Heathkit IO 4550

Heathkit IO 4550

Preliminary Information
Release Year	1978
Leak Risk	Unknown
Batteries	None
Mounting Technology	Through Hole, DIP Sockets, Flying Lead
Capacitor Types	Electrolytic, Tantalum, Paper, Ceramic, Polymer, Mylar, Mica
Destructive Entry	No
CRT Guides
General Information · Repair Cautions

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

Cause

• Several tantalum capacitors on the power rails fail short circuit

• 

  Visible and thermal images of a tantalum capacitor that has failed 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.

Original Parts

Vertical Circuit Board (85-1989-2)

Failed through hole tantalum capacitors

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.

Capacitor Specifications and Footprint Information

RefDes	Qty	Capacitance	Voltage	Mount	Type	Brand
C114-1, C114-2, C115-1, C115-2, C116-1, C116-2, C123-1, C123-1, C124-1, C124-2	10	10 μF	20 V	Through Hole	Tantalum	KEMET

Replacement Parts

Vertical Circuit Board (85-1989-2)

Heathkit IO 4550 before and after capacitor replacement, failed tantalums circled in red.

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.

Capacitors

RefDes	Qty	Compatible Part Number	Order Links
C114-1, C114-2, C115-1, C115-2, C116-1, C116-2, C123-1, C123-1, C124-1, C124-2	10	T356E106K020AT	Digikey

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.