by pebe » Thu May 29, 2014 2:59 pm
Here are the steps I would take to find the fault.
Switch on. Set voltage to around 15V. Set current to max. Connect a load that will bring on the fault (ie, reduce current to 5mA).
Bearing in mind its age, probably the most likely fault is that the main electrolytics have dried out. Measure volts across C30 – should be at least 30V. If not, most likely C30 and C31 have dried out. Measure voltage across C26 – should be at least 24V. If not C26 has dried out.
If OK so far, measure voltage between pins 4 & 8 of TAA243. Should be close to 18.6V. If 12.4V then check for voltage across 6.2V zeners GR28, GR29 and GR38 in case one has gone s/c. If a randomly low voltage, suspect TS26 that gives a constant current supply, or a component around it.
If OK so far, remove TS28. Connect meter neg lead to +ve power output (Bu3 on circuit) so all measurements are relative to that rail. Measure volts on LH side of R54 - should be less than 20mV. Measure volts at TS29 base – should be about 0.65V higher. If lower, measure pin7 of TAA243 – should be about 1.5V. If higher, suspect TS27 has an o/c base or R31 is faulty.
If pin7 is 1.0V or lower then check voltages on pins 2 & 3 of TAA243. If pin3 is higher than pin2, then remove C27 in case it is s/c. If still the same, then TAA243 is faulty (but can be replaced with another type).
Hope that helps, and I would like to know what the fault was.