Many will remember the monthly stories of the servicemen's difficult jobs, as told in the pages of Electronics Australia magazine.
This is my frustrating servicing experience of 2010.
I was engaged to restore an AM in-dash car radio as part of a classic vehicle restore/rebuild.
The radio in question was an Australian made Ford badged "Super-Fringe" model; c 1960/70's
A standard Superhet design of the time but with an RF amp stage and two IF stages (giving enhanced Sensitivity and Selectivity - as compared to lesser designs)
As most of these (and getting hard to find) car radios are 40+ years old, they will need servicing.
This particular unit, although working, wouldn't produce any audio o/p until several minutes after switch-on ??
My policy now, is not to bother to try and fault-find and repair, but to simply replace ALL active components; resistors, capacitors, electrolytic's, transistors etc.
Many components will be out of tolerance, noisy, leaky capacitors, low gain transistors etc.
Quality service components are still readily available and are inexpensive.
Total component replacement guarantees reliability and ensures that the radio is as good as new and won't need any more attention for a long time.
The original soldering was of a questionable 'standard' with many 'dry' looking joints, and excessive solder loading.
The permeability tuner and other controls were "de-gunked" and re-lubed.
Sourced from the 1967 XR Ford Falcon workshop manual: the schematic for this model showed PNP transistors, but by observation NPN devices had been (factory) fitted.
A redrafted circuit (below) shows NPN devices, a positive supply rail and additional info and measurements.
The transistors were early Australian made AWV branded metal enclosure types.
They bear type numbers like AS148, AS300, AS301, AS311, for which I could find no data anywhere ?
I have used the well known Silicon NPN BC548 as a suitable replacement.
Upon re-assembly the radio would not function at all, despite vigilant checking/testing of the repair work.
Eventually, I realised that VT4 (Mixer) having a 68K base bias resistor, would not be "biassed-on" - reducing down to 15K made the mixer work again!
Also VT5 (1st IF amp) originally has no base bias resistor at all?
When I fitted a 47K b/w base and supply rail, this stage functioned as it should!
How the original transistor amplified the IF without any bias is a mystery to me,
maybe a leaky type of transistor (self biassing!!)
Hopefully this may help anyone carrying out a similar repair
This unit was on my bench for a very very long time until I "nutted out" all the problems!