Friday, December 18, 2009

Servicing Heathkit IO-12 Oscilloscope

Few months back I got an old Heathkit IO-12 Oscilloscope from my friend and at the time of delivery this unit is not under the working conditions. Heathkit IO-12 is Value based laboratory oscilloscope with following technical features,

Frequency Response : 3Hz to 4MHz ≅ 5MHz
Sensitivity : 0.025V/inch
Rise time: 8┬ÁS
Sweep Range : 10Hz to 500kHz

When I remove the casing of the oscilloscope I found several damaged components (specially capacitors) and wires. After making the careful observation and testings I decided to rewire the whole oscilloscope with new components except the following,

  • 12BH7 Dual Triode
  • 12AU7 Dual Triode
  • 6AN8 Triode Pentode
  • 6AB4 Triode
  • 1V2 ½ Wave Rectifier
  • 6X4 Full Wave Rectifier
  • 6J6 Dual Triode

  • Main Transformer
  • Front end controls (including switches, potentiometers, connectors and indicator)
  • Most of the inductors
  • Cathode Ray Tube, etc

As mentioned in the above list all the Values, transformers and some of the critical components are in good condition. Only the major problem occurs at the electrolytic capacitor replacement stage(s) because in original configuration there are two capacitors packs (4 Pin capacitor package with common ground) are available. It is bit difficult to obtain those capacitors from local market. So what I'm doing is installing 4 separate electrolytic capacitors, instead of one single electrolytic capacitor pack. To do this I'm slightly change existing component layout of the original IO-12 Oscilloscope. Except these capacitor modules all other parts are commonly available in local market. (with some slight differences in packages and values)

After the rewiring, this unit work nicely and produce results as same as today's professional analog oscilloscope. At the time of testing I couple this unit with one of my old Phillips GM4585 trigger delay unit and achieve successful results. The above photograph indicate this restored IO-12 oscilloscope with 50Hz - 12.75V peak to peak sine wave.
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