Heathkit IO 4550

The Heathkit IO 4550 is a dual trace 10 MHz oscilloscope sold as a kit for the end user to assemble themselves.

Known Issues

 * Unit emits smoke and/or a loud bang when powered on
 * Several tantalum capacitors on the power rails fail short circuit
 * Solution: Replace all faulty tantalum capacitors on the +/-15 V rails and repair any damage

Disassembly Notes

 * Warning: this device contains exposed mains and CRT drive high voltage components, ensure capacitors are discharged before working on the unit.
 * It is recommended you power the +/- 15 V rails with an external bench power supply prior to plugging the device into mains to be able to limit the current draw to prevent damage to shunt regulators from shorted tantalum capacitors.
 * The PCB does not need to be removed from the chassis to replace capacitors, it is difficult to desolder/solder some capacitors with the board still in the chassis however in order to remove the PCB a number of flying leads and the front panel knobs will probably have to be removed.

Vertical Circuit Board (85-1989-2)
While Healthkit devices come with an assembly manual and components list unfortunately digitized versions on the internet are missing page 27 which contains information about those faulty tantalum capacitors. My unit also included orange tantalum capacitors on other PCBs in the unit however those tested good. The failure of this unit was while it was in the hands of the previous owner, it failed slowly overheating and damaging resistors that form part of a shunt regulator until it failed catastrophically, the previous owner had unsuccessfully attempted a repair by only replacing the one tantalum capacitor that had exploded.

Vertical Circuit Board (85-1989-2)
Note that those capacitors are not in critical analog parts of the oscilloscope circuit but are used for power rail bulk capacitance. As a result you can pretty much substitute any modern 10 μF 20 V capacitor, I used wet electrolytics that were laying around as the unit is unlikely to be used much, however below are some equivalent tantalum capacitors that should be considered if doing a proper restoration.

If parts are not available or different selection is preferred, you can use the values in the Original Parts section to perform a parametric search.