Author Topic: Heathkit IO-12 recap and a question  (Read 5647 times)

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Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #50 on: May 13, 2021, 06:12:00 am »
Swapped V6 and V5 before crashing just for grins.  Same dot on Line SW, same vertical line with 1Vpp hooked up.

I also have a few "nasty firesticks"!  Ha!  My dad has like a 200watter he uses to solder up the ends of model airplane fuel tanks he makes out of tin.  Thing is ancient and the tip is about 5/8" wide!  :=\
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #51 on: May 14, 2021, 05:47:32 am »
OK, I reflowed all joints associated with V6 and did some more testing and I am utterly stumped.  V6A simply refuses to amplify.  I get a good 1.8VAC at the grid (pin 2) on "Line SW" and the plate voltage is good (125VDC).  Something is up at the cathode (pin 3).  I lifted a leg of C127 (even though it's new), and tested it for capacitance and leakage per the DVM resistance check and it's fine.  On pin 3 with C127 disconnected I get like 5VAC on "Line SW".  I think the horizontal gain pot itself might be toast.  I will NOT give up.
 

Offline james_s

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Re: Heathkit IO-12 recap and a question
« Reply #52 on: May 15, 2021, 02:38:58 am »
Well pots do fail, they're easy to test though. You might also try cleaning the tube sockets and tube pins, I've run into situations with old radios where dirty tube sockets/pins caused a malfunction.
 

Offline floobydust

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Re: Heathkit IO-12 recap and a question
« Reply #53 on: May 15, 2021, 03:41:12 am »
V6a is a cathode-follower, so it will naturally have no voltage gain, that configuration has a voltage gain of 1. It's buffering the output of the sweep oscillator with bootstrapping to increase the input impedance (R148-R150) so it doesn't load anything down. The grid and cathode (AC) voltages I would expect to be the same.
So if you have 1.8VAC coming in at the grid, the most you'll get is 1.8VAC at the Hor. Gain pot when it's cranked to max.
V6b is the (voltage) gain stage. I would look around there.

edit: multimeter loading is likely making V6a grid (signal) voltage measurement appear lower than it is. So V6a cathode can have more amplitude than the grid, as an illusion.
« Last Edit: May 15, 2021, 08:28:29 pm by floobydust »
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #54 on: May 15, 2021, 05:40:06 am »
While you guys were posting I was poking and probing...   Thinking maybe I had a bad ground on the horizontal gain pot or bad pot itself or dirty tube pins or loose sockets, etc...

Stared at the skit and think I found a better way to test all the "junk" on pin 3 of V6:

Pin 3 to chassis ground (from the tube side of pin 3 to eliminate any bad solder joints) - 56K - which makes sense since that 47K resistor has drifted up to 54K.  Don't think that would affect much?

Pin 7 to chassis ground (tube side again) - Horizontal gain pot min - 0K.  Horizontal gain pot max - 12K.

Cleaned V6 pins (again) and retentioned sockets 1-3.  Still 1.8VAC at the grid on "Line SW" and basically nothing (0.01VAC) at the cathode with the gain maxed. 

THANKS fellas for the help!  I STILL will not give up.
 

Offline Quarlo Klobrigney

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Re: Heathkit IO-12 recap and a question
« Reply #55 on: May 15, 2021, 05:43:18 am »
R254 → 10MΩ, 257 → 100Ω, 259 100Ω?

Pin 1 & 6 going to the proper CRT pin without corrosion?

Disconnect C128 off of pin 6 of the V6 plate. Connect a 1KHz audio source thru C128 without anything that would cause a hot chassis. The Horizontal should be too wide depending on the Horizontal gain pot.
Voltage does not flow, nor does it go.
“I do one thing at a time, I do it very well, and then I move on.”
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #56 on: May 15, 2021, 06:35:00 am »
R254, R257, R259, all good.  Squeezed those CRT connections with no affect.  Did find a sketchy V7 pin 6 to CTR wire rubbing on the chassis there and fixed that too, no affect.

I'm a scared of screwing around with 1300VDC after numerous beers so I tried something else - I should see increasing AC voltage on pins 2 and 7 of V7 as I turn up the Horizontal gain with the Horizontal selector set to "Line SW", no?  If so, I see nothing. 

So, it's either V6A or V6B. 




Or something else.
 

Offline james_s

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Re: Heathkit IO-12 recap and a question
« Reply #57 on: May 15, 2021, 06:37:28 am »
You shouldn't really be doing anything at all inside that scope "after numerous beers". Even the "low" voltages in vacuum tube devices can knock you dead, or at the very least cause you to jerk back and slice open your hand on something.
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #58 on: May 15, 2021, 06:53:04 am »
James, WELL aware here.  I've got about 1800 all-tube watts worth of vintage bass amplification here in my basement, and have done work on all of it.  Plus work on dozens of tube guitar amps for friends.  Just got done redoing a 60's Fisher 400 stereo receiver and a couple of Roberts 6W single ended EL84 RTR monoblock amps for a buddy.  I know my limits, and I'm done for the night. 

The only time I've ever really been nailed is when calibrating my BK 7474B tube tester in my early 20's (I'm 45 now).  I ate about 350VDC in one arm and out the other, right through the ticker.  Low amps, but it still knocked me on my ass for a good 15 minutes and taught me a lesson I won't ever forget.
« Last Edit: May 15, 2021, 07:11:12 am by Rickenbackerman »
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #59 on: May 16, 2021, 05:54:34 am »
Only had about 2 hours to "play around" tonight, the wife's got me on the honey-do list this weekend...

Reflowed a few somewhat sketchy looking joints on V7, rechecked some more components, tested a few voltages.

Thought maybe I hosed something up and the the power supply for V6/V7 had AC?  Checked that, nope.  Did notice the voltage on pin 2 of V7 has mysteriously jumped up to about 105VDC though.  Setting dependent maybe? 

So I tried Quarlo's test and yanked the V6 pin 6 wire and fed C128 to V7 with 1kHz next.  The horizontal gain pot has no affect since the signal is being injected after that point.  I do get a nice wide trace, and it is big time affected by the horizontal position pot.  The trace wants to be toward the left (and off the screen) more than it does center or right depending on the position of that pot.   And I need LOTS of voltage to feed the trace.  My sig gen is not calibrated, but I had to jack it up to the 10V setting to get a full-width trace.

Something is definitely up with V6/V7...
 

Offline Paul Moir

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Re: Heathkit IO-12 recap and a question
« Reply #60 on: May 16, 2021, 07:05:28 pm »
I can confirm you need a good strong signal at C128 to get a wide trace; I'm using the 60Hz "Line SW" setting and I get 24Vpp needed to get a trace that reaches the limits of the CRT.

Let me know if there is something you would like checked.

Pin 2 of V7 is going to be dependent on the exact position of the Horizontal Position pot.  It's not super twitchy but that's only a 10-15 degree turn of  the pot.

Have you tried shorting out the horizontal gain pot?  Connect pins 1 & 2 together (the two lugs closest the edge of the chassis) with the control turned fully clockwise?  I would like to rule out that suspected pot.

« Last Edit: May 16, 2021, 07:13:51 pm by Paul Moir »
 

Offline Quarlo Klobrigney

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Re: Heathkit IO-12 recap and a question
« Reply #61 on: May 16, 2021, 09:21:35 pm »
So I take it you have the same working scope to take measurements? That's a good thing. At least you can break the bad circuit in half at least, and half again until as they say in France viola...
« Last Edit: May 16, 2021, 09:31:50 pm by Quarlo Klobrigney »
Voltage does not flow, nor does it go.
“I do one thing at a time, I do it very well, and then I move on.”
 

Offline Paul Moir

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Re: Heathkit IO-12 recap and a question
« Reply #62 on: May 16, 2021, 09:28:54 pm »
Yes, have you not read any of my replies?  The problem is at V6A.
« Last Edit: May 16, 2021, 09:31:05 pm by Paul Moir »
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #63 on: May 17, 2021, 04:54:54 am »
Paul, thanks for confirming you DO need a nice strong signal to drive V7, I was unsure about that.  And thanks for having yours opened up and taking measurements for me!  A working test mule is the best mule!

In post #54 I said about V6 - "Pin 7 (tube side) to chassis ground - Horizontal gain pot min - 0K.  Horizontal gain pot max - 12K."

I would think that would rule out the pot, but I'm desperate so I went and tried your "shorting out the pot" test out just now anyway.  First I had to reconnect the gray wire coming from V6 pin 6 that I disconnected last night and had a little bit of an AHA! moment, as I had a bit of a tough time getting a good joint (dirty wire I reckon) and though maybe that joint was bad in the first place.

Powered on, same result.  Powered off again.  Pins 1 and 3 jumped, powered on again, same result.  Reflowed all joints on the horizontal gain and horizontal position pots and checked the connections on the gray, purple and brown wires (gray from V6 pin 6, purple and brown to coming off the horizontal position pot) from end to end and everything looks good there too.

V6 still simply refuses to conduct.  I'm out of ideas short of finding a nice tall building and gently heaving this thing off the top of it.
 

Offline floobydust

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Re: Heathkit IO-12 recap and a question
« Reply #64 on: May 17, 2021, 05:57:08 am »
Clearly you've offended the oscilloscope gods. Usually that tells me I need to change my technique lol.
If things aren't making sense then something got missed or a previous measurement is wrong.

In your pictures, what is that film cap 683K at V6 "Phase" going to the potentiometer and (-) of 47uF C127? There's no cap there on the schematic and none are that small 0.068uF?

edit: I found a pic of another IO-12 and it has a 0.05uF at V6 as well.
*Can you check your replacement capacitors are going to the right terminals on the two pots R52, R72? Looks different...
« Last Edit: May 17, 2021, 06:15:17 am by floobydust »
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #65 on: May 17, 2021, 06:27:37 am »
CLEARLY I've offended the 'scope gods!

Hmm....  ok, on the skit, over by the PT, two lead go to "to all tube filaments", then below that two leads go to FIL (A) and FIL (B).  I just looked over the heater wiring and it looks nothing like the skit.  The heater wires go right to the heaters and that 0.068uF cap goes from pin 9 of V6 to the phase pot.  Is that cap C41?  No idea where R73 and C42 are or if they even are.  On the skit I don't see where FIL (A) and FIL (B) go...  you are getting tired... and sleepy....   :=\
« Last Edit: May 17, 2021, 06:41:01 am by Rickenbackerman »
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #66 on: May 17, 2021, 06:31:24 am »
"*Can you check your replacement capacitors are going to the right terminals on the two pots R52, R72? Looks different..."

Check and check.  R52 left lug, R72 right lug as looking at your pic.
 

Offline james_s

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Re: Heathkit IO-12 recap and a question
« Reply #67 on: May 17, 2021, 07:28:34 pm »
Are we absolutely certain that the new capacitors are all the correct value? It's easy enough to select the wrong order of magnitude, especially when converting from vintage to modern nomenclature. It has happened to me more than once.
 

Offline floobydust

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Re: Heathkit IO-12 recap and a question
« Reply #68 on: May 17, 2021, 07:45:22 pm »
Why is there no wire connected to H-OUT ?

At this point, I would re-check all the wiring and recap capacitors, there's a mistake somewhere.
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #69 on: May 17, 2021, 07:49:44 pm »
At this point I am certain of absolutely nothing.

flooby, there is a wire at H-out - it's a gray wire that goes to C128 and comes up from the bottom.  You just can't see it from the top.

Later tonight I'll recheck AC and DC voltages around V6.
 

Offline floobydust

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Re: Heathkit IO-12 recap and a question
« Reply #70 on: May 17, 2021, 08:17:43 pm »
Once I found a component lead clipping floating around underneath a tube socket, you have to leave literally no stone unturned. It could be a problem from the original build, the solid hookup wires do break if flexed too much.
One approach that works for me is to trace through the schematic by using an ohmeter to verify each connection. Just red mark with a pen each connection. Old flux is not good for high voltages and high impedances, so don't leave chunks on the pcb bottom.

During re-cap projects I sometimes forget where I was at, distracted phone rings etc. and the wrong cap or wire happens. This is without any beer  :popcorn:
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #71 on: May 17, 2021, 08:47:26 pm »
Definitely found lead clippings floating around and lodged into places where they shouldn't be before!

The horizontal amp worked perfectly before, so unless I disturbed something we can be sure the original build was A-ok. 

Check on the old flux, too.  I use a tiny flat blade screwdriver and scrape it off between the traces and socket pins on the bottoms of the boards.  Did that a couple weeks ago all around V6/V7 already.

Paul if you get a chance can you take a close look at the pic I took that flooby reposted above and see if anything looks out of place if you haven't already?  The shot in post #46 is better, actually, but If you need any better shots just let me know.
« Last Edit: May 17, 2021, 08:49:33 pm by Rickenbackerman »
 

Offline Quarlo Klobrigney

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Re: Heathkit IO-12 recap and a question
« Reply #72 on: May 17, 2021, 10:04:09 pm »
1 question, in all these posts, have you cleaned all the pots with something like D5?
Also, check all pots in question with one lead of the ohmmeter on the wiper and the other to each extent. 2 tests.
I have seen pots go bad on one end of the rotation many times, believe it or not. The well traveled road and such, it wears out. Also the metal covers on those pots look like there is some corrosion.
« Last Edit: May 17, 2021, 10:07:28 pm by Quarlo Klobrigney »
Voltage does not flow, nor does it go.
“I do one thing at a time, I do it very well, and then I move on.”
 

Offline Rickenbackerman

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Re: Heathkit IO-12 recap and a question
« Reply #73 on: May 18, 2021, 02:51:59 am »
I never mentioned that I cleaned the pots?  Jeez...  Yeah, I D5'd 'em weeks ago, they all needed it badly.

Checked all connections and components around V6A for the 17th time.  Reflowed a few more joints and scraped a little bit more nasty old flux off the bottom of the board between traces.  All looks good.

V6A DCV:
Pin 1 - 126V
Pin 2 - 12.7V
Pin 3 - 62.2V

V6A ACV with the horizontal selector set to "Line SW":
Pin 2 - 1.7V
Pin 3 - 0V

The horizontal gain pot is fine, I'm AM certain of that.  I tested it about three different ways by now.

edit: I also retentioned V6 socket pins 1-3 and hit all of the tube pins with a spritz of G5 gold (the hi-temp stuff).
« Last Edit: May 18, 2021, 03:14:47 am by Rickenbackerman »
 

Offline Quarlo Klobrigney

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Re: Heathkit IO-12 recap and a question
« Reply #74 on: May 18, 2021, 03:47:47 am »
OK. If you wish to answer, what part of the US are you in?
Voltage does not flow, nor does it go.
“I do one thing at a time, I do it very well, and then I move on.”
 


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