Heathkit HO-10 monitor scope

The photos

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Most people don't know but the voltages lurking inside an 'oscope can knock you on your ass. Be careful.

The problem

Heathkit loved to build oscilloscopes. They carried this tradition on when designing their station monitors. The classic HO-10 station monitor is basically just a stripped down audio oscilloscope.

Heathkit continued to offer station monitors up to the HW-5400 series with the HO-5404 station monitor.

The best looking of the model is (in my opinion) the SB610. And this is where the story begins.

This particular monitor had but one problem. The horizontal width of the trace was only about one inch long. No matter how you adjusted the horizontal sweep or position, the trace remained the same.

The first place I looked is the horizontal amplifier. Since I had a trace, had sweep and had vertical gain, this seemed logical to me.
The horizontal amplifier consists of 1/3 of a 6C10 tube. The tube was replaced with a known good tube with no joy.

A quick check of the plate voltage showed the problem. Instead of 400 vdc, I had about 35 vdc. And just to make things more interesting, my HP 34401A would click. This meter has relays inside that select the proper range to be displayed. This told me that the voltage was changing from a low voltage to a high voltage. A quick check with my scope showed the voltage changing.

My first thought was the .25 coupling capacitor from the plate was oscillating and drawing the voltage low. A quick shot of the soldering iron pulled one end of the capacitor out of the circuit. A second voltage check showed the problem remained.

The only other part connected to the plate is a 330K resistor. This resistor feeds voltage to the plate directly from the power supply. On the power supply side, I had the required 600 vdc. The other side of the resistor was 40 vdc. This is what you call a “ah ha” moment!

A new resistor was installed, the .25 cap soldered back and the scope powered up. Now I had a nice LONG trace that I could move and change with the horizontal controls.

Problem fixed!

Then two weeks later the scope went kaput again. This time the trace was really faint and was out of focus.

Back out of its case, the problem was really clear, a 330K resistor had smoked almost in two. A new resistor was installed and the scope powered up. NO trace this time at all.

Using my high voltage probe, the voltage is MINUS 1500 vdc, I read only 75 volts. Switching the meter to read AC, the transformer was still working ( It’s a known problem with Heathkit station monitors that the high voltage winding of the power transformers go open or short to ground) I was lucky as I still had the +600 vac on the output.

The only thing it could be was a open diode. Removing one lead of the diode out of circuit and checking it with my meter showed the diode had opened up. The second diode was checked and it too was open. ( NOTE! During assembly, someone forgot to solder one end of the diode to the solder strip. It was just crimped on the lug, but no solder was holding the connection. Was like that from the day the scope was assembled!)

Not having the original high voltage diodes to replace it, I used two 1N5408 diodes (rated at 3 A @1000 vdc ) in series. The new diodes were installed and the scope powered up. Within a few seconds, the new 330K was smoking.

I removed the plug from the scope tube. This keeps any problems with the tube away from the rest of the circuit. At power up the resistor still smoked.

The only other componet connected to this part of the circuit was one 1 Mfd cap at 1600 vdc. Removing this capacitor allowed the voltage to pop up. A quick check with the meter showed the capacitor was almost a dead short.

I replaced it with a pull out cap I had in the junk box. Will need to track down suitable new capacitors and replace the entire string.