Heathkit HW-7 part 3 – resistors

Before I start doing the fun stuff like adding a new mixer or tuning the audio filter I thought that I had better get down to basics.

Earlier I mentioned that vintage carbon resistors tend to fail by slowly increasing their resistance value and the only resistor I had removed so far showed signs of that. It also had an intermittent fault on the receiver where the mixer appeared to cut off when the PCB was pressed.

Today I removed all the old carbon 10% tolerance resistors and replaced them with modern metal oxide 1% resistors. I measured the value of each old resistor to compare it to it’s nominal value.

It took a bit longer than my lunch time to do it, it felt like I was building the kit from new!

Only two low value resistors remained within the 10% tolerance rating. All the others were 20 to 30% higher in value than what they should have been.

It was worth doing as testing after the work revealed that the receiver sensitivity had increased considerably and the VFO stability was improved especially immediately from switch on. The intermittent fault had gone so it was probably a dry joint on one of the old resistors.

With the correct resistor values in circuit it’s probably now working as the Heathkit design engineers intended back in 1972.


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