This Heathkit IT-28 restoration article is intended to accompany a companion piece titled A Tester Every Restorer Should Own. You’ll find that article posted in the Restoration section of the website in the Restoration Tips subsection.
As you may have read in A Tester Every Restorer Should Own, I’m a big fan of this tester, primarily for its ability to test capacitors under working voltages up to 600 VDC.
Here I’d like to show those who aren’t comfortable working on test equipment just how easy a Heathkit IT-28 restoration can be. Have a look at the bottom side of the chassis in the photo below. Not too intimidating right? And the Heathkit manual does a good job of explaining the theory.
Like most Heathkit gear, this one arrived on my test bench in, well, less than perfect working condition. The eye tube lit up with a nice green glow, but the functions seemed erratic and I could only get the leakage function test to work on a few voltage settings.
If you’ve acquired many Heathkits, you probably already know that about 50% of them contain at least one wiring mistake, and 95% have at least one bad solder joint. I acquired a Heathkit Chippewa Linear a few years ago, and the guy made a nearly fatal wiring mistake. He messed up the regulator circuit consisting of a series string of OA2 tubes, which should have limited the screens on the linear’s 4-400 final output tubes to +800V. Instead, the tube screens got a full +3,000 volts applied. Not to put too fine a point on the observation, this was NOT GOOD.
The unit you’re looking at in this picture had no less than 7 bad solder joints on the voltage switch alone. Do you see the large rotary switch in the photograph? The guy who soldered those resistors only managed to get enough solder on the joint to cover the top resistor lead, leaving the resistor lead underneath completely unsoldered and twisting there in the breeze. Look at the 2-watt resistors between the main electrolytic capacitors in the photo above. Same problem. A desperate famine of solder. The only benefit here was how easy it was to replace the old electrolytic capacitors. I was able to remove them without even having to unsolder the leads! So if you acquire one of these testers, dig in and examine every solder joint with a good pair of magnifying glasses. The good news for beginners is that the circuitry is quite straightforward and the unit is easy to work on.
Here is an important restoration tip. Even if you decide not to replace all the old paper capacitors (which I HIGHLY recommend you do), at minimum replace the capacitor in the bridge circuit. It’s easy to spot on the schematic, as all the other bridge components are resistors. Even though my unit was one of the fairly late ones, Heathkit supplied two paper capacitors that were soldered in parallel for one leg of this bridge. That’s right. That last sentence was not an April Fool’s joke. What were they thinking? We’re talking leakage city! And that means an utterly useless, highly uncalibrated bridge. Get those parallel paper caps outta there and don’t ever look back. Put in a modern polystyrene or mylar. You will find this capacitor on the tube side of the chassis near the range switch.
There are only three potentiometers to adjust to align this unit (you can see them at the upper right in the partially restored chassis photo below). You simply watch the eye tube for a closure. It will take you about five minutes.
When you’re finished examining all the solder joints, replacing the old electrolytic and paper capacitors, and following the easy alignment procedure, you’ll be surprised how well this unit can work. Now, whenever I check a cap for leakage, I’ll flip the function switch to measure the capacitor’s value just to make sure it’s in the ballpark. If you are interested in better measurement accuracy, you might want to replace all the carbon resistors with metal film resistors, as many of the old carbon resistors have probably drifted significantly in value. My unit produced good ballpark values, so I didn’t go to the trouble of this extra step (my unit was quite clean and probably hadn’t been used for many hours).
I didn’t alter the design in any way, but some people replace the 6AX4 with a solid state diode to reduce heat. I didn’t do that, as my unit is usually on for only a few minutes to help preserve the life of the eye tube. Arden Allen suggested adding a 330K resistor across each of the power supply electrolytic capacitors (which are in series) to balance the voltage drop across each of them (as was considered good design practice with series capacitors to prevent over-voltage failures).