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
 DANGER This device contains a high voltage electrostatically deflected CRT with fully exposed high voltage PCBs. |
 WARNING This device contains a fully exposed open frame mains voltage power supply. |
- 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
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.
| 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 |
| 7 | 1000 μF | 35 V | Through Hole | Aluminum Electrolytic | Rubycon |
Horizontal Circuit Board
| RefDes | Qty | Capacitance | Voltage | Mount | Diameter/Size | Height | Lead Spacing | Temp | Type | Brand | Series |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 100 µF | 100 V | Through Hole Axial | 0 °C |
Replacement Parts
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.
| 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 / TAP106M035SCS / TAP106K035SCS | DigiKey Mouser LCSC |
| 7 | 35PX1000MEFC10X20 / UVR1V102MHD1TO / EKY-350ETD102MK25S | DigiKey Mouser LCSC |
Horizontal Circuit Board
| RefDes | Qty | Compatible Part Number | Order Links |
|---|---|---|---|
| 1 | 107TMA100M | DigiKey Mouser |
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.