Heathkit IO-12 recap and a question

[Rickenbackerman]

"X1 really means divide-by-1 while X100 means divide-by-100.  Not multiply-by.  So a 1Vpp signal fed to the vertical on my 'scope is biggest on X1, smaller on X10 and tiny on X100."

JEEZ, that's not confusing at ALL!  Good to know yours and mine work the same, though.

I was wondering about the "opening into a circle" too.  When I touch the probe to V6 pin 2 with the scope powered on, the trace jumps off the screen for a second or so due to the DC present there, comes back as a vertical line and then slowly widens into the diagonal circle in the image I posted above.  Not sure if that info is relevant, but thought I'd add it here.

Paul can you probe V6 pin 3 and see what you get? 

[Paul Moir]

It's funny how when you're young, your critical thinking is underdeveloped but your naivete kind of makes up for it.  The x as a divisor rather than a multiplier got locked into my young mind as "that's just the way things are."!

Picture is V6 pin 3 with the horizontal gain turned down all the way.  Increasing it to about 30% produces a 45 degree angle line since my horizontal amplifier is working.  I also tried this by routing the 1Vpp test signal into the horizontal input and setting to sweep to EXT. INPUT.  Results were pretty much identical.

[floobydust]

So OP has no signal present at V6A pin 3, but it's present at V6A pin 2?
I'd think that is an open connection to the triode at its grid. If you're measuring at a nearby part there might be a busted connection or dud tube socket pin.
The plate and cathode must be connected to give the cathode the OK DC voltage seen.
The V6A grid circuit is really high impedance, if not it would squish/distort the sweep waveform.
I would look really hard at the 2,200 ohm grid resistor if cracked or open.

[Paul Moir]

Floodbydust,
I've been trying to replicate Rickenbackerman's results on my own scope by adjusting resistors, opening things, etc, so far without much success.  I have lots of experience with electronics but very little with tubes; this has been an education for me.  Let me know if you have any specific failures you would like to see tested.   

I tried opening the grid resistor and probing, and I get a diagonal about the same size with the horizontal maxed out, which is very sensitive to how close I place my hand to the probing wire.  The thing is, it's a diagonal rather than an open circle, which tells me that the signal getting to the horizontal circuit is *different* than the one getting to the vertical circuit.  Can it be possible that the other side of the filament is somehow leaking through?

[james_s]

"X1 really means divide-by-1 while X100 means divide-by-100.  Not multiply-by.  So a 1Vpp signal fed to the vertical on my 'scope is biggest on X1, smaller on X10 and tiny on X100."

JEEZ, that's not confusing at ALL!  Good to know yours and mine work the same, though.

I was wondering about the "opening into a circle" too.  When I touch the probe to V6 pin 2 with the scope powered on, the trace jumps off the screen for a second or so due to the DC present there, comes back as a vertical line and then slowly widens into the diagonal circle in the image I posted above.  Not sure if that info is relevant, but thought I'd add it here.

Paul can you probe V6 pin 3 and see what you get?

All scope probes are like that. If you have a little more background then it makes perfect sense and is a lot more intuitive. Many old scopes don't have a separate set of numbers on the V/div dial for different probe types, they just have the X1 value. If you connect a X10 probe you have to multiply the voltage you read on the scope by 10 to get the true voltage of the signal you are measuring. It's pretty much as the tachometer on a car, it will typically say X1000, you take the displayed reading and multiply it by 1000 to get the actual RPM. The X setting isn't multiplying the signal, it's telling you what YOU need to multiply the reading by to get the true value.

[Rickenbackerman]

James that makes PERFECT sense.  I never thought of it that way!

Just now I soldered up the horizontal gain pot back to stock (I had it totally bypassed for testing), but I left the center tap wire going to pin 7 disconnected to separate V6A from V6B.  I thought maybe something was wrong with B and dragging A down...   I'll keep it this way until I can get V6A working, and so far no luck. 

flooby - that 2.2K resistor is fine, I posted that pic I took of pin 2 which is downstream of that resistor.  There's AC voltage there but none on pin 3.

Paul - HUGE thanks for helping me out with this, again!

This "opening into a circle" thing is curious.  Is something wrong upstream of the feed to V6A? 

[floobydust]

When the Horiz. Amplifier is switched to LINE SW. as a (sweep) source, it gets mains reference AC signal from FIL. (B) which is a 180° plus delay or phase-shifted (by the PHASE potentiometer), compared to FIL. (A) which is the 1Vpp ref.
Said another way, Line SW feeds in a phase-shifted sine-wave to the Horizonal (sweep) amplifier. So, compared to the 1Vpp ref, you'd see an ellipse or circle.

It's puzzling there is no AC coming out of the cathode follower, yet the grid has signal and the triode's three DC voltages are OK. When things don't make sense, it's usually a flawed measurement or something unexpected/overlooked. You might have a bunch of bad 12AU7's with a common cooked grid, or there is some bad connection or short to the triode on the PC board, wiring/recap mistake made there etc.

[Rickenbackerman]

OK fellas, some MAJOR progress tonight!  I swapped V6 and V7 for grins and rechecked some voltages and...  no AC at V6 pin 2 all of a sudden.  Huh? is what I said!  I think my DVM was lying all along.  AC reading are sketchy with this thing, which is why I only really look at DC with it.  I need to buy a Fluke.

Traced the FIL (B) wiring to the Horizontal/Freq Selector switch and eventually found a cold joint (my fault of course) between the wire coming off pin 10 of the switch and C126? (I think?  my skit is blurry - the 2.2K R147 that feeds the V6A grid.  Reflowed that and we have horizontal gain again!  WOOO!!!

I was able to recreate the 45 degree line that Paul talked about by probing V6 pin 3 while on "Line SW".  No "opening into a circle" now, however with the probe not touching anything it tends to want to do that still.

However, not totally out of the woods just yet.  I hooked up my signal generator and played around for a bit.  Everything looks good on preset 2, on other settings the trace is smashed to heck on the right side.  Here's a pic of 1KHz sine wave. 

I think there are still bad caps around the Horizontal/Freq Selector switch possibly?  There's still a couple of stock films there...  I'm wiped but will sleep happy knowing I made progress, finally!

[floobydust]

I did feel hell freeze over, this scope needs to be on a reality TV show lol.
Next would be figuring out if the horiz. sweep trace being squished or non-linear is the sweep oscillator, or stuff downstream of it. Not easy to do without a scope to look at the ramp waveform at V4 or V5A. If you've got a different 12AU7 in there then recheck the DC voltages and also see what the SYNC selector does as far if EXT has the same squished look.

With your multimeter not able to read ACV with DC present, you can add a blocking capacitor plus resistor in series with the probe.

[Rickenbackerman]

This scope doesn't need to be on a reality TV show, it deserves its own show.  This thing is possessed. 

Fired it up, still doing the "smashed on the right" thing.  So for the heck of it and because it took two seconds I swapped V5 out with a used good one from my stash and fired it back up.  1KHz sine pic below.  It was working perfectly.  Then put the original V5 back in.  Still working perfectly.  I was stoked.  Messed around with some other stuff, took out the trash, watched some TV, went to the bathroom, etc...   Came back to the scope, fired it up, smashed on the right again.  Pic of that here as well.  There's gotta be a bad V4/V5 connection somewhere, right?

[james_s]

You already found one bad solder joint, this looks an awful lot like another bad solder joint. I've also seen flaky tube sockets, usually cleaning the pins will do the trick.

[Paul Moir]

It's a crapshoot, but check the ground tab next to the frequency selector, the one that screws into the preset pot support and gets grounded through the frame via the horizontal position pot and the horizontal gain pot.  I think I could see a bad connection there doing that, and you would have been into it replacing your caps.
(For those following this is just below R84 150K in the schematic.  C21 and maybe C23 got replaced.  I ran out of time trying to replicate the problem.  I also am very slightly squished to the right like in the "working good" picture.)

[med6753]

(For those following this is just below R84 150K in the schematic.  C21 and maybe C23 got replaced.  I ran out of time trying to replicate the problem.  I also am very slightly squished to the right like in the "working good" picture.)

That design of Heath scope at certain sweep settings will have a slightly non-linear trace. That's normal. But not as bad as op's issue. 

[Rickenbackerman]

Just reflowed a bunch of joints around V4/V5, no change.  Still smashed on the right.

James I hit ALL of the tube pins with G5 Gold awhile ago, that doesn't mean they're clean though!  I've wiggled them all around in their sockets a bunch too.

Paul - good call on that ground - looks like a trouble spot for sure but mine is fine.  I checked it and it was good, then tightened it up anyway.  Can you check something else for me? 
SYNC Selector set to -INT
HOR/FREQ Select set to LINE SW
I get about an inch wide trace, HOR gain maxed, with nothing connected to the vertical input.  With 1VPP hooked up to the vertical input, I get a 45 degree line (it still opens up a bit) top right to bottom left with the Phase pot full CCW.  As I rotate the Phase pot clockwise, it opens into a circle and then goes top left to bottom right (still slightly open) as I reach the end of full CW rotation.

Maybe I haven't played around with the Phase pot on these settings before, but I don't recall the phase pot ever doing a dang thing.

edit: Crap, I think you mentioned earlier in this thread that your phase pot was froze up solid.

[Paul Moir]

Actually with all the messing around I got it freed up.

Definitely a difference there:
Same setup, I get a nice big horizontal line the full width of the CRT.  To get it down to 1" I need to set the horizontal gain to about 30%.  (for comparison I get full width sweeps with the gain set to about 70%).
With the 1Vpp hooked up, I get an open circle which collapses to a 45 degree line with the phase control half-way, which again opens "the other way" as I advance past half way to full.  I get roundish circles with the phase control at both the maximum and minimum.
Changing the sync selector does nothing, except a momentary jump in the trace when it goes through the line trigger setting.

The phase pot only affects the phase of the horizontal sine wave going into the horizontal amplifier in the "Line SW" setting, so it doesn't do much.  There's a reason mine was frozen I think! 

[Rickenbackerman]

IT WORKS!  (for now)...

Did some more measurements, things weren't making sense around V6A again - no gain.  Swapped it out with a used good one, working again.  Maybe it's eating tubes?  Scope-O-Saurus?  Although this 12AU7 and the 1V2 that was dead when I started are the only tube swaps so far - the rest all work fine even though the 12BH7 tests weak but seems to run the scope OK.

[Quarlo Klobrigney]

Bad tube sockets. DeOxit or change them to ceramic. "Troubles-Be-Gone"
You can find them everywhere online.

[Rickenbackerman]

Quarlo I hit all the pins with G5 Gold weeks ago.  And I've tried ceramic sockets, I don't like them because they don't play well with the high voltages in tube guitar and bass amps (which is what I work on).  They arc.  Doesn't make sense, but I swapped out the power tube sockets in a 1970 Orange 120 with ceramics once.  Two months later they were scorched and the tubes were still fine.  For something like this they'd be okay - but - I think it was the tube.  I'd test it (again) but I'm going to bed victorious.

[james_s]

It could be a mechanical fault in the tube I guess, but sockets or solder joints at the socket would be what I'd suspect first. I've seen tube sockets a couple of times that had bad connections not resolved by a squirt of DeOxit.

[Quarlo Klobrigney]

So a new socket wouldn't fix all of that then?

[Rickenbackerman]

Quarlo I don't think you read the whole thread.  And I don't blame you.  It's long and it's a whole damn saga.

I've seen some wasted sockets.  Melted, and I mean seriously melted sockets.  Like turned into molten lava and then rehardened - phenolic, bakelite, "plastic" or whatever you want to call it.  Black arcing lightning tracks, power tubes glowing bright orange...  10A fuses swapped in for 3A fuses, etc...  guitar players are rough on amps!  I also had an old stock GE6550A short out plate to screen and smoke the power transformer in one of my 1973 Ampeg SVT's once.  Of course it happened while I let a "friend" borrow it.  I had Heyboer rewind it.  Works great again.

[Quarlo Klobrigney]

Oh but I have been reading all along.
I'm just to busy to reply, however I just had to.

As far as the ceramic sockets, i use them all the time up to 500V and have had no problem.
I don't know where you are getting them from, but you should change sources. Even a 9 pin socket made of ceramic should be able to withstand 1KV.
I replace a lot of sockets from 50's vintage radios both PC and PCB mount. No problems so far.
Just saying, that scope is low voltage in the signal paths.

[Rickenbackerman]

I agree on all points.  Ceramic sockets would have been fine in this application, but a simple tube swap fixed the problem at the end problem (for now), so no use swapping sockets.  I just don't like ceramic ones, I had that bad experience with them once.  Left a bad taste in my mouth...

[Rickenbackerman]

Update - couple days later and...  IT STILL WORKS! 

HUGE thanks to Paul Moir for helping me out by using his scope as a test mule, and thanks to everyone else that chimed in on this.

So glad to have a working scope again.  It's not calibrated?  It's low bandwidth?  It's perfect for what I need it to do.  It works again, and I fixed it myself (with help from you guys).  And the trace is actually sharper now than I ever remember seeing it.  Sweet!

[james_s]

Good job, glad to see an old instrument like that still getting some use. It's every bit as capable as it was back when someone paid what was likely several weeks wages for it. People managed to accomplish all sorts of things with test equipment that is primitive by modern standards.