Electronics > Repair

Heathkit IO-12 recap and a question

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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? 

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.

Recheck your work.  Measure the power supply voltages.


--- Quote from: Rickenbackerman on May 02, 2021, 07:38:46 pm ---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?

--- End quote ---

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.

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.


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