note that the primary voltage on the schematic is listed as "110 - 130 VAC." That's a large range and it doesn't specify the line voltage that corresponds to the 36VAC reading.

[pchilson]

After this past go round with the UF4007s. Turns out D407 and D408 are still good and D405 and D406 are short.
I've had replacement parts for everything but the zener diode and the transistor. I can get those on Tuesday.
I've gone over the Vertical Amplifier board and can find no shorted components there. Can I check transistors for shorts with out taking them out of circuit?

[pchilson]

I picked up the transistor and zener that I needed. I replaced the electrolytics as well. Now the power supply is solid and not taking out fuses so thats where the dead short was.
I did find a short zener on the vertical amplifier. I can get that replacement tomorrow.
As it stands now the horizontal amplifier and sweep generator board seem to have escaped the failures.
Once I get the vertical amp working I should be set.

Thanks again.

[pchilson]

Hi,

I know this is a very vague and general question but when you don't know you don't know...

I have solved the power supply issues. Machine will power up and stay up without blowing the fuse. The horizontal and sweep circuits seem to be working properly. The attenuator and vertical amp boards seem to be the issue now.
My checking, probing and physical inspection doesn't turn up any problem that I can find yet the vertical controls produce nothing. I am getting power to these boards but the voltages I'm reading are off.
In the pic I've attached I have outlined two locations where I should see positive voltage on the top and negative voltage on the bottom (schematic view). I read positive voltage where I should but where I should be reading negative voltage I am getting positive voltage instead.

What in a general sense could be a cause for what should be negative to be positive?
I'm getting cross eyed trying to follow this cut up schematic...

[Tom Phillips]

...I'm getting cross eyed trying to follow this cut up schematic...

Since you really seem to be into this project have you considered buying a copy of the full Heathkit manual? If so, I think I ran across one for sale and I could try to re-find the link and post it if you are interested. As I remember, asking price was ~$20 for an original hard copy.
Tom