Heathkit IG-42 Signal Generator Restoration – Part 4

Cabinet Repair


Now the the IG-42 was in good electrical order, it was time to move on to the cabinet repair.

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Heathkit IG-42 Signal Generator Restoration – Part 3

Line Cord and Chassis repair —

To continue on with the restoration, I replaced the ceramic disk filter capacitors on the line cord input with safety capacitors.  These are designed to fail to an open state, rather than a shorted state.  We hope that they never fail, but if they do, it should leave the chassis safely isolated from the line voltage, regardless of which way the old style plug is plugged in to the wall.

Line Cord safety caps installed

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Heathkit IG-42 Signal Generator Restoration – Part 2

Electrolytic Can Capacitor Re-stuffing

Next on the repair plan is to pull the 3-section electrolytic can capacitor, and re-stuff it with modern electrolytic capacitors.  This will ensure reliable operation for decades into the future, while preserving the looks of the old equipment.  I unsoldered the two resistors, the .05 uf capacitor and the various wires from the terminals of the can, being careful to note what each terminal had for connections.  Once that was done, I was able to unscrew the two screws holding the can to the chassis, and remove the can.

Electrolytic Can Capacitor removed from chassis

The can bracket lifted off, after un-twisting the four tabs on the can.

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Heathkit IG-42 Signal Generator Restoration – Part 1

The first fix applied to the IG-42 Signal Generator was the replacement of the 0B2 regulator tube. With the new old stock replacement, the B+ voltage dropped from 131 vdc to about 107vdc. The 0B2 is a 108 volt regulator, so that should be about right.

The new tube voltage chart is displayed below. I have not replaced any resistors or capacitors at this stage, so these readings may change again in the future, but they are much closer to specifications, now.

TUBE Pin1 Spec Pin1 Actual Pin2 Spec Pin2 Actual Pin3 Spec Pin3 Actual Pin4 Spec Pin4 Actual Pin5 Spec Pin5 Actual
6AF4 65 71.9 10 – 30 Neg. -11 0 0 5 – 7 AC 5.9 0 0
6AV5 0 0 5 – 7 AC 5.9 12 12 0 0 100 112
12AU7 60 70.2 1.0 Neg. -1.2 2 1.86 5 – 7 AC 5.9 5 – 7 AC 5.9
OB2 100 106.8 0 0 0 0 0 0 100 106.8
TUBE Pin6 Spec Pin6 Actual Pin7 Spec Pin7 Actual Pin8 Spec Pin8 Actual Pin9 Spec Pin9 Actual    
6AF4 10 – 30 Neg. -11 65 71.6            
6AV5 0 0 0 0 100 106.7        
12AU7 100 112.3 30 34 33 35.7 0 0    
OB2 0 0 0 0          

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Heathkit IG-42 Checkout – Part 2

I put the IG-42 Signal Generator back on the bench to continue my initial inspection.  Today, I measured the voltages specified in the construction manual and measured the resistors (in circuit) to see if there were any obvious problems to address.

Some of the tube pins were very difficult to access from the bottom side, so I stuck a wire in the top side on those pins and re-inserted the tube to get the readings.  This allowed me to continue without tearing the unit apart.

Cheater Wire for measuring tube voltages

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