Heathkit IO-12 recap and a question

[Rickenbackerman]

Hey guys,

I swear I posted this last night and today it's gone?  Moderators - did someone delete it?  Or did I not actually post it?

Been dealing with "scope wars" lately.  Had this old Heathkit IO-12 since college (I'm 45 now), it finally took a crap about 5 years ago so I bought a Tek 2235 off the site and shelved the Heath.  2235 took a crap last year and I spent a time recapping it and trying to bring it back to life to no avail.  There's a three page thread on that pig here somewhere - it's going back on the site for parts, I'm done with switch mode power supplies in "more complicated than I need" scopes is the short story.

So, back to the IO.  I figure it's all I need, and simple.  I just want to look at waves to see where tube guitar amps clip, I don't need stupid bandwidth and I don't even need it to be calibrated or even close to it - I just use an RMS DVM across the amp output to measure wattage at clipping.  When I shelved it, the issue it has was that the Y-axis would go spastic every minute or so and the screen would turn to noise - I'd flip the sync switch back and forth to get it to stabilize and then it would do it again in another minute.

So I just recapped it.  The big multisection can and both of the 1600V guys on the bottom were leaking goo everywhere.  I swapped all of the electrolytics and all of the film caps but I didn't check any resistor values yet.  Fired it up and I get a trace (yay!), and a sine with a sig gen hooked up.  The issue is the trace is only about an inch wide with the horizontal gain maxed.  I have a BK747B so I tested V6 and V7 and they're fine.   DeOxited all of the controls (they needed that badly!) and they all seem to do their job.   Any ideas? 

[WattsThat]

At the top of my priority list would be soldering. It probably has something to do with the fact that my first job out of high school was a bench tech at the local Heathkit store. Soldering, soldering, soldering. It’s all about the soldering.

Then be on the lookout for resistors that have drifted upward and well out of tolerance. It’s common with the carbon composition parts used back then. It seems to be more common with higher initial value parts from say 100K and up. Any one and two watters get checked first. But, they all are suspect for upward drift.

The film caps would have been at the bottom of my list.

PS: site was down for a while earlier today. Maybe the DB got rolled back and posts were lost.

[bob91343]

Recheck your work.  Measure the power supply voltages.

[james_s]

Hey guys,

I swear I posted this last night and today it's gone?  Moderators - did someone delete it?  Or did I not actually post it?

I noticed several of my replies to various threads vanished too and wondered the same thing, I don't believe there was anything offensive or forbidden in them so I suspect technical problems.

[floobydust]

I saw the post last night too, and it vanished today along with a thread about post counts dropping 1-5%.

+1 Check the power supply voltages, shorted caps usually take out the resistor. Is this the schematic to use?
Is your H pos'n in the middle or do you need the control cranked over, which indicates a lop-sided drive to the H-defl plates. How's the focus?
If you suspect a tube, you can also do swaps i.e. V5, V6, V7 are all 12AU7 I think.

[Quarlo Klobrigney]

Check the resistors, especially the wrinkled 1/2 resistors. They look rough on the outside compared to the standard AB resistors. They are notorious for being way off. I'm quite sure Heath used any surplus they could buy back in the day.

As far as posts disappearing, you are correct. I had 2 unread replies that vanished although they were in my email notifications.

[Rickenbackerman]

+1 Check the power supply voltages, shorted caps usually take out the resistor. Is this the schematic to use?
Is your H pos'n in the middle or do you need the control cranked over, which indicates a lop-sided drive to the H-defl plates. How's the focus?

I'm actually using an O-12 skit, the one you posted looks closer to the right one except my scope has a 12BH7 installed, not a 12BY7.  Speaking of the 12BH7, I tested ALL of the tubes just now and they all test good except for the 12BH7 which is weak.  I might have one in the tube stash...

H position is the middle, focus is perfect.  Every control seems to do what it's supposed to do.  I can get it to give me a really decent looking sine wave with my UNcalibrated IG-18 sig gen (I need the scope to work so I can calibrate it!), it's just that it's only an inch wide!  Couple of pics...

[Rickenbackerman]

Cap list in case anybody's interested:

Electrolytics:
Can G - four section - 40/450 20/450 20/450 50/300 - replaced with a CE manufacturing 80/525 40/525 40/525 20/525
Can F - three section - 20/20/20/250 - replaced with a CE manufacturing 20/20/20/450 - swapping this one out was tricky, it's PCB mounted and the tabs on the factory can are in a triangle and on the CE can they're in a | _ | shape, so it's kind of like stuffing a square peg in a round hole!
2) 40/150 axial - replaced with 47/160
1) 100/50 axial - replaced with 100/160

Film caps:
2) 0.1/1600 replaced with same
3) 0.25/400 replaced with 0.33/630
6) 0.1/200 replaced with 0.1/630
1) 0.05/200 replaced with 0.068/630
1) 0.01/400 replaced with 0.01/630
1) 0.05/400 replaced with 0.068/630
2) 0.1/600 replaced with 0.1/630
2) 0.2/200 didn't bother replacing these since they go to the horizontal presets which I never use and they look pretty hard to get at too!

I got everything from AES with the exception of the two 0.1/1600 caps which I had to get from mouser. Film caps are all the generic yellow polyester tubular guys except for the the two mouser ones.

[bob91343]

If the components associated with V7 are good and the supply voltage is proper, there must be insufficient input signal to the horizontal amplifier.  If you have another scope, measure the CRT drive to the horizontal plates.

[Rickenbackerman]

Unfortunately I don't have another scope...

I did some more poking around.  Checked the obvious stuff first like double checked my work (solder joints, make sure the 'lytics are pointing the right way, etc), everything looks good.

I rolled V5/6/7 around a couple times with no change.

So I poked around V7 a bit.  The two 100 ohm grid feeders - one has drifted to 110K, the other 130K.  The 12K 2-watter from pins 8/3 to ground is 13K.  The big 33K 3-watters on the plates are 35K.

Voltages on V7 (and what's on the skit I have in parentheses):
Pin 6 - 254 (260)
Pin 1 - 267 (260)
Pin 7 - 38 (43)
Pin 2 - 74 (74)

Those look pretty good, no?  I will poke around V6 next...

[Rickenbackerman]

Measured all of the resistors on V6, they all looked good. 

V6 Voltages (with skit value in parentheses again):
Pin 1 - 128 (120)
Pin 2 - 13 (14)
Pin 3 - 64 (62)
Pin 6 - 73 (68)
Pin 8 - 2.2 (2.4)

It's funny how this thing never gave me any horizontal gain issues when I shelved it, it was the vertical amp giving me problems.  Now the latter is fine and former is broken!   

[bob91343]

The horiz amplifier seems to be okay so it's apparently not got enough input signal.

[Rickenbackerman]

So that begs the question then, where does the input signal come from?  i.e. where do I look next?  Working on tube guitar amps I'm good at.  Tube scopes, apparently not! 

[Quarlo Klobrigney]

Which input signal? The vertical amplifier or the horizontal sweep?
There are 2 more. The Z axis and the sync.

[james_s]

The problem is with the horizontal sweep, so I think it's obvious that's the signal he's talking about.

[floobydust]

Where is the SYNC SELECTOR set to, does it have any effect?

Do the Heathkit test - switch HOR/FREQ over to EXT. INPUT and connect 1Vp-p OUT to the HORIZ INPUT banana jack there (keep intensity low). This bypasses the sweep oscillator and would indicate whether the H-amp V6 and H-defl V7 are working- although those stage's voltage readings look fine. Max. HOR. GAIN is supposed to give 1-1/4" wide trace.

I would measure sweep osc. V4 and V5 voltages.

[Rickenbackerman]

flooby - you read my mind - I was just looking at the skit thinking "I need to look at V4 and V5 next"...

If that all looks good I will do the Heathkit test...

Oh and the Sync Selector is set to +INT - I flip it to EXT the trace kind of "jumps" vertically and then settles back down.

[Quarlo Klobrigney]

I never assume.
Is there 370V at the junction of the 2 33K 1W resistors feeding the plates of V7?
Are the 2 33K resistors good?
So on and so on...

[Rickenbackerman]

V6 and V7 pin voltages posted above...

So...  I was looking at the schematic earlier today and thought I MUST have messed something up with the three-section 20/20/20 can so either V4, V5 or both weren't getting juice.  So I just poked at some more pins...

V4 Voltages (with skit value in parentheses again):
Pin 2 - 19 (14)
Pin 5 - 0.4 (1.4)
Pin 6 - -0.5 (0)
Pin 7 - 2.0 (2.2)

V5 Voltages (with skit value in parentheses again):
Pin 1 - 100 (110)
Pin 2 - 88 (95)
Pin 3 - 91 (95)
Pin 6 - 62 (52)
Pin 7 - -1.5 (-0.5)

Looks like I hooked that cap up correctly and those look pretty good too, right?  So what gives?  Try the Heathkit "adjustments" test next?

[Quarlo Klobrigney]

What are V7's cathode (s) at? I can't seem to read the schematic that was posted, but it also looks like the individual anodes should be at 230V not 260V indicating lack of plate current to pull them lower (drive) or their cathodes are higher off of ground than they should be (off value 12K 2W). Also what is point AA at in voltage?
Here's a later schematic of the H sweep area with reference numbers and a bit clearer.

[Quarlo Klobrigney]

Insomnia a wonderful thing I had time to separate just the schematic:

[floobydust]

V6 and V7 pin voltages posted above...

You're missing voltage measurements    there should be 3 per triode, 6 per tube. I phoned the Psychic Alliance and they want $4.99 a minute to do crystal ball readings.
I thought the sweep oscillator's amplitude is reasonable because blanking is working.  But the trace seems squashed on the left so the ramp linearity might be poor.

[Quarlo Klobrigney]

I was going to say that but I didn't want to be the probe nazi

[Rickenbackerman]

GUYS!  I know I missed a couple, my fault!  But we're all looking at different skits and there's not a voltage reading on every pin on every skit!  I'm not that much of a nutball, I swear!   

Hang on, lemme check V7 cathodes...  I "think" those are the only ones I missed according to the skit I'm looking at (O-12).  I checked the 12K  2watter and read 13K but let me check voltages quick...

V7 Pin 8/3 - 86V

"Psychic Alliance" and "Probe Nazi" - I'm cracking up!   

flooby when I had my sig gen plugged in and I was playing around with the settings on both the scope and the gen I could definitely see signs of the trace being MASSIVELY squashed on the left at certain settings.  Is that a clue?

Oh and THANKS for the help here guys!  MUCH appreciated!  I will not give up, just yet...

[floobydust]

I'm using the IO-12 schematic, same as IO-12U and looks similar to the predecessor O-12? Voltages are listed for every pin.

Usually you start a repair by first measuring all the power supply voltages. V10 is a master shunt regulator so CC needs to be 135V and not far off, EE GG for the sweep osc. and AA FF for the H-amp.