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

[Clif Holland]

Look around and tighten the ground screw.  They didn't hardwire the ground. 
In their infinite wisdom the ground for the power supply is thru a screw 
that comes loose and causes all sorts of problems.

Clif Holland KA5IPF
www.avvid.com




----- Original Message ----- 
From: "John Hagle" <jhagle at madbbs.com>
To: "ICOM Reflector" <icom at mailman.qth.net>
Sent: Wednesday, November 01, 2006 9:28 PM
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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