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

[Don Moman VE6JY]

Well, the bridge in the R-71 is a common cause of exactly what you are
describing with your R-70.  The R-71 bridge is run close to the ratings
and one diode sometimes goes shorted, causing quite a strain on the
transformer, certainly making it run HOT.  It may also explain the 60 hz
ripple you saw.  I am less certain that it would short intermittently
but (almost) anything is possible.   If you are "in there" anyway, it
may be worth putting a heavier bridge in there, and making sure the
thermal compound is adequate.  

The other comments put forth certainly are valid concerns as well.

73 Don
VE6JY

-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net]
On Behalf Of John Hagle
Sent: Thursday, November 02, 2006 3:29 AM
To: ICOM Reflector
Subject: [ICOM] power supply trouble with ICOM IC-R70 Receiver


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





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