[ICOM] power supply trouble with ICOM IC-R70 Receiver

[John Hagle]

I am having some trouble with an old Icom IC-R70 receiver. From time to 
time it develops a loud hum and the S-Meter light goes dim. The problem 
is probably in the power supply.

The power supply is conventional, non switching. It uses a power 
transformer tapped for both 110vac and 220vac feeding a bridge 
rectifier, filter capacitor (47,000mfd) and a three transistor regulator 
circuit including what appears to be current limiting circuit followed 
by some additional RF bypass caps and a small electrolytic. Pretty 
standard stuff.

However, the condition is intermittent. It is not sensitive to a bump or 
to thermal changes. It occurs at completely random intervals, hot or cold.

At first I thought "electrolytic" but now I am not so sure. When in the 
working condition there are several volts of saw-tooth ripple at the 
output of the bridge. However, the ripple is at 60Hz rather than the 
120Hz I would expect. Again, this is when the radio is working. And when 
working, the raw B+ measures about 15 volts.

If a single diode in the bridge rectifier is open, then the bridge would 
function as a half wave rectifier and the ripple would be at the AC line 
frequency of 60Hz. If a second diode were to open, then the bridge could 
still function as a half wave rectifier.

With the hum condition in the radio, the raw B+ measures only about 7.5 
volts. And I believe that the saw tooth ripple voltage is still 60Hz. (I 
didn't get the scope probe on it in time).

Are these bridge diode packages known to be intermittent like this in 
older Icom receivers.

Or should I be looking elsewhere.

thanks in advance

de N2JH
ex AA2GV es WA2SXH